Start Capacitor Repeatedly Failing 1

[Yurtle]

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.electricmotorwarehouse.com/PDF/TROUBL~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

 

[itsmoked]

You're sure it's a start capacitor and not a run capacitor?

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

 

[electricpete]

I don't work with many small motors. I hope OP doesn't mind if I chime in with a question: why would speed change when centrifugal switch opens? Or is it perhaps just a change in electromagnetic sound instead of change in speed?

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[itsmoked]

I believe the field distortion, while increasing the available torque, also limits the motor's top speed. When the switch drops out the motor speeds up some recognizable finite amount.

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

 

[electricpete]

Thanks Keith.

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[Yurtle]

So there are 2 capacitors both relatively large. The first is a Cornell-Dubilier PSU16115 (161-193 uF, 125V) as mentioned above. This is the one that has ejected goo both times. The second is an Aerovox Z23P2425M21 (25 uF, 240V). The first is designated as a motor start cap by CDE and the second is designated as a run cap by Aerovox, unless I am missing something.

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?

 

[Yurtle]

I should note in response to your idea that it is difficult to hear the click when the vacuum pump is turned on (just checked a working similar model) as the motor gets up to speed pretty fast. Faster than 1 sec, contrary to my original post. When you shut it off though and the motor slows down you can clearly hear the click as the switch re-engages. I never really though about what that sound was until now. I will order another 163-191, 125V cap and replace it again and listen for the sound. I would also like to figure out when the cap is blowing so I will leave the cover off the housing. If the problem continues would a higher voltage cap help with this problem?

Jay

 

[Compositepro]

What is temperature of the motor while running under load? The capacitor may be getting cooked by slow starts due to too high a load and if the load stays high, the hot motor will cook the capacitor. Vacuum pumps can easily be overloaded by pumping to much air due to vacuum leaks. They are designed to pump down small volumes. Once pumped down the load on the motor will decrease substantially. Also, dirt and varnish build-up in the pump can cause friction which futher loads the motor.

 

[jraef]

What is turning your vacuum pump on and off? Cap start motors are sensitive to harmonics and rapid rise rates of voltage, i.e. solid state switching. So if for instance you have a soft starter ahead of this motor, that could be your problem. People often fail to read the instruction manual on single phase soft starters that says "No Capacitor Start Motors" which, unfortunately, comprises the greater majority of single phase motors.

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Yurtle]

Composite pro,
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://files.engineering.com/getfil...5c69-4ce6-ad07-cfc59129110f&file=Photo_32.jpg

http://files.engineering.com/getfil...02d1-4abc-9f03-5749da266344&file=Photo_31.jpg

Jay

 

[itsmoked]

jraef; how do you exceed 5A @ 120Vac with a measly half horsepower?? Check your numbers. 1/2hp = 375W

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Jeff I bet you thought it was a 1hp..

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

 

[jraef]

I just looked it up on a motor FLA chart.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Yurtle]

I suppose that the caps could be wired wrong but if they are then they were wired wrong for about 2 years before I had any problems. When I replaced the start cap I didn't touch the other cap to make sure I didn't screw anything up. Now again I know very little about all of this ( I enjoy learning though) but the difference between the 2 caps (25 uF vs. 161-196 uF) seems like alot to me. Wouldn't that cause immediate problems if it were wired wrong. I have no idea how to tell which is which other than that. I can tell you that the supposed start cap has 2 clips attached and the supposed run cap has 3. I will try and see if I can get a wiring diagram for the pump from Emerson.

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?

 

[waross]

I'll bet on two things.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[Yurtle]

My inspection of the start switch leads me to believe that it is mechanistically OK, it seems to work smoothly. That being said it very well could be that the switch is cycling during operation when the load on the pump in increased and the motor slows. I am going to replace the run and start cap and see what happens. If the start cap does go again hopefully it will blow while I am watching it. I will also listen to see if I can hear the switch engaging during operation. Now I just need to find a replacement for the Aerovox run cap as they do not have an exact replacement.

 

[edison123]

Why not just replace that centrifugal switch ? If the start cap goes off again, at least you will have a new switch.

Muthu

http://www.edison.co.in