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 (69).
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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 10hp 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 (69).
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This large current is approximately twelve times greater
than actual fullload 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.
The amount of starting resistance necessary to limit
starting current to a more desirable value is
calculated using Equation (610.
where
Example:
If the full load current of the motor mentioned previously
is 50 amps, and it is desired to limit starting current to 125% of this value,
find the required resistance that must be added in series with the armature.
The starting resistors are used in a DC motor by placing
them in the starting circuit of the motor controller that is used to start the
DC motor. Starting resistors are normally of variable resistances, with the
value of resistance in the circuit at any time being either manually or
automatically controlled. The maximum amount of resistance will always be
inserted when the motor is first started. As the speed of the motor increases,
counter EMF will begin to increase, decreasing armature current. The starting
resistors may then be cut out, in successive steps, until the motor reaches
full running speed.
