Deck 33: Three-Phase Motors

Question 1

The squirrel-cage rotor code letters indicate the shape (type) of rotor bars. Different types of bars have a large effect on the of a motor.

A) operating voltage
B) overload current
C) starting torque
D) synchronous speed

Answer 1

C

Question 2

A motor is operating at 3450 RPM and is producing a torque of 32 lb-in. What is the output horsepower of the motor?

A) 1.75 hp
B) 2.13 hp
C) 4.57 hp
D) 6.93 hp

Answer 2

A

Question 3

A squirrel-cage rotor induction motor has a synchronous speed of 3600 RPM and an actual rotor speed of 3450 RPM. What is the percent slip?

A) 4.17%
B) 4.35%
C) 24%
D) 23%

Answer 3

A

Question 4

The squirrel-cage rotor is constructed of and end embedded in steel laminations.

A) bars; bars
B) rings; rings
C) bars; rings
D) rings; bars

Question 5

Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the lower of the two voltages, the windings are connected in .

A) series
B) parallel

Question 6

The wound-rotor induction motor is popular in industry because it has starting torque and starting current.

A) low; low
B) low; high
C) high; low
D) high; high

Question 7

Some motors are constructed so that they will operate on either of two voltages. When they are connected to operate on the higher of the two voltages, the windings are connected in .

A) series
B) parallel

Question 8

The synchronous motor is an induction motor.

Question 9

If a piece of machinery needs a replacement motor and will be damaged if the new motor rotates the wrong way. The test equipment needed to test the rotation direction of the new motor is a .

A) phase angle voltmeter
B) MEGGER®
C) digital multimeter
D) phase rotation meter

Question 10

For most squirrel-cage induction motors the typical percent slip is about percent.

A) zero to one
B) two to five
C) five to seven
D) eight to ten

Question 11

In the consequent pole motor, if the number of poles is doubled, the motor speed will be .

A) stay the same
B) cut in half
C) double
D) go to zero

Question 12

Three lines supply power to a three-phase motor. If one of these lines should open, the motor will be connected to single-phase power. This condition is known as .

A) double-phasing
B) single-phasing
C) triple-phasing
D) free-running

Question 13

The frequency of the current in a squirrel-cage rotor as the rotor approaches synchronous speed.

A) increases
B) stays the same
C) decreases
D) is cut in half

Question 14

In a three-phase motor, the speed of rotation of the magnetic field is called the speed.

A) magnetic
B) slip
C) synchronous
D) rotor

Question 15

To reverse the direction of rotation of a three-phase motor, reverse .

A) any one lead
B) any two leads
C) any three leads
D) all leads

Question 16

Selsyn motors operate on voltage.

A) two-phase
B) single-phase
C) three-phase

Question 17

The torque of a squirrel-cage induction motor will be maximum when the phase angle between the stator and rotor fields is .

A) 0°
B) 90°
C) 180°
D) 270°

Question 18

When a squirrel-cage motor is first started, the value of the current draw is .

A) equal to the running current
B) smaller than the running current
C) twice the running current
D) several times the running current

Question 19

A synchronous motor operated with no load and used for power factor correction is sometimes called a .

A) synchronizer
B) capacitor
C) selsyn
D) synchronous condenser

Question 20

The set of squirrel-cage bars on the rotor of a synchronous motor is used to start the motor and is known as the _____.

A) amortisseur winding
B) starting bars
C) synchronous starter
D) rotor bars

Question 21

The differential selsyn has three stator windings connected in a wye and a rotor made of .

A) three windings connected in a wye
B) one winding