At standstill an induction motor is nearly a short circuit: it draws locked-rotor current of ~6–8× full-load. That inrush dips the network voltage, heats the windings, and shocks the driven machine.
Reduced-voltage starters (star-delta, soft starter, autotransformer) cut the current — but because torque falls with the square of voltage, they also cut the starting torque hard. You can only use them on loads that don't need much torque to get moving — like a centrifugal pump started against a closed valve.
A VFD sidesteps the trade-off entirely by ramping frequency, giving full torque at a fraction of the current — and then stays on to control speed and save energy. It is the most capable and the most expensive.
1 · Why starting current is a problem
An induction motor develops torque from the relative motion (slip) between the rotating magnetic field of the stator and the rotor. At the instant of switch-on the rotor is stationary while the field is already turning at full speed — so the slip is 100%. To the supply, a stationary motor looks almost like a short-circuited transformer. It draws a large current, the locked-rotor current (LRC), typically 6 to 8 times the full-load current (FLA).
That inrush itself isn't usually destructive — it lasts only seconds — but it causes four real problems:
- Voltage dip. A large current through the supply impedance drops the voltage across the whole busbar — lights flicker, other equipment may trip. Utilities and class societies set limits on how much a single motor may draw.
- Thermal stress. Inrush heating is measured in amp²-seconds. A long run-up, or frequent starts, cooks the windings — the limiting factor is usually starts per hour.
- Mechanical shock. Full-voltage starting slams full torque into the coupling, gearbox and driven machine — and into the pipework as a pressure transient.
- Supply sizing. Cables, breakers and generators must tolerate the inrush, not just the running load.
2 · The one trade-off behind every starter
Reduced-voltage starting is the oldest idea: feed the motor less than full voltage at start, and the current falls in proportion. But torque does not fall in proportion — it falls faster. These two relationships are the whole subject:
This is why you cannot simply turn every motor down at start. The motor still has to develop more torque than the load demands at every speed, or it never gets moving — it sits there drawing current and overheating until the protection trips. How much you can reduce the voltage is set entirely by how much starting torque the load needs.
This connects directly to the pump rule in Part 2 of the pump series: a centrifugal pump started against a closed discharge valve needs almost no starting torque (its load torque rises with the square of speed, so it's tiny at standstill). That low torque demand is exactly what lets you use a gentle reduced-voltage starter on a pump — the two ideas are the same physics seen from opposite ends.
3 · The four methods
Direct-on-line (DOL)
A contactor connects the motor straight to full voltage. Simplest, cheapest, most reliable, and gives full starting torque — but full inrush (~6–7× FLA). Standard for small motors (roughly up to 7.5–15 kW depending on supply stiffness) and for any load that genuinely needs full torque to break away.
Star-delta (wye-delta)
The motor's six winding leads are first connected in star, which puts only V/√3 across each winding. Both the line current and the torque drop to about one third of their DOL values. Once the motor nears full speed, a timer switches the windings to delta (full voltage). Cheap and effective — but the one-third starting torque rules it out for high-torque loads, and the open-transition changeover produces a brief current and torque spike. Classic choice for unloaded or closed-valve pump and fan starts.
Soft starter
Back-to-back thyristors phase-control the voltage, ramping it smoothly from a chosen pedestal up to full over a set time, usually with an adjustable current limit (commonly 3–4× FLA). No transition step, controllable acceleration, and the option of a soft stop to avoid water hammer. It still obeys T ∝ V², so torque is limited during the ramp — but you can tune the limit to the load. No speed control once running.
Variable frequency drive (VFD)
A VFD rectifies the supply to DC and re-synthesises a variable-frequency, variable-voltage output. By starting at low frequency and ramping it up while holding the voltage-to-frequency ratio, it keeps slip small throughout — so the motor develops full torque while drawing little more than full-load current (typically ≤ 1.5×). It breaks the current/torque trade-off, then stays in circuit to give stepless speed control and large energy savings on variable-torque loads. The most capable and most expensive option, and it introduces harmonics that may need filtering.
4 · Watch the starting current
Pick a method and run the start. The faint grey curve is always the DOL reference, so you can see directly how much each method shaves off the inrush. Note that star-delta shows the characteristic spike at the star→delta changeover.
Interactive 1 — Starting current vs time
Live modelLine current during start
5 · Torque has to beat the load
Cutting the current is only safe if the motor still out-torques the load at every speed on the way up. This is where reduced-voltage starting succeeds or fails — and it depends entirely on the shape of the load's torque curve.
A centrifugal pump or fan has a load torque that rises with the square of speed, so it is nearly zero at standstill — easy to start gently. A conveyor, crusher or positive-displacement pump demands close to full torque from the very first degree of rotation — a reduced-voltage start may never break it away.
Switch the method and the load below. Where the motor curve sits above the load curve, the machine accelerates (green). If the reduced-voltage motor curve drops below the load, it stalls.
Interactive 2 — Torque vs speed
Live modelMotor torque vs load torque
6 · Choosing a method
| Method | Start current | Start torque | Speed control | Cost | Best for |
|---|---|---|---|---|---|
| DOL | ~6–7× FLA | Full (~150–200%) | No | Lowest | Small motors; high-torque loads; stiff supply |
| Star-delta | ~2–2.5× | ~1/3 (~50–65%) | No | Low | Low-torque starts (pumps, fans) on a budget |
| Soft starter | ~3–4× (set) | Reduced, tunable | No | Medium | Smooth start/stop; avoiding water hammer |
| VFD | ~1–1.5× | Full at any speed | Yes | Highest | Variable-flow duty; energy savings; frequent starts |
Match the starter to the load's torque curve, not just to the motor size. A 200 kW pump on a closed valve may start happily on star-delta, while a 30 kW loaded conveyor may demand a DOL or VFD. The decisive question is always: does the chosen method still develop more torque than the load needs, at every speed up to running?
Key takeaways
- Locked-rotor current is ~6–8× FLA because a stalled motor looks like a shorted transformer.
- Reduced-voltage starting trades torque for current at a square-law penalty —
I ∝ VbutT ∝ V². - Star-delta gives ~1/3 current and ~1/3 torque — great for pumps and fans, useless for high-torque loads.
- Soft starters ramp smoothly with a tunable current limit and soft-stop, but no speed control.
- VFDs break the trade-off, giving full torque at low current plus running speed control — at the highest cost.
- The load's torque-speed curve decides everything. Always check the motor out-torques the load at every speed.