J2

Question 1

List the two ways to start polyphase, squirrel-cage motors

Answer 1

full-voltage (across the line)

reduced-voltage starting

Question 2

What are the advantages of full-voltage starting

Answer 2

simplest control
lowest component cost
max start torque
min acceleration time

Question 3

When is reduced-voltage starting used?

Answer 3

where full-voltage starting may cause problems

Question 4

What is the main function of reduced-voltage starters?

Answer 4

lower the inrush current drawn by the induction motor.

Question 5

Reduced-voltage starters are usually categorized as

Answer 5

open transition or closed transition

Question 6

Describe an open transition reduced-voltage starter

Answer 6

the motor is very briefly disconnected from the line as the control transfers from the start to the run position

Question 7

Describe a closed transition reduced-voltage starter

Answer 7

the voltage to the motor is maintained as the control transfers from the start position to the run position.

Question 8

Since reducing the starting voltage reduces both stator flux and rotor current, the starting torque is

Answer 8

directly proportional to the square of the starting voltage

Question 9

A motor that will not start a load under full voltage will

Answer 9

definitely not start a load under reduced voltage

Question 10

where a motor is started and stopped frequently

Answer 10

a larger-than-normal contactor must be used if the normal life expectancy of the contacts is to be obtained.

Question 11

the most significant factor in determining whether a motor should be started across the line, or reduced voltage is

Answer 11

whether the power source can handle it.ie

Question 12

State the advantages that reduced-voltage starting has over across-the-line starting for AC motors

Answer 12

reduced inrush current
reduced torque
less disturbance (line drop) to the power system

Question 13

Describe primary-resistance starters

Answer 13

a power resistor is connected in series with each of the three-phase lines
when the motor reaches a certain speed the resistance is removed giving the motor full voltage

Question 14

what is the major disadvantage of the primary-resistance motor starter?

Answer 14

the large power loss in the resistors during starting.

Question 15

Describe an auto-transformer starter

Answer 15

steps down the starting voltage using taps on an auto-transformer

Question 16

The auto-transformer starter provides more

Answer 16

starting torque per line ampere of current than any other type of reduce-voltage starter using transformer action

Question 17

Which three-phase auto-transformer connection can sometimes result in unbalanced starting phase voltages? Wye or Delta

Answer 17

Delta

See page J2 - 19

Question 18

Why are auto-transformers used for motor starting generally intended for intermittent duty?

Answer 18

if they are left in the cct too long they may overheat and become damaged

Question 19

the auto-transformer starter with a closed-circuit transition is sometimes called the

Answer 19

Korndorfer-connection

Question 20

What standard taps are available on motor-starting auto-transformer starters?

Answer 20

80%, 65%, 50%

Question 21

Auto-transformer starters are sometimes referred to as

Answer 21

compensator starters

Question 22

Describe the wye-delta motor starter

Answer 22

also called the star-delta
requires access to each phase winding
only six-lead or 12-lead motors can be used
has only one starting step
connects in wye for starting
connect in delta for running

Question 23

Since each phase lead is brought out from the motor the rating of the overload relay is based on the

Answer 23

phase value

Question 24

List the two common methods of automatic acceleration for wound-rotor motors

Answer 24

definite-time acceleration

speed-sensing acceleration

Question 25

a speed-sensing acceleration starter uses a frequency-sensing relays to sense the speed of the rotor and

Answer 25

time the removal of the secondary resistance according to the load

Question 26

A regeneration starter used on a wound-rotor-motor replaces the resistors with an electronic cct that

Answer 26

recaptures the energy and transfers it back into the primary cct.

Question 27

what happens to the magnitude and frequency of the voltage induced in the rotor as the motor accelerates

Answer 27

decrease

Question 28

most electrical problems fall into one of following categories

Answer 28

opens
shorts
grounds
high resistances

Question 29

List the parts of a synchronous motor starter

Answer 29

out-of-step relay
field discharge resistor
field contactor
polarized field frequency relay (PFR)

Question 30

Describe the function of the PFR

Answer 30

monitors the rotor speed to time the application of the DC field current at about 92% and 97% or sync speed
time application so the DC field will be aligned with opposite poles in the synchronous field
disconnects the DC field excitation should an overload or voltage dip cause the motor speed to drop and reapplies it should the speed return to within sync speed before the out-of-step relay opens its contact

Question 31

List the types of motor brakes

Answer 31

friction or electromechanical brakes
plugging
dynamic braking
regenerative braking
eddy-current braking

Question 32

Most friction motor brakes are

Answer 32

spring-applied and electromagnetically released

Question 33

Advantages of friction-type brakes

Answer 33

hold motor shaft from rotating after motor has stopped
few control cct components
automatically applied when a power failure occurs
brake cannot reverse the motor

Question 34

Disadvantages of friction-type brakes

Answer 34

friction parts require maintenance and replacement
tend to be bulky
initial cost of the brake is high

Question 35

Advantages of plugging

Answer 35

fastest deceleration method
no friction parts
suitable for large motors and severe duty

Question 36

Disadvantages of plugging

Answer 36

requires reversing controller
requires a zero-speed switch or consistent time-delay switch
will not operate if a power failure occurs
draws large peak currents
may require special bracing to handle the high stress
motor is free to rotate after stopping
if a time-delay relay is applied too long motor may start in reverse

Question 37

Dynamic Braking or DC injection braking

Answer 37

stopping the motor by making it act as a generator

Question 38

Advantages of dynamic braking

Answer 38

can not reverse the motor is applied too long
gives smooth positive retardation
braking torque is adjustable by varying DC applied to the stator
no friction parts

Question 39

Disadvantages of dynamic braking

Answer 39

DC excitation is required during braking
braking is not available if source voltage fails
motor is free to rotate after stopping
frequent stopping may cause motor overheating

Question 40

Regenerative Braking

Answer 40

makes the motor act like a generator and feed energy back into the system

Question 41

Disadvantage of regenerative braking

Answer 41

only effective above synchronous speed

Question 42

Advantages of eddy-current brake

Answer 42

cannot reverse the motor if applied too long
adjusted by varying the voltage across the brake coils
no friction parts

Question 43

Disadvantages of eddy-current braking

Answer 43

more expensive
not available if source voltage fails
motor is free to rotate after stopping
braking action drops off as the speed of the motor decreases