Guide to the Application of Soft Starters

FAIRFORD ELECTRONICS Guide to the Application of Soft Starters APPLICATIONS GUIDELINES FOR THE QFE, QFE PLUS AND STARTMASTER SOFT STARTERS This bulletin tries to answer many of the common questions which are asked by users who are not familiar with soft-starter technology but who have come to recognise the cost benefits which arise from fitting Fairford soft starters. The QFE and QFE plus ranges of soft-starters are based on the Fairford System of microprocessor-based optimising soft-starters which have been used world-wide in more than 100,000 critical and non-critical systems of all sizes. The StartMaster range of soft-starters has been designed to meet the requirements of a versatile, simple-to-install, easily set-up, but standards-compliant product at low cost. 1

INTRODUCTION Since 1983, Fairford System soft-starters have successfully operated in every type of load and environment. The design has proven to be most reliable and adaptable while giving the user a powerful new mechanism to control the fixed-speed induction motor. However, due to the fundamental differences between electronic and electro-mechanical starting systems, there are a number of simple rules to follow and observations to note when using a Fairford soft-starter unit. GENERAL RULES FOR ALL APPLICATIONS. (a) (b) (c) Suitability In principle, any equipment incorporating an induction motor can be started by a soft-starter. However, a few considerations have to be taken into account before selecting a soft-starter since the motor performance and load characteristics can alter the rating of the unit that should be fitted. Normally, the breakaway torque of the load should be less than the full-load torque of the motor, unless a motor with a high locked rotor torque characteristic is employed. As a quick assessment, any load which has a low or no-load start with a moderate starting time, or which can be started with a star-delta starter, auto transformer or other forms of reduced-voltage starting, can be considered to be a potential application for a Fairford soft-starter. This means that the majority of applications are suitable. Induction Motor Characteristics The performance of any soft-started drive is heavily influenced by the speed/torque characteristics of the motor being started. Happily, most modern motors are perfectly suitable for use with soft starters and need no further consideration. Nevertheless, situations can arise when it is useful to know how different characteristics can affect performance. Induction motors are required to provide sufficient torque to accelerate the motor and its load from standstill to full speed and to maintain full speed efficiently at all torque levels up to the design full-load torque (FLT). For soft starting purposes, a motor with a high locked rotor torque (2-3 x FLT) is preferable since it gives greater control over the starting process. However, motors that have a high LRT tend to have low full-speed running efficiency while motors that are very efficient tend to have a low LRT. Many manufacturers now produce motors which combine the qualities of high starting torque and good running efficiency in a design of motor known as a deep bar or double-cage type. This type of motor is ideal, but not essential, for use with a soft-starter. The only aspect of motor design to be of significant concern is that it does not have a low pull-up or saddle torque, or the load may not be accelerated correctly. But again, few modern motors have this characteristic. Very small motors, (less than 0.75kW) which are operated at light loads may draw insufficient current to maintain conduction in the thyristors. This can give rise to erratic or rough operation and a soft-starter is unlikely to be suitable for such a load. The primary function of a soft-starter is to act as a torque-regulating device and therefore cannot apply a torque greater than that which the motor generates. For this reason, problematic applications for which many different starting methods have been tried but failed, may need analysis of the motor or load performance before a soft starter can be successfully applied. Rating For most applications, the starting demands are so small, and the inertia of the rotating masses so insignificant, that no special consideration needs to be given to the rating of the soft-starter. Nonetheless, it is important to ensure that it is equal to or marginally greater than the rated voltage and current of the motor that is to be controlled. Alternatively, if the number of poles of the motor and the moments of inertia of the load (Jload) and motor rotor (Jmotor) are known, a QFE soft-starter can be selected on the basis of the motor rated current as follows: Number of Poles 2 4 6 8 Synchronous speed (rpm) 3000 1500 1000 750 (Jload) / (Jmotor) less than 5 15 20 25 A table showing the more common applications is given on page 8. 2

(d) (e) (f) (g) (h) (i) (j) (k) (l) Maximum Motor Cable Length Because of the influence of cable characteristics on thyristor operation, the length of the cable between the output terminals of the starter and the motor should not normally be greater than 150-200 metres. If longer cable lengths are essential and the controller cannot be located closer to the motor, advice should be sought from Fairford Electronics Ltd. Power Factor Correction Capacitors (PFC) A separate contactor fed from the SUPPLY side of the soft-starter controller MUST always be used to connect power factor correction capacitors that have been applied individually to a motor. Capacitors should be switched in after the top-of-ramp condition is reached and switched out of circuit before a stop is initiated. It is important for any total system PFC scheme that automatically corrects for a range of inductive loads is not operated in such a way as to leave it heavily over compensated. This might introduce oscillations leading to damaging over-voltages. Lightly Loaded, Small Motors Lightly loaded, small-sized, star connected motors (less than 2kW), can produce high voltages at the motor terminals when shut down by the simple tripping of a line contactor. As these voltages can damage the soft-starter, it is safer to control the opening of the line contactor through the controller run relay contacts. Motors Fitted with Integral Brakes Motors which include an integral, electrically operated brake, internally connected to the motor input terminals, can only be soft-started when the brake is re-connected to the supply through its own contactor. Older Motors The action of the soft-starter introduces harmonic currents and voltages to the motor. It is therefore, important to ensure that the motor employs techniques such as rotor skewing in its construction to suppress the effects of harmonic fluxes and avoid rough starting. This is rarely a problem with modern motors because nearly all motors designed in the last 20 years employ these techniques. Wound-rotor or Slip-ring Motors Slip-ring induction motors ALWAYS need some resistance in the rotor circuit to ensure that sufficient rotational torque is generated to overcome any alignment torque that is present at start-up. The resistance can be safely shorted out in the normal fashion with a contactor controlled by the programmable relay set as top-of-ramp contacts. There is a Fairford Application Bulletin available for help in selecting a suitable value of resistance. Enclosures Thyristors are not perfect conductors, and the passage of current through them causes heat dissipation in the body of the device, which in turn causes the heatsink temperature to increase. Typically, the heat generated is 1 watt/amp/phase, which equates to a dissipation of 30 watts from the heatsink for a line current of 10 amps. Consequently, care should be taken that all cabinets or enclosures that house soft-starters have adequate ventilation. The StartMaster products however, are IP43 rated and do not need to be fitted in an enclosure except in very dusty atmospheres or in outdoor situations. Efficiency Although the use of the soft-starter introduces a power loss, the system retains an overall efficiency of 99.5% (higher in the case of StartMaster). If the QFE optimising function has been selected, then the gain in motor efficiency at part loads is far greater than the loss of efficiency arising from thyristor heating effects. If prolonged operation at full load is expected, the thyristor loss can be eliminated (as in some matched motor/pump drives) by closing a bypass contactor around the soft-starter. (This contactor is readily controlled by the built-in relay contacts of both products - see page 9). High-Efficiency Motors Due to an inherently steep front to the speed/torque curve, high efficiency motors can exhibit instability when lightly loaded. In the QFE the optimising rate parameter P19 may need to be adjusted to compensate. StartMaster is not fitted with an optimising function and is not affected. 3

(m) (n) EU Compliance with the EMC Directive After the fitting of the unit, users and installers in countries adhering to the CENELEC directives must comply with the EMC Directive 89/336/EEC. The manufacturer of the soft-starter has a statutory obligation to provide a guide for compliance with this directive; this guidance is given in the technical handbooks for each product. It is essential that users and installers understand and comply with the requirements described in these handbooks. Fuses Generally, circuit protection fuses should be rated at twice the motor rated current for normal low-inertia applications. RULES FOR SPECIFIC APPLICATIONS (a) In-Delta Operation (QFE and QFE plus only). (Available for software type 51 onwards) The QFE allows the soft-starter to be placed within the delta connections, which can permit the use of a lower current rated module. However, this form of operation is phase rotation sensitive, so an incorrect phase rotation results in the QFE refusing to start the motor. Simply changing the motor connections remedies this situation and enables motor starting to occur. If motor rotation is incorrect, the supply connections should be changed. It should be remembered that SIX connections between the motor and soft-starter are necessary to execute this form of working. Before using the QFE soft-starter in delta mode, controller parameter P6 must be set. (b) (c) (d) (e) (f) (g) High Inertia Loads High inertia loads such as centrifugal and axial fans, grinders, flywheel presses, etc., may require a larger size of soft-starter for the size of motor fitted to the drive. For example, a 15kW soft starter might be needed for a 11kW motor. This over-sizing is necessary to accommodate the effect of prolonged over-currents in the thyristors due to the extended starting time. If very high inertia loads are involved, then an analysis of the ramp times should be made for different current levels. This will require accurate data on the motor speed-torque and speed-current characteristics as well as the load characteristics. For further information, consult the supplier or manufacturer. Consideration must also be given to thermal overload protection and fuses when extended ramp times are involved because these may operate or be aged, during the ramping process. In these circumstances, the internal QFE electronic overload can be set with dual settings: (a) during rampup and (b) at normal operation after top-of-ramp is reached. The StartMaster does not have an integrated inverse-time overload facility (See also general rules (c).) Phase Control Operation Certain applications such as voltage regulators, transformers, heaters, lighting banks require the load to be controlled manually or from a 4-20mA input. The StartMaster and standard QFE soft-starters are not designed for these installations. However, a special version of the QFE controller is available for this type of application. (Please refer to your supplier for further information.) Resistive Loads Resistive loads such as heaters, etc. may require different forms of control such as burst firing. Both the StartMaster and standard QFE soft-starters are designed for these applications. However, a special version of the QFE controller is available for this type of application. (Please refer to your supplier for further information.) Frequent Starting Very high starting frequencies require careful consideration of the soft-starter thermal capabilities, and may show that a larger soft-starter should be fitted. Optimising (QFE and QFE plus only) Drives which operate for long periods of time at less than 35% of their rated capacity can benefit from the energy saving function (optimising) which adjusts the thyristor triggering to reduce the excitation losses of the motor. This will lower the running temperature of the machine and helps to extend its life. Soft-Stopping for Pumping and other Applications Soft stopping will reduce positive surge pressures in pipelines on shutdown. However, it is necessary to ensure that the ramp-down time is long enough to remove the energy from the fluid 4

before the pulsing of the thyristors is stopped, otherwise the surge pressure may still be present. Over-sized motors fitted to centrifugal pumps are sometimes difficult to control with soft stopping. Soft stopping can be successfully fitted to high friction loads such as conveyer belt systems where sensitive loads such as bottles are being transported. (h) (i) (j) (k) (l) Reversing Drives and Plug-braking QFE and StartMaster soft-starters used in conjunction with contactor controlled reversing and plugbraked motors show considerable benefits to the user by reducing mechanical and electrical stresses, particularly when utilising the current limited start feature. However, it is good practice to insert a 150-350 millisecond delay between the opening of one contactor and the closing of the other, to allow any residual flux in the rotor to die away. When plug braking is employed there should be some form of zero-speed detector to trip the drive after braking has been completed, otherwise the drive will accelerate in the reverse direction. The QFE dual setting feature is particularly useful in plug-braking where the braking settings need to be different from the normal starting ramp time and current levels. (See page 11). Replacement of Fluid Couplings Soft-starters can very effectively replace fluid couplings to yield benefits of higher efficiency running and lower operating and maintenance costs to the user. In a few applications, the coupling may be used to magnify the available breakaway torque, and it may be necessary to replace the fitted motor with another of larger size or one with a high starting torque characteristic. Despite the additional cost involved, it remains a cost-effective option for the user. Two-speed Motor Applications Two speed motors, whether Dahlander connected or with dual windings, can be soft started at each speed, provided that the start is given when the actual motor speed is less than the synchronous speed for the winding selected. This is particularly important when changing from high to low speeds. Of particular benefit in the QFE is the ability to have different operating settings for high and low speeds and the ability to switch rapidly between them. Multiple Motor Starting Please see page 10.. Overhauling Loads Certain applications can over-speed the motor as part of normal operation. Power flow then is from the motor to the supply. For QFE and QFE plus It is important that the optimising is disabled during the over-speed condition and reinserted during normal conditions, the StartMaster is unaffected and requires no special action APPLICATION TABLE The table on the following page shows many common motor applications that suit the QFE and StartMaster soft-starters. It lists typical breakaway torque requirements as a percentage of motor full-load torque (FLT). For the most satisfactory soft-start in a given application, the motor should have a full-voltage locked-rotor-torque (LRT) that exceeds twice the breakaway torque. (E.g. For a reciprocating compressor, the motor LRT should be in the region of 200% FLT.) As a general rule, the higher the motor LRT is above the load breakaway torque, the greater the control over the starting process. 5

Application Breakaway Torque (%FLT) Remarks Agitator 35 Air compressor- rotary, unloaded start 25-35 Air compressor- reciprocating, unloaded start 50-100 Air compressor- screw type, unloaded start 30 Usually two-pole motor Ball mill 30-50 Eccentric load, needs high starting torque motor Carding machine 100 Often high inertia Centrifuge 50-90 Usually high inertia Centrifugal fan- dampers closed 10-25 Usually high inertia Centrifugal fan- dampers open 10-25 Usually high inertia, very long ramp times Centrifugal blower- valve closed 25-35 Centrifugal blower- valve open 30-40 Can have long ramp time Chillers 10-25 Usually started unloaded, two-pole motor Conveyor- horizontal, unloaded 10-50 Conveyor- horizontal, loaded 100-150 Conveyor- vertical lifting, unloaded 50-85 Conveyor- vertical lifting, loaded 100-175 Conveyor- vertical lowering, unloaded 10-40 Conveyor- vertical lowering, loaded 10-25 Crusher (not rock)- unloaded 25-75 Can be high inertia Drilling machine- unloaded 10 Escalator- unloaded 25-50 Optimising feature can be effective Fan, axial-flow propeller 20-40 Feeder- screw 100-175 Needs high starting torque motor Feeder- vibrating, motor driven 100-150 Needs high starting torque motor Grinder- unloaded 10-25 Usually high inertia Hammer mill 20-125 Eccentric load, needs high starting torque motor Mills- flour etc. 30-50 Mixer- dry contents 35-75 Mixer- fluid contents 10-40 Mixer- plastic contents 75-125 High torque motor offers advantage Mixer- powder contents 75-125 High torque motor offers advantage Pelletisers 50-100 Press, flywheel 50-150 Needs high starting torque motor Pump- centrifugal 10-25 Soft stopping useful Pump- positive displacement, piston type 100-175 Needs high starting torque motor Pump- vane type, positive displacement 100-150 Needs high starting torque motor Rolling mill 30-50 Saw, band 10-35 Saw, circular 25-50 May be high inertia. Plug brake may be useful Screen, vibrating 30-60 Travelators, walkways 25 Unloaded starting Transformers, voltage regulators Nil Change firing mode Tumblers 30-100 Can be eccentric load, may need high torque motor Washing machine, rotary- all types 30-100 Usually highly geared, soft start very effective Fig 1: Applications Table 6

MOTOR CONTROL CIRCUITS (a) Motor Bypassing A separate bypass contactor, connected in parallel with the soft starter unit, allows a solid connection of the motor to the mains supply and the heating effect associated with thyristor losses is eliminated. Soft-starting and -stopping remains active as normal. At the completion of the starting ramp, a bypass contactor is closed around the main connections to remove the thyristors from circuit. The contactor is controlled by the programmable relay set as a Top of Ramp relay. This configuration ensures that bypassing only occurs when the motor terminal voltage is at mains voltage. The adjacent diagrams show typical power and contactor control circuits for bypassing. L1 L2 L3 Mains Supply Busbar Circuit Contactor Control Supply Bypass Contactor L1 Soft Starter T1 L2 T2 L3 T3 Emergency Stop Motor Power Circuit STOP K3.2 START K3 X1 11 21 X2 14 24 S1 QFE, QFEplus S0, 00 K3 Contactor Control Circuit Fig 2: Motor Bypassing Power and Contactor Control Circuits Note QFE and QFE Plus only: If you use the bypass configuration, you must select the protection mode to either START + BYPASS or PHASE LOSS ONLY. (Refer to the technical handbook for details) 7

(b) (c) (d) Multiple Motor Starting Both the QFE and StartMaster are capable of starting parallel-connected motors simultaneously provided each motor has similar characteristics and load. For such configurations, the rating of the unit should meet or exceed the total power rating for all the motors. Alternatively, the QFE units can start motors sequentially using bypass contactors under the control of the programmable relay set as a Top of Ramp relay. The dual setting feature of the QFE enables the control of individual groups of motors with differing start-up requirements. (For further details on multiple motor applications, consult the supplier.) External Motor Protection Relay The rating for a motor protection relay should be as for Direct-on-Line (DOL) starting, and if it is an electronic relay, then the user should confirm its suitability for use with a soft-starter. If the motor load is high-inertia (extended start time), then a longer trip time may be required. Motor Reversing The diagram in below shows a typical motor reversing circuit using two contactors, and, to interchange two phases of the 3-phase supply connections. For this application, the soft-stop time must be set to zero. It is also recommended that: a period of 150-350 ms elapses between the FORWARD and REVERSE commands. If the reversing rate is high, the QFE or StartMaster current rating may need to be selected based on the starting duty. (See page 6). Contactor Control Circuit Power Circuit Mains Supply Busbar L1 L2 L3 Fuses Control Supply Isolator Emergency Stop Enable STOP L1 L2 L3 Soft Starter T1 T2 T3 FORWARD REVERSE Motor 21 22 13 14 13 14 21 22 X1 11 S1 QFE, QFEplus X2 14 S0, 00 A1 A2 A1 A2 Fig 3: Typical Power and Contactor Control Circuit for Motor Reversing 8