3 Phase Systems

Question 1

A four-pole, three-phase motor has _____ coils.

Answer 1

12

Question 2

The speed at which the magnetic field of an induction motor rotates is called the:

Answer 2

Synchronous speed

Question 3

On starting, the slip of the induction motor is ____% of the synchronous speed.

Answer 3

100%

Question 4

In general, the power factor of an induction motor increases with the load until the maximum power factor is reached at:

Answer 4

Full load

Question 5

In an induction motor, maximum torque is always developed when the

Answer 5

Reactance and the resistance of the rotor are equal

Question 6

Specifically, the core, the conductors, the end rings, and the shaft are the main parts of the squirrel-cage:

Answer 6

Rotor

Question 7

When taking air-gap measurements on large motors, you should:
- keep a record fo the measurements and compare regularly
- take measurements about 90% apart
- take one measurement in the direction in which the load bears on the bearing
- all of the above

Answer 7

ALL OF THE ABOVE

Question 8

In a squirrel cage motor, low starting torque is a result of

Answer 8

Low resistance and high reactance

Question 9

A wound-rotor motor is always started with _________ in the rotor circuit.

Answer 9

Maximum resistance.

Question 10

To improve a motors service life, you should do all of the following EXCEPT:

Answer 10

Remove the windings insulation

Question 11

A synchronous motor requires:

Answer 11

Both AC and DC power

Question 12

When DC power is applied to the rotor of a synchronous motor….

Answer 12

A set of north and south poles is produced.

Question 13

With a low DC field current, the AC current:

Answer 13

Lags the potential difference

Question 14

Slipping a pole causes a(n):

Answer 14

Surge of current in the DC line

Question 15

The power factor of a synchronous motor will be less than 100% if the DC current in the rotor winding is:

Answer 15

Either too high or too low

Question 16

A motor with a single winding can be connected for:

Answer 16

Either four-pole or eight-pole operation

Question 17

Variable torque motors are designed to give:

Answer 17

An increase in torque with an increase in speed

Question 18

In a constant horsepower motor, the torque:

Answer 18

Doubles when the speed is cut in half

Question 19

When you reconnect the coils of an induction motor designed to run on a potential difference of 240V so that it will run on 480V

Answer 19

You reconnect the coils in series

Question 20

A two-parallel-Y connection:

Answer 20

Requires a lower potential difference than a series-Y connection

Question 21

When removing grease and oil from a motor, you should avoid using too much solvent because it

Answer 21

Can damage the insulation

Question 22

40degrees C is the

Answer 22

Standard temp for measuring insulation resistance

Question 23

A lack of torque, a decrease in speed, and a growling noises are all indications of a(n)

Answer 23

Open rotor circuit

Question 24

Angular loads are caused by forces applied to the bearing

Answer 24

Both parallel and perpendicular to the shaft

Question 25

200degrees F (95C) is the

Answer 25

Maximum temp at which bearings can be operated safely

Question 26

The capacity of the power supply does NOT limit the

Answer 26

Normal running speed of the motor

Question 27

At full load a motor connected to the line will do all of the following except:

Answer 27

Decrease its internal temperature

Question 28

A motor rated at 2HP at full load can develop ___to_____ hp if it delivers at max torque

Answer 28

3 to 4.5

Question 29

An inductor connected in series with the motor windings is found in a(n)

Answer 29

Reactor starter

Question 30

To ensure that three phase motor control equipment is in good operating condition, you should:

Answer 30

Maintain a regular plan of inspections

Question 31

Which of the following stops a motor when the potential difference falls too low and automatically restarts the motor when the potential difference returns to normal?

Answer 31

Low-voltage release

Question 32

During plugging, the current in an induction motor is higher than the locked-rotor current causing a(n)

Answer 32

Power loss

Question 33

A controller that provides both surge and backspin protection for a motor that drives a pump has a(n)

Answer 33

On-delay relay plus an off-delay relay

Question 34

Reduced voltage starters for three phase motors change the potential difference across the motor by means of:

Answer 34

Auto transformers or resistors

Question 35

When smooth starting is necessary but more than two steps are required, ________ is used.

Answer 35

Resistance starting

Question 36

The supply winding in an alternator

Answer 36

Can be part of either the rotor or the stator

Question 37

If the field winding has 4 poles, how many poles does the armature of the alternator have?

Answer 37

4

Question 38

There is no current in the armature of the alternator when the

Answer 38

Regulator is disconnected

Question 39

The size of the conductors in the winding determines the

Answer 39

Current an alternator can conduct without overheating

Question 40

If the load driven by an alternator is purely inductive, the current:

Answer 40

Lags the potential difference.

Question 41

When interlocks are used in motor-starting circuits to overcome voltage drop problems, which motor in the circuit is started first?

Answer 41

The largest

Question 42

In most automatic installations, power failure relays act if the potential difference drops to _______ of the rated value

Answer 42

70% or less

Question 43

Whenever an engine starts, it should run until all parts reach:

Answer 43

Their normal operating temperatures

Question 44

In order to avoid damaging an engine when only one cranking period is provided, the cranking attempt should last at least _______ seconds.

Answer 44

15

Question 45

In establishing for inspecting generators you should match the schedule to the maintenance schedule of the:

Answer 45

Prime mover

Question 46

The potential difference where the electric system of the utility company and the plant are connected is called the ________ voltage.

Answer 46

Service

Question 47

All of the following are benefits of system grounding, EXCEPT:

Answer 47

Allows service to be maintained in entire circuit until the fault is located

Question 48

A relay with inverse-time characteristics operates:

Answer 48

Faster as the current increases

Question 49

In a cascade system having large air-gap circuit breakers, there should not be more than ___ cascade step(s).

Answer 49

2

Question 50

In general, three phase motors will give satisfactory service when operated at what percent of their rated potential difference?

Answer 50

90%