MOTOR START WINDING SWITCH

1352367 Starting A.C. motors FRANKLIN ELECTRIC CO Inc 16 Sept 1971 [16 Sept 1970] 43299/71 Heading H2J An electric motor, having main and starting windings fed from an A.C. source, has a switch in series with the starting winding that is opened upon the attainment of a predetermined phase difference between the starting winding current and the source voltage. In the embodiment shown, the excitation of starting winding 43, Fig. 3, is controlled by the speed dependent phase angle between the applied voltage 40 and the starting winding current. For low speeds, capacitor 78, which forms part of a relaxation oscillator, charges as the applied voltage rises, until diac 76 breaks down at phase angle 61 (Fig. 2a) of the applied voltage, when capacitor 78 discharges providing trigger pulses 50 to turn triac 44 on, thus exciting the starting winding 43. Capacitor 78 recharges and the cycle is repeated to produce a burst of firing pulses to triac 44. In a diasbling circuit, capacitor 90 charges from a voltage divider 94, 98 across the supply until diac 88 breaks down at phase angle 63 (Fig. 2a) of the applied voltage, turning triac 86 on and shorting capacitor 78, thus the firing pulses to triac 44 are discontinued, and triac 44 turns off at its next current zero. Provided the starting winding current zero 60 (Fig. 2a) occurs before phase angle 63 is reached i.e. that the phase angle 62 between the starting winding current and applied voltage does not exceed a predetermined value, triac 44 is retriggered by pulses 50 and the starting winding excitation continues. Overload protection, e.g. against-a locked rotor condition is afforded by a divider across the supply, including a PTC resistor, that limits the voltage applied to diac 76, in an overload condition, to a value less than the diac breakdown value i.e. triac 44 cannot be triggered on. At full speed, and with triac 44 off, the voltage at 106 is the algebraic sum of the supply voltage and the voltage across the starting winding. This voltage sum, which depends upon the phase relationship between the two voltages and is thus related to speed, is used to control another relaxation oscillator, in which capacitor 116 charges until diac 118 breaks down, turning triac 86 on, preventing further excitation of the starting winding. The firing pulses 50 may be replaced by a single pulse. If the speed falls below a prescribed value, the voltage at 106 may be used to switch in the starting winding again, Figs. 5 to 8 (not sh