Question 1. Identify the following terms:a) Synchronous speed, ns

b) Rotor speed, nr

c) Slip speedd) Rotor current frequency, fr

e) Slip

Answer:

A) Synchronous speed, ns is the speed of the rotating magnetic field produce by the three phase stator windings.

B) Rotor speed, nr is the speed at which the motor is running or the speed of the rotor.C) The slip speed is simply the difference between the synchronous speed, ns and the rotor

speed, nr. Slip speed is given as ns-nr.D) Rotor current frequency, fr is the number of revolutions the rotor makes in one second.E) Slip or fractional slip is the ratio of slip speed, (ns-nr) to the synchronous speed, ns and is

given by Sli p , S=ns−nrns

×100 and is usually expressed as a percentage or per unit.

Question 2. Explain the principle of operation of a 3ø induction motor (with aid of a diagram)

Answer:

Fig.1

Fig 1 above shows a three phase supply connected to the stator windings producing a rotating magnetic field. An e.m.f is induced when the magnetic field cuts a bar on the rotor and thus current flows. The interaction between the magnetic field and the current flowing produces a force turning the rotor in the same direction as the magnetic field. Hence a torque is produced on the rotor.

Question 3. a) Explain the principle of operation of a 1ø split-phase induction motor (with aid of a diagram)b) State how the direction of rotation may be reversed for the motor above.

Answer: a) The split phase induction motor also called the capacitor start induction run motor,

consists of 2 windings, a main winding and an auxillary winding placed adjacent to each other. The rotor is positioned between these 2 windings and connected in series with the auxillary winding, is a capacitor and a centrifugal switch all connected across a single phase A.C supply.When a single phase voltage is applied the motor is started and the centrifugal switch closes and a current Is flows from the supply. This current then splits into two, I1 flowing into the auxillary winding and I2 flowing to the main winding. The capacitor in the auxillary circuit causes the current I1, to lead the current I2 by 90°. The split currents, I1 and I2 causes an interaction with the magnetic field produced by the two windings. This interaction produces a torque on the rotor and hence the motor starts. After starting the centrifugal switch automatically opens, this causes an increases in the current flowing to the main winding (since Is=I2). This results in the increase in the speed of the motor runs on its main winding only.

1ф Split Phase Induction Motor

b) The direction of rotation for this motor can be reversed by simply reversing the polarity of either the auxillary winding or the main winding

Is

I2

Auxillary windingMain Winding

Rotor

Centrifugal Switch

Capacitor1ф A.C Supply

I1

Question: 4. a) Explain the difference between a caged rotor and a wound rotor induction motor.b) State three advantages of wound rotor induction motors against caged rotors.

Answer: a) The main difference between the cage rotor and the wound rotor for induction motors is the presence of slip rings in the wound rotor and the absence in the cage rotor.

b) The advantages of the wound rotor motor compared with the cage rotor are that they: Have a much higher starting torque Have a much lower starting current Have a means of varying speed by use of external rotor resistance.

Question: 5. a) State three features of a synchronous motor.b) Identify the two types of synchronous motors.c) Explain why a three-phase synchronous motor is not self-starting (with aid of a diagram).d) Explain why salient pole rotors are not suitable for high speeds.

Answer: a) Three features of a synchronous motor: It runs at synchronous speed The motor can be made to operate with a leading power factor by exciting its winding The motor has no net starting torque

b) Two types of synchronous motor are salient pole rotor and cylindrical rotor.

c)

3ф Synchronous Motor

The diagram above represents a synchronous motor. When a three phase supply is applied, he rotor is lock due to inertia, when the stator windings are energized and creates a rotating magnetic field revolving at the designated motor speed. Thus the rotor is not able to follow the rotating magnetic field, hence the synchronous motor is not self starting and requires other means of starting.

d)Salient poles are used only with two speeds because it is difficult to build to withstand the stresses at high speeds. They also have high winding losses at high speeds.

Separate Starter

Rotor

Brown

Black

Grey

Stator Windings

3ф Supply

Question: 6.a) Explain why an induction motor cannot run at synchronous speed.b) State the value of Slip when the rotor of the above motor is at standstill.c) State the value of torque at synchronous speed.

Answer: a) If the three phase induction motor runs at synchronous speed, then there wil be no cutting of flux by the rotor conductors, hence no induced emf in the rotor conductors and therefore no torque would be produced causing the motor to stall at synchronous speed.

b)The value of Slip is equal to 100% when the motor is at standstill.

c)The value of torque is equal to zero at synchronous speed.

Question: 7. a) Explain why a reduction in applied voltage is necessary when starting large induction motors.b) State the value of starting current to full-load current (ratio) for a three-phase induction motor.

Answer: a) When starting large induction motors, it is necessary to reduce the starting voltage of the

motor since a small torque is required for starting and torque varies in squared proportion to applied voltage.

b) The starting current for a three phase induction motor may range anywhere between 4-7 times its full load current.

Question: 8.Explain (with aid of a diagram) one method of starting a 3ø synchronous motor.

Answer:

3ф Induction Motor

The synchronous motor is provided with a commutator for producing dirct current for self excitation, where the machine starts as a D.C motor with the ammature and field connected in parallel.

Brown

Separate Starter

D.C Starter

Rotor

Black

Grey

Stator Windings

3ф Supply

Question: 9. Explain (with aid of a diagram) Star-delta starting for 3ø Induction motors.

Answer:

Star-Delta starting is 1 method that is used for starting larger induction motors. The stator windings of the motor is connected in a star configuration during starting, here it receives about 0.577 times its supplied voltage. The reduced voltage is necessary to obtain the required starting torque after starting a change over switch converts the stator windings from a star connection to a delta configuration; here each winding receives the full supplied voltage. This causes the speed of the motor to increase and hence the motor runs on he delta configuration.

Question: 10. Explain (with aid of a diagram) Auto- transformer starting for 3ø Induction motors.

Answer:

An auto transformer is used for starting large 3ф induction motors. The motor is connected via the tapping on the auto transformer where it receives a reduced voltage based on the transformer tapping, this is necessary to obtain the required starting torque, after starting a double throw switch connects the auto transformer to the run position where the full supplied voltage is connected to the motor.

Question: 11. Explain (with aid of a diagram) Direct-on-line starting for small 3ø induction motors.

Answer:

Direct on line starting is used when starting small induction motors, the starting current is high since the motor is starting directly from the it’s supply; this may cause interference which supplies other consumers. A direct online starter is shown above.

Question: 12. a) State one application of a 3ø cylindrical rotor synchronous motor.b) State one advantage and one disadvantage of auto-transformer starting compared to direct-on-line starting.c) State the starting current in terms of full load current when using the following starters:i) Direct-on-lineii) Star-deltaiii) Auto-transformer with a 70% tapping

Answer: a) One application of a 3ф cylindrical rotor synchronous motor is an air compressor

b)One advantage of auto transformers starting compared to direct on line starting is that it can start large induction motors. One disadvantage is that the auto transformer starter cannot start small induction motor.

c)The starting current in terms of full load current when using the various starters are stated below:

i. Direct on line – 4:1ii. Star delta – 0.3:1

iii. Auto transformer with a 70% tapping – 1:0.7

Question: 13. State why D-O-L starting is sometimes limited in terms of the motor power rating.

Answer: Direct on line starting is sometimes limited in terms of the power rating of the motor, because higher ratings may cause prolonged speed on the motor which cause it to suffer damage from overheating.

Question: 14. State the condition for maximum torque in a 3ø induction motor.

Answer: Maximum torque occurs when rotor resistance and rotor reactance are equal. i.e. R2=Xr.

Question: 15. State the formula for slip when rotor resistance and rotor reactance is given.

Answer: S=R2X 2