Motor speed control of a hermetically sealed pump

[picclock]

Non professional newbie retired hobbyist here (NPNRH :cool: ?).

Need a hermetically sealed pump for a hobby project that will have a flow rate of around 1 cubic metre of uncompressed gas per minute. The compression of the gas is at relatively low pressures ~15psi so only low wattage required. Maximum external/internal gas pressure is likely 10 bar - well within normal refrigerator pump spec limits. Although many pumps specify the cubic capacity they do not seem to specify the pump rpm, which is likely to be a multiple of the mains frequency, I'm guessing 1500 or 3000 rpm. I'm thinking of a domestic type fridge motor for ease of availability and cost. Would a large cc pump running at lower speed be more efficient than a small one running much faster? I have a Samsung fridge freezer which has a digital invertor and runs at a much higher speed than a conventional synchronous motor yet claims better efficiency.

Is it possible to control a std domestic pump motor speed with a VFD motor controller ? Has anyone done it?

Failing that are small pumps available which can be driven from an external motor via a belt drive?

Any info or (polite) advice much needed and appreciated.

Best Regards :D

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[Brian_UK]

Have you looked at vehicle ac compressors?

They are normally belt driven, you should be able to get one from your local scrap yard.

[Dr._Fleck]

I've tried slowing down a ducted fan motor before and if I remember rightly the current went up as the motor slowed, then the motor suddenly stalled and overheated. It's probably because they are, as compressors, asynchronous motors that run at a set speed set by the mains frequency. You can get away with slowing them a little, in my experience, but probably not worth you going down the mains route.

[chemi-cool]

There are two basic ways to control electric motors speed.
By changing voltage and by changing frequency.
If you change voltage, watch the amps from going too high, frequency controlling is the best way and it will also save money spent on electricity.

[picclock]

Thanks for the help and replies. Sadly, I think varying the speed of a std mains hermetic compressor looks like its a no-no. Brians Idea of a car compressor sound like its well worth a try, and it makes controlling the speed simple. Not sure how good the shaft seals are likely to be on scrapyard compressors though, but I've seen a new scroll compressor on Amazon for £50 delivered so I will likely go for that, although the capacity is low at 86cc (revs revs revs :eek: ).

Best Regards

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@ Brian - Not a million miles from you in Ferndown, Dorset.

[chemi-cool]

Scroll is a good idea, but one phase will be a problem to control the speed.

[picclock]

Scroll compressor from amazon is a car one and I will drive it from a 3 phase motor.

Best Regards

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[picclock]

Hi All
Having purchased compressor as above, 3 phase motor, and VFD to drive it a question/option springs to mind. I had originally planned to use the compressor in the same orientation as in a vehicle, with the shaft horizontal. After a bit of thought I am wondering if there is any downside to mounting the compressor with the shaft vertical, pulley drive downward. I figure if mounted this way more lubrication will reach the bearings and shaft seal, prolonging its life. The inlet (S) would be below the outlet (D), though I'm not sure that matters. The dimensions on the pdf are reversed, but the drawing shows the correct port size arrangement.

14190

Can anyone see any snags with this arrangement ?

Many Thanks

Best Regards

picclock

[chemi-cool]

You can only install it as its manufacturer had planned. With the fixing bolts at the bottom.

[Brian_UK]

Bear in mind also that the oil sump is at the bottom of the compressor when it is horizontal.

If you mount it vertically you might wreck it by hydraulic lock in the cylinders due to oil ingress.

Also, it might be worthwhile looking into an oil separator to recover any oil pumped out of the compressor in normal running.

[picclock]

Hi Brian
No cylinders as its a scroll compressor. Basically its two scrolls with a thrust and radial bearing at the pulley end. Normal mounting position is as shown on the drawing with bolts from the side and suction/delivery ports on the side also. There is an overpressure valve which opens if the pressure is too high due to a high revving engine, but that will never occur in the way it is being driven. Have seen one disassembled on youtube and there does not appear to be a sump of any sort either. This one is Aluminium/Aluminium construction, but have seen much better ones using edge seals and stainless shim to improve the performance.

An oil separator is probably a good idea though the original car installations did not have one.

Best regards

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[Brian_UK]

An oil separator is probably a good idea though the original car installations did not have one.

Very true, I sort of assumed that you were not recirculating the air that you were pumping and therefore any oil mist pumped out of the compressor would be lost.