3 Point Starter of DC Motor — Construction, Working & Protection
A 3 point starter is a protective device used for starting and running DC shunt motors and compound wound DC motors. It limits the dangerously high starting current that flows through the armature when back EMF is zero at standstill.
Why DC Motors Need a Starter
When a DC motor is switched on, the back EMF (Eb) is zero because the armature is stationary. The armature current at starting is given by:
Since armature resistance (Ra) is very small (typically 0.5–1 Ω), the starting current can be 15–20 times the full-load current. This excessive current can damage the armature winding, commutator, and brushes. A starter connects external resistance in series with the armature to limit this inrush current during starting.
Construction of 3 Point Starter
The 3 point starter consists of the following components:
- Starting resistance — divided into several sections connected to brass studs
- Handle (lever arm) — spring-loaded, moves across studs from OFF to RUN position
- No Volt Coil (NVC) — electromagnetic coil connected in series with field winding
- Overload Release (OLR) — small electromagnet carrying armature current
- Soft iron piece — attached to the handle, held by NVC in RUN position
Fig: Connection diagram of a 3 point starter for DC shunt motor
Three Terminals — L, A, F
The name "3 point starter" comes from its three external terminals:
No Volt Coil (NVC)
The No Volt Coil is connected in series with the field winding. Its functions are:
- Holding function: When the handle reaches RUN position, field current flows through NVC, magnetizing it. The magnetic force holds the soft iron piece (attached to handle) against the spring force.
- Protection on supply failure: If supply fails, current through NVC drops to zero. NVC demagnetizes, and the spring pulls the handle back to OFF position — disconnecting the motor from supply.
- Protection on field failure: If field winding opens, NVC current becomes zero. Handle returns to OFF, preventing the motor from running at dangerously high speed (runaway condition).
Overload Release (OLR)
The Overload Release is a small electromagnet that carries the full armature current. Under normal load, the current is insufficient to attract the OLR lever. However, when armature current exceeds the rated value:
- OLR attracts the lever arm
- The lever closes contacts P-P (short-circuits the NVC)
- NVC loses its magnetism and releases the handle
- Spring pulls handle to OFF position
- Motor is disconnected from supply
Working of 3 Point Starter
The step-by-step working of a 3 point starter:
- Step 1: Initially, the handle is in OFF position. Supply is switched on.
- Step 2: The handle is moved slowly to contact stud No. 1. The entire starting resistance comes in series with the armature, limiting the starting current. Simultaneously, field winding receives full supply through a parallel path via NVC.
- Step 3: As the motor gains speed, back EMF builds up and armature current decreases. The handle is moved to stud 2, 3, 4, etc., gradually cutting out resistance sections.
- Step 4: When the handle reaches RUN position, all starting resistance is cut out. The motor runs at rated speed with full voltage across the armature. NVC holds the handle in this position.
At RUN position: Ia = (V - Eb) / Ra
Starting Current Formula
The design of starter resistance sections follows the principle that current should not exceed a maximum value (Imax) at each stud:
where k = Imax / Imin
This geometric progression ensures smooth acceleration with uniform current pulsations between Imax and Imin at each step.
3 Point Starter vs 4 Point Starter
Advantages & Disadvantages
Advantages:
- Limits high starting current, protecting armature winding
- Provides no-volt protection (auto-disconnects on supply failure)
- Provides overload protection via OLR
- Simple construction and low cost
- Prevents motor runaway on field failure
Disadvantages:
- Cannot be used where speed control by field weakening is required
- NVC and field winding are in series — reducing field current weakens NVC holding force
- Not suitable for DC series motors (no separate field winding)
- Manual operation — requires operator attention during starting
Applications
- DC shunt motors in lathes, drilling machines, and milling machines
- Compound wound DC motors in elevators and hoists
- Laboratory DC motor setups for educational purposes
- Small industrial drives where constant speed is acceptable
Frequently Asked Questions
1. Why is it called a 3 point starter?
It is called a 3 point starter because it has three external connection terminals — L (Line), A (Armature), and F (Field). These three points connect the starter to the supply, armature circuit, and field circuit respectively.
2. What happens if the starter handle is moved quickly to the RUN position?
If the handle is moved directly to RUN without pausing at intermediate studs, the full supply voltage appears across the armature with zero back EMF. This causes an extremely high inrush current (15–20× rated) that can burn the armature winding, damage the commutator, and blow fuses.
3. Why can't a 3 point starter be used with a field rheostat for speed control?
In a 3 point starter, the NVC is in series with the field winding. When a field rheostat increases resistance to weaken the field (for speed control), the current through NVC also decreases. If it drops below the holding value, NVC releases the handle and the motor disconnects — even though the motor is running normally.
4. What is the difference between NVC and OLR in a 3 point starter?
NVC (No Volt Coil) protects against supply failure and field failure by releasing the handle when current through it drops to zero. OLR (Overload Release) protects against overcurrent by short-circuiting the NVC when armature current exceeds the rated value, which causes the handle to return to OFF position.
5. Can a 3 point starter be used for a DC series motor?
No. A 3 point starter requires a separate field winding connection (terminal F) for the NVC to function. DC series motors have their field winding in series with the armature — there is no separate field terminal. For DC series motors, a two-point starter or drum controller is used instead.