Delta to star starter

Motor consumed power is a factor of two basic elements; load (shaft) power and losses. Any motor, no matter how connected, will only consume the load power that the load requires. So the only thing you are really changing by connecting it in star is the amount of losses. The losses break down into several sub groups, such as magnetic (iron hysteresis), copper (resistance or IR), friction and windage. The only parts of these losses that are affected by connecting it in star are the copper and iron losses, which make up about 60% of the total losses. When a motor that was designed to be connected in delta is connected in star, the effective voltage across the windings is lower by 58%. It consumes slightly less power because the iron losses are lower as a result of the lower magnetizing energy and the current is lower, so the IR losses drop as well.

But the current is lower only because it can be, and that's just because you have essentially a smaller motor when connected in star. Torque is related to the square of the voltage, so at 58% voltage, you get .58 x .58 or .33 or 1/3 of the normal torque. 1/3 torque and the same speed will equal a lower mecahnical HP (kW) output; 1/3 that of what it was originally designed to put out. If your load is such that it can continue to do the necessary work with 1/3 of the installed motor power capacity, then someone has grossly oversized that motor in the first place. The only caveat is if the over sizing was necessary to get enough starting torque to overcome inertia or friction. If that is the case, then by all means, you can run it in star after getting it to full speed. But remember, you will no longer have a 30kW motor, you will have a 10kW motor. If your operating load increases to require more than 10kW mechanically, that motor will stall.

Dear JRaef,

Thank you.

you will no longer have a 30kW motor, you will have a 10kW motor

I dont agree with this. In star 30kw motor would become 30/1.732=17.3kw.

Power= 1.732*V_L*I_L*P.F

The motor rating is decided by the maximum current flow through each winding.

In star I_L=I_p, I_L would be 1/1.732 times of delta. So the rated power would become 1/1.732 times.

But the starting torque of star would be 1/3 times the starting torque of delta.This is correct.

Kindly advise if i am wrong.

This is a mind bender. Delta is impedence parallel with respect to time and electrical degrees of rotation. Star is impedence series with respect to time and electrical degrees of rotation. At any instance of time a delta circuit splits current through two windings and exits through the two remaining legs and (T leads). At any instance of time a star circuit chooses one impedance path through the incoming path (one motor winding) and exits two legs (motor windings &T leads) in series with it through the two remaining legs. It took me years to see this. I hope I am explaining it well enough for you to understand. You may also note a star connected motor doesn't develop the same torque / horsepower as a delta connected motor. The difference is insignificant, but notable.

No. The motor windings should not be connected to delta at the time of starting.The rated voltage is applied across each winding in delta position and the high starting current may damage the windings.

Haven't you heard of D.O.L. starting?

If what you say is true, millions of motors around the would are having their windings damaged as we speak.

In the meantime & disregarding the appropriate size of the motor, it seems to me that this particular motor could be started in "star" & then run in "delta"...a typical scenario.

What is your main concern? Power consumption?

Dear elkangorito,

Yes, my concern is power consumption.DOL starter is currently used for this motor.This motor is delta connected. I took trail in star observed power saving. It is working satisfactorily in star at running. The main problem is it sometimes, unable to provide the required starting torque.That is why i am thinking about Delta/ star starter.i.e. Starting it in delta(to get higher starting torque), and running in star(to save power).

Thank you,