Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Nitin Sidhu]

Hello!

Yesterday I was introduced to a new concept. The rental companies crew could not get their 3-phase power distro to pass power. I was told, the unles L1,L2,L3 from the DG did not feed L1,L2,L3 to the distro, i.e. phases were mismatched, the distro would shut down. They did not know why or how, and even though you would think it is a fairly simple problem to resolve, the DG guy was feeding them 3 red cables, and 1 black. (there you go!). Eventual combination permutations seemingly got it working.

The only way I think this could be measure would be via phase cycles ? Where L1 would be at 0 degrees.

Question. What difference would it make ?
Question. What meter measures A/c phase ?

Wrong business im in!

Regards,
Sidhu

[Mike Sokol]

The only way I think this could be measure would be via phase cycles ? Where L1 would be at 0 degrees.

Question. What difference would it make ?
Question. What meter measures A/c phase ?

The only thing I can think of is phase rotation. The basic concept is that any 3-phase electric motor's rotation can be reversed simply by swapping any two legs of the three "hot" wires. While that's not a big deal if you're running a chain hoist with buttons to change from in to out, it's a real problem if you're connecting to a machine that can't tolerate any reversing of the 3-phase motor. In that case you have to verify correct phase rotation before you turn on the machine. You can get a phase rotation meter built into many high-end clamp meters such as the Amprobe AMP-310 http://www.amprobe.com/Amprobe/usen/Clamp-Meters/amp-200-300-series/AMP-310.htm?PID=79185

There's really no way that the 120 degree phase angles can change in a generator, unless someone has given you a "bootleg" single phase feed over 3 hot legs so that it LOOKs like it's 3-phase. But swapping wires wouldn't fix that.

Anyone with other ideas?

[TJ (Tom) Cornish]

Hello!

Yesterday I was introduced to a new concept. The rental companies crew could not get their 3-phase power distro to pass power. I was told, the unles L1,L2,L3 from the DG did not feed L1,L2,L3 to the distro, i.e. phases were mismatched, the distro would shut down. They did not know why or how, and even though you would think it is a fairly simple problem to resolve, the DG guy was feeding them 3 red cables, and 1 black. (there you go!). Eventual combination permutations seemingly got it working.

The only way I think this could be measure would be via phase cycles ? Where L1 would be at 0 degrees.

Question. What difference would it make ?
Question. What meter measures A/c phase ?

Wrong business im in!

Regards,
Sidhu

In the industrial world you need to know the phase rotation - clockwise or counterclockwise to make sure your motors turn the correct direction.  There are a number of testers that perform that function.  Here's an example: http://www.bkprecision.com/products/electrical-battery-testers/302-phase-motor-rotation-meter.html

Reversing the rotation is accomplished by swapping two of the legs. 

I don't know why a distro would care, and other than CW/CCW rotation, there isn't a measurable absolute phase characteristic that I'm aware of.  Maybe the distro was also a motor controller? 

Edit - Mike wins the post race.

[Mike Sokol]

Edit - Mike wins the post race.

Did I win a beer? It's 5 o'clock somewhere...

[TJ (Tom) Cornish]

Did I win a beer? It's 5 o'clock somewhere...

I will drink a Diet Coke in your honor.

[Nitin Sidhu]

wow...
Mike. Stop living here.

[Stephen Swaffer]

It is not uncommon-especially in equipment from Europe to have a phase protection relay installed in the control-essentially prevents the machine from powering up if the phase is reversed.  It is a safety feature-I could see it being useful if you were using a distro with chain hoists, since if you reverse the phase on a chain hoist, not only do the up/down buttons work in reverse, but the limit switches do not function correctly and that can indeed be a huge safety issue.

I am guessing that this is what you ran into-but phase reversal only has 2 permutations, so it its wrong switch any 2 (but ONLY 2 ) wires and you should be up and running.

[David Buckley]

Without disagreeing with anything anyone has said, there is perhaps a simpler explanation of why a phase failure relay might be fitted to the distro.

A phase failure relay typically does a couple of other things besides phase rotation; it checks for the presence of all three phases, and some also check phase/neutral voltage.  We know what happens if the neutral becomes unglued on a three phase rig....  This kind of relay is a cheap watchdog that can save a boatload of money.

[Mike Sokol]

Without disagreeing with anything anyone has said, there is perhaps a simpler explanation of why a phase failure relay might be fitted to the distro.

A phase failure relay typically does a couple of other things besides phase rotation; it checks for the presence of all three phases, and some also check phase/neutral voltage.  We know what happens if the neutral becomes unglued on a three phase rig....  This kind of relay is a cheap watchdog that can save a boatload of money.

You might be right. When a 3-phase motor under load goes single-phase (any one of the hot legs opening up) it can overheat quite rapidly, and even catch on fire if you don't have proper over-current protection.

[Stephen Swaffer]

Back to the OP-3 reds and a black.  Do we hope the neutral was black to differentiate it from phase conductors?

I guess in a haphazard situation a phase protection relay could be essential!  Still, if the neutral is known (should be simple) then you should only have 2 possibilities.  Rotation correct, or reversed.

Also, there are a number of phase rotation meters available.  Not real common, since you can usually double check motor rotation-but handy if reverse rotation can cause damage (some compressors) or the motor is not readily accessible.

[ProSoundWeb Community]