
D.C. Motor Drives 115
most 2-pulse ones. Overall, the motor then behaves much as
it would with pure d.c, though the I
2
R loss is higher than it
would be if the current was perfectly smooth.
In some small 2-pulse drives, however, there is insufficient
armature inductance to prevent the current from falling to
zero for part of each cycle. This is known as the 'discontin-
uous current' mode, and it is usually encountered when the
motor is lightly loaded. It is very undesirable because when
the current is discontinuous, the mean voltage from the con-
verter falls dramatically as the mean current drawn by the
motor is increased, and in addition th