A motor starter’s primary responsibility is to start and stop the motor to which it is connected. Starters are specially made electromechanical switches, similar to relays, that are used to protect the motor from overload. The starter supplies the motor with power either manually or automatically while also guarding against overloads and faults. The market offers motor starters in a variety of sizes and ratings depending on the type of motor. This article discusses the three point starter, which is employed in both industrial and commercial settings.

What is a Three Point Starter?

An electrical tool called a three point starter is used to start and maintain the speed of a DC shunt motor. This circuit’s series resistance connection reduces the initial high current and protects the apparatus from any electrical malfunctions. Here, the presence of back e.m.f. is crucial for driving the motor. This emf increases as soon as the motor’s armature begins to rotate in the magnetic field by producing an action that also opposes the voltage supply.

Construction of Three Point Starter

The DC motor based 3 point starter mainly includes three terminals namely L, A, and F. Here, L( line terminal) is connected to the positive supply, A (armature terminal) is connected to the windings of an armature terminal, and F (field terminal) is connected to the winding of field terminal.

The construction of 3-point starter includes a resistance ‘R’ for controlling the initial current. The “H’- handle in the circuit kept in the OFF condition with a spring ‘S’. The H-handle can be operated manually for motor operation.  At the beginning of the motor position, the motor field winding gets the total supply voltage, & the armature current is restricted to the particular secure value by the resistance R.

Working of Three Point Starter

To increase the motor’s speed until it is in the RUN position, the handle of the three point starter can be moved from one stud to another (contact positions). In this position, there are three main factors that are taken into account, including the following.

Three Point Starter

• The DC shunt motor gets the full speed
• The voltage supply in the circuit is straight across both the motor’s windings.
• The R-resistance is totally cut-out.

With the help of an electromagnet strengthened by an NVC, the H-handle in the circuit is kept in the RUN position (no volt trip coil). This NVC coil and the motor field winding can be connected in series. The NVC will become energised if it was turned OFF during the incident or if it fell below a fixed value. The handle-H is both released and pulled back to the OFF condition by the action of the S-spring.

H-handle will initially move in a CLK-wise direction toward stud1. When a DC supply is turned ON while it is in the OFF position. As the total resistance is initially connected in series with the armature circuit, the winding of the shunt field is directly related across the voltage supply.

The no-volt discharge coil is demagnetized. And the H-handle returns to the OFF position in the event that the voltage supply is abruptly interrupted. A supply failure would occur if the no-volt coil were not used. The final stud would continue to hold the H-handle. The DC motor will be openly allied across the supply. If the voltage supply is returned, producing an extremely high armature current.

When a DC motor is overloaded, it will draw excessive current from the power source, amplifying the current as it rotates off the excess release coil and pulls the armature, short-circuiting the no-volt coil. The S-spring pulls the H-handle close to the OFF position, demagnetizing this coil as well. The electric motor is therefore automatically disconnected from the current source.

Drawbacks of a Three-Point Starter

• The main drawback of the Three point starter is, it experiences from a major drawback of motors by a huge difference of speed with a modification of the field rheostat.
• To amplify the motor speed, the field resistance must be amplified. So the flow of current throughout the shunt field is decreased.
• Whenever adding high resistance to get a high speed will make the field current very low.
• When NVC (no volt trip coil) is associated in series by shunt field, then the minute current will decrease the power of the electromagnet.
• This magnet may liberate the arm of the H-handle through the usual motor operation as well as detach it from the power supply.
• Therefore, the 4 Point Starter can be used, where no volt trip coil is allied in the parallel field.

Three Point Starter and its effectiveness are the main topics here. It serves as a high initial current restriction and a no voltage, over voltage, and under voltage protector. An enormous difference in speed is this starter’s main flaw.