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AndrewEvans
02-09-2011, 05:34 PM
We have in our shop 3 phase 230 volt power,

We purchased an air compressor a year ago, It uses single phase 230 volt power

I hooked it up last year and its been working since with no problem.... Until Today. Pop it on and nothing happens, magnetic starter doesnt pull down, motor doesnt turn on

Troubleshooting I've done.

I get L1 to ground and ground to neutral 110 volts, not a problem, thats expected, but to go L1 to neutral (in reality L1 to L2 since I have 3 phase) I get 0 volts,

I reiterate, NOTHING Changed from last night to this morning, and suddenly my compressor doesnt work and the power to the compressor magically defies the laws of electricity.

Any guesses?

Al_The_Man
02-09-2011, 05:41 PM
Not quite following the logic?
If you have 3 phase star, then the star is usually at ground potential, you would then expect 110v from each phase to either ground or neutral.
If you have a Delta supply, (doubt it) things may be different.
What is the reading across all three phases?
Sounds like you have lost a phase, have you checked all fuses?
Al.

AndrewEvans
02-09-2011, 05:53 PM
No fuses, simple circuit breaker for "on/off"

what has been done in order to supply the compressor motor with its single phase in 230 volts is to use legs 1 and 2 of the 3 phase and also the ground

now, ostensibly that should give me no reading L1 or 2 to the ground and simply 230 from L1 to L2

mathematically and logically that works in theory, (and reality till a day ago)

but now I'm coming up with current L1 to ground and L2 to Ground but nothing L1 to L2

I'm leaning towards a combination of your theory of having lost a phase along with another one of a re-wired (without my knowledge) plug/receptacle

I tested the receptacle and I got L1 to Ground 230, L2 to ground 110, and L1 to L2 nothing, Guess I have to open the recep and see what the heck gives, not sure why at the compressor box I get 110 L1 and L2 to ground and recep something different though

so current best guess is recep is wired so ground is really L1, L2 is really ground and L1 is really common? (except in 3phase I didnt think there was truly a common

<-- idiot supplicant to the electrical Gods for a miracle

Al_The_Man
02-09-2011, 07:24 PM
No fuses, simple circuit breaker for "on/off"

what has been done in order to supply the compressor motor with its single phase in 230 volts is to use legs 1 and 2 of the 3 phase and also the ground

now, ostensibly that should give me no reading L1 or 2 to the ground and simply 230 from L1 to L2



Not by my reckoning, like I said it depends on what type supply you have, but normally I would expect L1 & L2 to read ~110v to ground, if your 3 phase secondary is not referenced to ground, there would be no point in taking a ground to the compressor, it is not going to achieve anything.
A 208 3 phase system would give you around 120v to star point.
http://www.allaboutcircuits.com/vol_2/chpt_10/5.html
Al.

Quokka42
02-10-2011, 03:26 PM
THere are two explanations for what you have measured:

1. L1 & L2 are now for some reason in phase.

2. You have lost one phase and are using a high impedance meter, which is picking up a stray voltage from some connected piece of equipment.

Obviously you have a supply problem, and 2. shows why in the electrical field we are taught to disconnect, then test again!

Nic7320
02-10-2011, 04:46 PM
Remember, when you lose the supply to any leg to a motor, the other leg feeds back through the motor winding and you'll read the L1 voltage on the L2 leg, or L2 on L1. If you check with an oscilloscope, and you'll find they are in phase and there is no voltage difference between the two.

The same is true with any light bulb circuit. If you interrupt the neutral return, and you read the same voltage on each side of the filament, but no difference of potential that will light up the bulb.

Easy way to diagnose: Disconnect the motor from the supply and then check the supply voltages. One of them is likely gone. When the motor is disconnected, you can also check DC resistance of the windings (should be only a few ohms).

AndrewEvans
02-10-2011, 05:11 PM
after much to-do and heming and hawing I have it resolved thanks to your huge amounts of help.

Well, the motor, the wiring (in the compressor & compressor plug) was just fine, but the receptacle wiring was a bit wonky & yep I lost my 3rd leg :eek: finding WHERE it was was a pain, turned out to simply be the breaker, guess thats what you get after 40 years

maybe it'll be a while before I have to touch the zappy stuff that makes my machines go *crosses fingers* I'm no grease monkey but I sure enjoy mechanical more than electrical troubleshooting