Star Delta Connection

First, please take off your Caps=Lock key, THAT IS THE EQUIVALENT OF SHOUTING!

With regards to increasing the time in Star: You can try that, but it is not likely to work. In Star, the motor is only developing 1/3 of its rated torque. Once the motor output torque reaches the load torque requirement, it will stop accelerating. So increasing the transition time may do nothing more than increase the heating in your motor, meaning that now your overload protection may trip out. Even if that doesn't happen, that does not guarantee that the transition spike will be low enough for your generator.

A better method of starting behind generators is Solid State Soft Starters, or even Autotransformer starters. Star-Delta is kind of a "lie" with regards to reduced voltage starting. The effective voltage across the windings is reduced, so the flux does not fully penetrate during Star. So when you transition to Delta, the re-establishment of that magnetic field will likel;y be just as high as it would have been with DOL starting. The only thing that changes is the duration of the spike, being shorter using Star-Delta. In the case of saturating a generator winding, that doesn't help much.

THANKS FOR THE REPLY ,I WILL TRY OUT SOLID STATE STARTER BUT SINCE THE MOTOR IS IN A PRODUCTION LINE , THE PLC WHICH CONTROLS THE MOTOR WILL START THE MOTOR MANY TIMES IN AN HOUR , SO WILL SOLID STATE STARTER BE EFFECTIVE FOR FREQUENT STARTS ???

Once again mate, check your CAPSLOCK.

Re the solid state starter, yes it is a good option for multiple starts and a VVVF drive may be the best option to match load inertia to the prime mover torque.

A soft starter is often bridged by bridged by a contactor when fully started so the motor runs at full speed with no drive losses. Consider another control regime, depending on the application, lets say a pump which can be difficult to start via Star Delta. If the motor is run at variable speed via the PLC and VVVF you can reduce or even eliminate the starts per hour problem. Applications such as Washplant Codisposal pumps use this regime successfully. Variable speed V/s stop start prevents water hammer and sedimentation.

HI Guru!

Good suggestion.

But price of soft starter or VFD will be too much higher that star-delta starter. Is there any cost effective solution?

*I could not understand the abriviation "VVVF". Please explain.

dear sir, many thanks for the reply

we sorted out the problem, the culprit was the AVR in our generator unit which was creating all the problem , we changed new AVR and the motor is starting without any problem , many thanks for yyour reply sir

Good work.

Just so you are aware, the AVR in your generator was most likely damaged by the severe voltage spike that happens when you transition a Star-Delta starter from Star to Delta. So it is likely to happen again.

yes you increase the star timing

Probably not, at least not without further investigation. Quite often extended running in star will not accelerate the system further as only 1/3 torque is available. The large step when engaging delta usually signifies this situation.

Further to my previous post. Lately I have been doing some research into Asynchronous or induction alternators. It is clear to me now that part of my success in starting drives was because I started the biggest drives with the most inertia up first they then acted as asynchronous alternators and transferred part of their momentum to subsequent drives at initial start up. Because the scrap fans were far closer to the printing machines than the alternators the line impedance ensured some of the inrush was supplied by the scrap fans.

There are several sources of info on the net and Lindsay Publications have a good booklet called "How to Run Three Phase Motors on Single Phase". Unfortunately it does not have an ISBN but just check their website.

The explanations are good and their method 3 explains the rotary phase converter which is an induction alternator.