Dec 23, 2014

Speed Control Methods of DC Shunt Motor

The speed control of a DC Shunt motor can be achieved by the following methods:
  1. Armature control Method and
  2. Voltage Control Method

Flux Control Method


This method is very frequently used because it is very simple and economic. In this method the speed control can be achieved by varying the flux Φ, because the motor speed N ∝ (1/ Φ) and therefore it is named flux control method. Along with the shunt field winding an adjustable resistance is connected in series as shown in the figure. This adjustable resistance is also known as shunt field rheostat. This shunt field rheostat moderates the shunt current and the flux. As a result, we can only increase the speed beyond the rated speed of the motor. Normally, it allows the speed to increase in the ratio of 3:1 varied speed choices have a tendency to create instability and meager commutation.


Armature Control Method


In this method the speed control can be achieved by varying voltage across the armature and the back EMF Eb. This is obtained by connecting an adjustable resistance in series by way of the armature shown in fig. This adjustable resistance is also known as controller resistance Rc.
N∝ {v-Ia(Ra+Rc)}

In this method is decreased due to voltage drop in the Rc. Since speed is directly proportional to back EMF. By using this method the speed can only control lower than the normal speed because the maximum speed is that corresponding to Rc=0 i.e. normal speed.

Voltage Control Method

In this process the supply voltage is providing the field current is not the same from that which deliveries the armature. By using this technique we can avoid the drawback of meager speed regulation and efficiency as we have seen the armature control method. But this method is fairly expensive. So this method is engaged for bigger size of motor and where the importance is given for efficiency.

Speed control of DC Shunt Motor using voltage control method can be achieved in two ways. They are  (i).Multiple Voltage control and (ii).Ward-Leonard System.

i) Multiple Voltage Control Method

In this method a fixed voltage source is permanently connected across the shunt field of the motor. The armature can be connected across quite a lot of diverse voltages over a right switch gear. The speed of the motor can be controlled by applying the various voltages across the armature. Since the speed is directly proportional to the applied voltage across the armature will be nearly proportionate to the applied voltage across armature. By using shunt field regulator intermediate speed can be obtained in this method.

ii) Ward-Leonard System


In this system, the field circuit is delivered from a isolated source. From a variable voltage generator the adjustable voltage for the armature is obtained. The fig above shows the typical Ward-Leonard system. In this system the armature of the DC Shunt motor is directly connected to a generator driven by a constant speed AC motor. The field of the generator is supplied from the exciter (E). The field of shunt motor is supplied from a constant voltage exciter. The Generator voltage can be varied by means of generator field regulator. The voltage applied to the motor may be reversed by backing the field current of generator through controller FC. On occasion, a field regulator is contained within the field circuit of DC shunt motor for additional speed adjustment. By means of this method, the motor can be worked at any speed up to its maximum speed.


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