When you need a star delta panel — for 15HP and above
Short answer: A star delta starter is needed for motors above 10 HP. A large motor draws six to eight times its normal current at the moment of starting — which trips your MCB, dims the lights across the premises, and stresses the motor itself. A star delta starter roughly halves that inrush.
This is why the 33NXT 10HP is ₹14,700 while the 33NXT Star Delta (15HP) is ₹33,600 — the latter contains a complete starter mechanism.
The problem at starting
When a motor is stationary its winding behaves almost like a short circuit. At the instant of starting it draws enormous current — this is called starting current or inrush current.
| Motor | Normal current | Starting current (DOL) |
|---|---|---|
| 5 HP | ~7 A | ~45 A |
| 7.5 HP | ~11 A | ~70 A |
| 10 HP | ~14 A | ~90 A |
| 15 HP | ~21 A | ~135 A |
| 20 HP | ~28 A | ~180 A |
A 15 HP motor draws around 135 amps at start. It lasts only a few seconds, but that is enough to trip an MCB and load the supply heavily.
What DOL means
DOL stands for Direct On Line — connecting the motor straight to full supply. It is the simplest and cheapest method and works well up to 10 HP.
- Advantages: inexpensive, simple, few components, full speed immediately
- Limitation: very high starting current on larger motors
How star delta works
It starts the motor in two stages:
- Starts in star connection — the motor sees reduced voltage, so starting current falls to roughly one third
- Switches to delta after a few seconds — now full voltage and full torque
The changeover is automatic, on a timer, typically after 5 to 10 seconds.
DOL vs star delta
| Feature | DOL | Star Delta |
|---|---|---|
| Suitable up to | 10 HP | Above 10 HP |
| Starting current | 6–8 times | 2–3 times |
| Starting torque | Full | One third |
| Cost | Lower | Higher |
| Components | 1 contactor | 3 contactors plus timer |
| Panel size | Small (190x222) | Large (400x325) |
| Effect on motor life | Reduced by repeated shock | Longer |
Which one — the chart
| Pump HP | Starter | Astero model |
|---|---|---|
| Up to 2 HP (1 phase) | DOL | 11NXT |
| 5 HP | DOL | 33NXT |
| 7.5 HP | DOL | 33NXT 7.5HP |
| 10 HP | DOL (at the limit) | 33NXT 10HP |
| 15 HP | Star Delta | 33NXT Star Delta |
| 20 HP and above | Star Delta or soft starter | Please ask us |
Running 15HP without one
- MCB trips repeatedly — on every start
- Voltage dip — lights dim across the premises and other equipment is affected
- Shock to the motor — reduced winding and bearing life
- Objections from the electricity board — in some areas a starter is required by regulation on large motors
- Mechanical stress on coupling and impeller
Star delta has a drawback too
In star mode the torque is also reduced to one third. That means the motor has to be started on a light load.
For a pump this is normally fine — you start with the discharge valve partly closed and open it gradually afterwards, so full torque is never required at start.
Watch the cut-out size
A star delta panel is physically larger — 400H x 325V against only 190H x 222V for a standard 33NXT. If you are fitting it into an existing board, measure the space first, or the panel will arrive and not fit.
Soft starters and VFDs — a third option
For motors above 15 HP there are two further options:
- Soft starter: ramps the current up gradually, smoother than star delta
- VFD: also controls speed and saves energy, but considerably more expensive
On plants where flow varies, a VFD often pays for itself in energy savings within two to three years. Please contact us separately for these.
Choose the right starter for a large pump
Send your motor HP and nameplate details — we will confirm whether DOL is adequate or a star delta is required.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
At what motor rating is a star delta starter necessary?
Generally above ten HP. Up to ten HP direct on line starting works acceptably, but for fifteen HP and larger motors a star delta starter is needed, otherwise the starting current is very high.
What does star delta actually achieve?
It reduces the starting current from around six to eight times normal down to two or three times. That prevents MCB tripping, avoids voltage dips across the premises, and reduces the shock to the motor so it lasts longer.
Why is a star delta panel so much more expensive?
It contains three contactors and a timer instead of a single contactor, along with more wiring, and the panel itself is physically larger. That is why the price rises sharply between the ten HP and fifteen HP models.
Can star delta be used on any motor?
No. The motor must bring out six terminals so that it can be connected in both star and delta. If the motor has only three terminals, star delta cannot be used.
How should a star delta pump be started?
With the discharge valve partly closed, opening it gradually afterwards. Torque is reduced in star mode, so starting fully open means the motor cannot reach speed and there is a jolt when it switches to delta.