GB1297379A - - Google Patents

Abstract

1297379 Automatic control of speed SEVCON ENG Ltd 4 Dec 1970 [4 Dec 1969 22 Dec 1969] 59330/69 and 62338/69 Heading G3R [Also in Divisions H2 and H3] A polyphase wound rotor induction motor is dynamically braked by disconnecting its stator 40 from the supply 44 and energizing at least one of its stator windings with rotor current, rectified at 48, and controlled by a variable markspace ratio pulse controller. The controller comprises a main thyristor 90 and also operates during driving to vary the effective rotor resistance by alternately short and open circuiting the rotor windings. Thyristor 90 is periodically triggered by an oscillator 112 whose frequency is controlled by a voltage derived from a transistor circuit, Fig. 5 (not shown) in which comparison is made between the rectified output of a tachogenerator and a reference voltage. This circuit also effects energization of contactors 42 and 126; when the speed is below reference value these are energized for driving, whereas when it is above they are de-energized so that the stator is disconnected at 42 and two of its windings 120, 122 are connected in parallel, via 126 and thyristor 90, across rectifiers 48. Winding 124 is short circuited. The comparator circuit can be over-ridden to provide a maximum driving speed. Thyristor 90 is turned off by a commutating thyristor 98, triggered by a circuit 118, Fig. 4 (not shown), responsive to the voltage across commutating capacitor 94. Thus the pulse length depends upon the rate at which this is charged by the rotor current. An auxiliary supply on terminal 116 maintains operation when this is low. A smoothing circuit comprises a capacitor 72 and inductor 70. During driving, 72 is charged by the rotor current when 90 is off via an auxiliary thyristor 82 (contacts 78 being closed by the comparator circuit), and is discharged via the path 74, 76, 78, 92 and 90 when 90 is triggered on. The charge on 72 is also transferred to commutating capacitor 94 via 100, 102, 96. After commutation the charge on 94 is reversed by the inductance of the circuit, and by the rotor current (82 now being turned off by the charge on 72). When this reverse charge is sufficient, the voltage across diode 80 and thyristor 82, detected by circuit 114, causes 82 to be triggered on and the cycle is repeated. During braking contacts 78 are opened, 72 discharges into the stator winding when 90 is on and recharges from the rotor current, via the stator, when 90 is off. Thus both charging and discharging of 72 assists dynamic braking.