Hello Woon Sum Kam,

To find the resistor value, you need to know the line-to-line impedance of the motor in wye connection. You can get this information from the motor nameplate or datasheet, If you don’t have these documents, you can use Ohm’s law to calculate the impedance from the voltage and current ratings of the motor.

What are your thoughts on this answer ? The resistance of the resistor is calculated by multiplying the voltage drop across the resistor (30% or 0.3) by the voltage of the circuit (660 V) and then dividing this result by the starting current (1400 A). This gives us a value of 0.14 ohms.

What is the assumed start sequence here -The softest option would be

star with series resistor

Star direct connnected

delta with resistor.

delta direct wired

What current do you want in each state ?

Start resistors can either be estimated as per earlier answer from dropping some fraction off the supply at the full running current, or instead by considering the desired maximum that the supply should never exceed. After all if we did not care about inrush, mechanical stresses and the lights going out, all motors could be delta direct on line and bang start. But of course we do care very much - so we soft start.

Mike.

Hello Mike,
My answer was in reference to the PDF document that is attached. I would like to confirm if my understanding is correct that Woon Sum Kam was inquiring about the selection of 0.15 ohm resistors.

Hi there, yes - the 3 resistors are in a position where they would be in common when contactor 'SC' closes - which I assume is the 'star' centre - which was why I wanted to clarify when in the run up sequence they get switched in.

Actually a 595kw motor is quite a beast, (rounding the maths bit and calling it it 200kW per phase and each phase 400V to ground I get ~ 500 Amps/phase and in delta each winding 500A/(root 3) or about 300A  and that is before the KVA to KW conversion )

So both the contactors and the resistors will need to be large - 300A and 0.15ohm is 45V and  about 15kW to sweat off in each resistor - I can see that going bang if the start up is set too slow  ;-)

I'm intrigued as to where you get the 1k44 amps from as starting current though as I see it the absolute upper limit on currents would be  to put the 0.15ohms across phase to phase 660V  direct you would have 4k4 amps in each resistor or in star any phase more like 2k5.

But without motor data- and perhaps you recognise this type or there is something else on the diag I'm missing, and If so I'd appreciate an indicator, I am less clear how you get to such a precise figure. Of course it is always possible I have missed some vital step - answers composed on the hoof in coffee breaks are not always ideal.

Mike

Hello Mike, I appreciate your feedback and acknowledge your superior expertise on the topic. I applied the BS 88 fuse value of 1400 A as the input for the calculation.

Hello Mike, I appreciate your feedback and acknowledge your superior expertise on the topic. I applied the BS 88 fuse value of 1400 A as the input for the calculation.

Thanks - so you too assumed an available input limit really then, good to know there is not some other trick - as you may reaslise from my responses on other threads, I can be a bit of a sponge for random info, and any new technique/ rule of thumb is of interest.

Mike