Start Capacitor Repeatedly Failing
Start Capacitor Repeatedly Failing
(OP)
Let me preface this by saying I have very little knowledge of electrical engineering, but I have learned a lot since trying to fix this problem.
I have a Welch Vacuum Pump which contains an Emerson 8917A motor (0.5 HP, single phase, 120V, 4.6 amps). The start capacitor (161-193 uF, 125V) failed (black goo everywhere, motor would hum but not turnover) and I replaced it and then it failed again (brown goo everywhere) after about 1 week of use. In searching for a solution I have found this:
http ://www.ele ctricmotor warehouse. com/PDF/TR OUBL~1.PDF
I have opened the motor up and the centrifugal switch seems to be functioning properly as far as I can tell. The weights swing out without much trouble and ring that contacts the switch moves away from it freely when they swing out. I am pretty sure it is not a cycling issue as the pump is turned on and left on for ~12-24 hrs, and never repeatedly turned off/on. The voltage of the motor as noted above is 120V and the capacitor is rated for 125V. I suppose that the capacitor could be experiencing more voltage but if it is I do not know why. When the motor does start (with the fresh capacitor) it does not seem to take more that a second or two to come to full speed. I do not know if the second capacitor failed while the motor was running or if it failed when switched on.
I am contemplating buying a start capacitor with the same capacitance but a slightly higher voltage rating (165V) to see if this fixes the problem. Do you think this would be a good idea?
I get the feeling that there is some underlying issue that is causing the start cap to fail and doing this would not fix that problem. Do you guys have any other ideas as to what could be the cause?
Thank you in advance for your help.
Jason
I have a Welch Vacuum Pump which contains an Emerson 8917A motor (0.5 HP, single phase, 120V, 4.6 amps). The start capacitor (161-193 uF, 125V) failed (black goo everywhere, motor would hum but not turnover) and I replaced it and then it failed again (brown goo everywhere) after about 1 week of use. In searching for a solution I have found this:
http
I have opened the motor up and the centrifugal switch seems to be functioning properly as far as I can tell. The weights swing out without much trouble and ring that contacts the switch moves away from it freely when they swing out. I am pretty sure it is not a cycling issue as the pump is turned on and left on for ~12-24 hrs, and never repeatedly turned off/on. The voltage of the motor as noted above is 120V and the capacitor is rated for 125V. I suppose that the capacitor could be experiencing more voltage but if it is I do not know why. When the motor does start (with the fresh capacitor) it does not seem to take more that a second or two to come to full speed. I do not know if the second capacitor failed while the motor was running or if it failed when switched on.
I am contemplating buying a start capacitor with the same capacitance but a slightly higher voltage rating (165V) to see if this fixes the problem. Do you think this would be a good idea?
I get the feeling that there is some underlying issue that is causing the start cap to fail and doing this would not fix that problem. Do you guys have any other ideas as to what could be the cause?
Thank you in advance for your help.
Jason





RE: Start Capacitor Repeatedly Failing
Most of the time when a single phase motor with a centrifugal switch starts you can hear a definitive 'CLICK' as the motor reaches full speed. Often you hear the motor speed change noticeably too, as the switch opens. If you hear no difference at all during a start-up then you probably do have a switch problem.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Start Capacitor Repeatedly Failing
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RE: Start Capacitor Repeatedly Failing
I have seen this many times. It's especially easy to see with bench grinders. We have one that has a sticky start switch. The motor will spin up and if we don't hear the click we just "tap it on the head" with a closed fist and you hear the "CLICK" and the motor gains another um.. 300RPM? A very obvious gain.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Start Capacitor Repeatedly Failing
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RE: Start Capacitor Repeatedly Failing
I do not know if the run cap is bad (it has not exploded like the other one) but I will see if I can get my hands on a meter and test it. If it is bad could that be what is causing my start cap to blow?
RE: Start Capacitor Repeatedly Failing
Jay
RE: Start Capacitor Repeatedly Failing
RE: Start Capacitor Repeatedly Failing
If that's not an issue, check your voltage drop. When you measure voltage, do it when the motor is starting, not when it is off line or even running. You may have a voltage drop from starting current. A common cause of this is using an extension cord that is too small for the distance.
but in typing that, I see another potential issue. 4.6A on a 1/2HP 120V motor? Something is seriously wrong there. At 120V you should be somewhere around 9.3A, at 230V you would be at 4.9A. So either you are reading the nameplate incorrectly or you are applying only 1/2 voltage to your motor!
If it is a 120V motor and you are just reading the nameplate wrong, then you will be drawing more like 9.3A and then if you are plugged into a 16ga common household extension cord, you may be creating a VD that is frying your capacitors. Get yourself a 12ga extension cord if you have to go any distance.
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RE: Start Capacitor Repeatedly Failing
I have wondered whether the temp is too high as the start cap has a temp rating of 65 degree C. but until recently I have had no problems (+2 years) and I have actually increased the air flow around the pump by removing a door. The pump is generally operating at full vacuum (0.1 mmHg). I can not deny that it could just be that the motor is old and there is too much load.
Jraef,
No soft starter, plugged directly into the wall. I double checked the specs and i have included 2 pics one is a sticker on the back the other is on the side of the motor.
http://
http://
Jay
RE: Start Capacitor Repeatedly Failing
Keith Cress
kcress - http://www.flaminsystems.com
RE: Start Capacitor Repeatedly Failing
Yurtle; I'm having a problem with this because the start cap is out of the circuit after that one second.... So, it has no power applied, no load, it's just a 'capacitor sitting on a bench' as it were. So it has no business failing at times other than the actual start. If the start circuit is staying in, the motor windings will usually incinerate looooong before the cap.
Makes me think we're actually talking about the run capacitor. Is there some way to confirm which is which without referring to the caps themselves? They could've been mis-applied or mis-wired.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Start Capacitor Repeatedly Failing
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RE: Start Capacitor Repeatedly Failing
I may have noted this before but I am not sure when the cap blew out but I do remember that the pump was running and then I turned it off and then tried to turn it on ~30 min later and it would not start which is when i found the second blown cap. Could the cap have blown upon starting but still start and run until I turned it off at which point I would not be able to start again?
RE: Start Capacitor Repeatedly Failing
1> The starting switch contacts are welded. The starting capacitor stays in the circuit and fries.
2> The motor is being slowed down by either too much load or low voltage or both of the above. The starting switch is cycling on and off. It could be that as the vacuum is increased, the load on the motor increases.
A cycling starting switch will fry a start capacitor.
Bill
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Jimmy Carter
RE: Start Capacitor Repeatedly Failing
RE: Start Capacitor Repeatedly Failing
Muthu
www.edison.co.in