Page 621 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 607
During acceleration, a motor adds mechanical energy to the load. It acts as a motor.
During deceleration, the motor takes away energy from the load. It acts like a brake
or generator. This means that energy is put into the load inertia during acceleration, and
energy is taken from the load inertia (returned to the drive) during deceleration (Figure 8.2).
Some drives can convert the generated electric power and put it back to into the
electric supply line while others dump the regenerative energy as heat through resisitors.
The amount of regenerative energy depends on the load inertia, deceleration rate, time
period, and load forces.
There are two different motion conditions where regenerative energy exists and
satisfies the T ⋅ w < 0 condition:

1. During deceleration of a load, where the applied torque is in the opposite direction
to the speed of inertia.
2. In load driven applications, that is in tension controlled web handling applications, a
motor may need to apply a torque to the web in the opposite direction to the motion
of motor and web in order to maintain a desired tension. Another example for this
case is where the gravitational force provides more than the needed force to move an
inertia, and the actuator needs to apply force in the direction opposite to the motion
in order to provide a desired speed.

Example Consider the electric motor driven load shown in Figures 8.3 and 8.4. Assume
that the load is a translational inertia and an electric motor is a perfect linear force generator.
Consider an incremental motion that moves the inertia from position x to position x using
2
1
a square force input. For simplicity, let us neglect all the losses. We will assume that the
motor-drive combination converts electrical power (P (t)) to mechanical power (P (t))
m
e
with 100% efficiency in the motoring mode, and mechanical power to electrical power with
100% efficiency in the generator mode.
P (t) = P (t) (8.4)
e
m
DC bus

Load
R regen C cap x
3 Phase
AC source M m
F

Rectifier Inverter

Non–regenerative Regenerative drives
drives
R ≠ 0 R regen = 0
regen
FIGURE 8.3: Regenerative energy in motion, its storage and dissipation.