Three-Phase Motors- Test 2

Question 1

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

Answer 1

maximum

Question 2

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

Answer 2

reverses

Question 3

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

Answer 3

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

Question 4

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

Answer 4

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

Question 5

Define the term synchronous speed.

Answer 5

The synchronous speed is the speed of the rotating magnetic field

Question 6

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

Answer 6

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

Question 7

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?

Answer 7

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

Question 8

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?

Answer 8

through electromagnetic induction

Question 9

Define the term slip speed.

Answer 9

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

Question 10

Define the term percent slip.

Answer 10

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

Question 11

Define the term speed regulation.

Answer 11

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

Question 12

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

Answer 12

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

Question 13

Define the term full-load torque.

Answer 13

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

Question 14

Define the term breakdown torque

Answer 14

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.

Question 15

Define the term locked rotor torque

Answer 15

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.

Question 16

Define the term starting torque.

Answer 16

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

Question 17

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

Answer 17

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

Question 18

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

Answer 18

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.

Question 19

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

Answer 19

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%

Question 20

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.

Answer 20

Speed regulation = (No-load speed - Full-load speed)/ Full-load speed X 100%
= 1191-1171/ 1171 x 100%
= 20/1171 × 100
= 1.7%

Question 21

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

Answer 21

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

Question 22

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?

Answer 22

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%

Question 23

Which part of the linear-induction motor actually moves?

Answer 23

23. the stator

Question 24

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

Answer 24

metres per second

Question 25

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

Answer 25

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