Basics of Industrial Motor Control: QUIZ

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Practical level

+++1 Name four types of circuit diagrams and describe the purpose of each.

+++2 Without referring to the text, describe the operation of the starter shown in ---16b, and state the use of each component.

+++3 Give the symbols for a NO and a NC con tact, and for a thermal relay.

+++4 Identify all the components shown using the equipment list given in Table A. Where are contact T and coil A situated physically?

+++5 If the start and stop pushbuttons are pushed simultaneously, what will happen?

+++6 If contact Ax in parallel with the start pushbutton were re moved, what effect would it have on the operation of the starter?

+++7 If a short -circuit occurs in motor M, which device will open the circuit?

+++8 A partial short-circuit between the turns of the stator winding of motor M produces a 50% increase in the line current of one phase. Which device will shut down the motor?

+++9 Under what circumstances is reduced voltage starting required?

+++10 In which quadrants do the following torque-speed operating points occur?

a. + 1650 RPM. 100 N m

b. +3150RPM, - 100N m

+++11 Calculate the mechanical power [hp] of the motor when it runs at 450 RPM.

+++12 A standard 3-phase, 4-pole squirrel-cage induction motor is rated at 208 V, 60 Hz.

We want the motor to turn at a no-load speed of about 225 RPM while maintaining the same flux in the air gap. Calculate the required voltage and frequency to be applied to the stator.

+++13 What is the current in the stator under the following conditions, knowing that the stator is energized at 460 V, 60 Hz?

a. Machine running as a motor at 1650 RPM and developing a torque of 100 N·m

b. Machine running as a brake at 300 RPM

c. Machine driven as an asynchronous generator at a torque of 120 N m

+++14 State in which quadrants a machine operates

a. As a brake

b. As a motor

c. As a generator

+++15 A machine is turning clockwise in quadrant 3. Does it develop a clockwise or counterclockwise torque? Intermediate level

+++16 A thermal relay having the tripping curve given has to protect a 40 hp, 575 V, 3-phase, 720 RPM induction motor having a nominal current rating of 40 A. If the relay is set to 40A, how long will it take to trip if the motor current is:

a. 60 A?

b. 240 A?

+++17 a. If the control circuit is used in place of that shown, show that the motor will start and continue to run if we momentarily press the start button.

b. Show that if we press the jog button, the motor only runs for as long as the button is depressed.

+++18 A magnetic contactor can make 3 million normal circuit interruptions before its contacts need to be replaced. If an operator the motor so that it starts and stops once per minute, after approximately how many working days will the contacts have to be replaced, assuming the operator works an 8-hour day?

+++19 a. Referring to ---24a and assuming that the motor is initially at rest, explain the operation of the circuit when the start button is momentarily depressed.

h. If the motor is running normally, what happens if we momentarily press the stop button?

+++20 a. Explain the sequence of events that takes place when the start button in ---25c is momentarily depressed, knowing that relay RT is adjusted for a delay of 10s.

b. With the motor running, explain what hap pens when the stop button is depressed.

+++21 Describe the sequence of events that takes place when the start button is momentarily depressed, knowing that relay RT is set for a delay of 5s. Draw the actual circuit connections, in sequence, until the motor reaches its final speed.

+++22 A 100 hp, 460 V, 3-phase induction motor possesses the characteristics given by curve 1. The full-load current is 120 A, and the thermal relays are set to this value. If the relay tripping curve is given, calculate the approximate tripping time if the load current suddenly rises to 240 A. (Assume that the motor had been running for several hours at full-load).

+++23 Neglecting windage and friction losses, calculate the power Pr supplied to the rotor when the machine runs:

a. As a motor at 1650 RPM

b. As a brake at 750 RPM

c. As a generator at 2550 RPM

+++24 In Problem 23, calculate the value of the rotor PR losses in each case.

+++25 Referring to ---39, calculate the volt age and frequency that must be applied to the machine so that it runs as a relatively high-efficiency motor:

a. At a speed of 1200 RPM. developing a torque of 100 N m

b. At a speed of 2400 RPM. developing a torque of 60 N m

+++26 Calculate the voltage and frequency to be applied to the stator so that the locked-rotor torque is 100 N m at a current of 40 A.

+++27 a. It’s impossible for a machine to instantaneously change from a point in quadrant 1 to point in quadrant 2. Why?

b. Can it move instantaneously from quadrant 1 to a quadrant 4.

+++28 A 4-pole, shunt-wound de motor has an armature circuit resistance of 4 n. It’s connected to a 240 V de source, and the no-load speed is 1800 RPM; the corresponding armature current is negligible.

Assuming constant field excitation and assuming that armature reaction effects can be neglected, calculate the following:

a. The armature current at 900 RPM

b. The mechanical power output [hp ] at 1200 RPM

c. The torque [N·m] at 300 RPM

d. The starting torque [ft lbf]

e. Draw the torque-speed curve that passes through quadrants 1, 2, and 4

+++29 a. Draw the torque-speed curve if 60 V is applied to the armature, while maintaining the same field excitation.

h. What is the frequency of the current in the armature coils at a speed of 300 RPM?

Advanced level

+++30 a. The curves in ---26 relate to a 100 hp.

460 V, 1765 RPM, 3-phase, 60 Hz induction motor, whose full-load current is 120 A. Calculate the breakdown torque for curves 1 and 2 [ft·lbf]

b. Calculate the torque developed when the resistors are in the circuit and the line current is 450 A [ft·lbf].

+++31 The motor having the T-n characteristic given is running at a no-load speed of 1800 RPM. The total moment of inertia of the rotor and its load is 90 [lbf]. The speed has to be reduced to a no-load value of 1200 RPM by suddenly changing the voltage and frequency applied to the stator.


a. The voltage and frequency required

b. The initial kinetic energy stored in the moving parts

c. The final kinetic energy in the moving parts

d. Is all the lost kinetic energy returned to the 1-phase line? Explain.

+++32 A 15 hp, 460 V, 3-phase, 60 Hz induction motor has the torque-speed characteristic.

a. What is the new shape of the curve if we apply 230 V. 60 Hz to the stator?

b. Calculate the new breakdown torque [ft·lbf]

+++33 In Problem 32 calculate the stator volt age needed to reduce the breakdown torque to 60 N·m.

Industrial application

+++34: A 30 hp, 1780 RPM, 200v, 3-phase cage motor driving a compressor is protected by a thermal relay having the time/current characteristic shown. Curve 3 relates to normal 3-phase operation, and curve 2 applies when the motor runs single phase.

The plant electrician set the relay at 82A, which corresponds to the rated full load current of the motor. Under normal operating conditions, a hook-on ammeter indicated that the motor draws a current of 71 A from the 200 v line.

Due to a fault on another circuit, the fuse in the distribution panel associated with phase C of the compressor motor suddenly blew, causing it to run as a single phase motor. As a result, the current in phases A and B rose to 135 A. What is the maximum possible time it took for the thermal relay to trip the contactor? (The thermal characteristic corresponds to cold start conditions.)

+++35 According to the manufacturer's specifications, it’s known that the motor in Problem 34 draws a locked-rotor current of 465 A at 200 v, 3-phase. The per-unit starting torque is 2. --- The motor is started during a particularly low voltage-sag in the electric utility system. This sag, combined with the line voltage drop caused by the large starting current, causes the voltage across the motor terminals to fall to 155v. In turn, the reduced torque causes the motor to accelerate very slowly and it doesn't reach full speed before the thermal relay trips out.

a. What is the per-unit starting current and per-unit starting torque under these abnormal conditions'?

b. Estimate the time it took for the relay to trip.

+++36 The stator winding of the motor in Problem 34 has a line-to-neutral resistance of 23 ohm.

(Multiple of current setting)

a. Calculate the stator copper losses when the motor runs normally on the 3-phase line, driving the compressor.

b. Calculate the stator copper losses when the motor runs as a single-phase motor. Does single-phasing tend to overheat the motor?

+++37 The holding coil of a 13 kW, 230 V, 3-phase 60 Hz contactor has a rating of 120v. According to the manufacturer's catalog, when the contactor is in the open position, the coil draws 100 VA at a power factor of 0.75. In the holding position, the coil absorbs 3 W and 115 VA. We want to excite the coil directly off the 230V line. To achieve this result, calculate the resistance and power rating of the resistor that should be connected in series with the coil a) when the contactor is open and b) when the contactor is closed.

Also see: Generating Electrical Power