# Three-Phase Motors- Test 2 Flashcards

1
Q

The flux created by any winding will be at a maximum when the current through that winding is at a ___________

A

maximum

2
Q

When the current through a winding reverses, the flux direction created by that winding
_____________

A

reverses

3
Q

What effect does increasing the frequency of the supply have on the speed of the rotating magnetic field?

A

Increasing the frequency of the supply increases the speed of the rotating magnetic field.

4
Q

What effect does increasing the number of poles have on the speed of the rotating magnetic field?

A

Increasing the number of poles in the machine decreases the speed of the rotating magnetic field.

5
Q

Define the term synchronous speed.

A

The synchronous speed is the speed of the rotating magnetic field

6
Q

What would be the synchronous speed of an eight-pole motor connected to a 60-hertz supply?

A

n = f x 60/P
n - 60 Hz x 60 / 4
= 900 rpm

7
Q

What would be the number of poles in a motor with a synchronous speed of 720 rpm when the motor is connected to a 60-hertz supply?

A

P = f x 60/n
P = 60 Hz x 60/ 720 rpm
= 5 pairs of poles
= 10 poles

8
Q

Since there is no electrical connection between the stator and the rotor, how is current made to flow in the rotor of a squirrel-cage induction motor?

A

through electromagnetic induction

9
Q

Define the term slip speed.

A

Slip speed is the difference between synchronous speed and rotor speed.

10
Q

Define the term percent slip.

A

Percent slip is the slip speed expressed as a percentage of the synchronous speed.

11
Q

Define the term speed regulation.

A

Speed regulation is the difference between no-load speed and full-load speed of the rotor.

12
Q

How is the direction of rotation reversed in a squirrel-cage induction motor?

A

By reversing the direction of the rotating field. This is accomplished by interchanging any two of the stator line connections to the source.

13
Q

A

The full-load torque of a motor is the torque necessary to produce rated horsepower at rated full-load speed.

14
Q

Define the term breakdown torque

A

The breakdown torque of a motor is the maximum torque the motor will produce. It is sometimes referred to as the stalling torque because, if the motor load is increased beyond that point, the motor will quickly slow down and stall.

15
Q

Define the term locked rotor torque

A

The locked rotor torque of a motor is the maximum torque the motor will develop at rest for all angular positions of the rotor with rated voltage and frequency applied to the stator.

16
Q

Define the term starting torque.

A

The starting torque of a motor is the same as the locked rotor torque.

17
Q

At what rotor speed will the stator current be the highest?

A

When the rotor is at a standstill (100% slip)

18
Q

What type of loads is a NEMA design class C motor suitable for?

A

Loads with starting torques equal to or greater than their breakdown torque. Examples are loads such as large centrifugal blowers, flywheels, crusher drums, loaded compressors, loaded piston pumps and loaded conveyors.

19
Q

Find the slip speed and percent slip of a six-pole, 60-hertz induction motor when the rotor is turning at 1158 rpm.

A

Synch speed, n =60 × f/ P
n = 60 x 60/ 3
= 1200 rpm

Slip speed = Synch speed - Rotor speed
= 1200-1158
= 42 rpm

Percentage slip = Slip speed/ Synch speed x 100
= 42/ 1200 X100
= 3.5%

20
Q

A six-pole, 60-hertz induction motor has a no-load speed of 1191 rpm and a full-load speed of 1171 rpm. Calculate the percent speed regulation.

A

= 1191-1171/ 1171 x 100%
= 20/1171 × 100
= 1.7%

21
Q

A motor is rated at 55.95 kW. How much horsepower is this?

A

Power (in hp) = Power (in watts) / 746
= 55.95 x 10^3 watts /746
= 75 hp

22
Q

A 25-horsepower, three-phase, 575-volt motor delivers rated horsepower to the load. The motor is connected to a 600-volt, three-phase source and draws 24 A at a power factor of 87% lagging. What is the efficiency of the motor?

A

P(in) = √3 V(L) I(L) cos 9
= √3 x 600 x 24 x 0.87 / 1000
= 21.7 kW

P(out) = (P (in) hp) x 746 / 1000
= 25 x 746 / 1000
= 18.65 kW

Efficiency = Output P/Input P X 100

= 18.65/ 21.7 x 100
= 85.94 = 86%

23
Q

Which part of the linear-induction motor actually moves?

A
1. the stator
24
Q

In what units is the synchronous speed of a linear-induction motor measured?

A

metres per second

25
Q

Relative to the direction of the moving magnetic field, in what direction does the linear- induction motor move?

A

The stator travels in the direction opposite that of the moving field.