Motor capacitor sizing

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S'mise

S'mise

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Michigan
I come across someone who has single phase 7.5hp Baldor motor driving an air compressor.

It will run a few seconds and then the breaker trips.

It has 5 capacitors wired in series.
I've seen 2 start caps in series with a run cap (totaling 3 caps) before, but I have never seen 5 caps in series.

I don't trust the wiring at this point and was hoping someone could point me to what values I should have, and perhaps a diagram.

Questions:
Why is it even necessary to put caps in series as opposed to just putting in one cap with a lesser value.
Is this tripping problem indicative of a run cap problem or perhaps the centrifugal switch not releasing?

Thanks
 
S'mise said:
I come across someone who has single phase 7.5hp Baldor motor driving an air compressor.

It will run a few seconds and then the breaker trips.

It has 5 capacitors wired in series.
I've seen 2 start caps in series with a run cap (totaling 3 caps) before, but I have never seen 5 caps in series.

I don't trust the wiring at this point and was hoping someone could point me to what values I should have, and perhaps a diagram.

Questions:
Why is it even necessary to put caps in series as opposed to just putting in one cap with a lesser value.
Is this tripping problem indicative of a run cap problem or perhaps the centrifugal switch not releasing?

Thanks
Click to expand...
Does the motor sound like it comes up to speed?
Have you put an amp clamp on it?
Did it ever work?

Cap problems are usually obvious. Blown, leaking or bulged indicate issues. Check them.
 
I don't know first hand (Fellow sparky at another location asking).
Sound like it's coming up to speed. "Runs for about 4 to 5 seconds"
Never measured with amp meter but it trips 40a breaker.
No leaking or budging caps.

Since it seems to start, I assume there's a problem with the run cap or cent switch.

Just strange to see 5 capacitors.

Most meters can't read higher uf range capacitors so I told him to bring them to me for testing. He said they were marked 216-259uf.
 
Thanks, I was hoping there was a Baldor link that would tell me the capacitance requirements.

For the matter why not list it on the name plate like some manufacturers do?
 
S'mise said:
I don't know first hand (Fellow sparky at another location asking).
Sound like it's coming up to speed. "Runs for about 4 to 5 seconds"
Never measured with amp meter but it trips 40a breaker.
No leaking or budging caps.

Since it seems to start, I assume there's a problem with the run cap or cent switch.

Just strange to see 5 capacitors.

Most meters can't read higher uf range capacitors so I told him to bring them to me for testing. He said they were marked 216-259uf.
Click to expand...
S'mise said:
Thanks, I was hoping there was a Baldor link that would tell me the capacitance requirements.

For the matter why not list it on the name plate like some manufacturers do?
Click to expand...
S'mise said:
Thanks, I was hoping there was a Baldor link that would tell me the capacitance requirements.

For the matter why not list it on the name plate like some manufacturers do?
Click to expand...
S'mise said:
Thanks, I was hoping there was a Baldor link that would tell me the capacitance requirements.

For the matter why not list it on the name plate like some manufacturers do?
Click to expand...
With the motor model number you might be able to search for the capacitor box kit, and get the capacitor requirements from that. Here is a link to the kit that goes with one of Baldor’s 7.5hp single phase general purpose motors. https://factoryaircompressorparts.c...-parts-36cb5005a03-36cb3800-oc3040f09-l1410t/
 
zbang said:
That'll be to get a higher voltage rating for the entire stack*- prob a 480v motor and the caps aren't rated for that (not factory original, I'd guess; maybe?).

*the voltage rating is additive but the capacitance is a reciprocal function like resistors in parallel.
Click to expand...
The only 480V single phase motors I've ever seen are inside of air handlers on A/C equipment.
 
zbang said:
That'll be to get a higher voltage rating for the entire stack*- prob a 480v motor and the caps aren't rated for that (not factory original, I'd guess; maybe?).

*the voltage rating is additive but the capacitance is a reciprocal function like resistors in parallel.
Click to expand...
Are you absolutely positive that someone hasn't mis-wired it while changing out a failed capacitor ?? i've never seen series'd caps in a motor....parallel lots of times, but series, never.

For capacitors in series, equalizing resistors (typically 100,000 ohms at a wattage dependent on the voltage involved) are required across each capacitor in the string to equalize the voltage across them, or else the capacitor with the lowest leakage current will have the highest voltage impressed upon it and be the first to let out the Magic Smoke (lowest leakage current implies highest internal resistance (OK, for the purists, this is actually REACTANCE since there is some inductance involved in the leads)

Here's an article that explains it

Balance Resistors for Series Capacitors | VA1DER

va1der.ca va1der.ca

Hope this helps, I've dealt with caps in series in High Voltage ( 6-10 KV ) DC supplies in broadcast transmitters.
 
OldBroadcastTech said:
For capacitors in series, equalizing resistors (typically 100,000 ohms at a wattage dependent on the voltage involved) are required across each capacitor in the string to equalize the voltage across them, or else the capacitor with the lowest leakage current will have the highest voltage impressed upon it and be the first to let out the Magic Smoke (lowest leakage current implies highest internal resistance (OK, for the purists, this is actually REACTANCE since there is some inductance involved in the leads)
Click to expand...

The need for equalizing resistors is for distributing a DC voltage that is applied across a set of series capacitors.

When driven with AC, the voltage distribution across series capacitors will be determined by their individual impedances at the AC frequency. In a motor starting electrolytic capacitor the equivalent shunt resistance for the leakage will be quite large compared its capacitive reactance at the AC frequency. Therefore how evenly an applied AC voltage is distributed across series capacitors will be dominated by how well their capacitances match each other, and not their individual leakages (unless, of course, the leakage is severe because the capacitor is defective).

Non-polarized electrolytic capacitors such as those used in motor starting are really two polarized electrolytic capacitors that are placed in series, but with reversed polarities (cathode-to-cathode). The capacitor is constructed such that a common cathode is physically shared between the two anodes.
 
OldBroadcastTech said:
Are you absolutely positive that someone hasn't mis-wired it while changing out a failed capacitor ?? For capacitors in series, equalizing resistors (typically 100,000 ohms at a wattage dependent on the voltage involved) are required across each capacitor
Click to expand...
No, that was my hunch from start.

The oem motor was a Weg motor that had two 340-408uf start caps in series (yes, with bleeder resistors) which would mean 170-204uf to start.
Perhaps the installer of new motor was trying to copy the series wiring?

Installed just one 216-259uf start cap (as specified by baldor) and replaced a warn centrifugal switch and it's running fine.

This ends the 5 cap mystery.

Thanks to all for your help.
It confirms that one shouldn't trust the guy that comes before you.

PS: I still think capacitance should be marked on the nameplate.
 
ActionDave said:
The only 480V single phase motors I've ever seen are inside of air handlers on A/C equipment.
Click to expand...
You don't work on much AG equipment then. Starting to see more 240/480 volt single phase motors in recent years than ever seen before. Particularly above 7.5 hp and in for specific equipment in odd ratings like 12 and 16 HP.
 
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