# Single Phase Start/Run Capacitors



## Mobius87 (May 20, 2019)

A farmer client of mine brought me a motor for a grain auger that he bought at an auction, supposedly new-in-box. When he tried it out at home, it would spin with no load but would trip a 2P 50A breaker after a few seconds. The motor is a WEG 1 Phase 230V 7.5hp.

Everything looked new and in good condition, so I stuck it on the bench and sure enough it spun but sounded awful, and was drawing almost 200A. I figured it was a bad start capacitor or a stuck centrifugal switch, so I told him to leave it with me.

It has two 330uF 250V start capacitors in parallel which tested good (330uf each, 660uF together), and a 60uF 440v run capacitor which also tested good. 

I pulled the start capacitors out and used the 60uF run capacitor as the start capacitor for a test. The motor starts and runs normally, and I can confirm the centrifugal switch opens (both audibly and via continuity test). It doesn't have much starting torque in this configuration obviously.

When I put the start capacitors into the system, with or without the run capacitor, the motor spins up but I haven't been able to confirm if the switch did or didn't open, as I ended up overheating and popping the start capacitors on my last test before my meter could respond. I suspect that it didn't open though. At that point I called it a day to have a thought about what was happening, as motors are not my strong suit.

Does this sound like a case of the switch being sticky and not opening soon enough? Are the start capacitors sized appropriately for this motor?


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## dspiffy (Nov 25, 2013)

I generally work with motors smaller than this, but from what I can tell, those capacitor values sound correct. 200a sounds high even for brief starting current. 

Capacitors can test good and still be bad. Capacitors can fail from being energized after years of sitting. Centrifugal switches also get sticky from sitting.

Without the start capacitors connected, it should still run if started by hand. Does it, and does it draw the correct amount of current?


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## micromind (Aug 11, 2007)

Connect a basic switch in series with the start caps. After it has spun up a bit, open the cap circuit. 

With a capacitor start/capacitor run motor, usually the run caps are always in the circuit and the start caps are in parallel with them until the start switch disconnects them. That's what you're doing with the manual switch, disconnecting the start caps after it starts. 

Since this is a new motor, it could be misconnected at the factory........rare but it does happen. 

Also, are the start caps actually connected as start caps or are they connected as the run caps?


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## paulengr (Oct 8, 2017)

The start and run caps are different types. The start caps are electrolytic types. Very high rating (uF) for the size but only able to be energized for a few seconds and as mentioned if they sit for too long they go bad. Run caps are oil filled and designed for continuous load but not the high starting voltage that probably destroyed it. You would have been better to energize without the start caps and flick the shaft with your hand to start it. When you replace a vastly more reliable method than an inertia switch is a potential relay and the price is the same. Caps for motors all run about $20 each.

The run cap is there for power factor. It helps make the motor pull less current and boosts torque. It’s not necessary to run.

The starting caps are for starting. A single phase motor does not have a rotating field. It’s more like a piston…once the motor starts turning it will “pump” the rotor and keep it going but it can’t start from a dead stop. There are additional coils though. They need a starting voltage that leads the normal voltage by about 35 degrees. The start caps phase shift the voltage to create the 35 electrical degrees offset and cause the motor to start spinning as a “two phase” motor. Once it gets started they are no longer needed and the whole starting system is disconnected. A centrifugal switch is one way. You can even use just a timer. A potential relay measures the voltage drop across the motor that changes as the motor builds up a counter EMF. Once it reaches a threshold the potential relay shuts off the starting circuit. This has the advantage of being just as simple but with almost no moving parts and it can be hermetically sealed.

Other approaches are possible. Shaded pole motors just have a second pole that uses inductance to achieve the two pole starting. Permanent split pole capacitor motors just leave the starting circuit powered up. Neither of these types are very efficient but they are good for say a cheap box fan motor where efficiency doesn’t matter.

For the record I work on motors all the time. Some are thousands of horsepower. And I despise single phase motors. I’ve gotten to the point where it costs roughly $60-80 for every part that can go bad and takes 20 minutes to change. It’s faster to replace everything than to waste time troubleshooting them,


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## dspiffy (Nov 25, 2013)

micromind said:


> Connect a basic switch in series with the start caps. After it has spun up a bit, open the cap circuit.


This is such a smart idea I dont know why I never thought of it in 15 years of working on motors.

There are even motors from ~100 years ago with manual start switches that do something similar.


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## gpop (May 14, 2018)

I worked on a 10hp single phase last week I think that was a weg.
It has 6 starting caps that were controlled by a 40 amp starter (looked like a air con starter except it had a 240v coil). Problem was burnt tips on the starter causing high resistance. Looked like run power closed the starter then the centrifugal switch opened the starter once up to speed. It was tripping the breaker and sounded horrible trying to start. 
I was going to change caps and starter but vendor did not have the caps so i replaced the starter and its working fine now. It was on a hydraulic trash compactor that cycles probably 20 times a day so it wasn't a surprises to find burnt contact tips on the starter. It was a surprise to find the starter in the pecker head as i have never seen that design before (then again i have never seen a 10hp single phase before)

Maybe your problem is a dirty switch that is not allowing the motor to get up to speed.


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## Mobius87 (May 20, 2019)

dspiffy said:


> I generally work with motors smaller than this, but from what I can tell, those capacitor values sound correct. 200a sounds high even for brief starting current.
> 
> Capacitors can test good and still be bad. Capacitors can fail from being energized after years of sitting. Centrifugal switches also get sticky from sitting.
> 
> Without the start capacitors connected, it should still run if started by hand. Does it, and does it draw the correct amount of current?


It started and ran normally when I used the 60uF run cap to start it. IIRC the no-load current was 17A. I was not aware the caps could fail from storage and that they could still test good. I wonder if this is the culprit as I imagine this motor, while new, was likely stored for a long while before it was sold at auction.

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## Mobius87 (May 20, 2019)

micromind said:


> Connect a basic switch in series with the start caps. After it has spun up a bit, open the cap circuit.
> 
> With a capacitor start/capacitor run motor, usually the run caps are always in the circuit and the start caps are in parallel with them until the start switch disconnects them. That's what you're doing with the manual switch, disconnecting the start caps after it starts.
> 
> ...


This is a good idea. I have to get new start caps anyway since I popped the ones that were in it. If they don't work right off of the bat, I'll give this a try.

And yes, it appears they were wired properly from the factory.

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## Mobius87 (May 20, 2019)

paulengr said:


> The start and run caps are different types. The start caps are electrolytic types. Very high rating (uF) for the size but only able to be energized for a few seconds and as mentioned if they sit for too long they go bad. Run caps are oil filled and designed for continuous load but not the high starting voltage that probably destroyed it. You would have been better to energize without the start caps and flick the shaft with your hand to start it. When you replace a vastly more reliable method than an inertia switch is a potential relay and the price is the same. Caps for motors all run about $20 each.


Thanks for the info, I appreciate it. Just for clarification, the run cap is still fine. The start caps I popped likely from overheating during my tests, from being connected to the circuit for too long.


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## Mobius87 (May 20, 2019)

gpop said:


> I worked on a 10hp single phase last week I think that was a weg.
> It has 6 starting caps that were controlled by a 40 amp starter (looked like a air con starter except it had a 240v coil). Problem was burnt tips on the starter causing high resistance. Looked like run power closed the starter then the centrifugal switch opened the starter once up to speed. It was tripping the breaker and sounded horrible trying to start.
> I was going to change caps and starter but vendor did not have the caps so i replaced the starter and its working fine now. It was on a hydraulic trash compactor that cycles probably 20 times a day so it wasn't a surprises to find burnt contact tips on the starter. It was a surprise to find the starter in the pecker head as i have never seen that design before (then again i have never seen a 10hp single phase before)
> 
> Maybe your problem is a dirty switch that is not allowing the motor to get up to speed.


5, 7.5, and 10hp single phase motors are as common as dirt around here, used for grain augers and aeration fans. Usually connected to an 80A service with #4 triplex or #2 USEB feeding one or more running fans, two houses, and a shop.


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## John Valdes (May 17, 2007)

Whats the nameplate current? Sure you have the correct breaker? I don't see how this would cause your issue, but since you did not list this value its a great place to start.
BTW. Time is money and caps are cheap. Replace both and make sure the breaker is right.


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## Mobius87 (May 20, 2019)

John Valdes said:


> Whats the nameplate current? Sure you have the correct breaker? I don't see how this would cause your issue, but since you did not list this value its a great place to start.
> BTW. Time is money and caps are cheap. Replace both and make sure the breaker is right.


Nameplate is 31.3 FLA Was plugged into a 50A breaker. Draws 196A for 5ish seconds before the breaker trips, way longer than it should take to start with no load.

Using the 60uF cap to test (as I have no other caps to try at the moment), it starts almost immediately and settles in around 17A no load.

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## paulengr (Oct 8, 2017)

Mobius87 said:


> Nameplate is 31.3 FLA Was plugged into a 50A breaker. Draws 196A for 5ish seconds before the breaker trips, way longer than it should take to start with no load.
> 
> Using the 60uF cap to test (as I have no other caps to try at the moment), it starts almost immediately and settles in around 17A no load.
> 
> Sent from my SM-G991W using Tapatalk


Breaker should be 250% of FLA. That’s around 75 A. You are hitting 6.26x FLA, typical start current. So right off we know the breaker is undersized and will likely trip on startup. What is overload relay set to?


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## Mobius87 (May 20, 2019)

paulengr said:


> Breaker should be 250% of FLA. That’s around 75 A. You are hitting 6.26x FLA, typical start current. So right off we know the breaker is undersized and will likely trip on startup. What is overload relay set to?


I understand that, if the motor were under load requiring full starting torque. This is sitting on the bench, plugged into a 50A welder outlet, rigged up to a 100A switch.

No load on the motor, no overload relay, not the final installation. Should easily start this motor as is.


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## emtnut (Mar 1, 2015)

Mobius87 said:


> I understand that, if the motor were under load requiring full starting torque. This is sitting on the bench, plugged into a 50A welder outlet, rigged up to a 100A switch.
> 
> No load on the motor, no overload relay, not the final installation. Should easily start this motor as is.
> 
> ...





Mobius87 said:


> Nameplate is 31.3 FLA Was plugged into a 50A breaker. Draws 196A for 5ish seconds before the breaker trips, way longer than it should take to start with no load.
> 
> Using the 60uF cap to test (as I have no other caps to try at the moment), it starts almost immediately and settles in around 17A no load.
> 
> Sent from my SM-G991W using Tapatalk


I agree it should start no problem on the 50A breaker with no load. It should be on an 80A breaker when installed (I know you know that  )

Don't know why those start caps are bad, but I'd think they are. If the centrifugal switch is working, I don't know what killed them. Just replace them, and I think you'll be good to go.


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## micromind (Aug 11, 2007)

From what I've seen, there are basically 2 things that will blow a start cap;

1) Drying out with age. This can take several years and often instead of blowing up, you'll see reduced starting torque.

2) Staying in the circuit too long. This can be because the load is too much for the motor to accelerate and it stays in start or speeding up to where the start winding is disengaged then slowing back down and re-engaging. This often happens when the ambient is very cold and/or a 230 volt motor is supplied with 208. 

2A) Since this is a new motor, it's possible that the start and run caps are swapped. If so, the motor would certainly draw a bunch of current and eventually blow the start caps. I've never seen it but it can happen.


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## paulengr (Oct 8, 2017)

Mobius87 said:


> I understand that, if the motor were under load requiring full starting torque. This is sitting on the bench, plugged into a 50A welder outlet, rigged up to a 100A switch.
> 
> No load on the motor, no overload relay, not the final installation. Should easily start this motor as is.
> 
> ...


That’s not how it works. The motor starts at stall and produces full torque at around 5-10x FLA. The current is inherent to the motor. All the load does is change how long it takes to get up to speed where the current drops off. Once at speed above the peak torque point, then you get the usual relationship where current and torque go hand in hand.

Many electricians miss this subtle point. With some kind of soft starter or VFD you can alter this relationship by altering the torque produced by the motor but normally you can’t.

Where load torque matters is that if you stay irises at high current too long the motor protection trips, or the motor burns up. With capacitor start/run the starting capacitor that is only rated for operation for a few seconds boils the electrolyte and blows out the pressure relief port if it has one or explodes if it doesn’t.


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## Mobius87 (May 20, 2019)

paulengr said:


> That’s not how it works. The motor starts at stall and produces full torque at around 5-10x FLA. The current is inherent to the motor. All the load does is change how long it takes to get up to speed where the current drops off. Once at speed above the peak torque point, then you get the usual relationship where current and torque go hand in hand.
> 
> Many electricians miss this subtle point. With some kind of soft starter or VFD you can alter this relationship by altering the torque produced by the motor but normally you can’t.
> 
> Where load torque matters is that if you stay irises at high current too long the motor protection trips, or the motor burns up. With capacitor start/run the starting capacitor that is only rated for operation for a few seconds boils the electrolyte and blows out the pressure relief port if it has one or explodes if it doesn’t.


I understand what you're saying and I don't disagree. What I'm saying is that an 80A breaker is not going to change the outcome, this motor should start fine on the 50A circuit as configured. Just because we're allowed a maximum of 250% breaker size, doesn't mean it won't start on less. With no load on the motor, the stall/start current should drop off fast enough as to not exceed the trip curve of the breaker. I'm trying not to confuse the issue by delving into irrelevant rabbit holes.

Test #1:

Started the motor without changing anything. The motor spins up but makes a lot of noise and I don't know if the switch opened. It settles in at 196A and stayed there until I shut it down after a few seconds (5 Mississippis). Its possible the motor is not spinning fast enough to open the switch as it did before. It did not trip the breaker.

Test #2:

Removing the START capacitors and using the RUN capacitor as the START capacitor for this test, I wanted to see if the motor would get up to speed and open the switch. It does, quickly and quietly settling in at 17A. Switch is open, disconnecting the capacitor from the circuit.

Test #3:

Reconnecting the START capacitors and leaving the run capacitor in the circuit, confirming that they are wired properly. Repeat of results from Test #1.

Test #4: 

Using the START capacitors and leaving the run capacitor out of the circuit, the motor does the same thing as test #3 and #1.

Test #5:

Repeat of test #2, same result. Motor starts fine.

Test #6:

Repeat of test #4, except this time the capacitors popped from overheating, which is my fault for not allowing them to cool down enough between tests.

I was confused and frustrated at this point, so I called it a day. I did not know at the time that the start capacitors could show capacitance but still be bad? How does one determine if the capacitor is good or not if this is the case?


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## 210860 (Apr 12, 2021)

*This is straight out of one of my textbooks.. (came back to add, I'm not the author. )
When checking capacitors, they must be removed from the circuit and have their capacitance measured. Such measurements will detect either open or shorted capacitors.

For best results, measure the resistance of a capacitor w/ an ohmmeter; if the capacitor is shorted, the meter will read less than 10 ohms. however, if the capacitor is good, the resistance reading will be about 50 ohms or greater..

Maybe others can expand further in assistance.

*have a safe day..


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## micromind (Aug 11, 2007)

Try swapping the start caps with the run caps.


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## dspiffy (Nov 25, 2013)

Mobius87 said:


> I was confused and frustrated at this point, so I called it a day. I did not know at the time that the start capacitors could show capacitance but still be bad? How does one determine if the capacitor is good or not if this is the case?


I've had numerous cases where a capacitor will test good and still be bad. Only way I know of to confirm is to replace it with a brand new/known good capacitor of the same value. Capacitors are cheap, I work with a lot of motors so I keep a ton on hand.


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## Mobius87 (May 20, 2019)

Well, new caps got put in and it works like a charm. Thanks for the assistance.

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## micromind (Aug 11, 2007)

dspiffy said:


> I've had numerous cases where a capacitor will test good and still be bad. Only way I know of to confirm is to replace it with a brand new/known good capacitor of the same value. Capacitors are cheap, I work with a lot of motors so I keep a ton on hand.


Same here, a basic cap tester doesn't apply enough voltage to blow them up if they've been sitting discharged for too long.


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## paulengr (Oct 8, 2017)

Dell3c said:


> *This is straight out of one of my textbooks.. (came back to add, I'm not the author. )
> When checking capacitors, they must be removed from the circuit and have their capacitance measured. Such measurements will detect either open or shorted capacitors.
> 
> For best results, measure the resistance of a capacitor w/ an ohmmeter; if the capacitor is shorted, the meter will read less than 10 ohms. however, if the capacitor is good, the resistance reading will be about 50 ohms or greater..
> ...


Two issues with this. First it only reads “shorted” if it’s big enough. It normally does this then eventually reads a couple Megaohms. How fast it goes to open circuit depends on how big it is. In school they made us measure this and do some kind of calculation to determine capacitance. The best way is just read capacitance with a meter that does this. Start caps are lousy accuracy…+/-20%. As they dry out the capacitance drops so you can quickly pick up on marginal caps.

Second issue is voltage. Need a good Megger. As capacitors dry out the voltage can creep along the edges of the foil and arc across but you will never see this at low voltage. If you set the Megger to just above the rated voltage and it drops to dead shorted then stars there or bounces around erratically, it has failed. If it rises back up it’s good. Warning: make sure to carefully discharge after you do this. The sting at full voltage and charge is painful for quite a while.


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## CMP (Oct 30, 2019)

Then there is the no meters method that I saw an uncle do as a teenager. Energize them at rated line voltage, *for a second* with a chord set with some clips on the end. Unplug the chord and wait a minute. Then short the terminals with the ugly screwdriver. If you got a nice blue pop, they were considered good. If they didn't arc pop or blew their guts outs, they needed new ones. /S


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## Galt (Sep 11, 2013)

If the voltage is at zero when you pull the cord out the capacitor will not pop.


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## dspiffy (Nov 25, 2013)

CMP said:


> Then there is the no meters method that I saw an uncle do as a teenager. Energize them at rated line voltage, *for a second* with a chord set with some clips on the end. Unplug the chord and wait a minute. Then short the terminals with the ugly screwdriver. If you got a nice blue pop, they were considered good. If they didn't arc pop or blew their guts outs, they needed new ones. /S


My dad's stepdad was a Navy electronics instructor (WWII) and an old ham radio guy. Him and his friends used to charge up the largest capacitor they could find, then toss it to one of them when they werent expecting it and yell "catch!!!"


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## micromind (Aug 11, 2007)

paulengr said:


> Warning: make sure to carefully discharge after you do this. The sting at full voltage and charge is painful for quite a while.


Hopefully you didn't find this out the same way I did.............

Remember in electronics class when we would charge up a cap and toss it to someone......POW!!!

Lol. 

These days though, a lot of motor caps have discharge resistors across the terminals. Not all, just a lot of them.


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## micromind (Aug 11, 2007)

dspiffy said:


> My dad's stepdad was a Navy electronics instructor (WWII) and an old ham radio guy. Him and his friends used to charge up the largest capacitor they could find, then toss it to one of them when they werent expecting it and yell "catch!!!"


Rats! You beat me to it by less than a minute.......lol.


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