Q For An Electrician

[Bob Villa]

Sometimes I work on equipment with 3-phase (delta). If you mix up the wires...how do YOU figure out which wire goes where? Thanks.

[DerbyDad03]

I mark them before I take them off so that I don't mix them up.

I know, it's a novel approach. What can I say...I'm a rebel.

[Dean Hoffman]

Put a jumper from one line at a time to ground at the source of the circuit. Take a continuity test to ground where the wires end. Power off, naturally, and locked out.

[Bob]

If the motor spins in the wrong direction, swap the leads on phases 2 and 3. Otherwise, phasing isn't critical or you need to ask a more authoritative source than Usenet.

[RBM]

If your connections are 3 phase, it shouldn't matter, except for direction on motors. If the motor spins the wrong way, transpose any two wires. If you want to connect to something single phase on a high leg delta, you need to determine which is the wild leg, which you can do with a volt meter.

[The Daring Dufas]

When I install 3 phase refrigeration or HVAC equipment, I also install or order the factory option of a 3 phase protection module which shuts down the control voltage if there is any problem with the power. The cheap simple unit doesn't care about which direction a motor is turning in relation to incoming power but there is a more elaborate unit that I install on high end gear that will not operate if the 3 phase is not exactly L1 L2 L3 as seen on an oscilloscope.

TDD

[Dean Hoffman]

Do you ever use anything like this Amprobe phase tester?

[Bob Villa]

I would think you would want more conventioneers! "Qualified", as far as OSHA is concerned...I never touch the stuff!

[Bob Villa]

Thanks for the replies...I thought it might be that simple.

bob_v

[RBM]

Years ago we use to have a mechanical phase tester. We were supposed to use it when connecting 3 phase service equipment. When connecting a 3 phase motor, it doesn't help to know the phase rotation of the power supply, but not know the phase rotation of the motor leads. Ultimately, trial and error is probably the quickest method

[The Daring Dufas]

I've had a Sperry rotating disk phase sequence tester for many years. I believe Ideal bought Sperry out according to this picture.

TDD

[The Daring Dufas]

I use my phase tester even when the power company crew is there hooking up the drop. The linemen around here are OK with it and will make sure the sequence is right before making a permanent connection. Southern Company Power wires L2 as the high leg in a Wye secondary 3 phase. If I'm lucky, I can get Delta-Delta then everybody's happy. :-)

TDD

TDD

[LSMFT]

If you are particular go to the panel and put a toner on A phase then go to the wire end with the detector. Same for B phase, last one must be C. It really doesn't matter except for three phase motors motors.

[Mr.E]

The secondary is a delta if it has a high leg. A wye will have equal voltage on each leg with respect to the grounded conductor.

[krw]

ANd if the panel or the power company is wrong? After the panel it's tough to screw up the color coding.

Many moons ago (over 35 years) I designed a power phase detector that found its way into every IBM mainframe of the time. They didn't like the blowers running backwards, but the phase-controlled regulators really got pissed with reversed phase rotation. Before the three-phase contactors kicked in the service system would sense the phase rotation and lock up with an error if they were bad. It *did* change at times, particularly after major power changes and bad storms. Relying on the installers to test the rotation didn't work.

[The Daring Dufas]

DOAH! I stand corrected. I should have said center tapped delta has the high leg. I was reading my diagrams wrong and I don't even use recreational drugs. I got it mixed up. :-)

TDD

[The Daring Dufas]

Didn't some of those mainframes use 400hz AC power like aircraft so the power transformers could be smaller/lighter?

TDD

[krw]

Sure, they used M-G sets to get the 400Hz. The M-G sets were also used as the power-backup to do checkpointing or switchover to secondary power, on power failure. IIRC, all of the phase controlled regulators used 400Hz. It's been a long time, but I believe the blowers were all 50/60Hz (they could take a brown-out on power change).

[The Daring Dufas]

When I was out at the Kwajalein Missile Range, now The Reagan Test Site, back in 1987-88, There were computer systems on Roi Namur that had MG sets with big flywheels for UPS type power. The island power plant used big old EMD diesel generators putting out 4,161 volts into the island power grid. I suppose the flywheels could keep things up until another EMD was started but during a mission, I believe a backup was kept warmed up anyway. I was working for a contractor who built the new mission control center that housed a Cray X-MP super computer (high tech back then) and an atomic clock. I don't think the Cray used any special power because we only ran conventional 60hz electrical power and wiring.

TDD