Starting of DC motors.
A
starter may be a device to begin and accelerate a motor. A
controller may be a device to begin the motor, control and
reverse the speed of the DC motor and stop the motor. While starting the DC motor,
it draws the heavy current which damages the motor. The starter protects the
motor against damage because of short circuits within
the equipment, it protects the motor against damage from long-term
overloads, it protects the motor against damage from excessive starting
currents, and it provides a convenient manner during which to
manage the operating speed of the motor.
The dc motor has no back EMF. At the starting of the motor, the armature
current is controlled by the resistance of the circuit. The resistance of the
armature is low, and when the total voltage is applied at the
standstill condition of the motor, the armature current becomes very high which
damages the parts of the motor. Thanks to the high armature current,
the resistance of the machine is cut out of the circuit when the machine gains
its speeds. The armature current of a motor is given by
Thus, Ia depends upon E
and Ra, if V is kept constant. When the motor is first switched ON, the
armature is stationary. Hence, the back EMF Eb is also zero. The initial
starting armature current Ias is given by the equation shown below.
Since, the armature
resistance of a motor is extremely small, generally but one
ohm. Therefore, the starting armature current Ias would be very large
so we'll have large current this huge current would damage
the brushes, commutator, and windings.
Since at the time of starting the DC Motor, the
starting current is extremely large. At the time of popping
out all DC Motors, apart from very small motors, and
additional resistance must be connected nonparallel with the
armature. This extra resistance is added in order that a
secure value of the motor is maintained and to limit the starting current
until the motor has attained its stable speed.
The series resistance is split into
sections which are cut out one by one, because the speed of the motor
rises and also the back EMF builds up. The
additional resistance is cut out when the speed of the motor builds up to
its normal value, and that we can start DC separately excited motor with 4
point starter, 4 point starter could be a device whose main function
is starting and acts as a current limiting device while starting of the DC
motor. to begin dc separately excited motor the sector must
be energized with the rated field voltage before or simultaneously with
energizing the armature. i'm assuming that by "4-point
starter," you mean a starter that connects a set resistance nonparallel with
the armature so shorts out the resistance in 3 additional steps
ending with the armature connected on to the availability with
no resistance nonparallel.
The diagram shows in figure .1 the field connected to the DC source through a variable resistor. That would be considered to be a separate source. The starting lever connects the field to the source through the brass arc at the same time it is moved to the "1" terminal. If that arrangement is not suitable for the field, the field could be connected directly to some other source, but there should be some provision to assure that the field is energized when the armature is energized. If the field is not energized when the armature is energized, the motor could run to a high speed if the load is light. That could cause the motor to be damaged.
 |
| figure.1 |
This type of starter
had problems, as it largely depended on the person starting the motor not to
move its handle too quickly or too slowly. If the resistance were cut out too
quickly (before the motor could speed up enough), the resulting current flow
would be too large. On the other hand, if the resistance were cut out too
slowly, the starting resistor could burn up. Since they depended on a person
for their correct operation, these motor starters were subject to the problem
of human error. They have almost entirely been displaced in new installations
by automatic starter circuits.
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