Circuit Operation
The operation of the circuit in the circulating-current free mode is not
very much different from that described in the previous pages. In order to
drive the motor in the forward direction, the positive converter is controlled.
To control the motor in the reverse direction, the negative converter is controlled.
When the motor is to be changed fast from a high value to a low value in the
forward direction, the conduction has to switch from the positive converter
to the negative converter. Then the direction of current flow changes in the
motor and it regenerates, feeding power back to the source. When the speed
is to be reduced in the reverse direction,
the conduction has to switch from the negative converter to the positive
converter. The conduction has to switch from one converter to the other when
the direction of motor rotation is to change.
At the instant when the switch from one converter to the other is to occur,
it would be preferable to ensure that the average output voltage of either
converter is the same. Let the firing angle of the positive converter be aP,
and the firing angle of the negative converter be aN
. If the peak line voltage be U, then equation (1) should apply. Equation
(1) leads to equation (2). Then the sum of firing angles of the two converters
is p, as shown in equation (3).
In a dual-converter, the firing angles for the converter are changed according
to equation (3). But it needs to be emphasized that only one converter operates
at any instant.
When the speed of the motor is to be increased above its base speed, the
voltage applied to the armature is kept at its nominal value and the phase-angle
of the single phase bridge is varied such that the field current is set to
a value below its nominal value. If the nominal speed of the motor is 1500
rpm, then the maximum speed at which it can run cannot exceed a certain value,
say 2000 rpm. Above this speed, the rotational stresses can affect the commutator
and the motor can get damaged.
Next it is shown how the operation of motor can be represented by means of
a block diagram. This approach can be helpful in designing the closed-loop
system.
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