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Hi everyone,
I have a 250 kW compressor on site and due to lack of load, the compressor is recycling every 50 mins. The compressor is driving a compression chiller and is star/delta started. I understand that one restart every 45 minutes should not be a major problem but I presume that the integrity of the insulation is not designed to withstand such duty cycle permanently. I believe that it would compromise the integrity of the compressor. The OEM no longer exists. Thanks for your input.

RE: Starting

I don't see that as being particularly problematic. I recall a large building Trane Centravac that started Star-delta about every thirty minutes without issue.

Can you change any gear ratios?
Is there a VFD option?
If you're using chilled water can you increase or add volume?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Starting

A star delta start 50 minutes apart is not so bad for stator winding. Why is it time based start instead of pressure or flow based?


RE: Starting

Muthu I suspect it's not time based, it's just the load resulting in that particular cycling period.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Starting

I am not sure but I think the equivalent S1 continuous duty in the case of a 10 minute cycle time it could be 216 kW if the motor is 60% S4 duty but in 1 minute cycle time 60% duty the equivalent S1 power could be 360 kW. In this case the life time of the induction motor will be less than 50%.The manufacturer has to calculate it-if he has all the motor data.
I got some data from a catalog and I do not know how close the real engine is.

RE: Starting

{Refrigeration/air conditioning] compressor: centrifugal or reciprocating?

If centrifugal, any inlet control vanes? The Centra-Vacs and Carriers I operated in the past had them...

If reciprocating: any unloaders or clearance pocket increasers available to alter the volumetric efficiency?

Electrical problems can sometimes be mitigated with mechanical solutions...


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Starting

I hope that you have some form of unloading.
We had a couple of air compressors with possible staring issues.
1. 125 HP compressor with 3 starts per hour allowed.
A time delay stop so that once started, a twenty minute timer was started. The compressor was prevented from stopping automatically until the timer had timed out.
If the compressor was not needed, it ran unloaded until the timer timed out.
2. 150 HP with an additional problem. The unloader would not always go to the unloaded position when the compressor was stopped.
The compressor, once started, would not stop but would run unloaded until it was manually stopped and the unloader was checked to ensure that it was in position for an automatic start.
With this arrangement it was considered that starts per hour were not an issue.
To paraphrase CR;
"Mechanical problems can sometimes be mitigated with electrical solutions... "

"Why not the best?"
Jimmy Carter

RE: Starting

"Electrical problems can sometimes be mitigated with mechanical solutions... "

LOL, a company I used to work for that made Soft Starters had as their slogan; "Electrical solutions to mechanical problems".

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Starting

MarkE had a FAQ about some challenges that may appear for wye-delta start
FAQ237-613: Star Delta (wye delta) transients
He talks about the fact that the incoming voltage can be out of phase with the residual voltage, resulting in a high effective applied voltage. His conclusion "The current transient can be greater than 2 times the full voltage start current, and the torque transient can be greater than 4 times the full voltage start torque."

I'll add that the nature of the transition (closed or open, how much delay, electrical phase shift associated with reconfiguring delta to wye) may make a difference.

As far as DOL repetitive start limits in NEMA land (which I presume does not apply to op, but gives you something to compare), there is some guidance in NEMA MG-10, which you can access from the first link in the following FAQ
FAQ237-1285: Limits for repetitive induction motor DOL starting

250kw motor (input rating) will probably be at least 250hp (output rating). For 250hp motor, if speed were 3600rpm, column A suggests maximum repetitive start frequency of 1.8 starts per hour average. There is other relevant info in columns B and C for your perusal.

(2B)+(2B)' ?

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