hi all,

consider the situation, you have a 60hp motor running with a vfd. you get starting problem. vfd is ok. you suspect the motor.

resistance/lamp test gives ok. now you want to run the motor without vfd for testing.

now, (this is the demand of the question) you can run it on either star or delta, no star delta. and you can run it only once (for testing).
which connection will you choose and why?

 

Engineer... use the connection being used when it is connected to the VFD!

Regards, Phil Corso

 

by the way, this is not a contest, its to seek help. and i don't know the answer ..

 

Engineer,

NEMA rated industrial motors that can run WYE or Delta draw 1/3 the starting current when connected as a WYE.

Delta connected the motor draws 3 times the starting current as when it is connected as a WYE.

When you run a motor from a VFD, the starting current is typically limited to 1 1/2 times the full load rotor current.

This could explain your starting issues.

By the way, high efficiency motors have a higher locked rotor current than standard motors. This can also cause starting issues. I have had this issue with Allen Bradley smart motor controller (SMC) starters.

I hope this helps,

William Hinton
Sr. Electrical Engineer
Delphi Automotive

 

Engineer... looks like you misunderstood me. If the connection,when connected to the VFD, is a Star, then test it with the same connection.

But, if I misunderstood you and you want to know the what connection to use to achieve the highest reliability, then that's another story.

 

first i want to know that what testing you have to do? it is not preferred the 60hp motor start direct online. before installation megger test you can do for the motor. winding resistance also you can measure.

 

dear phil corso & william hinton,
thanks & regards,

@ phil, thank you for your authoritative suggestion. actually i wrote my second post in continuation to my first post. your first reply had not surfaced till then, so this is my first reply to you (there has been no misunderstanding at all).

now, as i understand, what you suggest would be the safest way.

earlier i was wondering whether a motor that runs good on star will run good for all conditions in this context. similarly for delta.

& thanks again to william hinton for the precise current relationship, that would definitely prove helpful in future troubleshooting.

 

Engineer…

First, your test question: I presume it was one of conjecture! What would, should, could happen if...? But, conjecture alone is meaningless without detail. For example, how does the problem manifest itself? How do you know the VFD is OK? Why test the motor alone without first knowing the VFD's supply voltages and currents? Similarly, what are the VFD's output voltages and currents during start as well as running speed?

Secondly, my reply as to the most reliable winding-configuration; lately research papers have shown VFD-driven motors are exposed to a phenomenon, referred to as neutral-displacement or neutral-shift, which unduly stresses a motor’s windings if wye-connected. This occurs because the there is no neutral-reference! This phenomenon does not exist in delta-connected motors!

I don’t know how helpful this answer is to you (although my previous reply has certainly awakened the PtPs!)

Regards, Phil

 

actually, in part its a situation that occured few days back. we had starting problems, and in part imagination (conjecture,as you say it), actually motor had winding fault.

its always nice to hear something from you people, it helps..

 

Engineer... I ask again, "How does the problem manifest itself?"

Phil

 

Engineer... further to my earlier post about Y-connected motors being more susceptible to problems, see:

http://www.control.com/thread/1325318215

Regards, Phil

 

@ roy, i hope the situation is clear by now.