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Discussion Starter #1
Hi
I solved my motor problem, and I posted the updated fix, but I'm very curious as to why it now works.
Please only respond if you actually know the answer.

I had a 3hp 220v single phase motor that would only try, but not start when plugged into a 20 amp outlet controlled by a 220v 20amp breaker. When I just replaced the breaker with a 30amp the motor started and ran fine.
Just another note, I had had the same thing happen with a 3hp table saw a while ago, same solution.
In both cases the breakers were not tripping, but the motors would only try to start.
Thanks in advance for your answers.
 

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Discussion Starter #3
doubtful
they worked the same compressor before the motor went under water.
the breaker was off at the time.
It was an old craftsman 2hp.
I even ran the old motor after it dried out, but the bearings were shot.
 

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What size wire is run to these (like 12 gauge?) and how long is the wire run (like 25 ft.)?

I'm thinking there may be a "voltage drop" problem?

Or perhaps a poor connection from the breaker to the panel bus bar and replacing with another 20 amp breaker might have done the same thing?
 

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Hi
I solved my motor problem, and I posted the updated fix, but I'm very curious as to why it now works.
Please only respond if you actually know the answer.

I had a 3hp 220v single phase motor that would only try, but not start when plugged into a 20 amp outlet controlled by a 220v 20amp breaker. When I just replaced the breaker with a 30amp the motor started and ran fine.
Just another note, I had had the same thing happen with a 3hp table saw a while ago, same solution.
In both cases the breakers were not tripping, but the motors would only try to start.
Thanks in advance for your answers.
Please tell me you didn't just upsize the breaker and leave the 12awg wire? Should be 10.
 

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Discussion Starter #7
The breaker swap was only a test.
I will up size the wire to 10 gauge.
the run is less then 4 feet.

Thanks for all the answers, but none of them explain why.

I guess I'll check with some electric motor shops.
 

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Keep the #12. See RJs post above regarding breaker sizing for motors.
 

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Lic Electrical Inspector
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According to the NEC the motor draws 17A @ 230V. #12 NMB is rated for 20A according to the 2011 NEC. The conductors have to be sized @ 125% of the motor FLA which is 21.25A. The BC conductors will have to be changed to #10 NMB. The breaker can be as high as 45A, but if a 30A is holding, leave it. Also check the installation instructions from the manufacturer for what they require. Keep in mind that they will state that it has to meet NEC requirements. Also, if the attachment plug is being used to start the motor, it has to be horsepower rated.

Motors are misunderstood by electricians, contractors and inspectors. Several of our amature posterz who frequent this forum feel that I am mixing commercial thoughts with residential thoughts (whatever that means), but my fellow professional electricians who understand motors will agree with what is stated above.

ALso follow the motor links in my signature for additional information.
 

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Electrical Contractor
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Hi
I solved my motor problem, and I posted the updated fix, but I'm very curious as to why it now works.
Please only respond if you actually know the answer.

I had a 3hp 220v single phase motor that would only try, but not start when plugged into a 20 amp outlet controlled by a 220v 20amp breaker. When I just replaced the breaker with a 30amp the motor started and ran fine.
Just another note, I had had the same thing happen with a 3hp table saw a while ago, same solution.
In both cases the breakers were not tripping, but the motors would only try to start.
Thanks in advance for your answers.
doubtful
they worked the same compressor before the motor went under water.
the breaker was off at the time.
It was an old craftsman 2hp.
I even ran the old motor after it dried out, but the bearings were shot.
Are we dealing with a 2HP or a 3HP motor here?
 

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Was it the same 20 amp breaker that got moved about the panel and that was coincidentally in use during both of your tests?

Perhaps just that breaker was defective, imposing enough resistance to prevent the machine (the load) from operating but not tripping. (The breaker would overheat when it absorbed that much power, if the machine's motor didn't have a safety cutoff that overheated and opened first.)

When you have a chance, flip off the breaker you are now using, unclip it, and inspect the fin (stab) underneath. A loose fit of the breaker over the fin is a common cause of panel damage and can cause the problem you described. Does the 30 amp breaker that "fixed the problem" fit more tightly? If the fin is discolored, it should be sanded clean (with the main breaker off). If the fin is deformed, you should not use that breaker slot any more.

It is not good to use a 30 amp breaker on a circuit with wire smaller than 10 gauge. They do make "slow trip" breakers for use with machines or tools that may have a longer than average startup time with a high current draw but where the normal draw is within the limit for the wire.
 

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Discussion Starter #13
Allan
As I said before, I will be up sizing the wire to 10 gauge before I put the compressor on line.
I checked the stabs for oxidation, none found.
The 30 amp breaker, is my table saw circuit. this is the same circuit I used to test the new motor which would not start on the 20 amp compressor circuit, but started when plugged into my table saw circuit.

I then took out the 20 amp breaker and moved the 30 to it's location and the motor ran fine on the compressor circuit.
I hope this clarified the situation.

As I also said before, when I set up the table saw, a while ago, which also has a 3hp motor, it too would not start with a new 20 amp breaker. Tech services at Powermatic told me to up the breaker to 30amps, which solved that problem. Wire size was up sized on this circuit after it all test ok.
 

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Discussion Starter #14
I will be picking up both a new 20 amp and 30 amp, just to see if it's the old 20 amp breaker or not.
I am still curious.
 

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Discussion Starter #15
Today I installed brand new square D 20 amp 220 breaker, holding the 30 amp in reserve.
Plugged in the compressor, turned on the breaker, AND-----------------


The motor started and ran fine. Shut it off and restarted numerous times.
I guess I had a marginally defective breaker.
The 30 amp unit is headed back to Home Depot.
 

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Electrical Contractor
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Hi
I solved my motor problem, and I posted the updated fix, but I'm very curious as to why it now works.
Please only respond if you actually know the answer.

.....
Today I installed brand new square D 20 amp 220 breaker, holding the 30 amp in reserve.
Plugged in the compressor, turned on the breaker, AND-----------------


The motor started and ran fine. Shut it off and restarted numerous times.
I guess I had a marginally defective breaker.
The 30 amp unit is headed back to Home Depot.
Hmmm ... I remember saying something to that affect ....

Bad breakers.
Do I win a prize?? :whistling2:
 

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Discussion Starter #17
You were right!

I still would like to know why the old breaker worked with the old motor.
Is it possible it was tripping internally, but not tripping the lever?? I never had to reset it, and voltage readings were ok.
 

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You were right!

I still would like to know why the old breaker worked with the old motor.
Is it possible it was tripping internally, but not tripping the lever?? I never had to reset it, and voltage readings were ok.
We're you checking voltage while trying to start the motor? I bet the voltage was sagging severely
 

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UAW SKILLED TRADES
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Hmmm ... I remember saying something to that affect ....

Do I win a prize??
:)

How about a 1lb bag of gummy bears? I'd send twinkies but alas they are no more ..:(
 

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I still would like to know why the old breaker worked with the old motor.
If there is a poor electrical connection, you get "voltage drop". The voltage needs to be measured at the device and with the device powered on.

This is like a cruddy battery connection on a car battery. You clean the connection, then it works.

And there would be 3 connections on a breaker. Where it connects to the load center bus, the contacts inside the breaker, and where the wire connects to the breaker.

Note aluminum will "oxidize". It is possible something is aluminum and was oxidizing. If that was the breaker to the load center bus connection, just removing the old breaker from the load center, then reinstalling probably would have made it work again for a while.

They sell "anti-oxidation goop" for aluminum wiring in electrical departments in stores.

FYI - You can also do a "voltage drop test" to find these problems.
 
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