GB1502067A - Electric motor overload protection - Google Patents

Abstract

1502067 Control of DC motors PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 3 June 1976 [6 June 1975] 22966/76 Heading H2J For use with a variable, cyclically switched power supply circuit which includes a rectifying bridge having at least two thyristors 32, 33, a thyristor firing circuit 41, and a high power dissipation resistor 28 connected between the bridge and A.C. mains supply terminals, a permanent-magnet electric motor overload protection circuit 20 comprises means for detecting small residual voltage pulses which appear across the motor terminals 34, 35 and which have a polarity which is opposite to that of the voltage supplied to the motor, said pulses occurring at twice the frequency of the mains supply and each commencing in one half cycle of the supply voltage and ceasing in the next half cycle, the duration of said pulses being longer when the motor is stalled than when it rotates, and means responsive to an output of said detection means, which is provided when the motor is stalled, to provide a control signal for application to the thyristor firing circuit so as to retard the firing of the thyristors until slightly before the end of each half cycle whereby the current in the motor 40 is maintained at a very small value in a stable manner. Figs. 2 and 4 show respectively the voltage appearing across the armature terminals of the motor in a stalled condition and in a rotating condition. In the stalled condition, the duration for which the voltage is slightly positive, i.e. between constants t 3 -t 4 and t 6 -t 7 is considerably longer than the duration for which the voltage is positive, i.e. between t 14 -t 15 and t 18 -t 19 , in the rotating condition. As shown in Fig. 5, the negative terminal 34 of the motor is connected to an information point 78 of the protection circuit 20 and the positive terminal 35 is connected to a line 76 which is used as a reference. The combination of transistors 80, 84 produce, at the collector of 84, a pulse signal in accordance with the voltage which appears at point 78. Point 78 and the collector of 84 are connected to an integrating capacitor 89 via resistors 90 and 87 respectively, and it is arranged that when the motor is running, the voltage at point 88 is negative, but that when the motor is stalled, the voltage at point 88 is positive. A positive voltage at point 88 causes a transistor 94 to conduct and thereby lower the voltage at a point 57 of the thyristor firing circuit to thereby retard firing of the thyristors 32, 33. A warning may be given by a L.E.D. 102.