Abstract A speed control circuit for a universal electric motor having series connected armature and field windings, wherein a controlled semiconductor, such as an SCR, is utilized. A circuit containing a fixed resistor and a variable resistor is connected to the supply voltage. The fixed resistor is connected in parallel with a capacitor and the armature windings forming a series circuit loop containing the fixed resistor, capacitor, and the armature windings wherein the back EMF generated in the armature windings affects the amount of voltage for charging the capacitor. The control electrode of the semiconductor is connected by a voltage triggered signal channel to the capacitor. A selected operating speed is obtained by adjustment of the variable resistor. In this control circuit the capacitor is charged during the positive half-cycle by a voltage (across the fixed resistor) which is a fraction of the supply voltage minus the back voltage generated in the armature windings until the triggering voltage for the signal channel is built up on the capacitor whereupon the signal channel will be made conductive to fire the semiconductor into a conductive state. Preferably a diode is connected in shunt relation with the field windings to provide controlled excitation of the field windings during the negative half-cycle so that a significant armature back voltage is generated in the armature windings.
A speed control circuit for an electric motor having series connected armature and field windings, comprising: a pair of terminals for connection to a source of alternating current, first circuit means connected with said terminals and including a controlled solid-state semiconductor in series with said armature and field windings, second circuit means connected with said terminals and including a variable resistor in series with at least one fixed resistor, a capacitor connected at one end to said first circuit means between said controlled semiconductor and said windings of said electric motor and connected at the other end to said second circuit means between said fixed resistor and said variable resistor, said controlled semiconductor having a control electrode, and a voltage triggered signal channel connected between said control electrode and said other end of said capacitor, whereby said capacitor is charged during the positive half-cycle by a voltage which is a fraction of the line voltage minus the back voltage generated in said armature windings, until the triggering voltage for the signal channel is reached to open the signal channel and fire the semiconductor to feed source current to the motor windings, third circuit means connected in circuit with said terminals and including said armature and field windings in series with first switch means whereby upon closing of said switch means, the speed control circuit is bypassed and the electric motor is operated with full cycle current, a diode is connected in shunt with said field windings and has its polarity reversed with respect to that of said controlled semiconductor to provide controlled excitation of the field windings during the nonconducting periods of said controlled semiconductor, said first switch means incLudes second switch means in the shunt connection of the diode with said field windings and sequential switch operating means are provided for said first and second switch means whereby the shunt connection of the diode with the field windings is opened prior to establishing said bypass.
Claim 2
Dependent Claim
326 words
A speed control circuit for an electric motor having series connected armature and field windings, comprising: a pair of terminals for connection to a source of alternating current, first circuit means connected with said terminals and including a controlled solid-state semiconductor in series with said armature and field windings, second circuit means connected with said terminals and including a variable resistor in series with at least one fixed resistor, a capacitor connected at one end to said first circuit means between said controlled semiconductor and said windings of said electric motor and connected at the other end to said second circuit means between said fixed resistor and said variable resistor, said controlled semiconductor having a control electrode, and a voltage triggered signal channel connected between said control electrode and said other end of said capacitor, whereby said capacitor is charged during the positive half-cycle by a voltage which is a fraction of the line voltage minus the back voltage generated in said armature windings, until the triggering voltage for the signal channel is reached to open the signal channel and fire the semiconductor to feed source current to the motor windings, third circuit means connected in circuit with said terminals and including said armature and field windings in series with first switch means whereby upon closing of said switch means, the speed control circuit is bypassed and the electric motor is operated with full cycle current, a diode is connected in shunt with said field windings and has its polarity reversed with respect to that of said controlled semiconductor to provide controlled excitation of the field windings during the nonconducting periods of said controlled semiconductor, said first switch means incLudes second switch means in the shunt connection of the diode with said field windings and sequential switch operating means are provided for said first and second switch means whereby the shunt connection of the diode with the field windings is opened prior to establishing said bypass.