
130 Shipboard Electrical Power Systems
The steady-state full-load line current I
L
drawn by the dc motor from line voltage
V
T
is given by the following equation:
V I
HP
Motor Efficiency
T L
⋅ =
⋅746
(5.22)
Example 5.12
A 50 hp, 240 V dc motor runs at 2350 rpm at rated conditions. If its field current is
decreased by 20% while keeping the load current the same, determine its approxi-
mate and exact speed.
SOLUTION
The motor is assumed to be the shunt type, as it is the most widely used dc
motor. Its speed changes inversely with the field current under the same load-
ing. Therefore, a 20% decrease in the field current will increase the motor