Page 620 - Mechatronics with Experiments

606 MECHATRONICS

Torque (T)

T peak

T cont

Quadrant II (Generating) Quadrant I (Motoring)
Speed - Speed +
Torque + Torque + w max

Speed : (w)
Quadrant III (Motoring) Quadrant IV (Generating)
Speed - Speed +
Torque - Torque -

FIGURE 8.2: Four quadrant region torque-speed characteristics of an electric motor.

8.1.1 Steady-State Torque-Speed Range, Regeneration,
and Power Dumping

Electric motors can act either as a motor, that is to convert electrical power to mechanical
power, to drive loads, or as a generator, that is to convert mechanical power to electrical
power, when driven externally by the load. Let us consider the steady-state torque versus
speed plane (Figure 8.2). Motor-drive combinations that can operate in all four quadrants
of the torque-speed plane are called four-quadrant operation devices and can act as motor
and generator during different modes of an application. In the quadrants I and III of the
torque-speed plane, the mechanical power output is positive.
P = T ⋅ w > 0.0; motoring mode (8.2)
m
When the motor speed and torque are in the same direction, the device is in motoring
mode. In quadrants II and IV of the torque-speed plane, the mechanical power output is
negative. That means the motor takes mechanical energy from the load instead of delivering
mechanical energy to the load. The device is in generator mode or regenerative braking
mode.
P = T ⋅ w < 0.0; generating mode (8.3)
m
This energy can either be dissipated in the motor-drive combination, stored in a battery
or capacitor set, or returned to the supply line by the drive. This is precisely the energy
recovered (and stored in batteries) while braking in a hybrid car, where the electric motor
torque applied to provide the braking effect is in the opposite direction to the speed. By
storing this energy in the batteries and using it later, instead of wasting it, the energy
efficiency of the car is improved. Similarly, in industrial and factory applications, anytime
this condition occurs for a motor operation (motor operating in II or IV quadrant, T ⋅ w < 0),
the energy can be stored in battery/capacitors, returned to the utility supply line or wasted
as heat via resistors.