Starting methods compared

Reference notes

Use Next → on the narrator above to walk through the six classical starting methods for 3-phase induction motors, from direct-on-line to the modern VFD.

The starting problem

At standstill, slip s = 1, so the rotor circuit reactance s·X₂ is at its maximum. From the equivalent circuit:

This inrush current causes three problems:

• Voltage dip on the supply (annoying other loads).
• Excessive winding heating if start-up takes more than a few seconds.
• Mechanical shock on the load: torque pulses can damage couplings and gearboxes.

The various starting methods are all ways to reduce one or more of these effects.

1. Direct-on-line (DOL)

Just close a contactor. Full voltage hits the motor. Simple, cheap. Acceptable for motors up to ~5–10 kW where the supply can tolerate the inrush. Above that, utilities and good engineering practice mandate a soft start.

2. Star-delta starter

For motors normally run in delta connection, start with the windings in star (Y) and switch to delta (Δ) when up to ~75 % of synchronous speed. In star, each winding sees V_line/√3 — so:

Both starting current AND starting torque drop to one-third. Cheap, simple, common for medium-sized motors (5–50 kW). Only works if the motor is designed to run in delta.

3. Autotransformer starter

A tapped autotransformer reduces the voltage applied to the motor at start. If the tap is at fraction k (e.g. 0.65 → 65 % voltage):

Both supply current and starting torque scale with k², which is more aggressive than star-delta's 1/3. Multiple taps allow staged starting. Used for large motors (50–500 kW). Bulky and expensive.

4. Slip-ring rotor with external resistance

For slip-ring motors only. Insert external resistance via the slip rings at start, tuned so R₂ + R_ext ≈ X₂ → peak (breakdown) torque available at s = 1. As the motor accelerates, cut the external resistance out in steps. Result: maximum starting torque per ampere — best dynamic starting performance of any method. Used for cranes, mills, hoists where high starting torque is essential.

5. Soft starter (thyristor)

A phase-controlled thyristor stack ramps the applied voltage smoothly from a chosen starting value (~30 %) up to 100 % over a programmed time (typically 5–30 s). Modern simple choice for small to medium motors that need controlled start without full VFD complexity. Bypasses itself after start to avoid harmonic distortion in steady state.

6. Variable frequency drive (VFD)

Standard for new installations. The VFD outputs a 3-phase voltage at any chosen frequency f, keeping V/f constant to maintain the air-gap flux at rated. Starting:

• Begin at very low f (a few Hz) and low V (proportionally low).
• Ramp f from 0 up to rated over a programmable time.
• At every instant the motor sees only enough voltage to drive rated current → starting current never exceeds rated.

Plus: full rated torque is available at any speed from 0 to base. Plus: speed control "for free" in steady state. Plus: bidirectional torque, regenerative braking, programmable accel/decel ramps. The dominant choice for medium and large motors today.

Comparison at a glance

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