We will consider a fully controlled converter supplying a permanent-magnet motor, and see how the motor can be regeneratively braked from full speed in one direction, and then accelerated up to full speed in reverse. We looked at this procedure in principle at the end of Chapter 3, but here we explore the practicalities of achieving it with a converter-fed drive. We should be clear from the outset that in practice, all the user has to do is to change the speed reference signal from full forward to full reverse: the control system in the drive converter takes care of matters from then on. What it does, and how, is discussed below.
When the motor is running at full speed forward, the converter delay angle will be small, and the converter output voltage V and current I will both be positive. This condition is shown in Figure 4.7(a), and corresponds to operation in quadrant 1.
Figure 4.7 Stages in motor reversal using a single-converter drive and mechanical reversing switch.
In order to brake the motor, the torque has to be reversed. The only way this can be done is by reversing the direction of armature current. The converter can only supply positive current, so to reverse the motor torque we have to reverse the armature connections, using a mechanical switch or contactor, as shown in Figure 4.7(b). (Before operating the contactor, the armature current would be reduced to zero by lowering the converter voltage, so that the contactor is not required to interrupt current.)
Note that because the motor is still rotating in the positive direction, the back e.m.f. remains in its original sense; but now the motional e.m.f. is seen to be assisting the current and so to keep the current within bounds the converter must produce a negative voltage V which is just a little less than E. This is achieved by setting the delay angle at the appropriate point between 90° and 180°. (The dotted line in Figure 4.6 indicates that the maximum acceptable negative voltage will generally be somewhat less than the maximum positive voltage: this restriction arises because of the need to preserve a margin for commutation of current between thyristors.)
Note that the converter current is still positive (i.e. upwards in Figure 4.7(b)), but the converter voltage is negative, and power is thus flowing back to the mains. In this condition the system is operating in quadrant 2, and the motor is decelerating because of the negative torque. As the speed falls, E reduces, and so V must be reduced progressively to keep the current at full value. This is achieved automatically by the action of the current-control loop, which is discussed later.
The current (i.e. torque) needs to be kept negative in order to run up to speed in the reverse direction, but after the back e.m.f. changes sign (as the motor reverses), the converter voltage again becomes positive and greater than E, as shown in Figure 4.7(c). The converter is then rectifying, with power being fed into the motor, and the system is operating in quadrant 3.
Schemes using reversing contactors are not suitable where the reversing time is critical, because of the delay caused by the mechanical reversing switch, which may easily amount to 200"400 msec. Field reversal schemes operate in a similar way, but reverse the field current instead of the armature current. They are even slower, because of the relatively long time-constant of the field winding.
Double-converter reversing drives
Where full four-quadrant operation and rapid reversal is called for, two converters connected in anti-parallel are used, as shown in Figure 4.8. One converter supplies positive current to the motor, while the other supplies negative current.
Figure 4.8 Double-converter reversing drive.
The bridges are operated so that their d.c. voltages are almost equal thereby ensuring that any d.c. circulating current is small, and a reactor is placed between the bridges to limit the flow of ripple currents which result from the unequal ripple voltages of the two converters. Alternatively, the reactor can be dispensed with by only operating one converter at a time.
The changeover from one converter to the other can only take place after the firing pulses have been removed from one converter, and the armature current has decayed to zero. Appropriate zero-current detection circuitry is provided as an integral part of the drive, so that as far as the user is concerned, the two converters behave as if they were a single ideal bidirectional d.c. source.
Prospective users need to be aware of the fact that a basic single converter can only provide for operation in one quadrant. If regenerative braking is required, either field or armature reversing contactors will be needed; and if rapid reversal is essential, a double converter has to be used. All these extras naturally push up the purchase price.
