Page 167 - Electric Drives and Electromechanical Systems

160 Electric Drives and Electromechanical Systems

The maximum energy is returned when I R is zero, when the returned regenerative
energy is equal to the total rotational energy of the system.
The regenerative energy returned to the drive will increase as the regenerative
current decreases.

The flow of regenerative energy has a significant impact on the design of the power
supply for a PWM amplifier. In motoring operations there is a bidirectional current flow;
the average direction is from the supply to the motor. At certain times the average motor
current will be higher than the average current drawn by the power supply; this is
accommodated by the energy-storage capability of the power-supply capacitors. The
capacitor, acting both as a filter and an energy reservoir, needs to be sized to prevent the
d.c. voltage rising to unacceptable levels during the free-wheeling periods of the
switching cycles. However, during periods of sustained regeneration, energy is returned
to the supply which causes the capacitor voltage to significantly increase. Using the
anticipated energy requirements, it is possible to determine the size of the capacitor
which is necessary to ensure that the maximum bus voltage, V max , is restricted to a value
below the continuous voltage rating of the main power-bridge devices. Hence,

2E r

C (5.31)
V max V norm
2 2
where V norm is the d.c. link voltage under steady-state conditions. As noted earlier, some
of the returned energy, E R , will be dissipated in the motor and in the power electronics of
the drive system; hence the capacitor will be conservatively sized. In cases where the
load being controlled is high speed inertia, or requires sustained regenerative braking,
the value of this capacitor will become excessive; therefore, a bus-voltage regulator will
have to be considered. In some applications, for example electric vehicals, this energy
can be fed back to the battery, thus increasing the available range.
The conventional solution to the use of large power-supply capacitors is the use of a
shunt voltage regulator (shown in Fig. 5.20). The regulator consists of a resistor and a
semiconductor switch fitted directly across the d.c. bus; switching of the device is
controlled by a comparator. The comparator’s threshold voltage must be selected to
ensure that the regulator is not switched on when the supply voltage is at its maximum
permitted value during normal operation. In the determination of this value, the effects of
the supply’s voltage regulation must be considered. The value and the ratings of the
dissipative element and the semiconductor switch need to be selected to ensure
satisfactory operation. Since the peak ratings of the semiconductors in the bridge must not
exceed V max , during deceleration, the resistor must be capable of sinking the peak
regenerative current at that voltage. The energy flow within the drive is shown in Fig. 5.21,
the amount of energy lost is largly determined by the device losses in the power elec-
tronics and the overall efficancy of the transmission system. The maximum size of the
resistor is given by,
V max
R (5.32)
I R