Transistorized Switching Control of a Variable-Speed DC Motor

Where a dc motor is supplied energy from a storage battery or other fixed voltage source, variable-speed operation is usually accomplished by adding resistance in series with the motor supply. In this type of control, only 3 part of the total energy delivered by the source is available at the motor to do useful work. The balance of the energy is converted into heat by the series resistance and is lost. A more desirable approach to speed control, as presented in this report, is to insert a switch rather than a resistor in series with the motor's supply. Here the switch would be opened and closed at regular intervals which would represent infinite or zero series resistance, respectively. In either state, the switch would present no energy losses to the circuit. The motor speed varies with the percentage of time the switch remains closed. The latter system has the advantage that for a given speed setting, variations in load will not cause fluctuations in speed due to IR drop in any series resistor. The transistorized switching circuit described in detail in this report represents an efficient method of switching the motor supply current for good speed control. The laboratory model, which gave very satisfactory performance, was designed to control 20 amperes at 24 volts, or approximately a one-half horsepower motor. This rating may be increased with only minor modifications to the circuit.