Booster Speed Control of DC Motor:
A booster in series with the armature of the main motor may be employed for varying the voltage across the driving motor. The booster speed control is a separately excited dc generator, which adds or subtracts the voltage in the motor feeder. The adding (boosting) or subtracting (bucking) depends upon the field current and its direction, which is achieved by parallel resistance arrangement.
By this method the speed can be varied over a wide range depending upon the size of the booster. Since booster generator is to provide only the variation in the voltage for boosting and bucking its output, therefore, need not to be as much as that of the main motor inspite of the fact that it has to carry the main motor current.
The exact size of the booster however, depends upon the range of speed control required.
The chief disadvantages of this method is that it requires two extra machines which is expensive, Generally this method is employed where only a small variation in speed is required.