Speed control system for a-c motors, which slows motor down when in overspeed condition and stops motor when in underspeed condition

Claim 1

Control system for a-c electric motors in which the motor is designed for operation at a predetermined a-c supply voltage derived from an a-c source (13), the motor having an armature winding (12), said system comprising a bridge network which includes three impedance arms (15, 16, 17) and the armature winding (12) as the fourth arm thereof, the impedance branches being dimensioned with respect To the armature winding and the supply voltage such that the bridge will be in balance at a predetermined supply voltage and at a predetermined command speed and provide a diagonal sensing voltage of effectively zero; a phase angle control circuit (25) in series with the motor armature (12) controlling supply of power to the motor, the phase angle control circuit including at least one semiconductor switch (252; 254, 255), a timing capacitor (250) connected to the semiconductor switch and determining the firing angle thereof, a further capacitor (24) and a controllable variable resistor (221, 222, 400); and a sensing circuit (19) sensing unbalance of the bridge which includes a threshold switch (190, 191, 194) having an upper threshold level (FIG. 2: B) and a lower threshold level (FIG. 2: D), the threshold switch being controlled by said bridge circuit and becoming conductive if either the upper, or lower threshold level thereof is passed upon unbalance of the bridge, the circuit being connected to and controlling said controllable variable resistor (221, 222, 400) to change resistance and thus decrease the timing constant of the phase angle control circuit and decrease the phase angle of firing from said angle as determined by said timing capacitor to thereby limit supply of power to the motor upon a. change in supply voltage from the predetermined value in excess of the range which causes the threshold level of the threshold switch circuit to be exceeded upon unbalance of the bridge; b. change in command speed in excess of a predetermined value; c. change in command speed below a predetermined value.