DOL vs star delta starter: which starter for which HP motor in Kishanganj, Purnea and Katihar

Use a DOL (direct-on-line) starter for small three-phase motors, in general trade practice up to about 5 to 7.5 HP, where a hard start is fine. For bigger motors, like rice mill elevators, blowers, compressors and large pumps, use a star delta starter: it cuts the starting current to about one third of a DOL start. Always check the motor nameplate, the load and your electricity supplier’s rules before you finalise.

This guide is for electricians, panel builders and contractors working on rice mills, tea factories, cold stores, borewells and workshops in Kishanganj, Purnea, Katihar and Araria. The same advice applies across North Bengal, Bihar and Sikkim.

What is the difference between a DOL starter and a star delta starter?

A DOL starter connects the motor straight to the full supply when you press START. It is simple: one contactor plus an overload relay, and a START/STOP push button. The motor gets full voltage at once, so it pulls a big rush of current and gives strong starting torque.

A star delta starter starts the motor in star connection first, so each winding gets lower voltage. After a set time (a timer does this), it switches the motor over to delta and the motor runs at full power. It uses three contactors, an overload relay and a timer.

According to the Electrical Engineering Portal comparison, a DOL start draws 300% to 800% of full-load current, while star delta gives “only about one third” of the DOL starting current. The trade-off: star delta also cuts starting torque to about one third. The Lauritz Knudsen (L&T) guide puts it simply: DOL is “high current, high torque”, star delta is “reduced” current and “lower” torque.

Point	DOL starter	Star delta starter
Starting current	High (300–800% of full load)	About one third of DOL
Starting torque	High	About one third of DOL
Parts	1 contactor + overload relay	3 contactors + overload relay + timer
Wiring to motor	3 leads (one 3-core cable)	6 leads (two 3-core cables or one 6-core)
Motor needed	Any 3-phase induction motor	Motor that runs in delta at 415 V, with 6 terminals
Panel space and cost	Less	More
Best for	Small pumps, fans, workshop machines	Bigger pumps, blowers, compressors, mill machines

Which starter for which HP motor?

There is no single number in the standards that every site follows. The table below is general trade practice we see on sites, not a rule. The motor maker’s recommendation, the load and the local supply conditions decide the final choice.

Motor size (3-phase, 415 V)	Usual starter	Typical uses
Up to about 5 HP (3.7 kW)	DOL	Small borewell and monoblock pumps, exhaust fans, small lathes
About 5–7.5 HP (3.7–5.5 kW)	DOL or star delta (depends on load and supply)	Pumps, small blowers, flour mill chakki
About 7.5–50 HP (5.5–37 kW)	Star delta	Rice mill elevators and hullers, tea factory fans, air compressors, cold store compressors
Above about 50 HP	Star delta, soft starter or VFD as per the design	Large pumps, big blowers, process machines

A long rural line with low voltage, or a small transformer far from the site, makes DOL starting harder. The big inrush can dip the voltage and trip other loads. In such places, contractors often move to star delta at a lower HP. Ask the electricity supplier (NBPDCL in this part of Bihar) about their rules for motor starting on your connection.

Can I use a star delta starter on any motor?

No. Star delta works only on motors built to run in delta at the supply voltage, and the motor must have six terminals brought out (U1, V1, W1, U2, V2, W2). The Lauritz Knudsen (L&T) guide also notes star delta is “typically used for motors designed to run in delta configuration”.

Many small motors are wound for star only, with three leads. They cannot be star delta started. Check the nameplate. It will show “Δ 415 V” or similar if it runs in delta.

Also remember: because star delta gives low starting torque, it suits loads that start light, like fans, blowers, and pumps with the valve partly closed. A machine that must start under full load (a loaded conveyor, a crusher full of material) may struggle in star. For such loads, ask the motor or machine maker.

Which contactor and overload relay size for a motor starter?

Pick the contactor by the motor’s kW rating in AC-3 duty (the normal duty for starting and stopping squirrel-cage motors). The contactor’s AC-3 rating must be equal to or above the motor kW.

Set the overload relay to the motor’s full-load current (FLC) from the nameplate. A common site rule of thumb for 415 V three-phase motors is roughly 1.4 to 1.5 A per HP, but always use the nameplate figure.

In a star delta starter, the relay is usually fitted in the motor phase (winding) circuit. There it carries less current, so it is set to about 58% of the line FLC (FLC divided by 1.732). If your starter’s diagram puts the relay in the line, set it to full FLC. Follow the starter maker’s wiring diagram.

Checklist for a starter BOQ or order:

• Motor HP or kW, voltage, and full-load current (from nameplate)
• Starter type: DOL, star delta, automatic or semi-automatic
• Enclosure: sheet-steel box, IP rating for dusty mills or wet pump rooms
• Contactor make and AC-3 rating, overload relay range
• Coil voltage (415 V or 240 V AC)
• Extras: single-phasing preventer, ammeter, voltmeter, indicating lamps, timer range
• Upstream protection: MCCB or MPCB and its rating
• Motor cable size and length (two runs for star delta)

What cable do I need for a star delta motor?

A DOL starter needs one 3-core cable to the motor. A star delta starter needs six cores: usually two 3-core cables run side by side, or one 6-core control cable on smaller motors. Each run carries the phase current, which is lower than the line current, so contractors often size each run for about 58% of the motor FLC. Size it properly for length and voltage drop, and use armoured cable where it runs underground or across a mill floor. See our guide on armoured vs unarmoured cable.

Earth the motor body and the starter box properly. Never run a motor without earthing.

Why does my motor starter keep tripping?

Common reasons we hear about from sites:

• Overload set wrong — set too low, or set at line current when the relay sits in the phase circuit (or the other way round).
• Low voltage — long rural lines drop voltage during evening peak, the motor draws more current and the relay trips.
• Star to delta time too short — the motor has not reached speed when it switches to delta, giving a second current spike.
• Single phasing — one phase lost due to a loose lug or blown fuse. A single-phasing preventer helps.
• Wrong breaker curve upstream — a lighting-type MCB in place of a motor-duty breaker. Our MCB, RCCB, RCBO and MCCB guide explains curves.

Get a licensed electrician to test the motor windings and settings. Do not bypass the overload relay or jump the contactor to keep a machine running.

Where can I buy motor starters and contactors in Kishanganj, Purnea and Katihar?

Shree Shyam Suppliers in Siliguri is an authorised dealer for Lauritz Knudsen (L&T) and Havells, and we also deal in Siemens and Schneider, so you can get starters, contactors, overload relays, MCCBs and cable from one place. We deliver across North Bengal, West Bengal, Sikkim, Bihar, Bhutan and Nepal, and we cut cable to length.

Buying from an authorised dealer means genuine stock, the right contactor and relay range matched to your motor, and an itemised quote you can put straight into your BOQ. Browse our switchgear range.

Ordering tip

WhatsApp Shree Shyam Suppliers your list with:

1. Motor HP or kW and full-load current for each motor (a nameplate photo is best)
2. Starter type you want (DOL or star delta), or ask us to suggest
3. Brand preference, if any
4. Upstream MCCB or MPCB, and cable size and length
5. Quantity and site location

We will send an itemised quote with today’s rate.