Enter a motor and get the whole starter: contactor size, overload relay setting range, short-circuit protection and the current your cable has to carry. Sized to IEC 60947-4-1, including star-delta arrangements where each contactor is a different size.
The nameplate on the motor has everything you need. If you can read the current from it, enter that — it is always better than any estimate.
Best answer. Leave blank and we will estimate it from the kW.
11 kW 400V 21.5A cos φ 0.85. The amps figure is the one worth having — it comes from the manufacturer's own test, not from a formula.This changes the contactor arrangement completely — star-delta needs three contactors of two different sizes, a drive needs none at all in the classic sense.
One question, and it is about what happens after a short circuit rather than before it.
This is the complete starter for your motor. Send it to us for a price on the contactor, relay and enclosure.
This calculator gives indicative starter component ratings for budgeting and for checking a quotation. It is not a design document. Final selection must be confirmed by a qualified electrical engineer against the motor nameplate, the manufacturer's verified coordination tables, the prospective short-circuit current at the board, and the wiring rules in force at your site. Mega Standard accepts no liability for decisions made on these figures alone.
The current is calculated from first principles, not read from a table. The contactor and breaker ratings are rounded up to standard IEC frame sizes. Type 2 coordination must come from the manufacturer's own tested combination table.
You can calculate a motor's full-load current from its rating, and this tool does exactly that when it has to. But the number stamped on the nameplate came from the manufacturer testing that actual design, and it can differ from the calculation by several percent in either direction.
Several percent matters, because the overload relay is set to that current. Set it from a calculation that ran high and the relay will not protect the motor properly. Set it from one that ran low and the motor will trip on a hot afternoon for no reason.
The plate is riveted to the side of the motor and takes ten seconds to read. Use it.
Contactors are not rated by a single current. They are rated by utilization category, which describes what the contacts have to interrupt.
AC-3 is the normal squirrel-cage motor duty. The contactor closes onto starting current, but it only ever opens while the motor is running normally, breaking roughly full-load current. Pumps, fans, conveyors and compressors are all AC-3.
AC-4 covers inching, plugging and reversing. Here the contactor may have to open while the motor is still drawing locked-rotor current — six or seven times more. That is far harder on the contacts, and a contactor used on AC-4 is commonly derated to about half its AC-3 rating.
The practical consequence: a crane and a pump of identical kW need very different contactors. Choosing AC-3 for an application that jogs will burn the contacts out.
A star-delta starter has three contactors, and they are not all the same size. The line and delta contactors each carry the motor current divided by the square root of three — about 58 % of full-load current. The star contactor carries only about 33 %.
That means a star-delta panel uses smaller contactors than a direct-on-line starter for the same motor, which is part of why it is cheaper to build than the starting current alone would suggest.
But here is the trap. The overload relay sits in the delta leg, so it sees 58 % of the motor's full-load current, not 100 %. Set that relay to the motor's nameplate amps and it will never protect anything — it will sit at 58 % of its setting all day and only trip long after the motor has cooked. This calculator sets it correctly and shows you the figure.
Short-circuit coordination is about what state the starter is in after a fault has been cleared, and IEC 60947-4-1 defines two levels.
Type 1 accepts damage. The starter must not endanger anyone, but the contactor and relay may be destroyed and need replacing before the motor runs again. It is cheaper.
Type 2 permits no damage other than light welding of the contacts, which must be easily separable. The starter goes back into service after a check. It costs more, and it is the right answer wherever a stopped process costs more than the components.
One thing to understand about Type 2: it is not something you calculate. It is a tested combination of a specific breaker, contactor and relay, published by the manufacturer. Ask for the coordination table, and buy the combination that is on it.
At 400 V three-phase an 11 kW motor draws roughly 21 A, so an AC-3 contactor rated 25 A suits normal starting and stopping. If the machine inches or reverses, the AC-4 category applies and you need a considerably bigger frame. Enter your figures above for the exact answer including the overload relay range.
To the motor's full-load current from the nameplate — with one important exception. On a star-delta starter the relay sits in the delta leg and sees only about 58 % of that current, so it must be set to the lower figure. This calculator works out which case applies and gives you the number.
Either works if it is coordinated. A motor protection circuit breaker combines overload and short-circuit protection in one device and is usually the tidiest choice. aM motor fuses are rated to let starting current pass while still clearing a fault fast. What matters more than the choice is that the whole combination appears on the manufacturer's coordination table.
No. The motor must be wound so that delta connection matches your supply voltage, and it must have all six winding ends brought out to the terminal box. The load also has to allow the motor to accelerate at roughly a third of its torque — fine for a fan or an unloaded compressor, hopeless for a loaded conveyor or a crusher.
The drive provides electronic motor overload protection, so a separate thermal relay is often omitted. What you must never do is open a contactor between the drive and the motor while the drive is running — it can destroy the output stage. If a contactor is fitted there, it has to be interlocked with the drive's run signal.
We supply Schneider and Siemens contactors, overload relays, motor protection breakers and complete starter panels across Iraq. Send us the result and we will come back with matched components from a verified coordination table, not a guess.