Speed Regulation in DC Drives

We know,

From this equation we can say that with constant field flux and load on the motor, any change in incoming supply voltage will change the voltage delivered to the armature of the motor. 

Now if the incoming voltage supply and the field flux are steady but load on the motor fluctuates then, this will alter the current taken by the armature. 

Change in the load changes the current taken by armature which in turn changes the Eb level.

Since N α Eb, Changes in load causes speed to deviate from the set or required value. 

If we want the speed to remain steady at the set value even with fluctuations in incoming supply voltage or load on the motor or with both, then we shall need and electronically controlled variable speed D.C. drive with actual speed and current feedback

Figure : Simple Block Diagram

Provision to vary the speed of motor with soft start/stop circuit called Ramp Function Generator.

A Speed control loop for correcting the speed and hence regulating it called speed controller.

A current control loop for correcting the speed and hence regulating it called current controller.

  

Circuit to provide gate pulses to trigger SCRs in power converter called gate control set.

Ramp Function Generator:

Whenever we give the facility of speed variation we should control the rate of change of the speed i.e. we must control acceleration and deceleration for the safe operation.

It is achieved by a circuit which is called as Ramp Function Generator (RFG).

The Output of the RFG is the control input to speed controller

 Figure : Controlled Acceleration and Deacceleration

Following are the reasons for possible deviation of the speed from the set value.

1.  Supply Voltage Variation.

2.  Variation in load on the motor.

To achieve an ideal speed torque characteristics, it is essential to incorporate a speed  feedback for speed correction and regulation. 

The circuit consist of a simple PI controller which has two inputs

1. Set Speed value.

2. Actual Speed value.