3 Point Starter | Diagram and Working Principle


What is 3 point starter in DC motor?

A three-point starter is an electrical device, used for starting as well as maintaining the DC shunt motor speed. The connection of resistance in this circuit is in series which decreases the initial high current and guards the equipment against any electrical failures.

3 Point Starter Diagram



3 point starter consists of a resistance which is divided into several sections and includes three terminals namely LA and F.

Here,  L( line terminal) is connected to the positive supply,
         A (armature terminal) is connected to the armature winding,
and    F (field terminal) is connected to the field winding.

Apart from these, OLR (Overload release) and NVC (No Volt Coil) are also provided in the motor. These two acts as a protecting device for the motor.


Field winding is connected with No Voltage Coil. So when field current flows, the NVC is magnetized. Now when the handle is in the ‘RUN’ position, a soft iron piece is connected to the handle and because of flow of current through it, it gets attracted by the magnetic force produced by NVC,  The NVC is designed in such a way that it holds the handle in ‘RUN’ position against the force of the spring as long as supply is given to the motor. Thus NVC holds the handle in the ‘RUN’ position.

Now in case of any supply failure, current will stop flowing through NVC and NVC will get demagnetized. Under the action of spring force, handle will move back to OFF position and motor circuit will be open. Thus protecting the motor.


Overload Release is just a small electromagnet. As the name suggests, it is used to give protection to motor in case of overload. Under the normal condition, OLR carries the armature current and this current is not sufficient to pull the lever (as shown in fig.). But when the armature current exceeds the normal rated value, it attracts that lever and in turn, the triangular iron closes the contact PP. Thus, NVC gets short-circuited and it results in the release of handle, which returns to OFF position and supply to the motor gets cut off.

Working Principle of 3 Point Starter

It can also be called a three-point starter. Its structure is shown in Figure. To start the motor, the handle is moved from off position to the first contact position. The total obstacle is in the armature circuit, the field connection is connected directly to the supply through a volt coil. Comes in as soon as the speed of the motor increases in the same way as the archete back emf emits and decreases in the value of current Now the handle is moved to the second step. Again this type of function will be done eventually at the last step of the handle. This time the armature will connect directly to the supply and this time the handle does not come with a no-volt coil. The magnet will hold the handle in the Walt coil, and the motor will not need to hold the handle to continue and the motor will rotate at full speed.

Mechanism of 3 points Starter is done so that if the magnetism of the nvc coil is exhausted for any reason, and the handle will be re-off position through the spring, then the supply will not be supplied directly to the motor, with the handle again All the process has to be done, the overloaded relay in the starter is also fitted. It also protects from overload. In the mechanism of which the motor is overloaded.

There is a 3-point Starter equipped with such safety systems

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