Help with a electrical problem

[Sawtooth]

I have a friend that has a 3phase compressor in his shop and when he turns it on you can hear the wires vibrate inside the conduit for 1 to 1 1/2 seconds, almost sounds like a short but he says it has done this for 15 plus years. Here is what I know, the breaker is 3 pole 50 amp, the wire going from said breaker to the compressor (about 75 feet) is #10, (for the amp it should be #6 copper). The motor kicks on and pegs my amp meter at 60 amps and then runs at 28 amps on each leg. So any idea what is causing the vibration sound in the conduit. It is a straight shot with no junction box's and I checked that all the lugs were tight/torqued. Is this being caused becaused the feed wire is under sized :headscratch:, thanks.

 

[rrrr]

The amount of current is making the wire "dance" in the conduit. The #10 wire is undersized. If you change the wire to #6 as you indicated, it will be fine.

Using a 50 amp breaker on #10 wire is also a fire hazard and a NEC code violation.

 

[Tom Brown]

Using a 50 amp breaker on #10 wire is also a fire hazard and a NEC code violation.

Also, be sure to use resistant duct tape to hold those throws in the "on" position.

 

[McRib]

i would check the flux capacitor. it could not be capacitating properly.






i hate anything electric:grumble:

 

[90 Laveycraft]

It could be turning backwards.....three phase motors are directional - change the phase, you change direction.

The compressor will have direction indicators.....CHECK IT.

 

[GRADS2009]

220....221....Whatever it takes.

 

[cola]

It could be turning backwards.....three phase motors are directional - change the phase, you change direction.

The compressor will have direction indicators.....CHECK IT.

It's been making air for 15 years, it's not running backwards. The #10 is undersized for the 50 amp breaker.
You would need the nameplate rating to tell if the wire is sub-par.
The start current could be 10+ times the FLA easy.
I would install #6 THHN and drop the sup-breaker to 125% of FLA.
Mike

 

[90 Laveycraft]

It's been making air for 15 years, it's not running backwards. The #10 is undersized for the 50 amp breaker.
You would need the nameplate rating to tell if the wire is sub-par.
The start current could be 10+ times the FLA easy.
I would install #6 THHN and drop the sup-breaker to 125% of FLA.
Mike

Understand, but it will make air running backwards, pull twice the amps - and vibrate like hell also.......probably not the issue but I have leaned.....check the basics before you re-engineer.

 

[Sawtooth]

It's been making air for 15 years, it's not running backwards. The #10 is undersized for the 50 amp breaker.
You would need the nameplate rating to tell if the wire is sub-par.
The start current could be 10+ times the FLA easy.
I would install #6 THHN and drop the sup-breaker to 125% of FLA.
Mike

Thanks everybody, yeah COLA I will have to check the motor nameplate current rating to get the full load amps. Thanks again.

 

[DaveH]

The amount of current is making the wire "dance" in the conduit. The #10 wire is undersized. If you change the wire to #6 as you indicated, it will be fine.

Using a 50 amp breaker on #10 wire is also a fire hazard and a NEC code violation.

this is exactly right. you have been lucky so far. give it enough time and you will "fry" the breaker. ( the damage is already done. your 50a breaker doesnt pop with 60a of load?) If you continue to be lucky, you wont have a fire.

 

[wishiknew]

It's been making air for 15 years, it's not running backwards. The #10 is undersized for the 50 amp breaker.
You would need the nameplate rating to tell if the wire is sub-par.
The start current could be 10+ times the FLA easy.
I would install #6 THHN and drop the sup-breaker to 125% of FLA.
Mike

EXACTLY RIGHT #10 is too small #6 and a new 50 amp breaker is in order !!!!!

 

[stokerwhore]

the wire is to small should be #8 (125% x FLA)for the FLA (full load amps). as for the breaker you are in fact allowed to go bigger in such instances where the motor needs it to start.(NEC table 430.52) another thing you can try is swaping two leads. although it goes the right direction there may be one other order of termination that can help the magnetic field rotate right for some motors. think typically you swap a&c phases for rotation. does this motor have a mag start? could be getting tired check the contact points. if they aren't clean and flat they vibrate and arc and do bad things.

 

[Sawtooth]

I went by his body shop this morning and this is what is on the motor plate- 10 Hp., 230 volts, 28/14 amps, 1760 rpm, 60Hz, 3 phase, Nom. Eff. 89.5%, PF 74%, usable @208V 28.2A.

I was basing the wire size on the existing breaker size (50amp) and the NEC says #6 copper wire, I know that the #10 it has is to small and cant believe he has never had a problem with it other than the buzzing sound in the conduit at turn on. The only reason he called me the other night is here over the last couple of weeks once the compressor builds up head pressure it wont start back up unless he bleeds off a bunch of air. He had the manufacture out and they told him the compressor/PSI switch are fine and working properly, I can't believe they did not say anything about the supply wire size. So my guess was that it cant get enough power threw the small #10 wires to start back up.

 

[stokerwhore]

sounds like the motor is tired. i dont know if it has start windings or a capacitor but when you drop to much voltage(line loss) and start the motor that way it will wear it out premature.especialy if you only have 208v to begin with. the part where it says 28/14 leads me to believe the voltage part said 240/480(or 460). if you have 480 available that would be the way to go.when you over heat wires for an extended period of time the wire becomes brittle and looses ampacity. looks like he'll just have to call a goodelectrician that knows motors to look at it.
another thing you can check after disconnecting the supply voltage as close to the motor as possible is that you still have all the phases at the motor. you have to disconnect the motor because you will read through the motor windings otherwise.ano0ther way to do this easily would be to turn off the breaker open the disconnect and check the fuses for continuity since thats likely were you would loose a phase. i doubt that is it but a remote posibilty. some wont start at all if you lost a phase and none will start under load if you have.
good luck. -Jas

 

[WYRD]

If this is a nine lead motor it sounds like he has it wired for high voltage (460) and is running it on 230. Have him make sure to check the name plate and make sure the proper leads are jumpered together.

Ive also seen a motor single phase (loose one of the three phases) and act as you say. At the peckerhead check voltage from each phase to ground.

Could be internal on the motor windings too.

NEC allows you to oversize the rating of the breaker for motor startup

 

[stokerwhore]

nec 430.22(A). = 125% X FLA= 35.25A= #8

 

[WYRD]

nec 430.22(A). = 125% X FLA= 35.25A= #8

:thumbsup

 

[OCMerrill]

unloaders

Hopefully used before the FST.