Motor Control

Question 1

Why is a 3-phase induction motor more commonly controlled by an across-the-line starter, rather than a reduced voltage starter?

Answer 1

The across-the-line starter has a high starting torque, low cost and simple construction

Question 2

Which breaking method will apply a constant waveform to the stator of a motor to bring it to a stop

Answer 2

DC injection

Question 3

Why are NEMA magnetic motor starters rated in horse power

Answer 3

To be able to safely interrupt the locked-rotor-line current of the motor

Question 4

During an overload condition of a SCIM, the line current to the motor is interrupted by what?

Answer 4

Main contactor power contacts

Question 5

Plugging on a 3-phase induction motor is accomplished by which method

Answer 5

Momentarily reversing the phase sequence to the stator

Question 6

Which reduced voltage starter is an open transition

Answer 6

Wye/Delta without power circuit resistors

Question 7

A paper winder is controlled by a VFD, what is the most appropriate torque setting for this application

Answer 7

Constant horsepower

Question 8

Which “quick stopping” method will generally decelerate a 3 phase induction motor in the shortest possible time

Answer 8

Plugging

Question 9

If automatic restarting of a motor after a power failure represents a safety hazard for a motor drive, then what should the control circuit provide

Answer 9

Low voltage protection

Question 10

When starting a WRIM with starting resistors, the highest starting torque achieved when the secondary circuit is what

Answer 10

At maximum resistance

Question 11

If an accidental ground fault develops just before the motor coil while the motor is running, what will be the most likely result

Answer 11

The motor will stop due to the fuse opening

Question 12

Which starter provided the lowest starting line current for a given motor

Answer 12

Wye/Delta (1/3 starting current)

Question 13

VFD’s can cause electrical problems? What is not one of the problems that can be caused by a VFD

Answer 13

High inrush current

Question 14

When initially energized, what percent of line voltage across each individual coil or a 6 lead 3 phase SCIM motor when using a Wye/delta starter

Answer 14

57.7%

Question 15

Which of the following is not one of the main components of a VFD

Answer 15

Contactor

Question 16

A simple stop start 3-wire motor control circuit will run the motor if the start button is held on but stops if the button is released, what is the likely cause of this problem

Answer 16

Defective holding contact

Question 17

Which method of starting a SCIM would have the highest starting torque

Answer 17

Autotransformer set to 80% tap

Question 18

Which starting method would lend itself to the smoothest acceleration?

Answer 18

Three-stage wound rotor

Question 19

What is one disadvantage of using a primary resistance starter

Answer 19

Power losses

Question 20

With regard to a workers electrical safety, what does the term isolate mean?

Answer 20

To disconnect all energy sources

Question 21

If automatic starting after a power failure does not represent a safety hazard for a sump pump, then the control circuit should be wired to provide what?

Answer 21

Low voltage release

Question 22

Yours asked to troubleshoot a conveyor that has stopped working, It is controlled by a VFD, what is the first thing you should do?

Answer 22

Check the VFD for faults

Question 23

With primary impedance starters, reactors are used instead of resistors for motors with higher currents because the resistors could overheat (T or F)

Answer 23

True

Question 24

Dynamic breaking on a 3 phase SCIM is achieved by what method

Answer 24

Disconnecting the AC supply and applying a DC voltage to the stator leads until the motor comes to a stop

Question 25

What does the term combination starter generally refer to?

Answer 25

A starter that has a disconnect, a contactor, with over current and overload protection

Question 26

How many SCR’s would be in each line of a typical 3-phase AC soft starter full wave controller

Answer 26

2

Question 27

What will a normally closed time contactor of an ‘ON’ delay timer do?

Answer 27

Be delayed on opening after the relay has energized

Question 28

In an across the line starter, the minimum number of conductors in the power circuit required to connect to a 12 lead non reversible 3-phase motor to the supply line is 6 (T or F)

Answer 28

False

Question 29

A pulse width modulation variable frequency drive controls the torque characteristics of a motor by varying the following electrical properties

Answer 29

Voltage and frequency

Question 30

A time-closing or timed-to-close contact for an off-delay timer is what type of contact

Answer 30

Normally- closed contact

Question 31

A pilot light indicating that a motor is off due to overload would normally be wired in series with what

Answer 31

A normally open overload relay contact

Question 32

A 3 phase SCIM branch circuit would require how many power line conductors between wye/delta reduced voltage starter and the motor

Answer 32

6

Question 33

What is the difference between a 3 phase contactor and a 3 phase motor starter

Answer 33

The starter has an overload relay and the contactor does not have an overload relay

Question 34

What will a normally closed time contactor of an ‘ON’ delay timer do

Answer 34

Be delayed on opening after the relay has energized

Question 35

What is the purpose of the discharge resistor

Answer 35

To prevent a high voltage build-up across the field