Chapt. 17 Review ?'s

Question 1

Upon what kind of field does the operation of all AC motors depend?

Answer 1

On a revolving magnetic field

Question 2

How is the revolving field speed of an AC motor figured?

Answer 2

Field RPM = 2 x CPM / Number of field poles
or
N = 120f / P
N = field RPM
f = frequency in CPS (Hertz)
P = number of poles produced by the three-phase winding

Question 3

What are the two general groups of three-phase motors?

Answer 3

1. Syncronous

2. Non-syncronous

Question 4

What is the synchronous speed of a motor?

Answer 4

3600 RPM on a two pole motor.

Question 5

What is the difference between a synchronous and non-synchronous motor?

Answer 5

Synchronous: shaft is magnetically locked in step with the revolving motor field.
Non-synchronous: not locked

Question 6

How are synchronous motors constructed?

Answer 6

Almost exactly like AC generators.

Rotor has salient poles.

Question 7

What are the common characteristics of synchronous motors?

Answer 7

1. Require a direct current excited field.
2. Operate at a constant speed regardless of load.
3. May be used for power factor correction & voltage regulation.
4. Require more control equipment and maintenance than non-syncronous.
5. Have no built-in starting torque

Question 8

How do non-synchronous or induction motors differ from synchronous motors?

Answer 8

The non-sync’s rotor pole coils each form closed loops and the coil loops get their current flow by induction from the revolving field rather than from a separate DC supply.

Question 9

Explain how induction causes the rotor of a three-phase induction motor to rotate?

Answer 9

The induction causes current flow in the closed rotor coils & creating a strong flux pole which causes the rotor to turn.

Question 10

What is the slip of a motor?

Answer 10

The difference between the speed of the revolving field of the stator & the actual speed of the rotor.

Question 11

What is wye-delta starting and how does it affect motor current?

Answer 11

Motor starts with the winding wye-connected & switched to delta when the motor is up to speed.
The starting connection impresses 58% of line voltage, while the running connection impresses 100% of line voltage.

Question 12

What effect does the resistance of the rotor of an induction motor have on the motor starting torque and efficiency?

Answer 12

It gives it a high starting torque and better speed control

Question 13

Describe briefly the series or universal type single-phase commutator motor, its advantages and disadvantages?

Answer 13

The armature (rotor) has a commutator for directing the current through the armature coils and the series-connected field (stator) windings.
Its speed can be regulated.
Has poor efficiency.

Question 14

Name and describe the 3 types of capacitively displaced split-phase motors?

Answer 14

1. Capacitor-start; the capacitor-displaced phase is in the circuit only during starting
2. Split-phase cap.; the capacitively displaced is in the circuit during both starting & running.
3. 2-value cap motor; there are different values of effective capacitances for starting & running.

Question 15

What are the advantages and disadvantages of each of the 3 types of capacitor motors?

Answer 15

1. Capacitor-start; inherent low power factor.
2. Permanent split-phase; high power factor rating, but starting torque poor.
3. Two-value split phase; high starting torque and high power factor rating, but cost is greater.

Question 16

Why is the use of shaded-pole motors limited and for what services are they commonly used?

Answer 16

They have a very weak starting torque.

They are used on small fans like ceiling fans.

Question 17

Of what practical use are the Selsyns?

Answer 17

They are used for remote indication of position, levels, and the like.

Question 18

Briefly describe single-phase synchronous motors and their use.

Answer 18

They have a laminated field forming two poles and excited by an AC current coil. Poles are divided & a shading coil of one turn of heavy copper is placed over ½ of each pole.

Question 19

What is rotor clearance and why is it important?

Answer 19

It is the clearance between the stator and rotor of a motor.

Important to obtain maximum torque.

Question 20

What are some of the common causes of loss of rotor clearance ?

Answer 20

Oil collects dust eventually locking the rotor.

Bearing wear causes rotor drop rubbing the stator

Question 21

What are the two general classes of AC motors?

Answer 21

1. Squirell-cage rotor type

2. Wound-rotor type