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1 SCR DRIVE TRAINING SYSTEM OPERATION AND MAINTENANCE MANUAL SECTION 4 SCR UNIT

2 TABLE OF CONTENTS 1. OPERATION SPECIFICATIONS Electrical Mechanical Forced-Ventilation System SCR Enclosure Circuit Breaker Overtemperature Rating MAINTENANCE TROUBLESHOOTING SCR LOGIC INDICATOR THEORY OF OPERATION SCR Bridge Circuit Breaker Current Feedback RLC Filter Firing Pulse Voltage Feedback Regulator Power Supply Surge Suppression Circuit DC Ground Fault Detection Circuit Contactor Control Logic DC Module DC Regulator SCR Firing Circuit DC MODULE OVERVIEW BASIC FUNCTIONS DC MODULE POWER SUPPLY MODULE INPUT CIRCUITS TYPICAL ASSIGNMENT FIRING CIRCUITS POWER LIMIT...38 Page 1 of 49

3 4.7 MOTOR SPEED REGULATION MANUAL BRIDGE PHASE UP TESTING SPEED CALCULATOR CIRCUIT DC SLIDE SCR CURRENT FEEDBACK METERING CALIBRATION VDC CONTACTOR SUPPLY REPLACEMENT AND REPAIR SCR REPLACEMENT SCR Clamp Tightening Procedure BLOWER REPLACEMENT...48 Page 2 of 49

4 1. OPERATION The rectifies the three-phase AC supply to provide a continuously variable DC supply to the traction motors. The SCR bridge, which performs the rectification, is isolated by a circuit breaker from the AC bus. The bridge output is assigned to one of the motors via contactors. The contactors are closed in pairs (DC + and DC-). The contactor logic and the DC voltage level are controlled from the Operator s Console. Electronic circuits in the DC control module regulate the voltage and current within preset limits. All s are typically identical. If one unit is down, another is usually available to maintain power to the motor. Similarly, the electronic DC Modules and SCR cells of the bridge are interchangeable. 1.1 SPECIFICATIONS Electrical Three Phase AC Input: Voltage: 600 VAC Frequency: 50 Hz DC Output per Voltage: x the rated AC supply voltage Current: 0-to current limit DC Amps continuous at stall through maximum volts. Different limits are selected to allow maximum horsepower and torque to be obtained from the Equipment without exceeding Manufacturers ratings. Refer to label on top of each DC control module to verify current limit settings Mechanical a) Cubicle The assemblies are mounted within the cubicle, on the side and door panels and a slide pan located below the blowers. b) DC Module Many of the electronic circuits associated with SCR rectification and control are assembled on a single printed circuit (PC) card. The card and associated components are housed in a module constructed from 12-gauge cold-rolled steel. The module has its own heat sink. Size: 100mm W x 300mm D x 300mm H Weight: 9.5 kg Page 3 of 49

5 1.1.3 Forced-Ventilation System The ventilation system consists of one or two forced air blowers located below the SCR bridge. a) Air Flow Rating linear feet per minute through each SCR cell. b) Each bridge requires a motor to power the blowers. Each motor turns two blowers connected at either end of its shaft. Newest designs require only one fan per cell assembly. Voltage: 600 VAC single phase, 3 phase, or 240/120 VAC single phase Current: Approximately 1.0 Amp at 600 volt or 3 amp at 120 VAC Speed: 1800 RPM c) Electrostatic Filters are located on the front or back below the bridge. They consist of expanded aluminum gauze enclosed in a metal frame. They have an MBS (Master Bureau of Standards) rating of 12 to 15 percent, which is sufficient to trap common dust particles. Note: Filters are used when required SCR Enclosure The SCR is mounted within a set of enclosures designed to provide insulation and heat transfer, and to dampen mechanical vibration. The entire assembly is called the SCR enclosure or cubicle. The innermost assembly consists of the SCR enclosed on either side by aluminum heatsinks. A two-bolt clamp is tightened to press the heatsinks into the SCR. Pressure exerted on the SCR is indicated by a gauge located on the top side of the enclosure Circuit Breaker The circuit breaker has an overcurrent magnetic trip circuit. It also has an undervoltage (UV) or shunt trip circuit which is triggered for fuse failure and SCR overtemperature Overtemperature Rating. The overtemperature switch mounted on each SCR stack is set at 74C and located at the furthest heat sink point from the airflow source. Page 4 of 49

6 2. MAINTENANCE This section contains specific functional tests to assure proper operation of the SCR unit. Perform the tests after repairing or replacing any of the unit assemblies. If the unit fails to perform as indicated, refer to the troubleshooting index in Section III to locate the malfunction. Refer to Section III Troubleshooting for explanation of the various SCR unit circuits, and Section IV Removal and Repair for identification of the components. The DC Module test select switch (Fig II-1) enables a quick check of the contactor (CONT) and throttle reference (REF) signals received from the control console. The switch and test meter are colour coded. For example, if the select switch is set to one of the yellow CONT positions, the test meter needle will deflect to the yellow band to indicate a normal condition. Fig 2.1: DC Module Front Panel The SCR bridge can be phased up for testing by setting the manual voltage switch to ON and rotating the knob clockwise. In the ON position, the switch opens all assignment contactors so that power is not applied to the DC traction motors. CAUTION NEVER SWITCH THE DC MODULE TEST SWITCH TO THE ON POSITION IF THE SCR IS ASSIGNED AND RUNNING A DC TRACTION MOTOR. Page 5 of 49

7 The SCR bridge voltage can be monitored at the SCR volts test pins on the DC module. The ratio is 15:1 such that the test pins will be at 50 VDC when bridge voltage is 750 VDC. SCR Amps test pins should register 2.66 VDC per 1000 amps bridge current. The zero throttle interlock light indicates operational status of a circuit in the module that sup-presses the firing reference for the SCR bridge. When the light is on, the bridge voltage will be 0V. The light is on under two conditions listed below: is on but not assigned to any of the DC functions. is assigned to a function before the throttles are set to 0. The light goes off when the throttles are set to 0 and assigned contactors pull in. Page 6 of 49

8 1. Preliminary ACTION a) Ensure that to be tested will not be assigned from the Control Console. b) Energise the AC bus, if required. c) Close the SCR circuit breaker. 2. SCR Bridge Check a) On the DC Module, set the spring-loaded Manual Volts switch to on. b) Rotate the Manual Volts knob slowly clockwise to Max. and back down to Min. 3. Contactor and Throttle Check a) Trip the SCR breaker. b) Jumper the auxiliary contact of the breaker c) Set the Control Console assignment switch to various positions. d) Remove aux. contact jumper when all CONT and REF signals are checked out. 4. Module Power Supply Check Energise the AC bus. 5. Firing Pulse Check a) Ensure that under test will not be assigned at the Driller s Console. b) Open the SCR circuit breaker. c) Set the spring-loaded Manual Volts switch to On and turn the Manual Volts knob a ½ turn. d) 4. After completing the test close SCR breaker. Be sure Manual Volts switch is off. RESULT a) SCR On light will glow. b) Blowers under the SCR bridge will switch on. c) On DC Control Module, Power On and Zero Throttle Interlock lights will glow. Observe the front panel DC voltmeter. The voltage will climb smoothly to 750 VDC as the knob is rotated clockwise, and return to 0 as the knob is rotated counterclockwise. This will allow us to close the assignment contactors without applying power to the motors. In each position, check contactor (CONT) and Throttle Reference (REF) signals from Control Console. Example: Suppose RT is assigned to the in 1 o clock position. Then RT CONT (pin 129) and RT REF 1 (pin 130) can be checked. Confirm following voltages at DC Module. Voltage Location 12 VAC Pins VDC Pin VDC Pin 154 SCR On light will glow and blowers will switch on. Check firing pulses to each of the 6 SCRs with an oscilloscope. Example: Place the ground lead on K terminal of the Firing Pulse transformer and Probe on the G terminal. Page 7 of 49

9 ACTION 6. Feedback Check Check ripple of the Voltage Feedback signal across SCR Volts test pins on front of DC Module. RESULT The feedback waveform provides an indication of firing on the SCRs. Note that there are six peaks. Each SCR contributes a peak. Bridge Voltage Feedback 50VDC from 101(+) to 102(-) for 800V across SCR 2ms per division Good Bad Bad Page 8 of 49

10 ACTION 7. Isolate Faulty SCR a) This is accomplished by comparing the Current Feedback ripple and SCR firing pulses on a dual trace oscilloscope while the is driving a load. b) During each 60Hz cycle, there are six peaks in the ripple. Each of the six SCRs in the bridge contributes a peak. c) If one of the SCRs does not fire, a peak will be missing. If the SCR misfires, one of the peaks will be distorted. d) To identify the faulty SCR, compare firing pulses (pins 140, 142, 144, 146, 148 and 150) to each of the six SCRs with the current feedback ripple (pin 131). e) Firing pulse which is in sync with the missing or distorted peak is the one going to the faulty SCR. Good RESULT Bad - misfiring SCR - check gate pulses Bad - SCR firing out of sync Page 9 of 49

11 ACTION 8. SCR Resistance Test a) Trip the SCR circuit breaker. b) Switch Multimeter polarity to + and the scale to R x 10,000. c) Test the meter by joining the + and - leads. d) Disconnect the SCR Gate (G) and Cathode (K) leads. The SCR does not have to be removed from the heatsink. e) Measure resistance across the SCR by touching one lead to the AC bus bar and the other lead to the DC bus bar. f) Measure resistance across the SCR in the reverse direction, interchanging the leads. g) If the SCR is not installed in heatsink, lacks pressure, it will not be possible to make resistance test. RESULT The meter needle will swing to indicate 0 ohms. It should be in the Meg ohms. The SCR is leaking if the resistance is less. You will see the charging action of snubber capacitor that is across SCR. Let meter stabilise before taking final reading. Again, the resistance should be in the Meg ohms. Page 10 of 49

12 3. TROUBLESHOOTING Troubleshooting consists of locating a malfunctioning component in the. The troubleshooting index in the back of this section provides specific instructions. The Theory of Operation section provides an explanation of the various circuits in the unit. 3.1 SCR LOGIC INDICATOR ZEEFAX SCR systems are fitted with SCR logic indicators as standard. These provide a mimic diagram of the SCR control and assignment logic which helps rapidly identify the cause of any problem. The top section contains LEDs which indicate the status of the blower, SCR fuses and temperature switches. The lower section shows the logic sequence from the DC module CONT signal (pin 114), through the circuit breaker, returning to the DC module via the assignment contactors and auxiliary interlocks. The SCR logic indicator is the first point of call when troubleshooting and SCR. Fig 3.1: SCR Logic Indicator 3.2 THEORY OF OPERATION SCR Bridge A 3Ø AC supply from the bus is applied to the SCR bridge through a circuit breaker. Each AC phase is connected to two SCRs. One SCR feeds positive AC portion to the DC (+) bus and the other SCR feeds the negative AC portion to the DC (-) bus. For example, ØA is connected to A+ and A- SCRs. A + SCR feeds the DC (+) bus while the A- SCR feeds the DC (-) bus. DC (+) and DC (-) buses are connected to loads via assignment contactors. The SCRs are switched on and off to vary the DC level through firing pulses applied across the GATE (G) and CATHODE (K) terminals of each SCR. The firing pulses are generated in the DC Control Module. Refer to unique devices section for a general description of an SCR Circuit Breaker. The breaker is equipped with a set of auxiliary contacts. A normally open contact is interlocked with the front panel SCR ON light. Another normally open contact is a part of the Assignment Contactor logic. The breaker also has an undervoltage (UV) trip coil. The breaker is tripped automatically if the power supply to the coil is interrupted. Positive terminal of the coil is permanently connected to a +14Vdc power supply. The negative terminals is connected to a -14Vdc supply through various normally closed switches which signal the following hazardous conditions. Page 11 of 49

13 a) SCR Overtemperature. There are two temperature sensors. Sensor TS11 is mounted on the DC (+) bus and TS21 is mounted on the DC (-) bus. The sensor contact is designed to open when the SCR junction temperature exceeds 220 F (125 C). b) Blown SCR Fuse. Fuse protection for the SCR consists of high speed indicating silicon fuses. The UV Trip circuit is wired through trigger fuse microswitches of all the SCRs. A trigger fuse is connected across each Main fuse assembly. If a main fuse blows, the micro-switch trigger activates. The trigger fuse has a long plunger which is set close to the micro-switch arm. When the fuse blows, the plunger pushes into the arm, causing the micro-switch contact to open. c) Emergency Off. The UV Trip circuit is also interlocked with normally closed EMERGENCY OFF button on the Driller s Console. Page 12 of 49

14 3.2.3 Current Feedback Three current transformers are used to sense current flowing into the SCR bridge. On the PC1 Drillers Slide PCB, the CT signals are rectified and the resulting DC output divided through a resistor circuit. One signal is used to drive the front panel DC ammeter. The other signal is applied to the DC Control Module as SCR Amps (Pin 131). SCR Amps signal is 2.66 V per 1000 amps out of the SCR bridge RLC Filter A ferrite core is used for each SCR along with the RC circuit connected across the SCR, to form an LRC filter. The filter is designed to reduce rate of change of voltage (dv/dt) through the SCR. Excessive dv/dt or di/dt can cause the SCR to misfire and burn out Firing Pulse A pulse transformer is used to invert and double the SCR firing pulse supplied by the DC control module. The current pulse rises in less than 20 n Sec. to approximately 1 amp to hard fire the SCR. It then descends to a 0.5 amp backporch to force more and more of the SCR to turn on through a regenerative process Voltage Feedback The DC (+) and DC (-) buses are connected through a resistor divider network on PC2 (Voltage Feedback PCB) to develop a 1:15 analogue of the SCR bridge volts. Signals from the bus are dropped through a set of 3.9 kω resistors used to drive the front panel SCR Voltmeter. The other is a differential signal Vbr+ - Vbr- which is applied to the DC Control Module (pins ) for use in the DC regulator circuit. When the bridge voltage is 750 VDC, voltage across Vbr+ - Vbr- is 50 VDC Regulator Power Supply Transformer T5 supplies 46 VAC, 3Ø to PC1 board where it is rectified on PC1 to +60V for the contactor power supply. When added to the -14VDC contactor supply output from the DC module 74VDC is available for energising the coils of the main DC contactors. The star winding of transformer T4 supplies six 12VAC phase voltages to the module (Vca, Vcb, Vab, Vba, Vbc and VAC - pins 103 through 108). These signals are used to synchronise firing pulses for the six SCRs and derive the +14VDC power supplies Surge Suppression Circuit This is a circuit which filters transient spikes on the AC bus. Loss of the circuit does not disable the drive system but does increase the likelihood of increased electrical noise on the main AC bus. Old Style The line input is fused and then rectified through a diode bridge. DC output from the bridge charges a capacitor bank to 1000 VDC. A 25Ω, 225W resistor limits the charging current to 35A. About 30ms after power is turned on, relay K1 energises to short out the resistor. It takes 30ms. for K1 contacts to close. Excess charge caused by a spike is discharged through the resistor bank. When power is turned off, the capacitors are also discharged through the resistor bank. Page 13 of 49

15 CAUTION THE CAPACITOR DISCHARGE TAKES 10 SECONDS. DO NOT TOUCH ANY PART OF THE CIRCUIT DURING THIS PERIOD. The front panel surge suppressor light is normally on. It will go out if any of the line fuses are blown. The line fuses are linked via trigger fuses to a microswitch (FS1) whose normally closed contacts are in series with the surge suppressor light circuit. When a line fuse blows, the corresponding trigger fuse blows, causing its plunger to trip the microswitch. New Style The new style surge suppression circuits utilises Metal Oxide Varistors ( MOVs) instead of an RC circuit. MOVs are much faster acting, and clip voltage spikes above a rated level by absorbing their energy DC Ground Fault Detection Circuit The circuit consists of three lights connected on one side to a phase of the AC bus and grounded on the other. A series-connected meter indicates percentage of the fault. The lights glow dimly during normal operation. A DC ground fault completes the circuit through all the bus phases, so that all the three lights glow brightly. A positive or negative reading on the %DC ground fault meter indicates a ground on one of the DC buses. CAUTION THE CIRCUIT IS ONLY AN INDICATOR. THE GROUND FAULT MUST BE QUICKLY LOCATED AND CORRECTED Contactor Control Logic The bridge output is assigned to one of several DC motors by closing the appropriate contactors. The contactor logic is set through the Drillers Console assignment switch. Single pole contactors are used to assign motors which turn in only one direction. For reversing motors, the outputs of the single pole contactors are applied to the armature of the motor via a double pole contactor. This contactor reverses the armature lead polarity to reverse the motor. The power contactor coils require a 74VDC power supply to energise. The positive terminal of all coils is hardwired to a +60VDC power supply. A -14VDC supply is applied to the negative terminals of the coils via a series of contacts to ensure that all conditions are satisfactory to power the assigned motor. If any one of the contacts in this control logic opens, the power contactors trip and the SCR bridge is phased down. Refer to schematics S and S Note that the RT can be assigned to SCR2 in either the 11 o clock or 1 o clock assignment switch positions. SCR2 bridge is connected to the RT motor through single pole contactors K3 and K4 and reversing contactor K5. Now observe K3, K4 and K5 coil connections in the schematic. Note that the positive terminals of all the coils are connected to + 60VDC Page 14 of 49

16 Next, trace the -14VDC control signal. The -14VDC power supply in the DC Module is first passed through the manual volts switch internally, and that is normally closed. It is opened for test purposes to phase up the SCR bridge without applying power to the traction motors. The signal emerges from the module as CONT PS at pin 134. It is routed through a normally open auxiliary contact of the SCR2 circuit breaker. The contact closes when the breaker is closed, thereby assuring that the SCR2 unit is turned on. Next, the control signal is sent to the Driller s Console where it turns on the SCR2 ON light. The signal is also connected to one side of the assignment switch. It emerges on the other side at the 11 o clock contact when the assignment switch is set to the 11 o clock position or the 1 o clock position. Next, the signal passes through the REV-OFF-FWD control switch to select forward or reverse operation. Coil of contactor K5 is energised accordingly. The RT assignment has only one reversing contactor (K5), so K5 has to be energised when RT is assigned to others SCRs as well. The control signal continues through auxiliary contacts from K5 ensure K5 has energised, and is passed through an auxiliary relay contact. This contact ensures that all the auxiliary motors associated with this assignment have started (e.g. blowers, lubrication pumps). Note that there is a bypass contact from K4. This means that once the assignment has completed initially, failure of one of the auxiliary motors will not cause the drive to shut down. However, the failure of one of these units will cause an alarm which must be investigated immediately before damage to the motor or connected equipment occurs. CAUTION DO NOT ALLOW AUXILIARY ALARM CONDITIONS TO PERSIST FOR MORE THAN A FEW MINUTES WITHOUT INVESTIGATION WITH THE DRIVE RUNNING. THE DRIVE ONLY CONTINUE TO BE USED TO ENSURE SAFE SHUTDOWN. Further, the control signal is routed through all the normally closed auxiliary contacts of power contactors in the SCR2 unit other than K3, K4 and K5. This assures that the bridge output is not connected to two motors at one time. Next, K3 and K4 coils are energised. To ensure that their contacts have closed as a result, the control signal is passed through their normally open auxiliary contacts. Finally, the signal is returned to the DC Modules as RT CONT (Pin 129). In the module, the reference for the SCR firing circuits is disabled as long as all CONT signals (RT CONT, MP1 CONT, etc.) are not -14VDC. The schematic also highlights the RT reference and current limit control signals which originate in the Driller s Console DC Module The DC Module contains electronic circuits for the. These can be grouped into three assemblies. a) DC Regulator Page 15 of 49

17 b) SCR Firing Circuits c) DW Dynamic Brake Refer to the Dynamic Brake section for information on the brake circuit. A functional block diagram of the DC module is shown on Fig 3.2: Fig 3.2: DC Module Block Diagram DC Regulator The regulator is a feedback control circuit which automatically matches the motor speed and torque to the throttle command from the Control Console. The regulator output is a firing reference (TP7) to the SCR firing circuits. Inputs to the circuit consist primarily of the speed reference, speed feedback and current feedback. The regulator consists of two control loops, an outer voltage (speed) and an inner current (torque) loop. A speed reference from the control console is compared with the speed feedback signal to derive a current command which, in turn, is compared with the current feedback signal to derive the firing reference. Speed Reference This signal originates in the control consoles. The consoles are equipped with handwheels which the operator rotates clockwise to control the traction motors. Each handwheel is linked to a rheostat which outputs a 0 to -8.2VDC speed reference signal representing zero to maximum throttle. The regulator may receive the speed reference from more than one location. For example, the MP1 reference may come from either the Driller s Console or the Mud Pump Console. Page 16 of 49

18 Speed Feedback This is an analogue of the motor speed. It is 0 to +5VDC for maximum speed. The signal is designated N because the letter is a symbol for speed in conventional motor speed equations. In a shunt motor, the speed is directly proportional to the armature voltage. Therefore, the differential voltage feedback signals (Vbr+ - Vbr-) are simply compared in op amp Z701 to derive the single level N. In a series motor, however, the speed is a function of the armature voltage divided by the magnetic flux. The flux, in turn, is a function of the armature current. Therefore, N for series motors is derived by dividing the Voltage Feedback signal with the shaped Current Feedback signal in Z703. Current Feedback This is an analogue of the motor torque, since torque is directly proportional to the armature current. It is derived from the rectified output from the three current transformers measuring the 3-phase AC input to the SCR bridge. DC Foot Throttle Operation A Foot Throttle is provided for the Drawworks for quick response from the SCR bridge during tripping. The DW Foot Throttle reference (pin 114) is applied directly to the current regulator summing the junction, skipping the speed feedback junction. Therefore, it acts as a current command. When the Driller presses the foot throttle, current to the Drawworks motors rises quickly and the voltage follows. The DW speed reference from the Driller s Console hand wheel (pin 117) and the DW foot throttle reference are auctioneered through D10 and D59 to select the higher (more negative) throttle command. When the driller begins a tripping operation, he first sets the Drawworks to cathead speed by slightly cranking the hand throttle. This enables the foot throttle via a microswitch on the hand throttle. When the driller presses the foot throttle to lift a heavy load, the foot throttle reference quickly supersedes the DW speed reference. The foot throttle reference goes to zero when the driller removes his foot from the throttle. As a result, the DW speed reference takes over, and the motor speed and torque return to cathead values. Slaving Circuits This circuit achieves load sharing between two motors which are parallel-connected to drive a common shaft but are powered by separate SCR units. Such an arrangement is normally utilised for Drawworks. Load sharing is accomplished by applying the same current command to regulators of both SCR units. A diode or FET auctioneering circuit is used to select the higher (more negative) current command. Several safety interlock and limiting circuits are tied into the regulator. Contactor Interlock In the OFF mode, the speed reference signal is disabled by a hardwired positive current flow (+14VDC / 2.7kΩ). This positive flow is cancelled when the corresponding CONT signal switches from +5VDC to -14VDC. Page 17 of 49

19 Zero Throttle Interlock This circuit protects the SCR bridge and the traction motor from sudden starts. It disables the firing reference if the CONT signal switches to -14VDC while the corresponding speed reference is also high (negative). Consequently, the operator has to set the throttle to zero before switching the assignment. Current Limit This signal prevents the Speed Reference signal from demanding excess current. It is simply a negative current flow produced by applying a -10VDC across a selected resistor. To lower the Current Limit, the negative current flow is decreased by selecting a resistor with higher value. If the current limit desired is 1000 amps, R sel is approximately 390 kω. Speed Limit This signal prevents the speed reference from demanding excess speed. It is particularly useful for series motors. In shunt motors, an adequate field current prevents overspeeding. Power Limit This signal prevents the current command from demanding excessive power, and thereby overload the engines. It is excessive at about 85% to 90% to the engine-generator capacity on line. The power limit signal is derived by processing the current feedback, KW feedback and KVAR feedback from generators on line. See the Generator section for further information. Manual Operation During testing, it is often convenient to phase up the SCR bridge without applying power to the motor. A manual operation circuit makes this feasible. When the manual volts switch (S1) is set to ON, the -14VDC CONT Power Supply to the assignment contactor logic is disconnected and the manual volts rheostat is inserted into the regulator circuit. Now the bridge can be phased up by rotating the manual volts knob clockwise. Power is not applied to the traction motors since the assignment contactors remain open. CAUTION SCR Firing Circuit NEVER SWITCH THE DC MODULE TEST SWITCH TO THE ON POSITION IF THE SCR IS ASSIGNED AND RUNNING A DC TRACTION MOTOR. These circuits generate firing pulses for the SCR bridge. There are six identical firing circuits, one for each SCR. A Firing Pulse waveform is shown in Fig 3.3. The waveform actually consists of two pulses, a main pulse followed by a backup pulse. The back up pulse is essential for re-firing the SCR at low DC output when the current is discontinuous. The time difference between the main and backup pulses is constant. Page 18 of 49

20 Fig 3.3: SCR Firing Pulses The main pulse is synchronised with one of the 6-phase signals from the AC bus (Vab, Vbc, etc.) and Firing Reference from the DC Regulator. The backup pulse is synchronised with a main pulse signal from one of the remaining firing circuits. SCR Operation An SCR turns on when it is forward biased, and secondly, when its gate terminal is fired with a current pulse. If the gate is fired as soon as the SCR is forward biased, the SCR acts as an ordinary diode. The firing is delayed to vary the DC output. In the SCR firing circuit described above, the 6-phase reference indicates when the SCR is forward biased, and the firing reference indicates when the SCR should be fired to achieve the desired DC output level. For a general description of an SCR. Diode Bridge See Fig 3.4. The diodes turn on and off automatically as the bias changes. This process is called commutation. The waveform shows the commutation process through a single cycle. The 360 cycle is divided into 30 sections. Observe that between diode A+ is more positively biased than either B+ or C+. Similarly, diode C- is more negatively biased than either A- or B- during 90 to 210. Refer to the table for the commutation sequence. Note that each diode is on for 120. At any one time, two diodes are on. One feeds the positive AC portion to the DC (+) bus and the other feeds negative portion to the DC (-) bus. Page 19 of 49

21 DEGREES TURNS ON TURNS OFF 30 A+ C+ 90 C- B- 150 B+ A+ 210 A- C- 270 C+ B+ 360 B- A- Fig 3.4: Diode Commutation Page 20 of 49

22 SCR Bridge In a SCR bridge, the commutation does not occur automatically. It must be forced through firing pulses. Observe that in Fig 3.4 diode A+ is positively biased between 30 and 150. Similarly, the SCRs are biased for forward flow for 120 during each cycle. Thus the SCR can be fired anytime between 0 to 120. This is defined as a firing angle (α) range. When the firing angle α = 0, the SCRs are fired as soon as they are forward biased and the bridge output is maximum Fig 3.5: SCR Firing Angles In figure 3.5 the bridge output is also shown for firing angles α = 30, 60 and 90. The firing angle is restricted between 120 to 10. This corresponds to a speed reference of 0 to -8.2VDC, firing reference of 0.5 to -2.5VDC and bridge voltage of 0 to 750VDC at no load. Page 21 of 49

23 Sprocket Slip Circuit This circuit provides overspeed protection for two series motors that are driven in parallel from a single SCR bridge. Such an arrangement is normally used for Mud Pumps. If either one of the motors exceeds a preset speed limit, due to a malfunction in the chain drive, the circuit cuts off power to both motors by tripping the assignment contactors. It also turns on the front panel sprocket slip light. Overspeed protection for shunt motors is achieved through a field loss relay. A shunt motor cannot overspeed unless its field is removed or significantly weakened. The field loss relay, located in the field supply, monitors the field current. It opens to trip the assignment contactors of the motor if the current is below 35% of the rated value. Overspeed protection for series motors is normally provided through the overspeed circuit in the DC Module. The circuit compares the voltage feedback signal from the motor with the current feedback signal. It phases back the current command if the V/I ratio exceeds a preset level. Recall that in an overspeeding motor, voltage is high and the current is low. Fig 3.6: Parallel Motors The overspeed circuit functions effectively for all configurations of series motors except where two motors are driven in parallel from a single SCR bridge. Refer to figure 3.6. Suppose MOTOR A breaks its chain drive. The unloaded motor will overspeed. It will draw the full voltage, but little current. Most of the current will flow into MOTOR B. The overspeed circuit will not detect the overspeed because the current feedback signal indicates the total current drawn by the two motors. The Sprocket Slip circuit measures the DC current drawn by each motor through Hall Effect Devices (HEDs) and compares them to the armature voltage to detect overspeed. Page 22 of 49

24 Refer to Fig 3.7 for the installation diagram of the HEDs. An HED, mounted on the DC cable, measures current to the MPA motor. HED 2 measures current to the MPB motor. Fig 3.7: Sprocket Slip Connections Refer to the schedule diagram of the Sprocket Slip card, figure 3.8. Differential voltage signals from HED1 and HED2 are compared on Op Amps Z1 and Z2 to obtain I 1A and I 1B respectively. The currents are auctioneered via D1 and D10 to select the current with the lower value. (Since the motors have equal armature voltage, the motor with the lower current has the higher speed.) The lower current signal is summed in op amp Z4 against the voltage feedback current from op amp Z3. Output Z4-6 is amplified via Q1 and applied to the coil of relay K1. K1 is energised during normal operation and de-energised under fault conditions. Normally open contacts from K1 are mounted in the assignment logic of both mud pump motor assignments. The contacts open to trip the main assignment contactors, thereby cutting off power to the motors. A normally closed contact of K1 closes to turn on the front panel Sprocket Slip light. The light can be switched off by pushing the integral reset button. The button re-energises the coil of K1. Page 23 of 49

25 Sprocket Slip Test Procedure Fig 3.8: Sprocket Slip Circuit Perform the following tests to verify correct operation of the Sprocket Slip circuit. a) Verify that the mud pumps are in a satisfactory condition to run. b) Jumper pins and on the sprocket slip card. Electrically disconnect and run motors slowly one at a time to verify proper rotation. Verify that the voltage feedback to the sprocket slip PC board is present while motors are running, and that the voltage at pin 6 of the sprocket slip PC board is positive with respect to pin 5. After the rotation of each motor has been verified as correct, reconnect both motors. Remove jumpers from pins and of the sprocket slip PC. c) Have rig personnel bring up the mud pump throttle slowly. The SCR bridge voltage must exceed 200VDC in order for sprocket slip to occur. If one of the HED outputs are reversed, a sprocket slip condition will be occur, the mud pump will shut down and the sprocket slip light will illuminate. Page 24 of 49

26 d) Reset the mud pump by turning the hand throttle to zero and push the sprocket slip reset to turn out light. e) Run the pump again slowly. Monitor the voltages at the cathode of D1 and the cathode of D10. Verify that the DC level at D1 and D10 is approx. +1V/100 amps in the mud pump armature. If lower or negative polarity, it may indicate that HED wiring is incorrect. Bring mud pump up slowly and observe voltage change at cathode of D1 and D10. If one is going negative, reverse the HED input to the sprocket slip card, or reverse lead at HED if possible. f) After proper voltage feedback polarity and HED polarity have been established, run the mud pump up to greater than 250 VDC on motors. Remove one of the HED inputs to the sprocket slip PC and verify that pump stops and sprocket slip is indicated. Repeat for other HED. Perform test on all SCRs that can be assigned to the mud pump. Page 25 of 49

27 PROBLEM 1. SCR Circuit Breaker Trips Emergency stop. All SCR breakers trip. SCR/ Fuse Failure Overcurrent trip. SCR Overtemperature Field loss 2. SCR Bridge Inoperative Zero throttle interlock Throttle signal not reaching the module. Contactor logic defective. DC module defective. SCR misfiring Table 3.1: Troubleshooting (1) ACTION Driller may have pressed the Emergency Stop button on the Driller s Console. To resume operation, close the breakers. a) If an SCR fails, some fuses are also blown. Check fuse trip indicators or trigger fuses. b) Test which SCR fuses are blown. c) Replace all blown fuses and defective SCRs. Close the breaker to resume operation. a) Overtemperature switches will reset after SCR junction temperature drops below 74 C. Close the breaker to resume operation. b) Make sure SCR blowers are running. If not, check fuses. If the motors are shunt-type, ensure that field current is approximately 50 amps. Check Zero Throttle LED on DC module. To resume operation set throttle at control console to zero. Check for Speed Reference signal from the control console at the DC module. Check for -14VDC motor CONT signal at the module. If absent, trace the control signal. See Contactor Control Logic section for more information. a) Check SCR firing pulses. b) If pulses are absent, check module power supplies. If these are present, replace the module. Check current feedback ripple. Page 26 of 49

28 PROBLEM 3. Motor Speed Does Not Regulate No feedback signals to the module In case of mud pumps slaving, slave signal may be absent. Defective DC regulator ACTION Check V br + (pin 101) - Vbr - (pin 102) and SCR Amps (pin 131) signals. Check Slave inputs (pin 133 or 136) Replace the module. 4. Insufficient Power- DW, MP Check ammeters of SCR units driving two Mud Pump of Drawworks motors. One of them may read low, indicating that only one motor is running. Page 27 of 49

29 Table 3.2: Troubleshooting (2) PARAMETER PIN VOLTAGE TROUBLESHOOTING 1. Power Supply AC Voltages 2. Power Supply DC Voltages 3. Bridge Voltage Feedback Signal (Vbr+ & Vbr-) 4. Current Feedback Signal (I feedback) 5. Contactor Signal 6. Hand Throttle Reference VAC to Ground If absent, the + 14Vdc supplies will be low. As a result, one or more SCRs may not file. Check safety fuses Ground VDC VDC 101 & 102 The Voltage across the pins is 50VDC when the bridge voltage is 750 VDC. Polarity: 101 is positive for series motors, negative for shunt motors VDC when bridge current is 1000 Amps. All must be present for normal operation. Check safety fuses if absent. If absent, hand throttles on the Driller s Console will be extremely sensitive. If absent, the motors will oscillate, and may blow SCR Fuses. 116 (DW) -14 VDC When SCR breaker is closed, and DW assignment selected on Driller s Console. 117 (DW) 0 to -8.2VDC for zero through maximum throttle. Refer to the following pages for information regarding other functions. Page 28 of 49

30 PIN NAME FUNCTION SCR CB OPEN Vbr+ Vbr- 103 Vca 104 Vcb 105 Vab 106 Vba 107 Vbc 108 VAC 109 Spare CP Ref 2 Table 3.3: Troubleshooting (3) BREAKER CLOSED, DC CONTACT- ORS OPEN NORMAL RUNNING TROUBLESHOOTING SCR Bridge Voltage 0V 0V for SCR Bridge off. Measure voltage across Feedback, positive lead 1/15 of Bridge output voltage. pins 101 and 102. If SCR Bridge Voltage Polarity: 101 is positive for absent, hand throttle on Feedback, negative lead. series motors. 101 is negative Driller s Console will be for shunt motors. extremely sensitive. Power Supply AC Voltages 12 VAC If absent or low, check F1, F2 and F3. TB4-10 should read 0 ohms to ground. Hand throttle ref. from CP Console for Cement Pump PROP DB Propulsion Dynamic Brake phase back signal. 112 CP Cont Cement Pump contactor signal. 113 CP Ref 1 Hand throttle ref from Cement Pump Console for Cement Pump 112 CP Cont Cement Pump contactor signal. 113 CP Ref 1 Hand throttle ref from Cement Pump Console for Cement Pump 114 DW Ft Th Throttle reference from the Foot Throttle used to drive Drawworks. 115 DC Pwr System Power limit input. Lim 0 to -8.2VDC for zero through maximum throttle. 24 VDC Coil: -14VDC +8 VDC when CP 74 VDC Coil: contactor +60VDC closes. 0 to -8.2VDC for - through maximum throttle 24 VDC Coil: -14VDC +8 VDC when CP 74 VDC Coil: contactor +60VDC closes. 0 to -8.2VDC for - through maximum throttle 0V for off. 0V to -8.2V for zero through maximum throttle. -8.2V for system generators at no load, close to 0V at full load. 116 DW Cont DW Contactor signal Same as pin VDC must be present to drive DW. 117 DW Spd Reference from the 0V for off. 0 to -8.2VDC for zero through Ref Drawworks hand throttle on maximum throttle. Driller s Console. 118 P Ref ERC Reference from the Propulsion hand throttle on the Engine Room Console. As Above If absent, trace signal from CP Console hand throttle rheostat. Signal is present only on SCR units driving CP. If absent, trace signal from the Cement Pump Console hand throttle rheostat for CP. Signal is present only on SCR units driving CP. If absent, trace signal from the Cement Pump Console hand throttle rheostat for CP. If absent, trace signal to the Foot Throttle rheostat. Troubleshoot the Power Limit card which is located in GEN I cubicle. Signal present only on SCR units driving DW. If absent, trace signal to the DW hand throttle rheostat. Note: Propulsion Console assignment switch must be set to Drilling. If absent, trace signal to Prop hand throttle on the Engine Room Console. Page 29 of 49

31 PIN NAME FUNCTION SCR CB OPEN 119 P Ref WHC Ref from the Propulsion hand throttle on the Wheel House Console. As Above BREAKER CLOSED, DC CONTACT- ORS OPEN NORMAL RUNNING 120 P Cont Propulsion contactor signal Same as pin VDC must be present to drive propulsion thrusters. 121 MP2 Cont Mud Pump #2 contactor Same as pin VDC signal when MP2 assignment contactors in this SCR unit close. 122 MP Ref Mud Pump # 2 hand throttle Same as Pin 117 DC reference from the Driller s Console 123 MP2 Ref Mud Pump #2 hand throttle Same as Pin 117 MPC reference from the Mud Pump Console. 124 MPI Cont Mud Pump #1 contactor Same as Pin VDC signal. when MP1 assignment contactors in this SCR unit close. 125 MP1 Ref Mud Pump # 1 hand throttle Same as Pin 117 DC reference from the Driller s Console. 126 MP1 Ref MPC Mud Pump #1 hand throttle reference from the Mud Pump Console. Same as Pin DB Field Dynamic Brake Logic signal. Same as Pin VDC for DB contactor open. - 14VDC when it is closed. 128 RT 1 Lim Rotary Table current limit 0 to -8.2VDC for 50A to maximum signal from the Driller s current limit Console. 129 RT Cont Rotary Table contactor +14VDC -14VDC signal Linked to DW Cont if when RT RT driven by a DW motor. contactors in this SCR unit close. 130 RT Ref Rotary Table hand throttle Same as Pin 117 reference from Driller s Console. TROUBLESHOOTING if absent, trace signal to Prop hand throttle on the Wheel House Console. signal present only on SCR units driving thrusters. Signal present only on SCR units driving MP2. if absent, trace signal to the MP2 rheostat on the Driller s Console. if absent, trace signal to the MP2 rheostat on the Mud Pump Console Signal present only on SCR units driving MP1. If absent, trace signal to the MP1 rheostat on the Driller s Console. Note: Propulsion Console assignment switch must be set to Drilling. if absent, trace signal to the MP1 rheostat on the Mud Pump Console Present only on units driving DW. If absent, trace signal to 1 Limit rheostat. Present only in units which drive RT If absent, trace signal to RT rheostat. Page 30 of 49

32 PIN NAME FUNCTION SCR CB OPEN BREAKER CLOSED, DC CONTACT- ORS OPEN NORMAL RUNNING TROUBLESHOOTING fdbck Armature current feedback from SCR bridge CTs. 0V V for 1000 Amps out of the bridge. if absent, motor will oscillate, bridge fuse may blow. 132 RT Ref 2 Same as Pin 117 if absent, trace signal to RT rheostats. 133 MP1 Slv MP1 slaving signal to assure load sharing between two MP1 motors. 0V +5.0 V for 1,000 Amps when driving a MP1 motor. 134 Cont PS Assignment contactor logic control signal. 0V if Manual Switch on DC Module front panel is set to On. Otherwise, -14VDC. 135 DB CONT Dynamic Brake Logic signal. 136 MP2 Slv MP2 slaving signal to Same as pin 133 assure load sharing between two MP2 motors. 137 Do Not Use 138 DBø Dynamic Brake Logic signal A + Gate A + Cath A + SCR Firing Pulse 0V with Manual SW off. Refer tofig 3.3 Monitor the pulse across Gate (139) and Cath (140) pins. 141 A - Gate A - SCR Firing Pulse As Above 142 A - Cath 143 B + Gate B + SCR Firing Pulse As Above 144 B + Cath 145 B - Gate B - SCR Firing Pulse As Above 146 B - Cath 147 C + Gate C + SCR Firing Pulse As Above 148 C + Cath 149 C - Gate C - SCR Firing Pulse As Above 150 C - Cath Spare Spare V -14 V A DC power supply + 14VDC - 14VDC If absent, check safety fuses GND Do Not Use Ground, Chassis 0 V 0 V to chassis frame. Page 31 of 49

33 4. DC MODULE OVERVIEW 4.1 BASIC FUNCTIONS The basic function of the DC module is to control the DC motor, whether series wound or separately excited shunt wound style motor. The ZEEFAX DC control module has the built in ability to operate multiple, selectable assignments. To perform this properly the DC module must have a means of selecting different current limits for different assignments. The module must also provide for speed regulation of the motor and also motor overspeed protection (speed catching). The ZEEFAX system will also use the DC modules to carry out the power limiting function of the SCR power assemblies. The hand throttle, foot throttle, or computer generated speed input to the DC module is a speed reference. The particular input reference effective in controlling the module output is determined by a contactor (assignment) input. The contactor assignment -14 volts goes through a resistor to +14 volts. With no -14 volt contactor input, the +14 volt through the resistor turns on a diode to turn off a gate, therefore blocking the speed reference input. When the proper assignment is made, and -14 volts is applied to the contactor input at the module, the diode is biased off, thus allowing the speed reference input to be compared to the speed feedback ( N ). When running shunt motors the module will utilise a reduced value of voltage feedback, as speed feedback ( N ). The formula : SP = VA (FIELD) expresses the speed of a DC motor as armature volts divided by the flux of the field. Since field current is held constant, (flux) is constant and speed is basically armature voltage. N feedback must be positive so that the input to the differential amplifier must have the correct polarity input to produce a positive N. If the voltage feedback is reversed, this would cause the shunt motor to go quickly to a very high speed, limited only by the load and current feedback. When controlling series motors, the basic speed equation still applies, but the field current does not remain a constant and the value of (flux) changes. Therefore, to determine the speed of a series motor, armature voltage must be divided by a voltage representing field flux. Z702 creates the voltage for field flux from the current feedback. The polarity of the voltage feedback is reversed (from that of shunt motors) so that the output of AD532 will be positive. The voltage at test point TP8 is the current command. The more negative TP 8 goes the more current will be delivered from the SCR bridge. A current limit is provided on each module assignment inputs to prevent excess current (torque) from being obtained. The current command signal (input to Z8) is compared is compared to the current feedback to produce the firing reference for the firing circuits. Page 32 of 49

34 4.2 DC MODULE POWER SUPPLY The DC module obtains it s power from the AC bus through safety fuses. The safety fuses are mounted as close to the bus as we can safely mount them to eliminate long, unfused, wiring runs to the bus. Power is supplied to the primary of the DC regulator transformer through 6/10A fuses. 6 phases are produced in the secondary of the regulator transformer that are 12VAC with reference to ground. The transformer also has 3 separate secondary windings rated at 120VAC for use in the peripheral auxiliary systems such as the throttle circuits and contactor logic supply. The 6 phases are applied to diode bridges DB1, DB2, and DB3 to create the + & - 14 volt power supply. Notice that the + & - 14 volt s is not zener regulated as the zeners (D723 & D724) in the supply are 18 volts. 4.3 MODULE INPUT CIRCUITS The DC module has 6 different types of input functions available. They are all basically the same with minor differences. To develop the operational concept, one input will be examined then minor differences will be discussed. 4.4 TYPICAL ASSIGNMENT Pin 124 on the module (MP1 INPUT) will be positive 8-10 volts coming out of the module when the contactors are not assigned. R11 brings the positive voltage to pin 124, the MP1 contactor input, and also puts a strong positive at the junction of the D2 cathode and R53. When the module is assigned (-14 volts applied to pin 124) the reference input will cause junction D2 and R53 to go negative, this is a request for current. The amount of current that will be applied depends upon the load on the motor because R53 brings in the positive N feedback to sum with the negative going speed reference. R47 and R56 (select) connect to the +10 volt supply. This pulls positive on the speed comparison junction putting a positive offset at the junction which forces the reference to go -1 volt before a current command is obtained. This positive offset is called the dead band, requiring that the operator rotate the throttle 15 to 20 degrees before the bridge begins to phase up. The smaller the value of this resistor the more voltage from the throttle is required to phase the SCR up. The current limit setting will affect the dead band. The dead band allows the bridge to phase down and stop the motor even if the throttle voltage does not drop completely to zero. Reference ZEEFAX DC Regulator Schematic MP1 input. Assume that D48 is not in the circuit. Voltage at R34 (D48 cathode) can be calculated by the following (assume ½ volt drop for D48). I R34 = 9.5 = K + 22K I R34 = x K E R34 = 22 x 10 3 x x E R34 = -0.37V Cathode of D47 is -0.5V more negative than cathode of D48 D47 cathode (-0.5) = V Page 33 of 49

35 The current limit resistors R802 and R57 at -10V hold the cathode of D47 at the maximum negative voltage of about -1Volt. A -1Volt at the cathode of D47 would be -.5Volt at pin 3 of Z7. Z7 has a gain of 11 from the non-inverting input. Gain Z7 = R33 + R32 or = 11 R33 10 Therefore output of Z7 is approximately -0.5 x 11 = -5.5V. This is -5.0 at TP8 due to the diode drop of D20. To find the trim for R802, (resistor R57). TP8 equals V for 800 amps. Output of Z7 is: (-0.5V) = V. Input to Z7 is V/11 = V (E R34). E R34 / R34 = I R / 22K = x 10-5 amps. At the current limit D47 is reversed biased and this current is also the same value through R57 and R802. The voltage across R57 and R802 = 10V - ( V)/ V. The parallel combination of R57 and R802 is 485K ohms for a mud pump current limit of 800 amps. NOTE : Pin 131 (current feedback) is 2.67 volts per 1000 amps The output of Z7 is applied to pin 2 of Z8 via R25 (178K) + R39 (22.1K) or 200K ohm so that the current command is reduced by twice the feedback. This is the feature for two motor pump applications. In this case TP8 is 2 x (2.55/1000A) or 5.10V/1000A. TP8 value is -5.1V/1000A Z7 pin 6 value is -(5.1V/1000A + 0.5V) Z7 pin 3 value is -(5.1V/1000A + 0.5V)11 D48 value is -(5.1V/1000A) V Op amp Z7 has a small offset on the inverting input pin 2 established by R58, a 2.2meg resistor to a -10V. The output of Z7 is the current command to the firing circuit network. This is a voltage of -5.10/1000A at TP8. Pin 6 of Z7 is a diode drop more negative than TP8. The current command at TP8 is compared to the current feedback at the input to Z8, creating the firing reference at the output of Z8 pin 6. The negative current command of 5.1V/1000A is applied to pin 2 of Z8 through R25 and R39 (200K). Current feedback, a positive 2.66V/1000A, is coming into Z8 pin 2 via R37, a 100K resistor. When Z8 has an output of 0.5V, the limit or maximum, is established by D21. This corresponds to the maximum bridge output. Z8 s negative limit is set by voltage the voltage divider R76, R77 and diode D72. Setting the junction of R76 and R77 at -.5V will allow R76 to drop 14.5V at Z8 s maximum negative output. I R76 = 14.5V = ma. 16K Page 34 of 49

36 I R77 = E R77 = x 10-3 x 4.7 x 10 3 = 4.25V The maximum negative output of Z8 is approximately, E R77 = -4.25V + (-0.5V), present at junction of R76 and R77, therefore the output of Z8 voltage, pin 6 TP9, is -4.75V. Notice the module has an input for mud pumps for both the main console and remote console. Also notice that the input resistor for the remote console is 47K while the input for the main console is only 22K. This is to limit the speed from the remote console by ½ of that from the main operators console. This is due to the fact that the remote console is only for maintenance purposes. The rotary table (top drive) has 2 inputs. The second input is available for third mud pump operation or top drive. RT input pull down to negative 10 volts is used but two others, R35 and R23, go to pin 128. This input can also be pulled negative by voltage coming from the operators console (variable 0 to -8.2V). Since both sets of resistors go to a negative power supply, they are in parallel by varying amounts depending on the position of the current limit pot on the operators console. This current limit varies from 50 to 1200 amps and lets the operator adjust the torque of the motor. The propulsion input pin 111, during regeneration of the bridge, allows us to set a current limit of 500A maximum, or what ever R66 is adjusted to. The braking control signal input goes through R66 to set the limit. Notice R19 to N feedback is omitted, it can be reconnected if this assignment is to be utilised for some other function besides propulsion. Speed feedback is not required as the prop does not come out of the water. The foot throttle input is terminal 114. The foot throttle pot is typically a 200 ohm, 90 degree type, not a 300 degree as typically used. The throttle uses -8.2V source, so pin 114 normal input range is the same as the other speed reference inputs, 0 to -8.2V. Since foot throttle usage is actually more like that of a switch, full off or full on, the input circuit resistor, diode and capacitor arrangement on op amp Z9 (C5, C6, R52) creates a ramp for increasing the load. R51(300K) with D55 allows for current to decrease faster, but still not an immediate shut down of the current. This allows the drawworks load to function without a severe application of torque. It also allows smaller or older engines to pick up the load slower and helps to prevent overshooting when the load is suddenly dropped. Before the output of Z8 is fed to the firing circuits, there is a signal that interrupts the command for lack of assignment enable signal and, also, zero throttle confirmation after the assignment is made. Diode D42, D43, D44, D45, D46 and D73 go to the contactor inputs. If the contactors are not assigned, the 2.7K resistors to +14V will establish +8 to +10 volts at the cathode of the diodes. This will turn off the diodes so that +10V on the end of R43 will bring the base of Q1 to a positive level, thus Q1 turns on. This allows -10V on emitter of Q1 to go to the collector where through D56 it can pull off the firing reference and keep the bridge from phasing on. Q1 is used as a switch and can be called the firing inhibit transistor. Q1 is on when the module is not assigned, or has been assigned but a throttle reference was detected before the assignment was confirmed(zero throttle interlock). If the throttle input for the assignment is -0- and the assignment is made, the contactor input is pulled to Page 35 of 49

37 -14V which will eliminate the effect of +10V through R43 that is keeping Q1 turned on. Now Q1 can turn off, thus removing the -10V pull down voltage from the firing reference, however, C13 (5uF) must be discharged to remove the -10V effect on the firing reference. The output of Z8 may now control the firing reference (TP7). On the output of Z8 there is another diode arrangement of D38, D39 and D58 (D58 is omitted unless regenerative braking is utilised). When the bridge is first phased up in manual mode, notice the voltmeter goes negative. This indicates the SCRs will accept current that goes opposite to normal current output. SCRs will allow current flow from the motor back into the main bus for regenerative braking. When regenerative braking is used, we cannot stop firing the SCR bridge. This motor current is inductive current, so it cannot be stopped abruptly. The voltage swings way up due to Ldi/dt, if the current flow were to stop. D58, D38 and D39 and resistors keep TP7 voltage from going below the firing ramp regardless of what Z8 output does. The reference is held about -1.8V during regen. D58 should be removed if regeneration is NOT used. If it is in and resistor R82 is not selected, it will keep the firing reference from coming up properly. One easy way to tell if the SCR bay is regenerative is that the volt meter will be center scaled with both + and - scales. 4.5 FIRING CIRCUITS Firing circuits start with a capacitor to ground and a resistor to -10V. The + and -14V supply in the module is not zener regulated. 18 volt zeners are used which act as crowbars to keep spikes from damaging the op amps. Op amps will take 18 volts. Refer to B+ firing circuit, for the following. Voltage on C302 is setting at +0.5V. Input AC, sync, is filtered slightly by C301 and R301 to get the noise out of it. AC is then applied to diode D301. When AC is positive D301 is biased off and the charge on C302 has not changed. When AC goes negative it will now pull down on 22K resistor R303, removing the effect of +10V through D303, since it is now biased off. C302 will now charge towards -10V to create a ramp. Due to the RC time C302 will only charge to about -2.3V during the negative half cycle of sync. Time = RC 0.47 x x x 10-6 second (1 RC) So in 19.83ms the charge on C302 may reach 63% but the time of one ½ cycle is /2 or 8.33ms. We have less than ½ of 1 RC time constant for charging C302 (42% of 1 RC). Not using log ratio to determine the exact charge on C302, we have 8.33ms/19.83ms = 0.42 of 1 RC. 1 RC = 63% of 10V which is 6.3V. Assuming we would charge to 42% of 6.3V, C302 would reach 6.3 x 0.42 = approximately 2.65V. The firing angle is determined by the firing reference moving up and down the firing ramp. Whenever ramp pin 3 is above pin 2, Z3 s output will be positive. As firing ramp crosses the reference, pin 3 can swing the output of Z3 in the negative direction. When ever the ramp is less negative than the reference, Z3 s output will be positive. Page 36 of 49

38 Z3 will generate a variable width square wave, but instead of applying this pulse directly to the firing transistor, it goes through a resistor diode arrangement and capacitor C303 that is tied through R308 to +13V. The output of C303 is clamped above 0V by 2 diodes, D312 and D313. The output of Z3 is a nice square wave but C303 will cause a pulse to be developed. D307 will bypass R312 (4.7K) to pulse harder during negative output of Z3 then when Z3 s output goes positive, the capacitor almost sees a short circuit when diode D307 is on, but sees 4.7K ohms when D307 is off. Why is this done? The output is clamped at +1V. When Z3 s output swings back positive, the charge on C303 is dumped into diodes D312 and D313 to 0V. So the op amp would be facing a short circuit (when it goes positive) not during negative. Early op amps would be damaged by this but new op amps can take it. This is still left in for the extra protection since the circuit can swing from +13V to -13V, over a 25V swing. Since we are only concerned with the negative transition of Z3, the circuit provides a negative transition that moves up and down. As the firing reference gets down near the bottom of the ramp, firing pulses would become very narrow. This can be damaging to large SCR devices. They need at least a 50 microsecond pulse in order to turn on the entire device. If the SCR is not turned completely on all current must pass through a small area, damaging the SCR. In order to ensure a negative output that is always long enough, D302 goes back to the input to ensure the output of Z3 is held negative for at least 300 microseconds. This causes a glitch on the initial ramp and negative comes back to pin 3 (positive feedback) preventing sliver pulses. The negative signal from the output of Z3 goes to the base of PNP transistor Q301. The negative input will turn on Q301. This will drop -14V towards 0V, which is in the positive direction, causing Q302 to conduct into the pulse transformer. A back up pulse is used so that the SCRs can commutate. Remember when we deliver current to an inductive load we do not want to break the current flow at any time during the conduction cycle. The SCR on the + side sends a back up pulse to the SCR on the - side. The SCR on the - side sends a back up pulse on the + side. The firing cycle is A+ to B-, B+ to C-, C+ to A-, C- to A+, B- to C+ and A- to B+. The main firing pulse into Q301 is followed up 60 later by a back up pulse. Either the main or the back up can pulse Q301. The main pulse will be around 300 microseconds with the back up pulse around 200 microseconds in duration. The back up pulse comes in through diode D306, this shortens the time it turns on Q301 due to the diode drop. The time differential is not a problem since the SCRs only need 50 microseconds to turn on for full surface area conduction. D310 across the pulse transformer is a suppression diode, needed since the transformer can put out a large inductive pulse when Q302 turns off. The suppression diode protects the transistor from damage. The pulse transformer is mounted as close as possible to the SCR device. Wiring from the DC module to the pulse transformer is via twisted shielded pair wire. The shield must be grounded at the source end (module). The step down pulse transformer matches the impedance of the SCR to the module. It needs gate current to fire the SCR. A 33 ohm Page 37 of 49

39 resistor is used to keep the transformer from ringing and the resistance should not go below 27 ohms. 4.6 POWER LIMIT The power limit signal comes into pin 115 of the DC module. This signal originates from the power limit circuit external to the DC module which is connected to the individual AC modules and calculates the load in KW and KVA from the engine / generator sets that are on line. At TP8 in the DC module we have a negative current command. The SCR bay in use delivering the most current will be cut back as the power limit signal goes positive. This pulls TP8 less and less negative in proportion to the incoming power limit signal being applied to pin 115 of the DC module, thus limiting the maximum phase angle of the negative current command signal to the firing circuits. 4.7 MOTOR SPEED REGULATION Higher than normal RPM (ie: cement pump) is accomplished by bringing in a negative 14V from the contactor line to offset N feedback. Normal RPM high limit is This adaptation allows for RPMs as high as Diode D13 is installed when the module is controlling a series motor for drilling and a shunt motor for propulsion. The module will calculate overspeed and attempt to shut down the shunt motor as the module thinks it is a series motor. D13 brings in a negative to offset N when propulsion contactors are pulled in. A shunt motor will not overspeed with a field so we do not need N for motor speed catching. 4.8 MANUAL BRIDGE PHASE UP TESTING Signal firing reference comes from Z9. You may want to fire the SCR bridge manually to verify if the bridge operates properly. When you lift the manual phase switch, it interrupts the -14V source for the contactor line at pin 134. This is a safety feature in the manual phase up mode so that you cannot directly operate the motor from the manual phase up pot. TP7 firing ramp range is from -2.5V to +0.5V. Voltage divider network will give the manual pot -5V to 0V giving the bridge phase up from 0 to approximately 900V. Why is this more than 750V? Since the bridge has no load, capacitors in the snubber circuit network build the voltage to near peak of the AC input. 4.9 SPEED CALCULATOR CIRCUIT AD532 is a multiplier chip that can be used as a divider. Z702 can be a plain inverting amplifier but current feedback is brought in from the board with a gain of 10 (240/24). As the output reaches a point determined by R721 and R722 the diodes D703, D704 and D705 turn on as their cathodes reach -0.5V. This parallels the 240K feedback resistor reducing the gain of Z702. The motor torque curve is approximated in the circuit in a three point curve. When Z703 turns on it hits the first break in the curve. When Z704 turns on it hits the second break in the curve. The second break circuit approximates the flux curve of the motor from IA feedback. The gain of Z702 is 10 until the first break point is reached. The output of Z702 is about -5V at the first break. Page 38 of 49

40 If the bridge phases down there is no If, but there is some residual, but no Ia. So Ø = 0 and Vbridge / 0 = but the output of the divider is not stable. There will be some Vbridge because the motor is spinning and residual flux causes voltage in the armature. So the offset provided by R716 gives about 50A to allow AD532 to create an answer ( N ) from the residual flux ( ). NOTE : The offset adjusted by R716 should be -1.7V + or - 0.3V. This offset may be increased, but should not go above -2.3V. For series motors we put in two positive signals into the divider chip and we get -X / 10Y out. The chip takes the answer, divides it by 10 and inverts it. Since we need A + N we put in negative flux because the chip is an inverting multiplier it will always give you -XY / 10, (always invert and divide by 10). The answer is divided by 10 to keep the output within the power supply range. When shunt motors are used N must be + so pin 101 of the module is tied to - side of the bridge so that N will come out +. Changing the value of R713 to a larger value should have no effect on AD532 but it will increase the time it takes for ( ) flux to build up due to the increased RC. The output of the speed calculation circuit is input to the gate suppression network to provide a safety protection against the possibility of motor overspeed, particularly with regard to series motor operation. The output is also utilised with the initiation and control of the motor braking circuit, when used. Page 39 of 49

41 5. DC SLIDE The DC Slide PCB (PC1) collects together a number of circuits which historically were mounted on a slide-out panel on the SCR (hence the name). It is mounted inside the cubicle door, just above the DC Module. The principle circuits are described in the following sections. 5.2 SCR CURRENT FEEDBACK Fig 5.1: DC Slide PCB The output from the 3 current transformers (T1-T3) are connected to pins 1-3 of the DC Slide PCB. A 3-phase diode bridge rectifies these signals. The output of the diode bridge is connected to two 1 ohm burden resistors to provide a DC voltage which is proportional to the SCR current. This signal provides the SCR current feedback to the DC module, and drives the current meter on the SCR cubicle door, and any other current metering associated with the SCR (e.g. Rotary Table current on the drillers console when assigned). 5.3 METERING CALIBRATION The SCR current feedback signal is passed through independent calibration resistors to provide meter drive outputs on pins 6, 7 and 8 of the PCB. One of the meter drives is allocated to driving the meter on the SCR cubicle door. The others are usually used for remote indications, e.g. the Rotary Table current meter on the drillers console (when assigned). Page 40 of 49

42 VDC CONTACTOR SUPPLY The three secondary voltages from the contactor supply transformer are connected to pins 16,17 and 18 of the PCB. These are then rectified by a 3-phase diode bridge to produce 60VDC which, when referenced to the -14VDC CONT supply from the DC module, provides a total of 74VDC to drive the main DC contactors. Page 41 of 49

43 6. REPLACEMENT AND REPAIR Please read ahead to section (SCR Clamp Tightening Procedure) before attempting to replace an SCR. The tightening procedure is a vital part of the process and must be followed exactly. 6.1 SCR REPLACEMENT 1 Trip the SCR circuit breaker and remove the DC Module safety fuses CAUTION LIVE VOLTAGES MAY BE PRESENT ON SOME TERMINALS EVEN WITH THE CIRCUIT BREAKER OPEN AND THE DC MODULE SAFETY FUSES REMOVED. SOME CIRCUITS, E.G. SPROCKET SLIP MUST REMAIN ENERGISED FOR CONTINUED RIG OPERATION. EXERCISE EXTREME CAUTION. 2 Remove the insulated panels covering the SCR stacks and identify the SCR to be replaced. Positive SCRs are on the left, negative on the right. The topmost SCR is A, and the lowest is C. Therefore the top-left SCR is A+. 3 Remove the black insulating caps covering the SCR clamp bolts. Tip: pinch the bolt cover between forefinger and thumb to help remove it. Page 42 of 49

44 5 Disconnect the bolt fixing the heatsink stab to the SCR fuse. It may be necessary to unbolt the fuse completely to facilitate easy removal of the SCR heatsink. 4 Disconnect the snubber resistor wires so that front heatsink is free for removal. 5 Remove the heatsink clamp bolts 6 Slide off the SCR heatsink to reveal the SCR puck suspended in it s Nomex positioning card. Make a note of the orientation of the SCR by observing the position of the cathode. The cathode is the face with a larger flange, and this also has one of the small, coloured, firing circuit control wires connected to it. Page 43 of 49

45 7 Make a note of the positions of the coloured, firing circuit control wires from the SCR to terminal block on the side component panel, paying particular attention to the cathode wire. The colour coding of the new SCR device may be different, so make sure you can identify the cathode connection irrespective of wire colour. Disconnect the wires, taking care not to lose or misplace the 33R resistor connected across the terminals. Carefully feed the wires through the panel and remove the SCR from the heatsink. 8 Place the new device in the locating card and fit to the heatsink assembly, taking care that the cathode of the device is correctly orientated. In this photograph the cathode is facing the rear heatsink. On the opposite stack it will be facing the forward heatsink. 9 Feed the gate and cathode wires through the component panel and re-connect to the terminal block ensuring that the gate and cathode wires are connected in the correct positions. Make sure the 33R resistor is re-fitted and is securely connected. 10 Slide the front heatsink back on to the assembly and fit the clamping bolts. Tighten the bolts until they are FINGER-TIGHT ONLY, then refer to the instructions below for final tightening. CAUTION IT IS ESSENTIAL THAT THE CLAMP BOLTS ARE TIGHTENED TO THE CORRECT LEVEL. THE SCR DEVICE MUST BE UNDER THE CORRECT PRESSURE TO OPERATE, BUT EXCESSIVE PRESSURE MAY DAMAGE IT OR DISTORT THE HEATSINKS. Page 44 of 49

46 12 Re-fit the SCR fuse ensuring the connections are firm. 13 Reconnect the snubber resistor 14 Re-fit the black insulated bolt covers. 15 Re-fit the insulated covers to the SCR stack assemblies and test the SCR bridge. Page 45 of 49

47 6.1.1 SCR Clamp Tightening Procedure By virtue of their design, this range of clamps can only be used in multiple sets, and each assembled for the same load for any clamping arrangement that they may be used. It is also imperative that the disc spring washers are lubricated with a suitable load bearing grease prior to assembly and that they are so arranged as specified for any given load requirement. The loading of any limb assembly must be followed as shown below in table 6.1. With the C-Clamp correctly assembled, fit to limb with a suitably long screw or bolt with a thread as specified in the table. Tighten with fingers only - NO LEVERAGE TOOLS - until the screws/bolts of all the C-Clamps are as tight as is feasible. If the bolt head cannot be reached, the use of a socket may be used. Ensure that the heatsink to semiconductor surfaces are fully seated and parallel. Using a suitable wrench, tighten the clamp assemblies progressively in sequence with one quarter of a turn each until the maximum turns shown in the table is attained. It is important that the clamps are loaded in small stages together so as to be able to load the semiconductor evenly. Semiconductor to heatsink contact will be severely impaired if this operation is not performed with enough due care and attention. Fig 6.1: Heatsink Clamp Assembly Page 46 of 49

48 Part Number Load per Number Thread Size C-Clamp of Turns C-CLAMP/B/2 2 kn 1 M6 C-CLAMP/B/3 3 kn ¾ M6 C-CLAMP/B/4 4 kn ½ M6 C-CLAMP/B/5 5 kn ½ M6 C-CLAMP/A/ kn 2 M8 C-CLAMP/A/ kn 1 ¼ M8 C-CLAMP/A/10 10 kn 1 M8 C-CLAMP/A/ kn 5/8 M8 C-CLAMP/A/ kn 5/8 M8 C-CLAMP/A/20 20Kn ¾ M8 C-CLAMP/A/25 25 kn 1/3 M8 Table 6.1: SCR Clamp Loading The clamp used on this system is C-CLAMP/A/13.5 Page 47 of 49

49 6.2 BLOWER REPLACEMENT 1 Trip the SCR circuit breaker and remove the DC Module safety fuses CAUTION LIVE VOLTAGES MAY BE PRESENT ON SOME TERMINALS EVEN WITH THE CIRCUIT BREAKER OPEN AND THE DC MODULE SAFETY FUSES REMOVED. SOME CIRCUITS, E.G. SPROCKET SLIP MUST REMAIN ENERGISED FOR CONTINUED RIG OPERATION. EXERCISE EXTREME CAUTION. 2 The blower assembly is located immediately below the SCR stacks. Disconnect the wires connecting the blower to the terminal block. 3 Slide out the blower assembly. Either replace the blower on the mounting frame, or slide in a complete spare assembly. Make a note of the blower full load current (FLC). 4 Reconnect the blower connecting wires. Test the blower for correct direction of rotation by either (a) checking that the blower motor current is at least 75% of FLC with and AC clamp meter, or (b) checking the amount of airflow through the SCR stacks. CAUTION WHEN CHECKING THE AIR FLOW THROUGH THE SCR STACKS TAKE GREAT CARE NOT TO TOUCH OR DROP OBJECTS INTO THE STACK ASSEMBLY. Page 48 of 49