Patent GB-1434576-A (GB)
Title: Supply system for an esynchronous motor
IPC: B66B5/02
Filed: 1973-11-05
Abstract: 1434576 Control of A.C. motors; inverting SAFT-SOC DES ACCUMULATEURS FIXES ET DE TRACTION 5 Nov 1973 [6 Nov 1972 8 May 1973] 51194/73 Headings H2F and H2J A supply system for an asynchronous motor comprises an A.C. source for use as a normal supply and a D.C. source for use in combination with an inverter as an alternative supply, the inverter having an oscillation frequency which is appreciably lower than the frequency of the normal supply and the ratio between the output voltage of the inverter and its normal operating frequency being approximately equal to the ratio between the voltage and the frequency of the normal supply, so as to maintain the torque provided by the motor substantially constant for both conditions of operation. In the described embodiment a three-phase asynchronous motor 1 is normally supplied through a changeover switch 3 from an A.C. supply 2 of mains voltage and frequency, e.g. 220 volts, 50 Hz, the switch 3 being changed over if the supply 2 fails whereby the motor is supplied from an inverter 5 fed by a D.C. source 4, the inverter voltage and frequency being 24 volts, 6 Hz for example, to meet the abovementioned frequency and ratio conditions. The arrangement is particularly suitable for applications requiring only brief motor operation after failure of the supply 2, e.g. driving a lift to a rest position. Inverter operation and control.-Each phase 5 comprises the circuit shown in Fig. 2 consisting of two almost identical branches 6 and 7 each composed of two power transistors in parallel controlled by amplification transistors responsive to complementary signals Pi, Ni produced by a control circuit 11, whereby the branches 6, 7 are alternately conductive to connect the terminal Ai to the positive or negative terminal of the source 4. Diodes 8, 9 are responsive to motor overvoltages to conduct power back to the source 4. The control circuit 11 is shown in Fig. 3 and comprises a time base circuit 12 producing, after an initial starting period, a constant frequency pulse train which is applied to the three binary units of a cyclic shift register 13, each of which controls a respective bi-stable flip-flop 14, 15, 16 whose outputs provide the complementary control signals P 1 N 1 , P 2 N 2 , P 3 N 3  for each phase 5 of the inverter. Typical waveforms for the pulse train to register 13, the input signals to and output signals from the flip-flops 14, 15, 16 and voltage across terminals A 1 A 2  are described, Fig. 4 (not shown). A relay 17, 18 reverses the connections between flip-flops 15, 16 and the corresponding units of the register 13 to reverse the direction of the motor. A monostable flip-flop 19 and feedback circuit 20, 21 produce an initial low frequency for the pulse train to register 13, after which a circuit comprising members 22, 23, 25, 26 progressively increases the frequency to the constant value corresponding to the operating frequency of the inverter, whereby a high starting torque is produced for the motor if it has stopped or slowed after failure of supply 2. The source 4 may be a battery or a transformer-rectifier unit, Fig. 5 (not shown), the latter embodiment also having a resistor or battery (37) connectible by a switch (38) across the rectifier (36) during braking of the motor, whereby power fed back from the motor can be absorbed. Should the supply provided by members 34, 35, 36 fail, the battery (37) can be used to supply the inverter. Fig. 6 shows the application of the invention to a motor 1 driving a lift (not shown) the normal supply 2 also feeding a charger 27 for the battery 4 supplying an inverter as shown in Figs. 2 and 3. Normally a relay 28 is energized by the supply 2 to connect the latter to the motor and de-energize the coil of a relay 29 controlling the inverter, relays 28, 29 being interlocked in the usual way. When supply 2 fails, relay 28 and a power responsive switch 31 close to operate relay 29 and connect the inverter to the motor, the current taken thereby being used to control a circuit 33 whereby the motor will run in one or other direction if the current is below or above a threshold value, the circuit 33 controlling the relay 17, 18 of Fig. 3 and including a time delay element to allow for motor starting current.