Need of starter and induction motor starting methods

A starter is a device that limits the starting current of an induction motor and gives overload and no-volt protection.

Why a starter is needed

At standstill, slip $s = 1$, so the rotor behaves like a short-circuited transformer secondary. The rotor emf is maximum and the only opposition is the small leakage impedance:

$$I_{2,st} = \frac{E_2}{\sqrt{R_2^2 + X_2^2}}$$

The stator draws 5–7 times full-load current at rated voltage. This causes:

• a large voltage dip in the supply line, disturbing other loads;
• heating and mechanical stress on the windings;
• tripping of protective devices.

Small motors (below about 5 kW) can be started direct-on-line; larger ones need a starter that reduces the voltage or adds rotor resistance during starting.

Starting methods

1. Direct-on-line (DOL) starter — full voltage applied through a contactor with overload relay. Simple and cheap; starting current 5–7 × FL. Used for small motors.

2. Stator resistance (reactor) starting — resistors in series with each stator phase reduce the voltage to $xV$. Current falls to $x$ times, but torque falls to $x^2$ times. Resistors are shorted after run-up. Wasteful; used rarely.

3. Auto-transformer starting — a 3-phase auto-transformer applies a tapped voltage $xV$. Motor current falls to $x$ times, line current to $x^2$ times, starting torque to $x^2$ times. Better than resistance starting; used for large motors.

4. Star–delta starting — for motors designed to run in delta. At start the winding is connected in star, so each phase gets $V/\sqrt{3}$; after speed builds up it is switched to delta.

$$I_{line,\,Y} = \tfrac{1}{3} I_{line,\,\Delta}, \qquad T_{st,\,Y} = \tfrac{1}{3} T_{st,\,\Delta}$$

Cheap and common for medium squirrel-cage motors.

 Supply ---[TPDT switch]--- Stator
          Start: Y          Run: Delta

5. Rotor resistance starting (slip-ring motors only) — external resistance in the rotor via slip rings reduces starting current and, by increasing $s_m$ towards 1, increases the starting torque. Resistance is cut out in steps as the motor speeds up.

6. Soft starter / VFD — thyristor (or inverter) control raises voltage (or frequency) smoothly. Low current, smooth acceleration; used in modern installations.