DC MOTOR OPERATION
DC Motors
DC MOTOR OPERATION
DC motors require special starting resistors for operation due to their unique
design. A knowledge of the operation of these starting resistors is necessary to
understand DC motor operation.
EO 1.11
EXPLAIN why starting resistors are necessary for large
DC motors.
EO 1.12
LIST the four nameplate ratings for a DC motor.
Starting of DC Motors
At the moment a DC motor is started the armature is stationary and there is no counter EMF
being generated. The only component to limit starting current is the armature resistance, which,
in most DC motors is a very low value (approximately one ohm or less), as shown in Equation
(6-9).
(6-9)
Ia
Et
ECEMF
Ra
In order to reduce this very high starting current, an external resistance must be placed in series
with the armature during the starting period. To show why this is essential, let us consider a
10-hp motor with an armature resistance of 0.4 ohms. If the motor were supplied by a 260 VDC
source, the resulting current would be as shown in Equation (6-9).
Ia
Et
ECEMF
Ra
Ia
260VDC
0
0.4W
Ia
650 amps
This large current is approximately twelve times greater than actual full-load current for this
motor. This high current would, in all probability, cause severe damage to the brushes,
commutator, or windings. Starting resistors are usually incorporated into the motor design to
limit starting current to 125 to 200 percent of full load current.
ES-06
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