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MEC 90 Electric motor,

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Andrew Tinsley12/08/2020 17:53:55
1169 forum posts


I have just been looking at a single phase motor which came out of my Centec, when I opted for a 3 phase motor,on first first commissioning it.

There is no makers or information plate on it and the "Box" which houses the terminals isn't there, simply a blanking plate.

Cast into the aluminium housing is MEC 90. Googling this, leads me to believe that it is intended for a 3 HP air compressor, so quite hefty for a single phase motor.

Removing the blanking plate reveals 4 wires flopping around. Checking these shows that they are two separate windings.. One is about 3.5 ohms which must be the main winding and the other around 25 ohms which must be the start winding.

I would wire this up as a capacitor start motor, using an 80 microfarad capacitor (simply because it is about the sort of value I would expect, but more honestly because it is to hand).

I have a niggling doubt about doing this, because I assume the start current surge is likely to be high on an air compressor, unless there is a system to bleed the pressure in the tank supply line.

Does anyone know if these air compressor motors are usually capacitor start? Or is there some other starting method for these "specials" I might as well put it into good order, although I can't think of any use for such a beast.


Edited By Andrew Tinsley on 12/08/2020 17:56:41

Jon Lawes12/08/2020 17:59:08
388 forum posts

Bear with me as I am a novice, I think compressors are often capacitor start, especially as unlike something like a lathe they have an immense resistance to start moving (I know a lathe has some inertia, but nothing compared to a compressor). I believe there is often something, mechanical or electrical, which reduces the pressure in the compressor stage to assist with the starting too.

As I say, purely a novice who has repaired one in the past and had to do some research at the time.

Clive Foster12/08/2020 18:29:00
2314 forum posts
76 photos


Its generally advised that single phase compressor motors be capacitor start and capacitor run machines with two capacitors fitted. The run one permanently in circuit and the start one providing extra phase shift to boost it as it runs up. When running these are pseudo 2 phase machines which cope much better with the fluctuating load of the compressor piston(s) than a basic single phase motor.

Also a good idea to feed the tank via a decent size manifold allowed to bleed down to atmospheric pressure between start ups so the initial run-up is not against full tank pressure. A non return valve seals off the main tank. Many of the pressure controlled electrical start - stop units incorporate a suitable bleed valve. Alternatively simple bleed back through the compressor will suffice.

My 16 cu ft / min Atlas Copco Vee twin had a manifold approaching 16" long by 4" diameter which seemed adequate to reduce starting loads. I have kept 3 of the small 340 size Calor gas cylinders to use as manifolds / collector tanks should I ever set-up another small compressor. Those particular cylinders are orphaned as Sievert, who took over that size of cylinder, won't re-fill Calor cylinders.


Emgee12/08/2020 19:23:10
1637 forum posts
224 photos


A few weeks ago there was a resistance type starter pictured, maybe you need something similar.
Have you checked to see if there is any kind of centrifugal switch fitted to 1 set of windings ?


larry phelan 112/08/2020 19:26:26
801 forum posts
14 photos

I remember looking at single phase motors in Machine Mart catalouge and they stated very clearly that they were NOT suitable for compressors.

Andrew Tinsley12/08/2020 20:23:11
1169 forum posts

Hello all and thanks for the feedback.

Larry, MEC 90 motors are designed for compressors and they come in single and 3 phase versions, so I have no doubt that this motor could be used on a compressor.

Emgee, I have not stripped it down, but I would strongly suspect there is a centrifugal switch. I . I don't know of any single phase motor, of the power type, that doesn't have one. It definitely isn't the very ancient type that was mentioned a few weeks ago. I had one powering my backup compressor and know them inside out!!

Clive you have hit the nail on the head. I had a niggling doubt about it being a capacitor start. That would explain the peculiar external wiring which I cut through when the machine was dismantled for transporting back to my place. There were no caps apparent, but then the previous owner never got the Centec working. I hasten to add that I have no intention of using it on an air compressor!. In fact I cannot see much of a use for a 3 HP, 2800 rpm single phase motor. Even so I will get somewhere near the correct caps and a box and make it a runner.

Thank you all,


Edited By Andrew Tinsley on 12/08/2020 20:24:51

SillyOldDuffer13/08/2020 10:12:50
6171 forum posts
1335 photos
Posted by Andrew Tinsley on 12/08/2020 20:23:11:


... MEC 90 motors are designed for compressors ...

Even so I will get somewhere near the correct caps and a box and make it a runner.


True MEC90 motors are sold for compressors, but I think MEC 90 is a Frame Size, not a particular motor type or application. The motor inside the frame could be any of the single-phase types, or 3-phase, and might be fitted to any equipment. Not sure where MEC comes from; I suspect Italian. One motor specification mentions IEC/EEC as equivalent to UNEL-MEC, in which case MEC is 'Mercato Comune Europeo', Italian for 'European Economic Community'. Can't find what UNEL is, Italian for IEC, is CEI (Commissione elettrotecnica internazionale)

Anyway, I suggest the compressor clue is a red-herring; all that's needed is to work out what type of single-phase motor it is and take it from there. Which is exactly what Andrew is doing!

Can you report back what worked and what didn't - always useful to understand how to go about getting an unknown motor running.

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