Air Compression Motor

[tablesawtom]

Thank you for all of your responses. What happened a few years ago is I put an air shut off valve at the tank and forgot to close it when I left the shop. I din't turn the motor off because the pressure switch would stop the motor from running. I don't remember what happened to my air line but when I got back to the shop a couple of days latter the compressor was running continuously and it was quite warm to say the least. And then it worked okay for a while but it has continued to go down hill ever since.

If, and I am sure it is true, that the compressor motor is not rated for continuous duty and yes my old motor is an open frame and so what happened is probably the reason for the motor going bad. Hind sight is always better than foresight so now I  close the air valve to keep the air in the tank and turn off the switch to the motor in case the pressure switch malfunctions. And it has become habit because of the problems I have encountered.

Being retired, I am not in the shop everyday or for long periods of time and because of cost I am sure that the motor will work just fine for my needs. And perhaps knowing why the motor failed in the first place helps.

So thanks again for all of your expertise and help in making my decision easier. The fear of RPM disparity bothered me but I am perfectly okay with it now.

Tom

[TDKPE]

Doesn't it also usually mean that it's an ODP (Open Drip-Proof) type motor as opposed to (for example) TEFC (Totally Enclosed Fan Cooled) like what one might use for a table saw?

ODP is almost always cheaper than a similar non-vented motor, for a number of reasons including higher insulation class (NEMA requires class F for TEFC), and more metal to get the heat out of the windings and into the frame where the external fan can blow it away.

[TDKPE]

 The fear of RPM disparity bothered me but I am perfectly okay with it now.

Yeah, it’s not really a disparity anyway.  It’s like electric devices like receptacles being rated 125V for use on 120V systems which typically run around 117V while NEMA motors are rated 115V while many folks still call it 110V, but it all can run on voltage as low as 108V (-10%).  

[Roly]

(voltage*frequency)/number of poles will give you rpm

Its a 2 pole motor

You use (120VX60hz)/2poles = rpm number of 3600
You use (125VX60hz)/2 poles = 3750rpm

Get your meter out and measure your voltage.

4 pole motors are used in 1800 or 1875 rpm motors

The numbers used are generic on the plate and your actual actual RPM are specific to your actual voltage (and any other factors in the motor itself).

A cheap Non Contact Tachometer will tell you real quick your actual achieved rpm.

The voltage is not a factor in this type of motor, just the frequency and number of poles.   Roly

[Roly]

Its the number the vendor is using to compute the rpm. A plate with 3600rpm listed is using 125V in their computation. So it is the ONLY factor they can change.

The name plate of the motor in question.  Harbor Freight 3 hp compressor duty motor.    Roly

[BloomingtonMike]

The name plate of the motor in question.  Harbor Freight 3 hp compressor duty motor.    Roly

Thank you for the picture Roly. Deleted my replies that mean nothing for sure.

[Alan S]

Before replacing the motor, have you tried replacing the capacitors?  

I have a similar cheap HF motor that has been driving my dust collector for a decade or so, that went out.  (It was labelled 5 HP, but is likely closer to 3.)  I tested the capacitors (a good capacitor when discharged, then tested for continuity initially reports low resistance, and gradually converts to an insulator.  If you then check for DC voltage, you will briefly see the voltage applied by your meter, and then nothing.)  My run capacitor was bad, so I replaced that, which did not entirely fix the problem; also replacing the start capacitor made the motor run like new.  

That fix can be done without opening the motor itself.  I opened it to check the centrifugal switch, and found that on these HF motors it's held by snap rings so it can't slip out of place.  Since it was open, I changed the bearings for some higher quality ones ($3 at Accurate Bearing), and figure the motor is now better than new.

[tomsteve]

new motor has to have the same shaft rotation direction as the old motor,too.