- 3 Point Starter Definition: The 3 point starter is a device that helps start and regulate DC motors by managing the initial high current.
- Diagram of the Starter: The starter’s diagram features labeled parts such as OFF, RUN, and connection points, illustrating its structure and functionality.
- Working Principle: The starter gradually decreases the resistance applied to the motor’s armature, facilitating a safe increase in speed.
- Safety Mechanism: The No Voltage Coil ensures the starter remains in the RUN position under normal conditions and releases it to OFF when power fails, enhancing safety.
- Comparison with 4 Point Starter: Unlike the 3 point starter, the 4 point starter can handle a wider range of motor speeds without losing connection, making it better for certain applications.
A 3 point starter puts extra resistance in the armature circuit of a DC shunt motor or compound wound DC motor at start, then cuts that resistance out as speed rises. A 4 point starter does a similar job with a different hold-coil connection.
Those types of DC motors need a starter because back emf Eb is missing at standstill. Back emf appears only once the armature turns in the magnetic field, and it then opposes supply voltage. At start that back emf is zero. It grows with speed.
The armature voltage equation is:
where E is supply voltage, Eb is back emf, Ia is armature current and Ra is armature resistance. At start Eb = 0, so E = Ia.Ra and armature current Ia is
The current would be very large at start because armature resistance Ra is small. A 3 point starter inserts extra resistance so starting current stays within a safe range.
The layout below shows how that extra resistance is switched. The diagram uses electrical symbols for the parts of a three-point starter.
Construction of 3 Point Starter
The starter is a variable resistance split into sections. The studs are marked OFF, 1, 2, 3, 4, 5, RUN. Three terminals give the device its name:
- ‘L’ Line terminal (Connected to positive of supply)
- ‘A’ Armature terminal (Connected to the armature winding)
- ‘F’ Field terminal (Connected to the field winding)
That is the construction of 3 point starter. Terminal L feeds an overload-release (OLR) electromagnet. The other side of the OLR goes to the conducting lever of the handle. A spring on that handle, and a soft-iron piece on it, return the handle to OFF if nothing holds it. From stud 1 a branch goes through the No Volt Coil (NVC) to terminal F. At start the extra resistance is all in series with the armature. OLR and NVC are the two protective coils.
Working of Three Point Starter
For the working of the 3 point starter, the handle sits at OFF when DC motor supply is applied. The handle is then moved slowly onto stud 1. The shunt or compound field is energised through the NVC path that bypasses the starting resistance. All of that resistance stays in series with the armature, so starting current is:
Moving the handle onto studs 2, 3 and 4 then drops more of the series resistance as speed rises. On RUN the extra resistance is out of circuit and the motor runs at its normal speed.
Back emf has by then grown with speed, so it opposes supply voltage and holds armature current down.
The extra resistance is then no longer needed. The handle is moved by hand from OFF to RUN as speed builds. The No Voltage Coil is what keeps it on RUN while the motor is supplied.
That coil works as follows.
Working of No Voltage Coil of 3 Point Starter
Field current also flows through the No Voltage Coil, so the NVC is magnetised. On RUN that magnet holds the soft-iron piece on the handle against the spring for as long as the motor is supplied. The NVC is therefore also called a hold-on coil.
If supply fails, NVC current falls and the magnet lets go. The spring then returns the handle to OFF and opens the motor circuit. That NVC-and-spring pair is the no-volt protection. It does not replace over-current protection from the OLR.
Drawbacks of a Three Point Starter
A 3 point starter is a poor match for wide speed control by a field rheostat. Raising field resistance to raise speed also lowers NVC current.
That weaker field current can leave the hold magnet too weak for the spring, so the handle can drop to OFF while the motor is still meant to run. A 4 point starter puts the hold coil on a separate supply path so field weakening does not drop the handle.
