The braking system using an induction motor is fed by a converter and is made to operate as an asynchronous incorporated

摘要

1532811 Control cf AC motors BRITISH RAILWAYS BOARD 6 April 1976 [14 April 1975] 15224/75 Heading H2J A three phase induction motor 2, fed from an A.C. supply through a D.C. source 1, contactor 5 and inverter 3, is braked by reducing the output of source 1 to zero, opening contactor 5 to cause induction generator operation and closing a contactor 6 to connect the motor to a controlled power-absorbing, unity power factor load 7, whereby real power flows through the load 7 and reactive power through the inverter 3. Thus the inverter rating may be reduced compared with a braking arrangement using a load connected across the D.C. side of the inverter. The load may be a switched resistor chain, a rectifier followed by a controller (e.g. D.C. chopper) connected to a battery to be charged, or a system to feed power back to the A.C. supply. During braking the inverter frequency f is controlled by a feed-back loop 10 responsive to the voltage of the motor or of the D.C. side ofthe inverter to maintain a constant flux density in the motor for producing a desired braking torque, for example maximum torque. To prevent high voltages being applied to the inverter during braking, it is connected to tappings 17, 18, 19 on the motor windings as shown in Fig. 3, the winding terminals 14, 15, 16 being connected to a load in the form of a resistor bank 7 which is progressively reduced, as the motor speed and voltage fall, to maintain the braking torque constant. The tappings 17, 18, 19 may also be used during high speed motoring, the operation for motoring with full winding or tapped winding connections and for braking with trapped winding connections and a tapped, switched-segment resistor 7 being described, Figs. 4(a), (b) and (c) (not shown). Alternatively each winding may comprise two sections connected in series for low speed motoring and in parallel for high speed motoring and braking. Figs. 5(a), (b) and (c) (not shown). Another method is to connect the windings in star for low speed motoring and in mesh for high speed motoring and braking. The loop 10 may be adjusted to obtain a desired braking torque below maximum by varying the constant of proportionality K 1 in the equation f = K 1 V.