Comparing motor starting methods — DOL, star-delta, soft starter, VFD

The motor replacement guide covers how to safely replace a motor, including the star/delta choice on the nameplate. This article covers a different aspect of the same motor: which starting method you choose, and why that directly affects inrush current, starting torque and the stress on cabling and switchgear.

The four common starting methods

Method	Typical inrush current	Principle
DOL (direct-on-line)	approx. 5–8× In	Motor gets full mains voltage immediately; simplest, but highest inrush current and shock.
Star-delta	approx. 1.5–2× In	Motor starts in star (each winding gets only the phase voltage, 1/√3 of the line voltage) and switches over to delta after a delay for full operation.
Soft starter	approx. 2.5–4× In	Thyristors (SCRs) ramp up the voltage gradually over an adjustable ramp time (typically 5–30 s).
Variable frequency drive (VFD)	approx. 1–1.5× In	Controls both voltage and frequency from a low level, keeping slip — and therefore current — low while still delivering full torque.

Note: these are typical orders of magnitude from practice, not guaranteed values for every motor — the actual inrush current depends on the motor design (cage type), the load during starting, and the soft-starter/VFD settings.

Why star-delta isn't always a good alternative

Star-delta reduces inrush current, but the starting torque drops proportionally with it — because torque is approximately proportional to the square of the applied voltage, the star position delivers only about one-third of the DOL starting torque. For a load with high starting torque (for example a fully loaded conveyor or a compressor already under pressure), the motor in star simply may not reach speed — it stays "stuck" until the current rises too high or the motor protection trips. Star-delta is therefore mainly suitable for low-starting-torque loads such as fans and centrifugal pumps that start relatively unloaded.

The switchover moment itself (from star to delta) also causes a brief, sharp current and torque spike because the motor is momentarily de-energised during the switch and then re-energised out of phase — with an incorrectly set switchover time (too early, before the motor has reached sufficient speed) that spike can become just as high as a DOL start.

Soft starter versus variable frequency drive

A soft starter only controls voltage (via phase-angle control with thyristors) and keeps the mains frequency fixed — suitable for limiting inrush current and mechanical shock at start-up, but offers no speed control during operation; after the ramp phase the motor is typically bypassed directly onto the full mains.

A variable frequency drive controls both voltage and frequency simultaneously (V/f ratio), keeping slip — and therefore drawn current — low throughout the entire start while motor torque remains intact. That makes a VFD suitable for applications that also need variable speed during operation (pumps, fans with flow control), but it brings its own consideration: see the frequentieomvormer-aarding-emc guide for the EMC earthing and bearing currents that come with PWM drive control.

Practical relevance

When choosing or replacing a starting method for a three-phase motor, it must be established: what starting torque does the load need (a high starting torque often rules out a simple star-delta start), is variable speed needed during operation (then a VFD), and is the cabling/motor protection (see also the motor replacement guide) rated for the actual inrush current of the chosen method.

Common mistakes

Applying star-delta to a load with high starting torque — the starting torque in star is only about one-third of the DOL torque; the motor then fails to reach speed.
Setting the star-to-delta switchover time too short — the motor has not yet reached sufficient speed at switchover, causing a sharp current/torque spike that defeats the intended current limitation.
Treating a soft starter and a variable frequency drive as interchangeable — a soft starter only controls the starting phase (fixed mains frequency, no speed control during operation), a VFD continuously controls both voltage and frequency.