star/delta motor failure when starting

[bob956]

I have a customer whose 200 H.P. 1185 rpm motor fails about every three months. It always fails after running half a day and is restarted. It is started by a star deta starter.

 

[GGOSS]

Hello Bob956,

Much has been said about motor failures and star/delta starters in previous threads however in order for anyone to provide you with useful information it would be beneficial to know what the failure mode is. That is to say, have you sent any of these motors to re-winders for evaluation/repair? Did you receive a report indication what type of fault the motor has suffered ie winding short, broken rotor bars, etc., etc?

This information would greatly assist others to provide you with 'quality' answers to your question.

Regards,
GGOSS

 

[bob956]

I'm sorry. New to this site. I inspected the motor and it showed no sign of heat build up. It shorted between phases opposite lead end.

 

[GGOSS]

Hi again bob956,

No need to appologise!

As a starting point I would suggest you go to:

http://www.easa.com

click on 'Industry Info' and then on 'Failures in Three Phase Stator Windings'.

There you will find great pics of motor failures together with descriptions of fault cause.

In addition it would be fair to expect further input and explanations from other forum members.

Hope this helps.

Regards,
GGOSS

 

[electricpete]

I agree w/ ggoss' comments. More details will help. Some more questions:

"fails about every three months. It always fails after running half a day and is restarted."

Can you explain this some more. Is the motor started daily?... then it always fails after about 1/2-day after starting? Or fails immediately upon starting?

What kind of failure occurs? Overload trip? Instantaneous trip? Have you measured current during operation?

Is it the same motor stator being rewound over and over? (in this case we may suspect you have a core which has been damaged by repeated winding stripping).

 

[stardelta]

If its continually blowing between phases It seems to me that the phase barrier insulation material is maybe not up to the job, or the phase coils are not being correctly insulated from each other during the rewind. Put this question to the repairer and get his comments, not all repair companies are experienced in rewinds of this size. I would expect a years guarantee on a rewind of this size as a minimum. I would also insist on VPI.

 

[GusD]

Hi Bob 956

If motor protections are operating as they should I see no reason why the motor should fail so periodically.
I don't know what type of control you have for switching from Star to Delta .If this malfunctions and it remains in
Star after load is applied for long ,you could have a toasted motor winding unless TEMP DETECT,turn it off.
Ask the repair shops if the failure is a total winding burn out or something else. GusD

 

[induction]

First thing is to ask the shop who wound it if they are core testing it. I have seen it large pumps where overburn cuase hot spots in the lamanations. I myself insulate every coil in a motor with wye delta start run. Just recently a 300 H.P. pump came in that I had wound 8 years prior. If your controls are good and voltages are are phase to phase shorts should not occur. By the way bearing was the cause of the motors return.

 

[Marke]

One of the problems can be the quality of the winding wire used. All winding wire has pinholes in the insulation, high quality wire has less than low quality. If low quality winding wire is used, and the phase-phase insulation is poor, then the high transients that occur during the star to delta changover can result in a break down of that insulation.

A question that I would ask is, does it breakdown on the transition, or when it gose into star at the beginning of the start. I suspect when it changes over to delta.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GusD]

Hi BOB 96

Is it a centrifuge application? Is it a type of load that normally requires very long accelerating times.
Some of us are in the impression that if you start your load faster, you don't have as much heat build up as when you have a long period of acceleration.
On these types of loads, it is better to stretch the starting time to allow motor to start dissipating heat.
If you start this load at reduced times and you have to have multiple starts,the motor does not have time to get rid of the heat build up and toasts in short order.
BOB96 . Are the windings cooked or do they fail some other way.

GusD

 

[Shortstub]

How frequently is it started?

 

[jbartos]

Suggestion: The motor may have a life-cycle three months for some reason, e.g. heavy load causing it to operate at higher temperature than it is designed for.

 

[GusD]

Hi bob956

I just came across some information that could have a connection with your custumer's problem regarding Y/D motors.He was loosing motor windings nearly every 3 months or so.A very hi rate of winding failures.
As a rule ,we set motor overloads as per Table 44 on the CECode or possibly any code .This rule says that for motors with a Sfactor of 1.15 OLoad is set at 115% of motor nameplate FLAmps or Table 44 and 125% for motors with a 1.25 Sf.This is true for most motors unless ,they have a Star/Delta starter.
The following example indicates how to determine the OLoad setting of 75 HP ,575 volts,3phase(SCIM) controlled with a Wye/Delta starter.The motor is rated for continuous Duty and has a Sfactor of 1.2.

Sorry about the Table info.Is in the CECode.Maybe not much different in the USA or VDE rules in Europe.
Step1= Use Table 44 to determine the FLA of the motor. Table 44?--75 hp,575 v,3Ph SCIM draws 77 Amps.

Step 2==Determine the Phase value of the current.
77Amps/Sroot3(1.73)=44.45 Amps

Step 3==Use rule 28-306(1)to determine the overload setting.44.45x1.25=55.56 Amps

Step 4 == Rule says to comply with the marking requirements outlined in Ru28-306 (2)

If I wasn't thinking about a Star/Delta motor and I set my
OLoads at FLA of 77x 1.25 =96.25 Amps.it would not be hard to burn the motor out,if one wasn't paying attention.

I will be honest and admit that until today, I had never thought about it.The reason might have been,because I didn't have a problem of this nature with a Star/Delta motor.
If you have a problem,it might be worthwhile to look at the OLoad setings.

Just to make my day,tomorrow I will find out that everybody else knew about this little rule.I hope that
there is at least one person that has never heard about it.

Thank you all



GusD

 

[Farkel]

Is starter closed transition or open?

 

[Shortstub]

Please elaborate on "... fails about every three months."

Do new motors fail? Rebuilt ones too? Has starting-duty been investigated? How many events constitute "failure?"

In general, star-delta starters are not to be used for "severe" starting duty. They are primarily intended to ameliorate power supply inadequacy.

That said, then, in order to resolve your problem, and here I concur with Jbartos' proposal, you should provide meaningful data so that, in turn, helpful comments will ensue.

I suggest the three "R's" of Electrical Problem Solving:

o R1, what are the motor's "R"ated parameters?
o R2, what are the application criteri"R"?
o R3, what are the power supply "R"estrictions?