Large Motor on Low load ? 1

[Crispin]

I have a 11000 volt 2540kW motor driving a compressor, the load on the compressor is decreasing over time and as a result it is being suggested that the compressor be "re-wheeled". Following this work the compressor load will be in the region of 700kW.

Are there any significant issues with running an exisitng 2540kW motor at or around 700kW?

regards

Crispin

 

[aolalde]

For an induction motor the PF and EFF will be lower than normal. Your electricity bill will increase. I estimate that a new 700 KW motor will have 0.91 EFF, and the big motor at 1/3 load 0.8 EFF.

InWatts1=700/.91 = 769.2 kW (a new 700 kW motor)
InWatts2=700/.8 =875 kW (your actual motor)

You will have a difference of 875-769.2 =105.8 kW

Depending on daily hours of work you will compare the investment of a new motor or the extra cost due to low efficiency.

If this is a Synchronous motor you could match the excitation close to unit power factor and the difference in power consumption will be reduced.

As a benefit, the life of the 2540 kW motor under 700 kW load will be extended due to very low operating temperature.

 

[jraef]

Although aolalde is correct in his assessment, the only minor clarification I want to offer is from that first statement about the electricity bill increasing. That is true only when compared to replacing the motor with one rated at 700kW because of the differences in efficiency that he pointed out, but of course technically your bill will DECREASE compared to what it is now. If you are charged a penalty for poor power factor it will cost you more in comparison as well, but that can be corrected with capacitors if ned be.

"Venditori de oleum-vipera non vigere excordis populi"

 

[RRaghunath]

The power factor will also be much lower for a part-loaded motor. This may not pose much problem in your case except the additional losses in the cables.

But there is another issue - the circuit breaker switching the motor off is likely to undergo greater stress (switching over voltage) with low load / low power factor motor compared to the one loaded to near-full rating. This is significant especially if the motor duty involves frequent start / stops and there is a possibility of switching over voltages causing failure of motor / circuit breaker.

 

[mc5w]

It would help a lot if you could state why air consumption decreases over time. If it is because production has dropped off then you might want to buy a second air compressor to save wear and tear on your big unit.

A modest second air compressor could also be used to keep the system pressurized when not doing production so as to avoid the heavy load that occurs when repressurizing the system. In 1 plant where I worked they had 6 or 7 air compressors so that they had backups and so that they could match the number or running air compressors to actual production needs.

Mike Cole, mc5w@earthlink.net

 

[5Why]

From a compressor perspective - that's a pretty big de-rate on a centrifugal compressor. For the cost of overhauling the big machine with small wheels it may be possible to buy one or two smaller machines to match the load. Not only will the motor efficiency of the large unit be reduced, but I suspect the the unit will not be able to maintain compressor efficiency with this drastic a change. New units may provide a more economical solution.