Hello folks, I just joined the forum and didn't want to start with a question in my intro post so here goes my second post! I recently got this CH twin tank air compressor for next to nothing with a motor that can't start itself, it just hums, loudly hums, it's very clean for its age and doesn't appear to be used that much. Anyways, I've dealt with this situation on previous compressors and the only two things I've ever come across is a start cap and a centrifugal switch, if it's not one of those I'm not quite sure where to go. This one has a 120 uf start capacitor which I changed because they're cheap and why not, and upon testing it with a capacitance test it read exactly 121.6 so I ruled that out. I can't get it to start even with the drain valve open on the tank which means there's zero tank pressure, I even removed the air line off the head and still nothing. I can turn the motor by hand with what I'd consider a normal amount of resistance for an air compressor, at least compared to the ones I've dealt with before, and I can kind of get it started by spinning it by hand, though twisting the fan while it's trying to start isn't something I enjoy doing, it takes a little more than a nudge, almost half a rotation. Initially when I replaced the capacitor it started just fine and sounded great but now I'm right back to square one. Any input on those who have dealt with this or highly educated in this subject would be greatly appreciated! Thank you folks and have a great evening!
Ominous Humming Motor
- Thread starter Mike1919
- Start date
fitter30
Well-known member
Good possibility that the centrifugal switch isn't closed to put the start capacitor in circuit. Kill power. Pull the end bell to inspect. There's a plastic / metal sheeve over the shaft that has to slide that moves arms to activate the switch. Even though it looks clean but rust can sometimes screw the mechanism up. Spray it with wd40 or something like it then move the plastic slide. Put back together.
That's what I was concerned about but I couldn't find the switch, is there a possibility it's on the other side of the motor, and if so, can you even separate the motor from the compressor assembly?
Attachments
micromind
Well-known member
You'll very likely need to remove a bolt/nut from the end of the crank to separate the motor from the pump.
are we sure there's a centrifugal switch on those motors? I'm thinking usually no?
Bolt #27
Not sure and since nothing shown in parts diagram, I think you are correct.
I would want to seperate the load from motor to further test. Need to establish if motor even able to run.
fitter30
Well-known member
120 mfd capacitor is a start cap. That contact is open should be closed. Never seen a motor like that one. Looked up that motor only saw a diagram like the one you posted spray it give it a tap. Where is it located?
kerrynzl
Well-known member
The compressor check valve could be stuck open . The motor cant start against pressure
That's what I was thinking but shouldn't that be negated if I tried starting it with the air line pulled off of the head?
walta
Well-known member
Seems to me 120mf is too large a number to be in the circuit continuously. I don’t see a centrifugal switch that leaves us looking for some electronics that will shut down the start windings.
Could there be a current relay hidden in the pressure switch. Are there more than 2 conductors don’t count ground wire going from the motor to the switch?
I had a similar compressor that would trip the overload on startup seemingly at random. Seemed to happen more when it was cold. It would just hum and trip the overload. It was worse when connected to an extension cord or a 15 amp curcuit. The fix turned out to be the previous owner had changed the oil and used dinosaur oil when synthetic was required.
We need to consider has the new cap failed after the first start.
Could there be a current relay hidden in the pressure switch. Are there more than 2 conductors don’t count ground wire going from the motor to the switch?
I had a similar compressor that would trip the overload on startup seemingly at random. Seemed to happen more when it was cold. It would just hum and trip the overload. It was worse when connected to an extension cord or a 15 amp curcuit. The fix turned out to be the previous owner had changed the oil and used dinosaur oil when synthetic was required.
We need to consider has the new cap failed after the first start.
kerrynzl
Well-known member
In normal circumstances YES
But it also depends on the compressor [pump] type.
Some can trap pressurized air inside pump.
I did find that suspicious too being such a large capacitor but didn't think too much of it as it seems pretty properitary to the compressor being threaded in and all, the old one was 120 and looked identical but who's to say someone didn't already change it before me with the wrong one. Upon further investigation of the pressure switch, it looks like there are only two wires not counting ground going to the motor itself and as for oil, that is a good idea because I did check it and found it to be very clean, surprisingly clean actually, as if it was just recently changed.
Attachments
justsam
Well-known member
Where do the leads or terminals of the capacitor go? If the capacitor has terminals, how many leads are on them? Two leads on one terminal and one lead on the other? The 120 uF is a reasonable value in my mind for the start cap. for this application which does have a minor start up load, (frictional loss of compressor). If there is an unloader valve to release air pressure for start up, make sure it is OK. I did not see a legend for the exploded diagram, and a wiring diagram would also be convincing. Any part in the legend call "timer" or "current relay"?
Last edited:
It looks like they just go into the windings which is where I become lost, the capacitor has two terminals consisting of a black and a white wire clipped on via spade terminals
Attachments
Thank you, I think I'm going to change the oil so I for sure know that there's a lightweight compressor oil in there and that they didn't put automotive oil in, then I'll put it together just enough to try to start it and start measuring voltages, another thing I found very odd is I couldn't find a circuit breaker, when the guy "demonstrated" it to me it hummed for a few seconds causing me to cringe but nothing ever happened. Here, when I try it, it just dims the lighting on my archaic household electrical system but still no breaker tripping if it even has one
micromind
Well-known member
It could be a PSC (Permanent Split Capacitor) motor. These have a capacitor but no start switch. Very common with fans.
The advantage is no start switch to fail. The disadvantage is very low starting torque.
The advantage is no start switch to fail. The disadvantage is very low starting torque.
Well, this is the final run, I'm putting new SAE 30 compressor oil in it, the oil that I drained was spotless, however it looked kind of thick but was hard to tell, I also noticed a bunch of this sealant around what looks like the pressure release switch which, to my knowledge, wouldn't have been factory, but I'm not sure. I'm trying another new capacitor just in case the other one was bad and have to put a new cooling fan on being I broke the other one, but that probably has nothing to do with starting. After this, I may just put it back together and use it to hold down a stack of manuals and receipts in my garage so they won't blow away in the wind.
Attachments
djbmw
Well-known member
Take the motor apart and look for burn marks on the windings, or on the contactor/centrifugal switch.
A humming noise means that something isn't getting the electrical pixies that it needs - likely due to bad contact (from burns, debris, damage, etc.). Pop it apart and snap some pics. My tire machine motor had a burnt out contactor (last pic below) and would hum/buzz in a certain position. A few passes with sand paper and it was up and running within 20 min (I also cleaned it before re-assembling).
A humming noise means that something isn't getting the electrical pixies that it needs - likely due to bad contact (from burns, debris, damage, etc.). Pop it apart and snap some pics. My tire machine motor had a burnt out contactor (last pic below) and would hum/buzz in a certain position. A few passes with sand paper and it was up and running within 20 min (I also cleaned it before re-assembling).
Here are the contactor points, I gave them a pretty decent sanding and am still getting the same results, it can start about 2/5 times regardless of pressure in the system or not, I'm going to try to get the rotor out but something is holding it to the compressor so I'm guessing the shaft is bolted straight to the connecting rodTake the motor apart and look for burn marks on the windings, or on the contactor/centrifugal switch.
A humming noise means that something isn't getting the electrical pixies that it needs - likely due to bad contact (from burns, debris, damage, etc.). Pop it apart and snap some pics. My tire machine motor had a burnt out contactor (last pic below) and would hum/buzz in a certain position. A few passes with sand paper and it was up and running within 20 min (I also cleaned it before re-assembling).
Attachments
djbmw
Well-known member
Those look good. How are the windings, rotor, stator, etc?
I dont see a start/run cap (im also on my phone and on the road so I cant really look at the images/tread in detail) so I imagine the motor uses a commutator. What is the condition of it, and the brushes?
Sanding may have killed what ever life was remaining in the contacts. Did you take the voltage readings mentioned in post# 16 before/after?
The pic in post# 4 shows a bolt - part# 27. Drain the pump and open it to get access to the bolt. Can probably drive it out with a wooden dowel or light impacts from a center punch.
djbmw
Well-known member
Did you look at his pic? The contacts look beautiful. As long as they make contact with eachother, they have life left (hundreds of thousands of on/off cycles).
Stuart in MN
Well-known member
The thing about electrical contacts is they're typically plated with silver or some other metal, so sanding can remove that plating which will shorten their operating life. They'll work, but they'll corrode and/or deteriorate quicker from the small arcs that happen each time they're operated.
manwithtools
ALLIANCE MEMBER
Be sure you have a full voltage and wire size for a full 15 amps supplying that compressor. I have an older Emglo compressor of similar design and it will not start if not supplied with adequate voltage and current, it will just hum. It's been that way since new, I've never been able to use it on anything other than a 25' 12ga extension cord.
djbmw
Well-known member
Perhaps they are on some motors (based on quality/power?),.. but certainly none that I recall working on. If you look up brand new replacement centrifugal switches you'll see they are bare copper (image attached for reference).
Edit: i was able to find a few silver plated ones (image attached). They seem to be more commonly used on older quality motors. Most mass produced motors that come with innexpensive equipment seem to be using straight copper though.
Attachments
Last edited:
dave*99
Well-known member
It's important to note that low current contacts are often plated too. And burnishing is easier on the metals than sanding:
Sanding and burnishing both clean relay contacts, but burnishing (using specialized files or cardstock) is generally preferred over sanding for removing oxidation because it's less abrasive and designed to clean without removing precious metal plating, while sanding (with sandpaper) is riskier as it can leave grit and easily strip protective gold/silver flashes, causing future issues, especially on low-current relays. Burnishing polishes and reshapes contacts slightly for better connection, whereas sanding can create irregularities, making burnishing the safer, more effective method for preserving contact life and reliability.
fitter30
Well-known member
Dressing or sanding contact is only for a emergency they will fail again welding themselves together. 3 volt drop across them 240 vac. Their either ready to be replace or in most cases the whole relay, switch or contactor.
djbmw
Well-known member
That certainly hasnt been the case across the electric motors that I've repaired. If i look at all of the 2hp to 8hp motors that I have sanded/polished the contactors of over the last 15 years, I still have 6 of those motors in operation today and they run quite well (2 tire changer motors, 1 fan motor for a high volume evacuation fan in my paint booth, 1 motor on my screw-drive vehicle lift, 2 motors for my zip-line lift tow).
fitter30
Well-known member
Haven't found a manufacturer that would recommend dressing contacts. Running service work can't take that chance. Has to be fixed right. What you do with your own equipment is different when getting paid. Electric work that i have done line voltage 94% 480, 3% 208/240, 3% 4160.
Sorry, I missed that one, here is what I got with the power on and the motor humming, line voltage read at 119 and the motor voltage straight off the contactor read 112 volts