Electrical Drives
DC Controllers
Dc motors are energized or started by manually operated switches or magnetically
operated contactors. Manually operated switches handle a modest
range of motor currents. Magnetically operated contactors usually
provide a safer and more convenient means for controlling motor power circuits,
particularly in large horsepower ratings.
The inrush currents of line-started dc motors are limited only
by the "at-rest" resistance of the circuit and may be ten times the
full-load current. Because dc motors have commutators that can be damaged by
excessive starting currents, full-voltage starting is not always possible, except
for small motors that have relatively high winding resistance. Compound-wound motors
up to 7 hp and other dc motors up to 2 hp can usually be line started.
A dc motor can commutate more current at low speeds than at high speeds.
With the armature practically standing still, most commutators handle currents
as high as six times the full-load rating of the motor without excessive sparking.
A starting or current-limiting resistor must be added for larger motors.
Resistance is gradually shorted out as the motor accelerates.
The motor counter-emf increases with speed, thus reducing the current.
When the motor reaches its base or normal operating speed, all starting resistance
is shorted out, and the motor draws its normal running current.
If magnetic starters are used for acceleration, starting resistance can be shorted
out in one or more steps, depending on motor size and the smoothness
of acceleration desired. There are three methods for obtaining automatic acceleration:
time-limit, current-limit, and speed acceleration.
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