Speed Control Of DC Motor(series/shunt) Electrical Machines Speed Control Of DC Motor(series/shunt) Created by: Parmar Janhvi (130200117069) Patel Kajal (130200117082) Vekariya Jinal (130200117122)

Speed Control Of DC Motor Speed control means intentional change of the drive speed to a value required foe performing the specific work process. Speed control is a different concept from speed regulation where there is natural change in sped due to change in load on the shaft. Sped control is either done manually of some automatic control device. One of the importance features of dc motor is that its speed can be controlled with relative ease. We know that the expression of speed control dc motor is given as,

Speed Control Method Speed (N) of 3types of dc motor –series,shunt,compound can be controlled by changing the quantities of the expression. (i)Terminal voltage of the armature (ii)External resistance in armature circuit R (iii)Flux per pole The first two cases involve change that affacts armature circuit and the third one involve change in magnetic field. Therefore speed control of dc motor is classified as: (1) Armature control method (2) Field control methods

Speed control of shunt motor Variation of Flux or Flux Control Method: By decreasing the flux, the speed can be increased and vice versa. The flux of a dc motor can be changed by changing Ish with help of a shunt field rheostat. Since Ish is relatively small, shunt field rheostat has to carry only a small current, which means I2shR loss is small, so that rheostat is small in size.

(ii) Armature control method : This method is used when speeds below the no-load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat in series with the armature circuit. As controller resistance is increased, voltage across the armature is decreased, thereby decreasing the armature speed. For a load constant torque, speed is approximately proportional to the voltage across the armature.

(iii) Armature voltage control The set up for the armature voltage control by using the rheostat as a potential divider.The advantages of this method is that it can very the dc shunt motor right zero upto the rated speed. When the moving contact of the point A in fig. the armature voltage Va is zero .Hence the speed is zero.

(iv) Voltage control In the applied voltage control of dc shunt motor the field winding is connected to a constant dc voltage .But the armature is applied with different voltages with the help of suitable switch gear as shown in fig. .

(v) Ward Leonard method This is basically the armature voltage control of a dc motor .The variable armature voltage is obtained from a dc generator.M1 is a coupled mechanically to a dc generator. The output voltage of the DC generator is applied to the armature of dc motor M2 the speed of which is to be controlled.This variable dc voltage acts as the variable armature voltage for M2 and its speed can be controlled.

Comparison of Rheostatic Control and Field Control

Speed control of series motor (a)Field control method: (a) Field Diverters: The series winding are shunted by a variable resistance known as field diverter. Any desired amount of current can be passed through the diverter by adjusting its resistance. Hence the flux can be decreased and consequently, the speed of the motor increased.

(b)Armature diverter : A diverter across the armature can be used for giving speeds lower than the normal speed. For a given constant load torque, if Ia is reduced due to armature diverter, the ɸ must increase (∵Ta α ɸ Ia ) This results in an increase in current taken from the supply (which increases the flux and a fall in speed (N α I/ ɸ ). The variation in speed can be controlled by varying the diverter resistance.

(C) Tapped Field Method This method is often used in electric traction. The number of series filed turns in the circuit can be changed. With full field, the motor runs at its minimum speed which can be raised in steps by cutting out some of the series turns

(d) Paralleling Field Coils this method used for fan motors, several speeds can be obtained by regrouping the field coils. It is seen that for a 4-pole motor, three speeds can be obtained easily.

Armature control method Armature resistance control: In this method armature circuit is provide with a variable rasistance .Field is directly connected across the supply so flux is not changed due to variation o series rasistance.This is applied for dc shunt motor. This method is used in priting press,cranes,hoists where speeds lower than rated is used for a short period only. Armature voltage control : This method avoids disadvantages of poor speed regulation and low efficiency of armature resistance control methods.The basic adjustable armature voltage control method of speed control is accomplished by means of an adjustable voltage generator is called Ward Leaonard system .This method is best suited for steel rolling mills, paper machine,etc.

Advantages of armature control of shunt motor Very fine speed control over whole range in both directions Uniform acceleration is obtained Good speed regulation Disadvantages of armature control of shunt motor A. Costly arrangement is needed ,floor space required is more B. Low efficiency at light loads

Merits of rheostatic control 1.It is possible to vary the speed smoothly. 2. The speed can be controlled right from 0 to N rated. 3. The potentiometer rheostat acts as starter. Demerits of rheostatic control 1.It is not possible to control speeds above the rated speed. 2. A large size rheostat needs to be used. 3. The System efficiency decreases due to power loss taking place in the external resistance.

Advantages and disadvantages of series motor All the advantages and disadvantages of the flux control technique, rheostastic control and applied voltage control for the DC shunt motor as it is applicable to these techniques used foe DC series motor as well.