Single current regulator for controlled motoring and braking of a DC-fed electric motor

Control apparatus for a DC-fed motor control system, comprising:

• an electric machine having an armature winding;
• a rechargeable source of DC normally connected to a DC bus;
• DC bus switching means connecting the armature winding to the DC bus, including transistor means for supplying current to the armature winding from the DC bus, and rectifier means for returning current from the armature winding to the DC bus;
• current control means for modulating the transistor means (1) during a motoring mode of operation to control the current supplied to the armature winding, and (2) during a braking mode of operation to control the current returned to the DC bus;
• an energy exchange capacitor connected across the DC bus;
• a load resistor;
• mode switching means effective when activated to interrupt the connection of the source to the DC bus, and to connect the load resistor across the DC bus in parallel with the capacitor;
• sensing means effective during the braking mode of operation for detecting an operating condition for which the modulation of the transistor means is ineffective to control the current returned to the DC bus;
• and
• means for activating the mode switching means in response to the detection of said operating condition by the sensing means, whereafter the capacitor takes the place of the source, the current returned to the DC bus is supplied to the load resistor, and the modulation of said transistor means become effective to control such returned current.