Hi all,

I have recently been given an air compressor that was in need of some repairs. I was hoping to be able to get this all done on my own, but I am totally stuck when it come to re-wiring the motor and capacitors.

The motor itself is a single phase, 240v, 3Hp unit. Its marked MEC 90 L, but I believe this is more of a description of the physical dimensions than the electrical side of things from what I have looked at on the net.

I have two old, and clearly blown capacitors and two new replacements. One is 70uF the other is 55uF. So I have assumed that this is a Capacitor start, capacitor run motor, and that the 70uF is the start and the 55uF is the run. I have searched all over the place for an internal wiring diagram and just cannot find one, and for the life of me I cannot figure out how to connect it up properly.

Coming from the motor are 3 wires, they are not marked in any way. So I will call them A, B and C for now. The resistances across them are as follows, approximately, I was using an analogue meter and cannot remember the exact numbers now:
A - C = 0.5 Ohm
B - C = 1.5 Ohm
A - B = 2 Ohm

So to me this suggests that A & B are the start and run windings, with C being common. I loosely understand the principals of wiring the motor, but I keep finding conflicting information on the internet as how it should be wired. Some places say both capacitors should be in parallel across both coils, with the Start cap being switched out by a centrifugal switch when the motor is up to speed, others seem to suggest that the run cap be in parallel with the start cap in series. Either way, I can't get my head around how to do this with just the 3 wires. In my head I need four.

So does anyone have any ideas or tips on what I could look at next? I would really love to get this going.

Thanks in advance

Chris

 

Hi Chris, First question. When you say "I have two old, and clearly blown capacitors and two new replacements. One is 70uF the other is 55uF." Do you mean that you just happen to have these capacitors in stock or do you mean that You know the two old ones belong to this motor ? Capacitor start / capacitor run motors have the run winding directly across the main. The auxiliary winding is connected in series with the run capacitor across the mains. The start capacitor is connected in parallel with the run capacitor via the centrifugal switch so that it is switched out of circuit when the motor is up to speed. You have not mentioned if this motor has a centrifugal switch. I think that the run winding is A to C and the start winding is B to C The smaller capacitor is normally the run capacitor. If the motor has a centrifugal switch then this is how I suggest connecting it. C to neutral. A to live. One end of the 55 uF cap to B the other end to live. One end of the 70 uF cap to B the other end to one side of the centrifugal switch. The other side of the centrifugal switch to live.

Les.

 

Les

Thanks for coming back to me so quickly. Sorry should have mentioned that, the blown capacitors came from the motor, and the new ones were included and have matching values. I have a feeling the old owner got to the same position I am in and got stuck. Just as I have.

I do not know if the motor has a centrifugal switch. There are only three wires coming from the motor housing itself into the capacitor box. Hence my confusion. I have removed the rear most cover from the motor housing but that only contained a fan, and i was not able to see past it into the motor to see if there was anything further. I really cannot see anything else so I would be inclined to suggest there are only the 3 wires in total, and no further connections. I will get a photo and attach it to see if that helps at all.

I assumed this was cap start cap run, due to there being two capacitors, but I could just be incorrect.

Thanks again for the help, really appreciated.

 

Pictures as promised.

53 = general image
54 = wiring
55 = old capacitors
56 = why i think at least one of the capacitors is bad

 

I have seen two alternatives to a centrifugal switch bu only on small motors such sealed fridge compressors. The first is a current operated relay with the coil in series with the run winding. (The coils is only a few turns of thick wire.) The idea behind this method is that the current through the run winding is very high before the motor is up to speed so the contacts close putting the start capacitor in circuit. The other is to use a large PTC thermistor. This just effectivly connects the start capacitor fo a preset time. When the thermistor is cold it has a low resistance so a lorge current passes through the start winding. The thrermistor will heat up quite quickly which increaces its resistance and thus reducing the current through the start winding. I have never seen these methods used on a motor as large as 3HP.

Les.

 

Right bit more info. I pulled the motor off the frame tonight so I could get it apart.

The 3 wires go directly into the windings, there does not appear to be much else in there. Certainly no centrifugal switch that I could see. I have attached pictures of the front and back end of the windings.

I tried to spin the motor up with my electric drill, it only goes to 2000 rpm max, so I would have been at less than that, all resistances stayed the same. Follows really, I did this before I got in to it.

One interesting thing to note if that of the 3 wires A + C are 2.5mm while wire B is 1.5mm.

I can buy a new motor, but it seems a shame, as it seems in good shape otherwise, the bearings are good etc. The rear end of the windings are rather black and nasty, but I do not know if this is just how these sorts of things are?

So any ideas what this actually is, and how to make it go again?

 

Those windings are fried.

 

He's dead Jim.
Max.

 

Oh well, thanks all.

I got the compressor for free, so even after getting a new motor I still got a good deal. Learnt some new stuff along the way too, so even better.

Thanks again.

 

Donate that to a trade school as a training aid.