GB1465681A - Control systems for electric motors - Google Patents

Abstract

1465681 Control of A.C. motors UNITED KINGDOM ATOMIC ENERGY AUTHORITY 21 Aug 1974 [5 July 1973] 32082/73 Heading H2J The stator 26 of a linear induction motor is supplied through solid state switches 19a ... 22a connected as shown to phases L1, L2 of a three phase A.C. supply L1, L2, L3, the switches being alternately fired in pairs 19a, 20a and 21a, 22a by respective control circuits 14, 16, 23, 19, 20 and 18, 15, 24, 21, 22 responsive to opposite phase pulse trains produced by a square wave oscillator 10, whose frequency and mark/space ratio are adjusted to desired values to respectively match the motor and its load and produce acceleration or deceleration of the motor, the switches 19a, 20a applying forward power to the motor and switches 21a, 22a reverse power and the forward to reverse ratio corresponding to the mark/space ratio. To accelerate the motor, a mark/space ratio of say 7 : 3 causes switches 19a, 20a to close to supply the stator 26 with seven cycles of a 50 Hz forward power from L1, L2, L3, after which switches 21a, 22a close to supply three cycles of 50 Hz reverse power and so on. Thus the net forward thrust causes the motor to accelerate, the forward thrust decreasing as the forward slip decreases (with increasing speed) and the reverse thrust increasing as the reverse slip increases. Eventually a constant speed is reached at which the integrated forward thrust exceeds the integrated reverse thrust by an amount equal to friction and other losses. The oscillator frequency may be 5 Hz for the 7 : 3 mark/space ratio or may be varied between zero and the frequency of the supply L1, L2, L3. Gates 15, 16 are controlled by respective inverters 13, 17 to prevent overlapping of the closed periods of switch pairs 19a, 20a and 21a, 22a, brief "no power" periods between power reversals thus being produced. The motor may be stopped by opening all the switches 19a ... 22a or by altering the oscillator mark/ space ratio to 1 : 1 or to less than unity for a brief period to produce increased braking. To protect against runaway due to failure of either the forward or reverse supply of power, the supply to the stator 26 may be monitored by coils connected to a fail-safe control circuit.