Single-phase motors:Hysteresis Motor

Hysteresis Motor

The operation of this motor depends on the presence of a continuously-revolving magnetic flux. Hence, for the split-phase operation, its stator has two windings which remain connected to the single-phase sup- ply continuously both at starting as well as during the running of the motor. Usually, shaded-pole principle is employed for this purpose giving shaded-pole hysteresis motor. Alternatively, stator winding of the type used in capacitor-type motor may be used giving capacitor-type shaded-pole motor. Obviously, in either type, no centrifugal device is used.

The rotor is a smooth chrome-steel cylinder** having

high retentivity so that the hysteresis loss is high. It has no winding. Because of high retentivity of the rotor material, it is very difficult to change the magnetic polarities once they are induced in the rotor by the revolving flux. The rotor revolves synchronously because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity. However, the rotor poles always lag behind the stator poles by an angle a. Mechanical power developed by rotor is given by Ph is hysteres loss in rotor. Also Th = 9.55 Pm /Ns. It is seen that hysteresis torque depends solely on the area of rotor’s hysteresis loop.

The fact that the rotor has no teeth or winding of any sort, results in making the motor extremely quiet in operation and free from mechanical and magnetic vibrations. This makes the motor particularly useful for driving tape-decks, tape-decks, turn-tables and other precision audio equipment. Since, commercial motors usually have two poles, they run at 3,000 r.p.m. at 50-Hz single-phase supply. In order to adopt such a motor for driving an electric clock and other indicating devices, gear train is connected to the motor shaft for reducing the load speed. The unit accelerates rapidly, changing from rest to full speed almost instantaneously. It must

do so because it cannot accelerate gradually as an ordinary motor it is either operating at synchronous speed or not at all.

Some unique features of a hysteresis motor are as under :

(i) since its hysteresis torque remains practically constant from locked rotor to synchronous speed, a hysteresis motor is able to synchronise any load it can accelerate–something no other motor does.

(ii) due to its smooth rotor, the motor operates quietly and does not suffer from magnetic pulsations caused by slots/salient-poles that are present in the rotors of other motors.

In Fig. 36.56. is shown a two-pole shaded-pole type hysteresis motor used for driving ordinary household electric clocks. The rotor is a thin metal cylinder and the shaft drives a gear train.

Example 36.6. A 8-kW, 4-pole, 220-V, 50-Hz reluctance motor has a torque angle of 30º when operating under rated load conditions. Calculate (i) load torque (ii) torque angle if the voltage drops to 205 V and (iii) will the rotor pulled out of synchronism ?

Tutorial Problems-36.2.

1. A 230-V, 50-Hz, 4-pole, class-A, single-phase induction motor has the following parameters at an operating temperature 63ºC :

r1m = 2.51 ohms, r2¢ = 7.81 ohm, X m = 150.88 ohm, X im = 4.62 ohm, X 2¢ = 4.62 ohms Determine stator main winding current and power factor when the motor is running at a slip of 0.05 at the specified temperature of 63ºC.

[3.74 Ð48.24º, 0.666] (AMIE Sec. B Elect. Machines (E-B) Summer 1991)

2. A fractional horse-power universal motor has armature circuit resistance of 20 ohm and inductance of 0.4 H. On being connected to a 220-V d.c. supply, it draws 1.0 A from the mains and runs at 2000 r.p.m. Estimate the speed and power factor of the motor, when connected to a 230-V, 50-Hz supply drawing the same armature current. Draw relevant phasor diagram.

[1726 rpm, 0.84] (AMIE Sec. B Elect. Machines 1991)

3. A universal series motor, when operating on 220 V d.c. draws 10 A and runs at 1400 r.p.m. Find the new speed and power factor, when connected to 220 V, 25 Hz supply, the motor current remaining the same. The motor has total resistance of 1 ohm and total inductance of 0.1 H.

[961 rpm; 0.7] (AMIE Sec. B Elect. Machines 1990)

QUESTIONS AND ANSWERS ON SINGLE-PHASE MOTORS

How would you reverse the direction of rotation of a capacitor start-induction-run motor ?

Ans. By reversing either the running or starting-winding leads where they are connected to the lines. Both must not be reversed.

In which direction does a shaded-pole motor run ? Ans. It runs from the unshaded to the shaded pole (Fig.36.57)

Can such a motor be reversed ?

Ans. Normally, such motors are not reversible because that would involve mechanical dismantling and re-assembly. How-

ever, special motors are made having two rotors on a common shaft, each having one stator assembly for rotation in opposite direction.

What is a universal motor ?

Ans. It is built like a series d.c. motor with the difference that both its stator and armature are laminated. They can be used either on d.c. or a.c. supply although the speed and power are greater on direct current. They cannot be satisfactorly made to run at less than about 2000 r.p.m.

How can a universal motor be reversed ?

Ans. By reversing either the field leads or armature leads but not both.

How can we reverse the direction of rotation of repulsion, repulsion-

induction and repulsion-induction

and repulsion- start-induction-run motors ?

Ans. By shifting the brush positions by about 15º electrical.

How can we reverse the rotation of a 1-phase, split-phase motor ?

Ans. By reversing the leads to either the running or starter winding (Fig. 36.58) but not both.

What could be the reasons if a repulsion-induction motor fails to start ? Ans. Any one of the following :

1. no supply voltage

2. low voltage

3. excessive overload

4. the bearing lining may be stuck or ‘frozen’ to the shaft

5. armature may be rubbing

6. brush yoke may be incorrectly located

7. brush spacing may be wrong.

What could be the reasons if a split-phase motor fails to start and hums loudly ? Ans. It could be due to the starting winding being open or grounded or burnt out.