Residual voltage starters for 75kW, 415V motors

[RRaghunath]

I understand I have the option of going for Star-Delta starter or Auto transformer starter for the purpose. Among these, I also can choose open transition type starter or closed transition type.

But, I am at a loss to find, of the above, which is the best techno economical option.

I would appreciate if some one can guide me.

Thanks in anticipation.

 

[aolalde]

I guess you mean Reduced Voltage Starters, if so, you must check the speed-torque curve and the inertia of the load against the motor speed-torque. This analysis will tell you how far could be the voltage be reduced and successfully start the motor. The motor torque reduces proportional to the squared reduction of voltage.
By the other hand if the motor starts unloaded and after starting the load it is engaged (clutch), your options are open to use almost any desired starting method.

 

[Marke]

Hello rraghunath

First, I assume that you mean "reduced voltage starters"
You can apply a reduced voltage starter to almost all applications, but the limitation is that the motor must be able to develop enough torque to accelerate the motor and load to full speed before changing to full voltage.
The torque reduces with the square of the voltage reduction when a reduced voltage starter is employed.

You have nominated two reduced voltage starters and also mentioned open and closed transition starters.
Both of the starters are possible in both open and closed transition, but you should never see an open transition auto transformer starter because a closed transition can be produced for the same price.
An open transition starter has a major problem. There is an FAQ on this subject. faq237-613 The switching transient can cause significant damage electrically and mechanically over a period of time.

Cost wise, the open transition star/delta is the cheapest and has severe transients at change over. A closed transition star/delta starter is more expensive and if it is done correctly, will have lower switching transients. The start torque of the star/delta starter is one third of the full voltage starting torque of the motor and can not be altered. The closed transition star/delta starter can be effective for high inertial loads where the start toque required by the machine is purely inertial.

The auto transformer starter is almost always closed transition and so does not have the switching transient. The start torque can be set by the tapping used on the transformer. Transformers for frequent starting or high inertia loads are expensive.

I would recommend that you also consider a solid state soft starter as an option.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

my take would be to review if a reduced start is necessary for a 75kW motor to beign with, depending upon the size of the system feeding it.

You can't beat economy and superiority of a DOL start, if it is acceptable otherwise.

If a reduced voltage start is necessary, in these day and age I would go with electronic soft starter. Except that they may not like the harsh or non-conditioned envioronment.

Among the star-delta and autotrasformer, the auto xfmr provides selection of 50%, 65% and 85% (normally) voltage tap, while star-delta has fixed voltage reduction equivlent to 58% voltage tap. Y-delta requires six leads to be brought out of a motor and wired to the strater, autoxfmr starter does not. But both have the same effect of causing transients during open transition. I would go with a closed transition for this size.

I think Y-delta starter would cost less than a comparable auto xfmr unit. Cost of electronic soft start may be very comparable (they are in the USA).

 

[RRaghunath]

At the outset, my apologies for the error in the title of the post. Thank you all for understanding and for the responses.

I am looking for numbers if some one can provide me.
Some sort of comparison among closed transition type Star-Delta vs Auto transformer type and electronic Soft starter (as suggested by rbulsara).

rbulsara,

the location of the installation is remote and depends for power on Gas engine generators. I guess that is the reason clients' consultants have stipulated reduced voltage starters for motors beyond 55kW rating.