How does a four point starter overcome the objections
of a three point starter?
The speed of the motor is controlled by means of the field rheostat. To
increase the speed of the motor it is necessary to set the field rheostat to a
higher resistance value. The current through the shunt field is reduced and is
the current through the Coil of the holding electromagnet.
The reduced current
through the coil reduces the strength of the magnet and makes it susceptible to
line voltage variations. In the weakened condition a slight reduction in line
voltage would further weaken the holding reduction in line voltage would
further weaken the holding magnet, releasing the arm of the starter and thus
disconnecting the mother from the line. Unscheduled stoppages of the motor make
the three point starter quite unpopular.
The four-point starter provides the motor with no-voltage protection.
Should the power fail, the motor must be disconnected from the line. If not
full line voltage will be applied to the armature without the benefit of the
starting resistors when power is restored. The holding magnet leeing connected
across the line, release the arm when the voltage drops below a specific value
, thus protecting the motor when power is restored.
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