DOL, star-delta or soft starter: sizing a starter for mill and crusher loads
Choose the starter by how the load starts, not by the motor's horsepower. A load that starts unloaded — a blower, a centrifugal pump, an empty elevator — suits star-delta. A load that starts against resistance — a crusher jaw with stone in it, a loaded conveyor, a huller — needs a soft starter or a VFD, because star-delta throws away two thirds of the starting torque along with the current.
- DOL6–8× FLC starting · 100% torque · up to ~7.5 kW on LT
- Star-delta2–2.7× FLC · 33% torque · unloaded starts only · needs 6 terminals
- Soft starter2–4× FLC, adjustable · 25–75% torque · bypasses when up to speed
- VFD≤1.5× FLC · full torque from zero · speed control as well
Why is starting current the whole problem?
An induction motor at standstill looks to the supply like a short circuit through the winding resistance. It draws six to eight times its running current until it gets near synchronous speed. On a 30 HP motor that is roughly 250 amperes for a few seconds against a running current of about 40.
Three things object to that. The distribution licensee objects, because the surge drags the voltage down for everyone on the feeder. Your own plant objects, because contactors and cables see it every start. And the driven machine objects, because full DOL torque arrives as a mechanical hammer blow into a gearbox or a belt drive.
What does each starter actually do?
| Starter | Start current | Start torque | Suits | Avoid on |
|---|---|---|---|---|
| DOL | 6–8× FLC | 100% | Small motors, hard starts, anything under about 7.5 kW | Anything the licensee will object to; belt drives that snatch |
| Star-delta | 2–2.7× FLC | ~33% | Blowers, aspirators, centrifugal pumps, empty conveyors | Loaded starts, crushers, high-inertia loads |
| Soft starter | 2–4× FLC, set | 25–75%, ramped | Crushers, loaded conveyors, hullers, pumps with water hammer | Where you also need speed control — buy the drive instead |
| VFD | ≤1.5× FLC | Full, from zero | Variable loads, blowers on throttled ducts, process lines | Fixed-speed constant-torque loads where payback never comes |
How does the load decide it?
Split every motor in the plant into one of two groups and the answer falls out.
Starts unloaded or nearly so. A centrifugal blower has almost no torque demand at zero speed because the air load rises with the square of speed. Same for a centrifugal pump against a closed valve, and for an elevator or conveyor that was emptied before shutdown. These are star-delta loads. A third of the torque is plenty to get them moving, and you save the cost of a soft starter.
Starts against resistance. A crusher jaw with stone still in the chamber. A conveyor that stopped fully loaded. A huller with paddy sitting in it. A rotary kiln feeder. These need real torque from the first instant, and they need it applied smoothly. Star-delta fails them twice: the motor may not accelerate out of star at all, and if it does, the changeover happens at high slip and produces a current transient no better than DOL, with a torque step through the drive train.
Star-delta starters fitted to crushers are the most common wrong starter we replace in this district. The symptom the owner describes is always the same: "it hums, then it bangs". What is happening is that the motor cannot accelerate in star, sits there drawing locked-rotor current for the full timer setting, then slams into delta at almost standstill.
The other one is a huller line where somebody has fitted DOL on everything because it was cheapest, and the mill trips the incomer whenever two motors are started close together. The fix is rarely a bigger breaker — it is staggering the starts and putting soft starters on the two biggest machines.
What does TPWODL expect on an LT connection?
Bargarh and Sambalpur are in the TP Western Odisha Distribution Limited licence area, and like most licensees TPWODL restricts direct-on-line starting above a threshold — generally around 7.5 kW, roughly 10 HP, on an LT service. Above that you are expected to fit reduced-voltage starting.
Treat that as the starting point for a conversation with your section office rather than a universal rule, because the limit is applied against your sanctioned load and the state of the feeder you are on. It is worth a phone call before you buy, because a starter chosen after the connection is refused costs you the starter twice.
Sizing the starter itself
Once the type is settled, three numbers matter.
- Contactor rating. Size to AC-3 duty for a motor, not AC-1. An AC-1 rating is for resistive loads and will be a much higher current for the same physical contactor. A contactor sized on its AC-1 figure will not survive motor switching.
- Overload relay range. The relay must have the motor's full load current comfortably inside its adjustment range, ideally near the middle. A relay set at the extreme end of its band protects badly. Set it to the nameplate FLC, not to the highest setting that stops it tripping.
- Star-delta contactor sizes. The main and delta contactors carry the phase current, which is the line current divided by root three — about 58 per cent. The star contactor carries about a third. Sizing all three as though they were full line current is common and simply wastes money; undersizing the delta contactor is the dangerous version of the same confusion.
The short version
- Under about 7.5 kW, and the licensee is content — DOL.
- Above that, and the load starts unloaded — star-delta, and check the motor has six terminals.
- Above that, and the load starts loaded — soft starter.
- Variable load where speed can be reduced for part of the duty — VFD, and see whether it pays for itself.
Tell us the motor rating and what it drives — the actual machine, not just the horsepower — and we will tell you which starter it needs and quote the contactors and relay to match.