Short answer: a star-delta starter needs three contactors and one thermal relay: a main contactor and a delta contactor sized at about 0.58 × motor full-load current, a star contactor at about 0.33 × FLA, and a thermal overload set to FLA when it sits in the line — or to 0.58 × FLA if it sits inside the delta branch. With the CJX2 range that means reading the AC-3 rating at 400 V, not the AC-1 figure on the side of the box, and using an NTE8 timer for the star period. Star-delta gives roughly one third of direct-on-line starting torque, so it only fits light-starting loads: pumps, fans, compressors off-load.
This page is the selection and wiring logic for three documented parts: the CJX2 contactor, the NR2 thermal overload relay and the NTE8 time relay. We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — so every rating below is quoted from published data for the exact series.
Why the contactors are not all the same size
In delta run the motor’s three windings sit across the line, so each winding carries the phase current: line current divided by √3, which is 0.577. Two contactors carry that winding current — the main contactor and the delta contactor — and they are in series with the winding for the whole run. The star contactor only carries current during the start, when the windings are in star and the starting current is about one third of the direct-on-line value, so its duty is 0.33 × FLA and it is loaded for a few seconds at a time.
A 40 A motor therefore does not get three 40 A contactors: it gets two devices rated at 23 A or more and one at 13 A or more. Fitting three identical 40 A contactors wastes money on the star leg; fitting three at FLA/3 leaves the main and delta legs undersized for continuous running.
| Device | Current it carries | Multiplier of motor FLA | Example at 40 A FLA |
|---|---|---|---|
| Main contactor (KM1) | Winding current in delta run | 0.58 × FLA | ≥ 23 A → CJX2-2510 |
| Delta contactor (KM2) | Winding current in delta run | 0.58 × FLA | ≥ 23 A → CJX2-2510 |
| Star contactor (KM3) | Line current during star start only | 0.33 × FLA | ≥ 13 A → CJX2-1810 |
| Thermal relay in the line | Full line current | 1.00 × FLA | set to 40 A |
| Thermal relay inside the delta branch | Winding current | 0.58 × FLA | set to 23 A |
Reading the CJX2 rating the right way
The CJX2 range covers 9 to 95 A of rated operational current in AC-3 at 400 V, roughly 2.2 kW to 55 kW at 380 V. AC-3 is the category for squirrel-cage motors switched during running — exactly the star-delta case. The AC-1 figure is a resistive-load rating and is much higher; quoting it for a motor is how a starter ends up undersized by two frames.
| CJX2 frame (AC-3, 400 V) | Rated operational current | Typical role in a star-delta starter |
|---|---|---|
| CJX2-0910 / 0901 | 9 A | Star leg of small motors, or auxiliary circuits |
| CJX2-1210 / 1201 | 12 A | Star leg up to roughly 36 A FLA |
| CJX2-1810 / 1801 | 18 A | Star leg, or main/delta on small frames |
| CJX2-2510 / 2501 | 25 A | Main and delta legs in the 25–40 A FLA band |
| CJX2-3210 / 3201 | 32 A | Main and delta legs to about 55 A FLA |
| CJX2-4011 / 5011 / 6511 | 40 / 50 / 65 A | Main and delta legs on larger frames |
| CJX2-8011 / 9511 | 80 / 95 A | Main and delta legs at the top of the range |
The four-digit suffix is the auxiliary contact configuration, not the current: 10 is one normally-open, 01 one normally-closed, 11 one of each. That matters here because the interlocking is built from auxiliary contacts — one NC on the star contactor to block the delta contactor, one NC on the delta contactor to hold the star contactor out after transition.
Setting the NR2 thermal relay
The NR2 is a three-pole bimetal overload relay covering 0.1 A to 150 A across the range, trip class 10A, rated 690 V and 50/60 Hz, with overload and phase-loss protection, manual or automatic reset and direct plug-in mounting onto CJX2 and NC1 contactors. Mounting bases MB-1 to MB-4 cover the cases where it has to sit apart.
Where the relay sits decides its setting, and this is the most common commissioning error on a star-delta starter. In the line, upstream of the split between main and delta legs, it carries full line current and is set to the nameplate FLA. Inside the delta branch, in series with one winding, it carries only winding current and must be set to 0.58 × FLA. A relay set to FLA in the delta branch will never trip: it is 73 percent too high, and the motor burns before the relay notices.
Trip class 10A is for normal starting duty; a high-inertia fan taking longer than about ten seconds at locked-rotor current needs class 20 or 30, which the NR2 range does not provide. The relay also protects against overload and phase loss, not short circuit — that is the upstream MCCB or fuses, chosen for the prospective fault current.
The NTE8 timer and the transition
The NTE8 is a DIN-rail time relay with a 0.1 s to 480 s range in the economy A/B/J/Y versions, AC 24–415 V or DC 24 V supply at 85–110 percent of rated voltage, and contacts rated AC-15 230 V 1 A, DC-13 30 V 1 A, thermal current 5 A. The multi-function M versions extend to 10 days with 16 A contacts. In a star-delta starter it supplies the star period: it holds the star contactor in for the set time, then drops it and picks up the delta contactor.
What the NTE8 does not provide is the transition dead time. Star and delta contactors must never be closed at the same instant, or two phases are short-circuited through them; the interlock is mechanical between KM2 and KM3 plus the NC auxiliary cross-wiring above. A dead time of roughly 30 to 100 ms is built into the control circuit with an off-delay, either a second timer or a dedicated star-delta relay. On large motors the back-EMF is still significant after a short dead time, and closing into it produces a current spike worse than a direct start.
| Phase | KM1 main | KM3 star | KM2 delta | What the operator sees |
|---|---|---|---|---|
| Start pressed | Closed | Closed | Open | Motor accelerates in star at about one third of DOL current |
| Timer expires | Closed | Opens | Open, waiting for dead time | Current falls; motor coasts for the dead time |
| Delta closes | Closed | Open, interlocked | Closed | Motor runs in delta at full winding voltage |
| Stop pressed | Open | Open | Open | All three open; motor stops |
When star-delta is the wrong starter
Star-delta delivers about one third of direct-on-line torque, so a loaded conveyor, a positive-displacement pump or a crusher will not accelerate on it: the motor sits at high slip drawing near direct-start current until the overload trips. It also requires six motor terminals with a delta winding rated for the line voltage. A motor with three terminals, or a 690 V delta winding, cannot be star-delta started at all. Heavy-starting loads need a soft starter or a drive.
What to send us for a quotation
Motor nameplate data — kW or HP, full-load current, voltage, frequency, poles — plus the starting duty, the prospective short-circuit current, the control voltage inside the panel and the destination country. We quote contactor, relay and timer as a matched set so the frames and relay range line up, and we say which certificate file exists for each code.
FAQ
What size contactors do I need for a star-delta starter?
The main and delta contactors carry winding current in delta run, about 0.58 times motor full-load current. The star contactor carries current only during the star start, about 0.33 times FLA. A 40 A motor therefore needs two contactors rated at 23 A or more and one at 13 A or more.
Where should the thermal overload relay be set?
In the line, upstream of the split, the relay carries full line current and is set to the motor FLA. Inside the delta branch, in series with one winding, it carries only winding current and must be set to 0.58 times FLA. A relay set to FLA in the delta branch will not protect the motor.
Can I use an NTE8 as the star-delta timer?
Yes for the star period: 0.1 s to 480 s in the economy versions, switching the star contactor. It does not provide the transition dead time, so a separate off-delay or a dedicated star-delta relay is still needed between opening the star contactor and closing the delta contactor.
Why do the contactors need a mechanical interlock?
If the star and delta contactors close at the same time, two phases are short-circuited through them. A mechanical interlock plus normally-closed auxiliary contacts cross-wired between the two coils prevents simultaneous closure; the electrical interlock alone can be defeated by a sticky contactor.
What starting torque does star-delta give?
About one third of direct-on-line torque, because the voltage across each winding is reduced by a factor of root three. That suits pumps and fans but not loaded conveyors, crushers or positive-displacement pumps, which may not accelerate and will draw near direct-start current until the overload trips.
Related: Contactors and motor control · AC-1 vs AC-3 vs AC-4 · NR2 overload relay setting · NTE8 on-delay and off-delay · Motor protection circuit breakers · MCCB range · Request a quotation