Page 164 - Electric Drives and Electromechanical Systems

Chapter 5 Brushed direct-current motors 157

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FIG. 5.17 The characteristics of the I t current fold back circuit. The time that the drive remains supplying current
above the continuous rating is limited by the curve shown, in this case peak current is limited to t 1 seconds, in
2
addition the drive can only supply a current of I 2 for t 2 seconds before the I t limit is applied.

5.4 Regeneration

Up to this point, consideration has been limited to drives which provide power to the
motor either to accelerate the load or to hold it against a torque at a constant speed. As
discussed in Ferreira and De Almeida (2018) one of the key approaches to energy
efficiency is return a system’s kinetic energy to the drive or supply using the principle of
regeneration. The rapid decelerations that are found in servo-type applications can
result in regeneration; that is, energy is returned to the motor from the load and is either
absorbed by the motor or it has to be dissipated externally. The energy can either be
dissipated as heat in the motor or returned to the supply. While this chapter is primarily
concerned with d.c. brushed motors, the principles discussed in this section are also
applicable to other forms of motor-drive systems. For the case of a motor being decel-
erated to a standstill, the motor’s terminal voltage, speed, and current are shown as a
function of time in Fig. 5.18. Prior to deceleration from a speed of u int , the terminal
voltage is equal to K e u int þ I a R a immediately following the application of the negative
braking current, the terminal voltage drops to K e u int I a R a . The terminal voltage will
decrease as the speed decreases until it equals zero when u m K e ¼ I R R a at t ¼ t o .
The motor will continue decelerating until reaching a standstill at time, t ¼ t z . During the
period of regeneration, the energy supplied to the drive by the motor is given by.
Z t z
E r ¼ v m ðtÞi a ðtÞdt (5.22)
0
where,
v m ðtÞ¼ K e u m ðtÞþ I R R a (5.23)