Introduction
This page describes how a separately-excited dc shunt motor can be operated
in either direction in either of the two modes, the two modes being the motoring
mode and the regenerating mode. It can be seen that the motor can operate
in any of the four quadrants and the armature of the dc motor in a fast four-quadrant
drive is usually supplied power through a dual converter. The dual converter
can be operated with either circulating current or without circulating current.
If both the converters conduct at the same time, there would be circulating
current and the level of circulating current is restricted by provision of
an inductor. It is possible to operate only one converter at any instant,
but switching from one converter to the other would be carried out after a
small delay. This page describes the operation of a dual converter operating
without circulating current.
As shown in Fig. 1, the motor is operated such that it can deliver maximum
torque below its base speed and maximum power above its base speed. To control
the speed below its base speed, the voltage applied to the armature of motor
is varied with the field voltage held at its nominal value. To control the
speed above its base speed, the armature is supplied with its rated voltage
and the field is weakened. It means that an additional single-phase controlled
rectifier circuit is needed for field control. Closed-loop control in the
field-weakening mode tends to be difficult because of the relatively large
time constant of the field.
The power circuit of the dual-converter dc drive is shown in Fig. 2.
Each converter has six SCRs. The converter that conducts for forward motoring
is called the positive converter and the other converter is called the negative
converter. The names given are arbitrary. Instead of naming the converters
as positive and negative converter, the names could have been the forward
and reverse converter. The field is also connected to a controlled-bridge
in order to bring about field weakening.
The circuit shown above can be re-drawn as shown in Fig. 3. Usually an inductor
is inserted in each line as shown in Fig. 3 and this inductor reduces the
impact of notches on line voltages that occur during commutation overlap.
TO THE TOP |
