The basic principle of the speed control is that the speed can be controlled by injecting the voltage into the secondary circuit of the motor. When more current flows through the secondary circuit the speed will increase and if the current is less then the speed will be low. These motors are generally classified as
- Scharge motor.
- Susceptor type.
Scharge motor
It is a 3 phase commutator motor of which the speed can be increased or decreased beyond normal effectively. It is a variable speed commutator motor. The stator is would as three phase induction motor. The stator has two windings: (i) the primary winding and (ii) the secondary winding or adjustable or regulating or compensating winding. The primary winding is connected across the three phase supply through three sliprings. A separate D.C. armature winding known as adjustable or regulating winding is housed in the rotor slots. The leads of the secondary coils are connected to the commutator. There are three sets of brushes which are placed on the commutator as shown in Fig. 20.11 and each stator winding is connected across two brushes. The brushes can be moved in any of thg direction around the commutator by the lever attached or by means of a small motor. The magnitude of the e.m.f. injected and its slip frequency injected in the secondary circuit, depends upon the spacing between the two sets of brushes and the angular displacement depends upon the angular position with respect to the centre of the rotor winding.
Action. Let three phase suplpy is given to the sliprings. A rotating magnetic field is produced by the rotor winding. Initially let the stator winding brushes are placed on the same segment, it means the stator winding is short circuited. Due to electromagnetic induction the voltage is induced in the stator winding which is short circuited and the motor is started as an induction motor. In the compensation or regulating winding the e.m.f. is induced due to transformer action. Now as the sets of the brushes are placed on the commutator and on the same segment, it means the stator winding carries only the stator winding current and not the injected current. Now if the brushes are separated then the e.m.f. induced in that particular portion of the compensating Winding is injected in the stator circuit. The speed depends upon the e.m.f. injected, if the injected voltage is in the phase opposition to the secondary voltage then the speed will fall. In other position if the votlage injected is obtained by moving the brushes in the opposite direction to that of first, the voltage will be in phase with the secondary voltage and the speed will increase. Thus by changing the direction of the current in the stator circuit the speed of the motor can be controlled.
The power factor can also be controlled, if the injected e.m.f. is in quadrature leadng the induced rotor e.m.f. the power factor of the motor will be improved.
Advantages
Following are the advantages of this motor:
- Good control and regulation of the speed is obtained. The speed can be varied from zero r.p.m. to above normal speed.
- Power factor can be improved.
Disadvantages
Following are the disadvantages:
- Due to wound rotor the cost of the motor is more.
- Maintenance cost is more.
- More frictional losses, so the efficiency is effected.
- Skilled worker is required to operate.
Uses. The variable speed commutator motors are used where a variable speed with good speed ægulation is required.