Speed control of a dc motor needs some feedback of speed signal. This could be from a tachogenerator or can be tapped from Armature Voltage. Know more about the two methods

Voltage feedback Vs Tachogenerator feedback....

Majority of applications of DC drives involve holding the speed of the dc motor to a fixed set value irrespective of variation in load, supply voltage etc. For controlling the speed precisely, measurement of speed is necessary. Unless we measure we can not control. Tachogenerators do the job of measuring the speed. They give voltage proportional to the speed of the motor. Accuracy of control will therefore depend to a great extent on the quality of the tachogenerator. Typical speed holding accuracies are 0.5 % to 1 % .

However, there are many applications which require that speed of the dc motor be varied from zero to full speed but accuracy is not necessary. In such cases, a cheaper control is possible using Armature Voltage Feedback instead of Tachogenerator feedback. The assumption here is that the speed of any dc motor is roughly proportional to the armature voltage applied. This is true if we neglect the resistance drop taking place in the armature winding. This drop in big motors is typically up to 5 % of the rated motor voltage. That means, by using armature voltage feedback, we can get speed holding accuracy up to 5 %. . However, there is a method called “IR compensation” by which a better control is possible. It is not as good as that achieved by using a tachogenerator. A signal porportional to the Current in the Armature is already available in the drive. It is derived from Current Transformers on AC side of the thyristor bridge. This is subtracted from the feedback of Armature voltage. Thus, actual feedback equivalent to (Armature Voltage- I x R ) is applied to the speed amplifer and a better control of speed is achieved.

Only precaution necessary while using the armature voltage feedback is to isolate the armature voltage Galvanically before it is fed to the electronic circuit. DC to DC isolators are available for this purpose. The isolation prevents circuit components from becoming "live" if feedback is not isolated.