Speed control of shunt motor

Flux Control Method

It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.

To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux. Field current is relatively small and hence I2R loss is small, hence this method is quiet efficient. Though speed can be increased by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation.

Armature Control Method

Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa. That is when supply voltage V and armature resistance Ra are kept constant, speed is directly proportional to armature current Ia. Thus if we add resistance in series with armature, Ia decreases and hence speed decreases.

Greater the resistance in series with armature, greater the decrease in speed.


Voltage Control Method

• Multiple voltage control:

In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.


• Ward-Leonard System:

This system is used where very sensitive speed control of motor is required (e.g. electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside.

M2 is the motor whose speed control is required. M1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M1. In this method the output from the generator G is fed to the armature of the motor M2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.

DC series motor starter

Construction of DC series motor starters is very basic as shown in the figure. A start arm is simply moved towards right to start the motor. Thus at first maximum resistance is connected in series with the armature and then gradually decreased as the start arm moves towards right. The no load release coil holds the start arm to the run position and leaves it at no load.

3 Point Starter

When motor is to be started, the lever is turned gradually to the right. When lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series. Hence at starting full resistance is added in series with armature. Then as the lever is moved further, the resistance is gradually is cut out from the armature circuit. Now, as the lever reaches to position 6, all the resistance is cut out from the armature circuit and armature gets directly connected across the supply. The electromagnet E (no voltage coil) holds the lever at this position. This electromagnet releases the lever when there is no (or low) supply voltage.
When the motor is overloaded beyond a predefined value, overcurrent release electromagnet D gets activated, which short circuits electromagnet E, and hence releases the lever and motor is turned off.

Important of starting method of a DC motor

Thus, to avoid the above dangers while starting a DC motor, it is necessary to limit the starting current. For that purpose, starters are used to start a DC motor. There are various starters like, 3 point starter, 4 point starter, No load release coil starter, thyristor starter etc.

The main concept behind every DC motor starter is, adding external resistance to the armature winding at starting.

4 Point Starter

In four point starters, the hold on coil is connected directly across the supply line through a protective resistance R. when the armature touches stud no 1.the line current divides into three parts 

i) Armature starting resistance and overload release. 

ii) A variable resistance and shunt field winding. 

iii) Holding coil and current limiting resistance.

The field gets directly connected to the supply, as the lever moves touching the brass arc. The no voltage coil (or Hold on coil) is connected with a current limiting resistance Rh. This arrangement ensures that any change of current in the shunt field does not affect the current through hold on coil at all. This means that electromagnet pull of the hold-on coil will always be sufficient so that the spring does not unnecessarily restore the lever to the off position. This starter is used where field current is to be adjusted by means of a field rheostat.

The basic difference between three point and four point starters is the manner in which the hold on coil is connected. The unnecessary tripping of starter can be stopped by connecting separately or parallel both magnetizing and field coil. They are connected in such a way that both should carry their individual current. Thus voltage drop in one coil will not affect the voltage in other coil.

Disadvantages of four coil starter:

The only limitation of the four point starter is that it does not provide high speed protection to the motor. If under running condition field gets opened; the field current reduces to zero. As there is some residual flux present and speed (N) is directly proportional to flux (ø) the motor will tries to run with dangerously high speed .this is called high speed action of motor. In three point starter as no volt coil is directly connected to across the supply; its current is maintained irrespective of the current through the field winding .hence it always maintain the handle in run position as long as supply is there .and thus it doesn’t protect the motor from field failure conditions which returns into high speeding of the motor