It is wound in spiral shape and placed on the porcelain or asbestos sheet. A number of tapping are connected with the studs. When the motor starts, full resistance is connected in series with the armature and as the motor attains the speed, it is gradually decreased to zero value.
A copper strip is placed under the handle to make contact with the studs. It has a soft iron piece which is attracted by the no volt and the handle remain in ON position as long as the current flows through the no volt coil. The function of a spring is to bring the handle in OFF position when the supply fails. Tension of the spring is placed in such a manner so that the handle comes at the OFF position immediately after the no volt coil is demagnetised.
It consists of a few turns of thick wire connected in series. It is current flowing in the armature. If due to any resons Fault or over load the load current is increased, the no volt coil terminals are shunt circuited by the over load coil lever.
When the motor is overloaded beyond a predefined value, 'overcurrent release electromagnet' D gets activated, which short-circuits electromagnet E and, hence, releases the lever and the motor is turned off. The field winding gets directly connected to the supply, as the lever moves touching the brass arc the arc below the resistance studs.
The no voltage coil or Hold-on coil is connected with a current limiting resistance Rh. This arrangement ensures that any change of current in the shunt field does not affect the current through hold-on coil at all.
This means, electromagnetic pull of the hold-on coil will always be sufficient so that the spring does not unnecessarily restore the lever to the off position. A 4 point starter is used where field current is to be adjusted by means of a field rheostat for the purpose of operating the motor above rated speed by reducing the field current.
DC series motor starter Construction of DC series motor starters is very basic as shown in the figure. The start arm is simply moved towards right to start the motor. Thus, maximum resistance is connected in series with the armature during starting and then gradually decreased as the start arm moves towards right. This starter is sometimes also called as a 2 point starter.
Why only 40 kHz used? Replacing a Light Fixture. Cable Size for switch board wiring. Timer instead of delay. Electrical 1. Why do we Need a starter in DC motors? Eddie Njeri says:. July 22, at pm. Leave a Reply Cancel reply Your email address will not be published. Leave this field empty. As the back EMF reaches its maximum, the starter progressively decreases the external resistance, R s , to zero.
Shunt wound and compound wound DC motors typically use either a 3 point or 4 point starter. The name comes from the number of terminals that connect to the motor, with 3 point starters having three terminals: the armature terminal, which is connected to the motor armature winding; the line terminal, which is connected to the positive supply pole; and the field terminal, which is connected to the field winding.
This protects against unnecessary tripping when the motor is run above its normal speed.
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