Speed control apparatus for electric motor

Abstract

A direct current obtained by rectifying an alternating current voltage by means of a diode bridge circuit is applied to a direct current motor connected to the diode bridge circuit. Use is made of the fact that the voltage developed across the diode bridge circuit varies, depending on the magnitude of the induced voltage across the motor. To that end, the interjunction of a capacitor series circuit coupled to the bridge circuit is connected through a voltage responsive bidirectional switching device, such as a diac, to a control electrode of a voltage controlled bidirectional switching device, such as a triac, for phase control of the alternating current voltage. One electrode of the voltage responsive bidirectional switching device is coupled to a variable resistor provided in a gang fashion with a controller, such as a foot controller pedal, so that the voltage obtainable from the variable resistor is superposed on the voltage in the switching device, such as a diac. In operation, therefore, if and when the induced voltage across the motor decreases, an electric charge in the capacitor is discharged to be supplied to one electrode of the diac, thereby to control the voltage controlled bidirectional switching device, such that the conduction phase is increased and thus, the speed of the motor is increased. Preferably, the above described capacitor is provided with a series connected discharging resistor.