Operating Principles of Split-Phase Induction Motors

Question 1

Flux produced by a winding is:

a. in phase with the voltage applied to the coil
b. in phase with the current flowing through the coil

Answer 1

b. in phase with the current flowing through the coil

Question 2

What is the physical displacement between the main and auxiliary windings of a two-pole split phase motor?

Answer 2

• In a two-pole motor it is 90 mechanical and electrical degrees
• In motors with more poles, it will still be 90 electrical degrees but less than 90 mechanical degrees

Question 3

How does a magnetic pole move?

a. from a main winding to an auxiliary winding
b. from an auxiliary winding to a main winding in the stator

Answer 3

b. The magnetic pole moves from the auxiliary winding to a similar polarity main winding. The higher inductance of the main winding causes the current and flux in this winding to lag behind the current and flux in the auxiliary winding

Question 4

How is the direction of rotation reversed in a split-phase motor?

Answer 4

by reversing the direction of current through the auxiliary winding

Question 5

Why does a standard split-phase motor (without a starting capacitor) have a low starting torque compared to a capacitor-start, split phase motor?

Answer 5

The standard split phase motor has a phase shift between the main and auxiliary winding of 30-50 electrical degrees. The capacitor-start, split phase motor has a phase shift of 90 electrical degrees, which corresponds better with the physical displacement of the poles in the motor

Question 6

Which type of split-phase motor combines good starting torque with good overload capacity?

Answer 6

the capacitor-start, capacitor-run split-phase motor

Question 7

Which type of split-phase motor operates at synchronous speed?

Answer 7

the reluctance motor

Question 8

What modification is made to the rotor in the motor that operates at synchronous speed?

Answer 8

Parts of the rotor core are cut away to produce salient poles that lock into step with the rotating field

Question 9

For a split-phase motor, at approximately what percentage of rated speed does the centrifugal switch open?

Answer 9

75%

Question 10

In a dual-capacitor type of split-phase motor, which capacitor is larger:

a. the starting capacitor
b. the running capacitor

Answer 10

a. The starting capacitor is approximately 10-15 times larger than the running capacitor