Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

ECT > Week 4 materials > Flashcards

Week 4 materials Flashcards

1
Q

List the advantages of AC Motors versus DC Motors:

A

-A readily available power supply
- Cheaper
- Maintenance cost
- Less weight

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What are two classes of Three Phase AC Motors?

A

Synchronous and asynchronous

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Synchronous motor

A

A synchronous motor is one whose rotor speed (Nr) is equal to the speed of the stator rotating magnetic field (Ns). In other words the speed of the shaft is rotating at the same speed as the rotating magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Asynchronous motor

A

An asynchronous motor is one whose rotor speed (Nr) is not equal to the speed of the rotating magnetic field (Ns). The asynchronous motor is more commonly called an induction motor.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

True or False: The asynchronous motor is more commonly called an induction motor.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What two main types of induction motors?

A

Squirrel-Cage and Wound Rotor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the 2 main parts of an AC motor?

A

Rotor and stator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the formula for calculating synchronous speed?

A

Ns = 60 * f/ P

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How will speed be affected by the increased pole pairs?

A

it would decrease the stator speed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

True or false: The purpose of the stator in both class motors is to create a rotating magnetic field.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the purpose of a stator?

A

The purpose of the stator in both class motors is to create a rotating magnetic field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is this formula used to calculate? Ns > Nr

A

asynchronous motor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Rotor

A

The rotating member of the motor and it provides the torque or power to do the mechanical work. It is connected to the
shaft of the motor.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is the difference in synchronous speed and rotor speed called?

A

Slip

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Why is slip necessary?

A

To maintain induction on the rotor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

True or false: The wound rotor induction motor can be used where the maximum torque is desired throughout the entire speed range

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Slip rings

A

allows the transmission of power and electrical signals from a stationary to a rotating structure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

True or False: The wound rotor induction motor can be used where the maximum torque is desired throughout the entire speed range.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

True or False: When R and XL are equal, the phase angle of the impedance of the rotor is 50°.

A

False. The phase angle of the impedance of the rotor is 45 degrees.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Starting torque for the wound rotor motor is with all the resistance at _________.

A

maximum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

In a wound rotor motor, to increase the speed, for full speed operation, the rotor winding resistance is _________ out through the contactor.

A

shorted

22
Q

True or False: Wound rotor motors can be used as variable-speed motors.

A

True

23
Q

How can we manipulate the torque of the wound rotor induction motors?

A

by changing the value of the externally connected resistances

24
Q

Can wound rotor motors can be used as variable-speed motors?

A

Yes

25
Q

For a 2-pole motor which has only ___ pair of poles per phase:

A

1 / Ns = 3600 rpm

26
Q

For a 4-pole motor which has ___ pair of poles per phase:

A

2
Ns = 1800 rpm

27
Q

For a 6-pole motor which has ___ pair of poles per phase:

A

3 / Ns = 1200 rpm

28
Q

For an 8-pole motor which has ___ pair of poles per phase:

A

4 / Ns = 900 rpm

29
Q

The percent slip at no-load is less than 1% while at full-load it is usually between _______.

A

3 to 5 %

30
Q

The losses in an induction motor consist of the ____ power losses and the _____ losses

A

Stray and copper

31
Q

True or false: Generally, the farther the code letter from A, the higher the inrush current per hp

A

True

32
Q

True or false: A power factor close to unity (100%) is most undesirable.

A

False. It is most desirable

33
Q

True or False: Most motors are Design A

A

False. Most motors are Design B

34
Q

True or False: A Design A motor has torque characteristics similar to those of the Design B motor, but there is no limit on starting inrush current. This may cause starter sizing problems.

A

True

35
Q

What is this formula used to calculate? (Output Power / Input Power) x 100

A

Efficiency

36
Q

True or False: The actual efficiency of the motor is guaranteed by the manufacturer to be within a tolerance band of this nominal efficiency

A

True

37
Q

The maximum allowed by NEMA standards represents an
additional ______ of motor losses from all sources, such as friction and windage losses, iron losses, and stray
load losses.

A

20%

38
Q

True or False: The service factor (S.F.) is required on a nameplate only if it is higher than 2.0

A

False. It is 1.0

39
Q

Define duty on a nameplate

A

This block on the nameplate defines the length of time during which the motor can carry its nameplate rating safely.

40
Q

What does CSA stand for?

A

Canadian Standards Association

41
Q

True or false: UL indicates that the manufacturing system and the motor components meet the standards of, Underwriters Laboratories.

A

True

42
Q

What does “INSUL CLASS” on nameplates mean?

A

It is an industry standard classification of the thermal
tolerance of the motor winding

43
Q

True or false: At higher altitudes, the motor would tend to run hotter because the thinner air cannot remove the heat so effectively, and the motor may have to be derated.

A

True

44
Q

What does “ENCL” mean on a nameplate?

A

Classifies the motor as to its degree of protection
from its environment, and its method of cooling

45
Q

True or false: The most common variations for ENCL on a nameplate are Open Drip-Proof (ODP) and Totally Enclosed Fan Cooled (TEFC).

A

True

46
Q

Open Drip-Proof (ODP)

A

ODP-An open drip-proof motor allows a free exchange of air from outside the motor to circulate around the winding while being unaffected by drops of liquid or particles that strike or enter the enclosure at any angle from 0 to 15 deg. downward from the vertical

47
Q

Totally Enclosed Fan Cooled (TEFC)

A

A totally enclosed fan cooled motor prevents free exchange of air between inside and outside the motor enclosure. It has a fan blowing air over the outside of the enclosure to aid in cooling. A TEFC motor is not considered air or water-tight; it allows outside air containing moisture and other contaminants to enter, but usually not enough to interfere with normal operation.

48
Q

True or false: The first two digits of the frame size divided by 4 defines the height of the shaft centerline from the bottom of the feet.

A

True

49
Q

What does the third digit in the frame size tell you?

A

determines the distance between the foot holes nearest the shaft and the opposite drive-end foot holes

50
Q

True or False: The easiest field test that prevents the most failures is the ground-insulation test

A

True

51
Q
A

ECT (3 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions