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In many high‐performance systems, drives are required to operate in all four quadrants of the torque‐speed plane, as shown in Fig. 2-16b.

Fig. 2-16 (a) Electric drive and (d) four-quadrant operation.

The motor drives the load in the forward direction in quadrant 1, and in the reverse direction in quadrant 3. In both of these quadrants, the average power is positive and flows from the motor to the mechanical load. In order to control the load speed rapidly, it may be necessary to operate the system in the regenerative braking mode, where the direction of power is reversed, so that it flows from the load into the motor, and usually into the utility (through the power‐processing unit). In quadrant 2, the speed is positive, but the torque produced by the motor is negative. In quadrant 4, the speed is negative and the motor torque is positive.