Failed start and run capacitors

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Jraef said:
I think this is a good point. I have started seeing harmonics issue showing up in traditionally residential areas too, in the same way Besoeker did; someone wants harmonic mitigation for a VFD and to do it right, you need to know from whence you start. I've been surprised by the existing THD when I get there, even though it doesn't appear to be an "industrial" area where one would expect it. Could be the cumulative effects of all the CFLs, electronic ballasts, PC power supplies, big screen TVs and "energy efficient" appliances that all have SMPS or VFDs on them..
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WhatHeSaid. There seems to be more and more crapola on the power grid. With SMPS's on everything from shavers to microwaves to laser printers, we're not in Sinewave Kansas anymore, Dorothy. And then there are the switched solar array inverters.
 
120626-0836 EDT

It is important to really know if both capacitors are failing.

If the capacitors were interchanged it is likely the motor would not start. The same capacitor used for both run and start, meaning a single capacitor motor, has very low starting torque.

So it is reasonable to assume the capacitors are wired correctly, or the motor might not start.

Going back to the second post I see a run capacitor voltage rating of 370. This is a ratio to line voltage of 370/240 = 1.54 . When I looked at the Dayton capacitor photograph it looked like a 330 V rating.

A properly rated run capacitor is probably much less likely to fail than the start capacitor.

In both run and start modes the capacitors are in series with an inductor. The impedance of the inductor increases with frequency so this should somewhat reduce the effect of harmonics. But one does have to consider resonance where impedance becomes a minimum, and equal to the series equivalent circuit resistance. On either side of resonance the impedance goes up.

Both capacitors failing might imply some large voltage spikes, possibly thousands of volts.

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Here is an update:

It was a bit messy in the control cabinet, and being a utility guy, I did not want to mess around in there too much. The monitoring data showed voltage THD between 2.2% and 5.2%, with almost all of it 3rd harmonic. There was one "spike " in THD up to 6.5%. There was both a run and start capacitor that failed at the same time. Data shows a typical looking motor start when compared to the others seen. I have not found out how the motor is controlled, still checking on that. I did notice that the contactor looks to be rated for 240 amps locked rotor (see picture). Data indicates LRA of 300 Amps, sometimes up to 350 Amps.
 

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120709-1453 EDT

If both capacitors failed at the same time, then this motor likely uses both capacitors for starting and drops out the start capacitor at speed.

This might imply a very large voltage spike during starting. How could this happen? One way is having a major contact bounce during start, when a large inductive current was flowing, causing a large inductive kick.

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That contactor is switching the start caps out based on the centrifugal switch position; at rest the contacts should be open, when energized the contactor should pull in, then when the motor gets to speed, it should drop out again. If that contactor is welded shut, the start caps are in the circuit all of the time and it can lead to the early demise of both sets of caps. Check to make sure that contactor is doing it's job properly. Mars contactors are the cheapest of cheap China made junk. They barely outlast a warranty period.

Here is a service manual for that motor, the internal wiring diagram is on the last page.
 
Jraef said:
That contactor is switching the start caps out based on the centrifugal switch position; at rest the contacts should be open, when energized the contactor should pull in, then when the motor gets to speed, it should drop out again. If that contactor is welded shut, the start caps are in the circuit all of the time and it can lead to the early demise of both sets of caps. Check to make sure that contactor is doing it's job properly. Mars contactors are the cheapest of cheap China made junk. They barely outlast a warranty period.

Here is a service manual for that motor, the internal wiring diagram is on the last page.
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I think your right about that start contactor, if you look closely on the left side of it in the photo when viewed in full size above you can see signs of arcing by the bluing on the Bakelite.
 
hurk27 said:
I think your right about that start contactor, if you look closely on the left side of it in the photo when viewed in full size above you can see signs of arcing by the bluing on the Bakelite.
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As well as the 'soot' all over the conductors.
 
Are the capacitors of spurious quality?

The capacitors installed now may be checked separately with applied voltage and their current measured. Their capacitance may be calculated and compared with name plate value.
 
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T.M.Haja Sahib said:
Are the capacitors of spurious quality?

The capacitors installed now may be checked separately with applied voltage and their current measured. Their capacitance may be calculated and compared with name plate value.
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I would think the caps are of average quality, plus there have been so many fail, it is different than getting a "bad batch."
 
wirenut1980 said:
I would think the caps are of average quality, plus there have been so many fail, it is different than getting a "bad batch."
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Well we pointed out you have a bad start contactor, the wrong size caps as per the posted PDF for this motor, what else you need, install the correct caps, and new 40 amp rated contactor and it should be good to go, and no you don't size the starting contactor on the LRA, it was sized by the manufacture at 40 amps so why change it, just use a better quality one.
 
In addition to what hurk stated, it is always good to the check the actual capacitance values against the name plate values in the site, using true rms meter for voltage and current measurement irrespective of whether the capacitors have spurious quality or not. It may reveal whether the capacitors are working too hard due to any non-linearity in the supply.
 
wirenut1980 said:
Thanks guys, good stuff. Do you think the contactor needs to be upsized at all? LRA measured was 300-350 Amps, nameplate rating on contactor was 240 LRA.
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The contactor is only switching/carrying the current that flows in the start capacitors not the entire load on the motor. That current is significantly less than the total load carried by the main supply conductors.
 
Recent posts indicate good reason for start capacitors to fail, why are run capacitors failing, they are in ciruit no matter what position starting switch or contactor is in.

Or maybe they fail once the start capacitors fail because they are attempting to take over starting duties?

I suppose if the load has low enough torque demand they will get it started. My experiences though are usually in case where starting torque needed is high enough the motor will not accelerate at all when start caps are failed and motor will draw locked rotor current. This will result in either overcurrent or overload protection being called on before the run capacitors get to the point of failure.
 
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