motor circuit

[sportzcoach]

I have a 3 HP motor that is 12.6 amps. I put my clamp meter on the wires at the breaker and at start up it draws 71 amps is that normal? It trips the breaker in about 3 seconds. I don't have much experience with motor circuits. What can I do to correct this?

 

[LarryFine]

Six times running current for starting is normal.

What is the voltage, and what size breaker and wire?

 

[jim dungar]

Check your breaker size.
Standard motor overload relays allow 6X FLA for 20sec. Most breakers have similar time current curves.

 

[SSDriver]

If it stays at that amperage for 3 seconds you have an issue. If your reading it on a regular meter not set for inrush, you have a problem. Sounds like your motor shaft is frozen. Can you spin it freely?

 

[retirede]

What is the application?

Is this a new installation, or is it something that used to work?

 

[texie]

I have a 3 HP motor that is 12.6 amps. I put my clamp meter on the wires at the breaker and at start up it draws 71 amps is that normal? It trips the breaker in about 3 seconds. I don't have much experience with motor circuits. What can I do to correct this?

It seems to me that more details are needed. Have you applied the requirements of Art. 430 to get an understanding of the correct OCPD size?

 

[sportzcoach]

The panel is an old FPE, Breaker is two pole 20 wire is #12 I actually didn't do the hook up another electrician did and I'm just trouble shooting it for the owner. It used to be a 480 volt 3 phase compressor that they changed out to a 240 volt single phase motor.

 

[don_resqcapt19]

What was the horsepower of the original motor?

 

[ActionDave]

You could use a bigger breaker but there's more details that needed.

Why was the motor changed?
What happens if you uncouple the motor from the load?
Does the motor come up to full speed and still pull 70A?

 

[texie]

Since the OP indicates this is a compressor I would want to be sure that the unloader is working. And is the HP of the motor correct for the compressor?

 

[retirede]

The panel is an old FPE, Breaker is two pole 20 wire is #12 I actually didn't do the hook up another electrician did and I'm just trouble shooting it for the owner. It used to be a 480 volt 3 phase compressor that they changed out to a 240 volt single phase motor.

I suspect that the required starting torque of the compressor is too high for a single phase motor.

What type of compressor and what type of unloading mechanism does it have?

 

[ptonsparky]

I suspect that the required starting torque of the compressor is too high for a single phase motor.

What type of compressor and what type of unloading mechanism does it have?

I wouldn't think a 3ph vs 1ph torque would be different enough, if any.
Totally wrong substitution of motor HP would not surprise me.
Compressor stuck, or unloader valve otherwise.
Always the possibility for someone to have totally messed up the wiring.

 

[ActionDave]

Rule #1 when someone calls because there's a problem with a motor-
It's usually not the motor.

 

[Jraef]

... It used to be a 480 volt 3 phase compressor that they changed out to a 240 volt single phase motor.

Bingo, we have a winner!
3HP 230V single phase motor is 17A in the NEC chart, so you would use a breaker of around 40A (250% of the motor FLC from the NEC chart). A 20A 2 pole breaker was probably OK when it was a 480V 3 phase motor, but it is way too small now.

You need to check your conductor sizes too. If they still have 12ga wire, that's possibly a no-go as well. 17A x 1.25 = 21.25A, it might need #10 conductors if there is no motor OL relay or the motor is not internally protected, you might even need #8 if the only thing on that circuit is the 40A breaker.

Side issue: Torque is torque, 3 phase or single phase. HP = Tq x RPM / 5250, it doesn't matter if the HP is derived from 1 phase or 3 phase. So if the old 460V 3 phase motor was also 3HP, the torque from the single phase 240V motor would be the same (assuming also that the RPMs are the same).

 

[LarryFine]

A 20A 2 pole breaker was probably OK when it was a 480V 3 phase motor . . .

 

[Jraef]

Yeah, OK, didn't think that through...

But we don't know what was there. Being 480V it might have had its own local combo starter with an OL relay so the 20A was just the feeder protection for #12 wires.

 

[LarryFine]

Yeah, OK, didn't think that through...

But we don't know what was there. Being 480V it might have had its own local combo starter with an OL relay so the 20A was just the feeder protection for #12 wires.

I was actually reacting to the number of poles, not the rating.

 

[4x4dually]

I was actually reacting to the number of poles, not the rating.

Just connect the extra wire to the bus bar. It'll be fine.

 

[augie47]

(a) I would double check that nameplate. 12.6 amps (if that # came from the nameplate) is low for a 3 HP
(b) assuming it is 3HP, the Code would allow a 40 amp (or 45) to allow for starting current. You need to increase that breaker size.
(c) Unrelated, but I would want to assure the motor had overload protection either internal or external.

 

[hillbilly1]

Amazing the FPE breaker tripped before a fire! Pretty tough to trip an FPE! LOL!