Function of Starter in Motor — Need, Working & Types
A motor starter is a device used to safely start and stop an electric motor. Without a starter, the motor draws dangerously high current at startup — often 5 to 7 times the full-load current — which can damage windings, brushes, and connected equipment. This article explains why starters are essential, how they work, and the different types used in DC and AC motors.
Why Does a Motor Need a Starter?
The armature current in a DC motor is given by:
Where:
- V = Supply voltage
- Eb = Back EMF (proportional to speed)
- Rₐ = Armature resistance (typically 0.3–0.8 Ω)
At the instant of starting, the motor speed is zero. Since back EMF is directly proportional to speed (Eb ∝ N), the back EMF is also zero at standstill. The starting current becomes:
For a 220V DC motor with Rₐ = 0.5 Ω, the starting current would be 220/0.5 = 440 A — far exceeding the rated current of perhaps 50 A. This is the fundamental reason every motor needs a starter.
Inrush Current Problem
The excessive starting current causes several problems:
- Winding damage — High I²R heating can burn armature insulation
- Commutator sparking — Excessive current causes heavy arcing at brushes
- Mechanical stress — Sudden high torque can damage couplings and gears
- Voltage dip — Large inrush current causes supply voltage to sag, affecting other equipment on the same bus
- Fuse blowing — Protective devices may trip unnecessarily
In AC induction motors, the problem is similar. At standstill, the slip is 1 (maximum), and the rotor impedance is at its lowest, resulting in high starting current — typically 6 to 8 times the full-load current.
Function of a Motor Starter
The primary functions of a motor starter are:
- Limiting starting current — Introduces external resistance or reduces voltage during startup to keep current within safe limits (usually 1.5 to 2 times full-load current)
- Gradual acceleration — Allows the motor to build up speed smoothly as back EMF develops
- Overload protection — Disconnects the motor if current exceeds rated value for a sustained period
- No-volt protection — Automatically disconnects the motor if supply fails, preventing sudden restart when power returns
- Under-voltage protection — Trips the motor if voltage drops below a safe threshold
As the motor accelerates, back EMF builds up and the current naturally decreases. The starter resistance is gradually cut out in steps until the motor reaches full speed and runs with only the armature resistance in circuit.
Types of Motor Starters
DC Motor Starters
- 3-Point Starter — Uses three terminals (L, A, F). Suitable for DC shunt motors. The no-volt coil (NVC) is in series with the field winding, providing both no-volt and overload protection.
- 4-Point Starter — Uses four terminals (L, A, F, N). Preferred when field speed control is used, as the NVC current is independent of field current.
AC Motor Starters
- DOL (Direct-On-Line) Starter — Connects motor directly to supply. Used for small motors (up to 5 HP) where inrush current is acceptable.
- Star-Delta Starter — Starts motor in star connection (voltage reduced by √3), then switches to delta at full speed. Reduces starting current to 1/3 of DOL value.
- Auto-Transformer Starter — Uses a tapped auto-transformer to provide reduced voltage at start. Offers flexible voltage taps (50%, 65%, 80%).
- Soft Starter — Uses thyristors (SCRs) to gradually increase voltage from zero to full. Provides smooth, stepless acceleration.
- VFD (Variable Frequency Drive) — Controls both voltage and frequency for precise speed and torque control from zero to full speed.
Comparison of Starter Types
How to Select the Right Starter
Choosing the correct starter depends on several factors:
- Motor rating — Small motors (below 5 HP) can use DOL; larger motors need reduced-voltage starting
- Load type — High-inertia loads (crushers, mills) need high starting torque; low-inertia loads (fans, pumps) can use star-delta
- Supply capacity — Weak supply networks cannot tolerate high inrush currents
- Starting frequency — Frequent starts favour electronic starters (soft starter/VFD) over electromechanical types
- Cost vs performance — VFDs offer the best performance but at highest cost; star-delta is economical for simple applications
Starting torque with star-delta = (1/3) × DOL starting torque
Frequently Asked Questions
1. Why can't we start a DC motor without a starter?
At standstill, back EMF is zero, so the armature draws V/Rₐ current — typically 5 to 7 times the rated current. This can burn the winding insulation and damage the commutator within seconds.
2. What happens if the starter resistance is not gradually removed?
If the full resistance remains in circuit, the motor runs at reduced speed with poor efficiency. The excess voltage drops across the starter resistance, wasting energy as heat and potentially overheating the starter itself.
3. Can small motors run without a starter?
Yes. Fractional horsepower motors (below 1 HP) have relatively higher armature resistance, so the inrush current ratio is lower. They can often be started with a simple switch (DOL). However, even small motors benefit from overload protection.
4. What is the difference between a 3-point and 4-point starter?
In a 3-point starter, the no-volt coil is in series with the field winding. If field resistance is increased for speed control, the NVC current drops and may release the handle. A 4-point starter has the NVC connected independently across the supply, so field current changes don't affect the holding mechanism.
5. Why are VFDs replacing traditional starters in modern industry?
VFDs provide soft starting with minimal inrush current (1–1.5× FLC), precise speed control, energy savings at partial loads, regenerative braking, and built-in protection features — all in one device. The declining cost of power electronics has made them economical even for medium-sized motors.