Compressors at 87Hz

[fridgey]

Has anyone tried running a copeland semi hermetic compressor at 87Hz on a inverter drive. I know the bitzer semi can do this and the have there own varispeed comprssors with attached inverter rated at 87Hz. According to the motor name plate ther should be no problem as they are rated at both 220-240 delta and 380-420 star. So this tells me that it possible from the motor side. The problem maybe the the mechanicals are not up to running at 2500RPM which is substacially above the standard 1440 RPM. I can find much on the net about this on refrigeration compressors apart from on Bitzers. Any expierience with this or feed back would be appreiciated.

Cheers

[FreezerGeezer]

Maybe I'm being silly here, but why not just ask Copeland?

[HVACRsaurus]

Hi Buddy,

I've seen screws & recips run up to 60Hz, mostly open drive stuff & within thier recommended speed tolerances.

At this stage I'd say anything up around the 87Hz would be OEM kit.

[fridgey]

I have asked one of our wholesalers who supplies Copeland compressors still haven't got an answer. This question has all come about in a quest to save a client power but utilising the majority of the existing plant. With the cost of power rising all the time people are becoming more concerned about the electricity they are using.

As there system has multiple evaporators running off 1 compressor the load on the compressor is constantly changes. The where VSD (inverter) comes in as it can it adapt to the load required, reducing power consumption and stop starting of compressor. I want to add some extra load to the system and decommission some smaller compressors.

The reason I was asking about the 87 HZ is that the capacity of the compressor is increased by about 70% from standard 50Hz. It is entirely possible to run the motor on the compressor to 87Hz as Bitzer do. But you can’t just drive a standard compressor motor to 87Hz. HVACRsarus you are quite right, compressors that I have set up previously with a VSD had an operating range from 30Hz to 60Hz as this is all the motor can do, otherwise it will overheat.

For the 87Hz technique it requires the motor has to be rated in star 400V 50Hz and delta 230V 50Hz. You reconfigure such motors from star to delta operation then connector to inverter. 230V 50Hz is set as the base frequency in the VSD or it is set to 400V 87Hz depending on the drive. What this set up does is it allows the motor to magnetic field current to stay constant up to 87 Hz, with the output torque of the motor also being constant ignoring the effects of motor loses at low speed. Above 87 Hz the magnetic field weakens consequently so doe’s motor torque. Increasing the voltage rating of the motor from 230 to 400 V in a Delta you will at the same time have to increase the rated frequency in the same ratio to keep the magnetic field current constant. If it is not the motor will break down because of a bigger loss of power in the stator.

This is exactly what Bitzer do with their Varispeed compressors. They are achieving about 1.7 times more power from the same motor and consequently 1.7 times the refrigeration capacity. This gives it a much bigger frequency range, from 208V 25Hz through to 400V 87Hz. So it obviously the Bitzer semi-hermetic is capable of this but is the Copeland semi hermetic? According to the name plate the motor will be, the question will the increased piston velocity and increased gas flow through the valve plate cause mechanical issues?

[HVACRsaurus]

I'll have to read through that information a couple more times before I can catch on..

If you are looking to retrofit an existing application & the compressor has the required capacity at 50Hz, why the need for 87Hz? Is there still opportunity for improved energy effeciency in the 30 - 60Hz range?

[fridgey]

If we stay with R22 for the plant it can just run to 60Hz and it will have the capacity for the extra load. We are looking at retrofiting the plant to a different refrigerant with R22 phase out. In my opinion all the replacement for R22 a crap. The are all blends of 3 to 5 different refrigerants and are non azeotropic. I was considering R507 but it still costs more than R22 at the moment because of it high GWP thank you carbon tax.

So that leaves R134A, its not usual considerered for a retrofit of R22 systems. For a few reaseons but mainly because the the compressor will lose to much capacity. After checking pipe sizes, evaporators and heatload R134A is an option and has the best COP of HFC refrigerants, lower GWP, and cheaper . But I need to run the existing compressor up to 87Hz to get the same capacity as you would for R22 at 60Hz. Hence my question. If it should be able to be done with the existing compressor, but if its not, i'll have to look into other options But replacing the compressor throws my pay back time for energy saving out to about 7 years and I don't think the client will do it.

[Coorsman777]

We have really good luck retrofitting R22 with R407F. We have gained capacity and saved energy with R407F versus R22.

[D.D.KORANNE]

at 87 hz , the current drawn by motor may be more than the max design ; whereby motor winding would overheat .hence the max frequency at which the comp can be operated is limited by the max current design of motor theefore advisable not to exceed .

[Justin_]

Considered R404a?
We retrofit 22 systems to 404. R404 is fairly similar to R507.
Do your existing comps have the ability to load/unload?

[Frikkie]

Running motors as high as 87Hz is very risky unless that motor was designed from the ground up to run at those speeds. There's many factors that will result in the motor failing catastrophically. The inverter output filtering would be critical at those high frequencies, the voltage harmonics of the output would normally be an issue because they increase at higher frequencies and cause internal heat. Motor construction would also be critical, stator air gaps and the layout of the stator slots would determine whether the internal I squared R losses run away to the point of failure. If the motor is not designed for higher speeds of operation the extra heat produced in the rotor will also possible cause bearing failure in the longer term. A higher grade of bearing lubricant may also be necessary due to higher hydraulic action and higher bearing running temperatures. Finally there would be a higher likely of inducing the rotor to a voltage that would discharge in pulses through the bearings to earth causing damage to the internal surfaces of the bearing which is leading to friction and failing. A rotor earthing kit would probably be a needed retrofit. I think it would be dangerous unless the manufacturers say you can by design but I would be interested to know if the motor survived if you decide to go ahead.

[AC_From_Hell]

I have ran 50Hz fixed speed compressors via VSDs a lot of times, to about 75Hz many times. If you carefully adjust the speeds according to the temperature of the compressor there are no problems.