ECET 211 Electric Machines & Controls Lecture 8 Motor Control Circuits (1 of 2) Lecture 8 Motor Control Circuits

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1 ECET 211 Electric Machines & Controls Lecture 8 Motor Control Circuits (1 of 2) Text Book: Electric Motors and Control Systems, by Frank D. Petruzella, published by McGraw Hill, Paul I-Hai Lin, Professor of Electrical and Computer Engineering Technology P.E. States of Indiana & California Dept. of Computer, Electrical and Information Technology Purdue University Fort Wayne Campus Prof. Paul Lin 1 Lecture 8 Motor Control Circuits Part 1. NEC Motor Installation Part 3. Motor Reversing and Requirements Jogging Sixing Motor Branch Circuit Reversing of AC Induction Conductor Motors Branch Circuit Motor Reversing of DC Motors Protection Jogging Selecting a Motor Controller Part 4. Motor Stopping Disconnecting Means for Plugging and Anti-plugging Motor ad Controller Dynamic Breaking Providing a Control Circuit DC Injection Breaking Electromechanical Friction Full-Voltage Starting of AC Brakes Induction Motors Part 5. Motor Speed Reduced-Voltage Starting Multispeed Motors of Induction Motors Wound-Rotor Motors DC Motor Starting Prof. Paul Lin 2 1

2 Lecture 8 Motor Control Circuits Part 1. NEC Motor Installation Requirements NEC Article 430 covers application and installation of motor circuits including conductors, short-circuit, and ground fault protection, starters, disconnects, and overload protection. Motor Brach Circuits include the final overcurrent device (disconnect switch and fuses or circuit breaker), the motor starter and associated control circuits, circuit conductors, and the motor. Figure 8-1 Basic elements of a motor branch circuit that the NEC addresses Prof. Paul Lin 3 Motor Control Circuits NEC Motor Installation Requirements Sizing Motor Branch Circuit Conductor NEC Article 430, Part II Article Installation requirements for motor branch circuit conductor A single motor used in a continuous-duty application must have an ampacity of not less than 125 percent of the motor s Full- Load Current (FLC) Article through Conductor ampacity must be determined by NEC Tables through and is based on the motor nameplate horsepower rating and voltage Full-Load Current (FLC) indicates the use of NEC table rating Full-Load Amperes (FLA) indicates the actual nameplate rating Article through Prof. Paul Lin 4 2

3 Lecture 8 Motor Control Circuits Part 1. NEC Motor Installation Requirements: Sizing Motor Branch Circuit Conductor Example 8-1 Problem: using your edition of the NEC, determine the minimum branch circuit conductor ampacity required for each of the following motors: (a) 2 hp, 230V single-phase motor (b) 30 hp, 230V, three-phase motor with a nameplate FLA rating of 70A Solution: (a) NEC Table shows the FLC as 12 A. Conductor ampacity required is 12 x 125% = 15A (b) NEC Table shows the FLC as 80A. Conductor ampacity required is 80 x 125% = 100A adc/shopping/catalog/motors Prof. Paul Lin 5 Lecture 8 Motor Control Circuits Part 1. NEC Motor Installation Requirements: Sizing Motor Branch Circuit Conductor Feeder Conductors supplying two or more motors must have: An ampacity not less than 125 percent of the FLC rating of the highest-rated motor, plus, The sum of the FLC ratings of the other motor supplies. Ampacity of the conductor => NEC Table (B)(16) => American Wire Gauge (AWG) Prof. Paul Lin 6 3

4 Lecture 8 Motor Control Circuits Example 8-2. Problem: Three 460V, 3Φ motors rated at 50, 30, and 10 hp share the same feeder (Figure 8-2). Using your edition of the NEC, determine the ampacity required for size the feeder conductors. Solution: 50hp motor NEC Table shows the FLC as 65A. 30hp motor NEC Table shows the FLC as 40A. 10hp motor NEC Table shows the FLC as 14A. Required ampacity of the feeder conductor is (1.25)(65) = A Prof. Paul Lin 7 Part 1. NEC Motor Installation Requirements Branch Circuit Motor Protection Nonmotor loads use circuit breaker that combines overcurrent protection with short-circuit and ground fault protection. Motor loads Draws up to 6 times of normal FLC of the motor. Best method of protection for motors separate the overload protection devices from the short circuit and ground fault protection Figure 8-3 Motor branch circuit protection Prof. Paul Lin 8 4

5 Part 1. NEC Motor Installation Requirements Branch Circuit Motor Protection NEC Article 430, Part IV Explains the requirements for branch circuit short-circuit and ground fault protection. The NEC requires that branch circuit protection for motor circuits must protect the circuit conductors, the control apparatus, and the motor against over current due to short circuit or ground faults. Table maximum values on the ratings or setting of these devices NEC Article lists the standard sizes of fuses and breakers Instantaneous trip circuit breakers Inverse time circuit breaker the higher the overcurrent, the shorter the time required for the breaker to trip and open the circuit Prof. Paul Lin 9 Part 1. NEC Motor Installation Requirements Branch Circuit Motor Protection Example 8-3. Problem: Determine the size of inverse time circuit breaker permitted to be used to provide motor branch circuit short circuit and ground fault protection for a 10 hp, 208V, 3Φ squirrel-cage motor. Solution: NEC Table => the motor FLC = 30.8A. NEC Table => maximum ratings for an inverse time breaker as 250 percent of the FLC x 2.5 = 77A Use 80A inverse time circuit breaker if a 70A s is not adequate. Prof. Paul Lin 10 5

6 Part 1. NEC Motor Installation Requirements Selecting a Motor Controller Motor controller Any device that is used to directly start and stop an electric motor by closing and opening the main power current to the motor. It can be a switch, starter, or other similar type of control device. Figure 8-4 Examples of motor controllers NEC Article 430, Part VII details the requirements for motor controllers see page 204 for some of the highlights. Prof. Paul Lin 11 Part 1. NEC Motor Installation Requirements Disconnecting Means for Motor and Controller NEC Article, Part IX covers the requirements for the motor disconnecting means. The Code requires that a means (a motor circuit switch rated in horsepower or a circuit breaker) must be provided in each motor circuit to disconnect both the motor and its controller from all ungrounded supply conductors. Separate disconnects and controllers may be mounted on the same panel or contained in the same enclosure, such as Figure 8-5 Combination fused-switch, magnetic starter unit Prof. Paul Lin 12 6

7 Part 1. NEC Motor Installation Requirements Disconnecting Means for Motor and Controller If a person is working on the motor, the disconnect will be where he or she can see. It protects the person from a motor accidentally starting. The NEC defines within sight as being visible and not more than 50 ft (15 m) distant from the other. Figure 8-6 The disconnecting means must be located within sight from the controller, and the driven machine location Prof. Paul Lin 13 Part 1. NEC Motor Installation Requirements Disconnecting Means for Motor and Controller For stationary motors rated more than 40 hp DC or 100 hp AC, a general-use or isolating switch can be used but should be plainly marked DO NOT OPERAE UNDER LOAD. An isolating switch Intended to isolate an electric circuit from its source of power No interrupting rating Intended to be operated only after the circuit has been opened by some other means. Example 8-4 Problem: determine the current rating of the motor disconnect switch required for a 460V, three-phase, 125 hp motor. Solution: NEC Table => Motor FLC = 156 A NEC => motor disconnecting means to have an ampere rating of at least 115 percent of the FLC rating of the motor 156A x 1.15 = 179A A 200 A disconnect switch is required Prof. Paul Lin 14 7

8 Part 1. NEC Motor Installation Requirements Providing a Control Circuit Has its load devices: coils of magnetic contactor, magnetic starter, relay, etc NEC Article 430 covers the requirements for motor control circuits The elements of control circuit all the equipment and devices concerned with the function of the circuit: Conductors, Raceways, Contactor coils, Source of energy supply to the circuit, Overcurrent protection devices, and all switching devices that govern energization of the operating coil Control circuit voltages and control transformers: 120V, 460V, 600V Ground fault NEC Article requires that motor control circuits be arranged so that they will be disconnected from all source of supply when the disconnecting means in the open position. Prof. Paul Lin 15 Part 1. NEC Motor Installation Requirements Providing a Control Circuit Figure 8-7 The design of the control circuit must prevent the motor from being started by a ground fault in the control circuit wiring Figure 8-7a. A ground fault on the coil side of the start button can short-circuit the start circuit and start the motor Prof. Paul Lin 16 8

9 Lecture 8 Motor Control Circuits Motor starting related issues Starting Current Counter EMF DC motors Locked-rotor current AC motors Figure 8-8 Starting current is reduced as the motor accelerates Prof. Paul Lin 17 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-9 Single-pole factional horsepower manual starter Figure 8-11 Three-pole manual motor starter Figure 8-10 Double-pole manual motor starter Prof. Paul Lin 18 9

10 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-12 Typical magnetic across-the-line starter Prof. Paul Lin 19 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-13 Connection diagram for motor pushbutton stations Prof. Paul Lin 20 10

11 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-14 Timed starting of two motors Prof. Paul Lin 21 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-15 Hand-Off-Auto (HOA) motor control circuit Prof. Paul Lin 22 11

12 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-16 Emergency stop motor control circuit Prof. Paul Lin 23 Full-Voltage Starting of AC Induction Motors: Manual Starters Figure 8-17 Combination Starter Prof. Paul Lin 24 12

13 Reduced-Voltage Starting of AC Induction Motors: Two reasons: 1) Limits line disturbances 2) Reduces excessive torque to the driven equipment When a motor is started at full voltage, the current drawn from the power line is typically 600 percent of normal full-load current The large starting inrush current of a big motor could cause line voltage dips and brown-out. Higher than full-load torque can cause mechanical damage such as belt, chain, or coupling breakage. Electric utility current restrictions, as well as in-plant bus capacity, may require motors above a certain horsepower to be started with reduced voltage. Typical reduced voltage starters: Primary-resistance, Autotransformers, Wye-Delta, Part-winding, solid-state starters Prof. Paul Lin 25 Table 8-1 Typical voltage, Current, and torque characteristics for NEMA Design B Motors Starting Method % voltage at motor terminals Motor starting current as a percent of: Lockedrotor current Fullload current Line current as a percent of: Lockedrotor current Fullload current Motor starting torque as a percent of: Lockedrotor current Fullload current Full voltage Autotransfo rmer 80% tap 65% tap 50% tap Partwinding Wye-delta Solid-state Prof. Paul Lin 26 13

14 Reduced Voltage Starting of AC Induction Motors Figure 8-18 Primary resistance starter Prof. Paul Lin 27 Reduced Voltage Starting of AC Induction Motors Figure 8-19 Autotransformer starter Prof. Paul Lin 28 14

15 Reduced Voltage Starting of AC Induction Motors Figure 8-20 Wye and delta motor winding connections Figure 8-21 Wye-delta starter Prof. Paul Lin 29 Reduced Voltage Starting of AC Induction Motors Figure 8-22 Part-winding starting Prof. Paul Lin 30 15

16 Reduced Voltage Starting of AC Induction Motors Figure 8-24 Soft start ramped-up voltage and current limiting Figure 8-25 Typical soft start starter Starting Modes Soft start Selectable kick start Current limit start Dual-ramp start Full-voltage start Liner speed acceleration Preset slow speed Soft stop Prof. Paul Lin 31 DC Motor Starting Figure 8-26 Across-the-line DC motor Figure 8-27 Definite-time reduced-voltage DC starter Prof. Paul Lin 32 16

17 DC Motor Starting Figure 8-28 Variable-voltage acceleration of a DC shunt motor Prof. Paul Lin 33 Summary & Conclusion Prof. Paul Lin 34 17