Air Compressor Problem - Motor Stalls
in [Repair]
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From: PeterD on 7 Mar 2010 18:20 On Sun, 07 Mar 2010 15:32:31 -0500, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: > >Sparks Fergusson wrote: >> >> Alan Douglas <alan_douglasat(a)verizon.net> wrote: >> >> >>And all those replies ignore the fact the unloader is operating (at a >> >>wrong, low pressure) which certainly doesn't point an evil finger at >> >>the motor at all... >> >> >> > >> >Hmmm, yes, I think that wasn't in the original post so I missed it. >> >That would account for gradual deterioration too. >> >> The unloader is functioning properly, as far as I can tell. It's a >> combination check valve/unloader. So, when the check valve is open, it >> allows the high pressure air into the tank. When the check valve >> closes, it vents the pump side to atmosphere. >> >> So, then the motor stalls, the check valve closes and the unloader >> part releases the pressure. When the presssure falls sufficiently, the >> motor restarts and the cycle continues. > > > It shouldn't. The unloader is supposed to open when the motor first >starts, Typical small compressors the unloader opens when the pressure switch activates and shuts off the compressor. This releases all pressure on the compressor head, allowing the motor to start when pressure drops down to the cutin PSI. > to let it come up to speed. Since the motor is stalling, you >either have a bad pressure sensor, or the run capacitor has lost some or >most of its capacitance. The pressure sensors deteriorate with age, and >the calibration goes to hell. IHMO it has not been said that the motor is stalling! A stalled motor (called locked rotor) will cause input current to swing sky-high, usually tripping the breaker, making a hell of a noise, and is very obvious. > > The capacitors have to pass a lot of current, and aren't made to last >forever. They are AC electrolytics to keep the size and cost within a >range you can afford to build and sell a product. A film capacitor >would be quite large, and can caost more then a new compressor. > > Some compressors have two capacitors. One in the start circuit, and >the other in series with one of the run windings to provide the required >phase shift.
From: PeterD on 7 Mar 2010 18:22 On Sun, 07 Mar 2010 20:59:24 GMT, hatespam(a)invalid.invalid (Sparks Fergusson) wrote: >"Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: > >> It shouldn't. The unloader is supposed to open when the motor first >>starts, to let it come up to speed. > >An electric unloader, yes. This is a mechanical one, integrated with >the check valve. It is working as designed. > >> Since the motor is stalling, you >>either have a bad pressure sensor, or the run capacitor has lost some or >>most of its capacitance. The pressure sensors deteriorate with age, and >>the calibration goes to hell. > >The pressure switch is OK. It is not what is stopping the motor. The >contactor never opens and the motor is receiving full voltage through >the stall cycles. The capacitor could certainly be the issue, though. So the original post is incorrect then? WHich is right, this or the other one? Is motor current high when the motor stalls? > >> The capacitors have to pass a lot of current, and aren't made to last >>forever. They are AC electrolytics to keep the size and cost within a >>range you can afford to build and sell a product. A film capacitor >>would be quite large, and can caost more then a new compressor. > >Indeed. This motor is probably 15-20 years old, so I can easily >believe the capacitor may be at fault. I'll have to disassemble the >motor and check it as soon as I get a chance. > >> Some compressors have two capacitors. One in the start circuit, and >>the other in series with one of the run windings to provide the required >>phase shift. > >I only see one capacitor, so I'm assuming it's in the starting >circuit. I don't have a wiring diagram, and GE has no information on >it's website for this motor. > >Thanks!
From: Sparks Fergusson on 7 Mar 2010 20:00 PeterD <peter2(a)hipson.net> wrote: >So the original post is incorrect then? WHich is right, this or the >other one? Is motor current high when the motor stalls? I measured about 5 amps at startup (0 psi) rising to close to 6 when it stalls. When it stops turning, the current actually drops back closer to 5 amps, then increases as the motor starts turning again.
From: Sparks Fergusson on 7 Mar 2010 20:04 PeterD <peter2(a)hipson.net> wrote: >Typical small compressors the unloader opens when the pressure switch >activates and shuts off the compressor. This releases all pressure on >the compressor head, allowing the motor to start when pressure drops >down to the cutin PSI. On this compressor, the check valve/unloader is completely separate from the pressure switch. The unloader is similar to this one: http://www.amazon.com/Control-Devices-CA121A-Unloader-Check/ >IHMO it has not been said that the motor is stalling! A stalled motor >(called locked rotor) will cause input current to swing sky-high, >usually tripping the breaker, making a hell of a noise, and is very >obvious. The voltage to the motor is never interrupted during the stop/start cycle. Above a certain psi (about 95), the motor starts bogging down and eventually stops. This occurs even if I bypass the pressure switch and attach the cord directly to the motor leads. Then, after the unloader vents the pressure, the motor will restart. Input current does not swing sky-high, though, nor does the breaker trip, nor is there a hell of a noise. It definitely comes to a dead stop, though.
From: Sparks Fergusson on 7 Mar 2010 20:08
PeterD <peter2(a)hipson.net> wrote: > >From the original post that started this thread: > >>... underpowered - it pumps up >> to about 95 psi ok, but then the motor stalls. The unloader bleeds the >> pressure off, the motor restarts, runs for a couple of seconds, and >> then stops again...repeat...repeat..." Yes, those're the symptoms. |