Motor strapping HV-LV

[gar]

130427-2338 EDT

A wye motor fed with 4 wires, meaning the mid-point of the wye is connected, operates as a 2 phase motor if one hot supply line is opened. This has a rotating magnetic field.

If the center point is just floating, then there is only single phase excitation and no rotating magnetic field, just a unidirectional field changing max + to 0 to maximum - to 0 and back to +. No rotation without a mechanical push.

A delta connected motor where one phase line is lost has only single phase excitation. In this case to one coil with two series connected coils in parallel with said one coil. Still only a unidirectional oscillating magnetic field.

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[Smart $]

Why not, they should have a retractable at the top of the ladder extended down so you can hook it to your D ring on your harness, if you fall it locks:thumbsup:

Yeah, if someone has already set up the ladder and been up there. Yo-yo's not gonna be there if you're the first one up. And what if there is no qualifying means of attachment within reach once up there. :thumbsdown:

Scissors or boom lift is preferable at that height and one has a bit of wiggle room once up there :thumbsup:

 

[GoldDigger]

If the center point is just floating, then there is only single phase excitation and no rotating magnetic field, just a unidirectional field changing max + to 0 to maximum - to 0 and back to +. No rotation without a mechanical push.
.

But that will change if the center point is not connected externally but the third phase line is held at 0 volts rather than being open. Both of the other two phases will then drive current through the center point via the third phase line in addition to the current from one phase line to the other through the two wye windings in series. This could potentially happen through mis-wiring or through a fault to ground in one phase line that caused it to disconnect from the supply but remain grounded.
I have not done the full vector analysis to figure out what the results would be, but I am not convinced that it would not rotate. (How's that for a strong statement for this time of night?)

 

[gar]

130428-0801 EDT

GoldDigger:

As soon as the center point is not floating, then it becomes a strange 3 phase motor based on your suggested circuit. Assuming the source was a wye and the center of the source connected to the bad phase wire.

There is a case where it would still be single phase. A three phase delta with a single phase neutral from the center tap of one phase.

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[don_resqcapt19]

130427-2338 EDT
A wye motor fed with 4 wires, meaning the mid-point of the wye is connected, operates as a 2 phase motor if one hot supply line is opened. This has a rotating magnetic field. ...

I have only seen one motor where T0 (center of the wye) was connected to a grounded conductor. I have no idea of why this was done.
The motor that I worked on was 500 hp, 3 phase, 480 volt, fed from a VFD.

 

[Open Neutral]

It's in a rotten place to even see; a through-the-roof ventilator. Made arrangements to use forklift to get up there to pull the motor, and will examine it on the bench.

Owner has clamp-on ammeter and reports it draws 2.23, 2.03, and 2.10 amps. That's 50% of the rated FLA at 208/240. That sounds way too low for even an unloaded motor, but it's been {large # of years} since I got an A in AC Machines Lab.

 

[GoldDigger]

Owner has clamp-on ammeter and reports it draws 2.23, 2.03, and 2.10 amps. That's 50% of the rated FLA at 208/240. That sounds way too low for even an unloaded motor, but it's been {large # of years} since I got an A in AC Machines Lab.

Those current numbers are not unreasonable for a lightly loaded motor which is wired for 480 and then run off either 480 or 240 or 208.
The bigger question is what happens when it is wired for low voltage and does not run at all. Be sure to measure all of the combinations of voltages at the leads which will be connected to the motor while you are up on the lift. I have a hard time believing that what you measure at the ground matches what is getting to the motor.

The fact that both the old motor, while it was working, and the new motor only seem to run when wired for HV makes me doubt that you will find the answer on the test bench.

 

[ActionDave]

Owner has clamp-on ammeter and reports it draws 2.23, 2.03, and 2.10 amps. That's 50% of the rated FLA at 208/240. That sounds way too low for even an unloaded motor, but it's been {large # of years} since I got an A in AC Machines Lab.

Now we have something real to chew on.

A lightly loaded motor wired for a higher voltage will run when supplied by a lower voltage and the amps will be lower.

 

[Open Neutral]

The bigger question is what happens when it is wired for low voltage and does not run at all. Be sure to measure all of the combinations of voltages at the leads which will be connected to the motor while you are up on the lift.

That may not be possible as is..... It looks like it's really designed for access from above. But a change I hope to make is adding a junction box so the motor can be wired to a pigtail on the ground, then mounted, THEN connected.

 

[GoldDigger]

That may not be possible as is..... It looks like it's really designed for access from above.

LOTO, remove the motor, then with the wires in a safe location energize and come back to measure the voltages? It is not essential to get the voltages under load, as you have the current readings.

But a change I hope to make is adding a junction box so the motor can be wired to a pigtail on the ground, then mounted, THEN connected.

Sounds like a plan!

 

[Open Neutral]

LOTO, remove the motor, then with the wires in a safe location energize and come back to measure the voltages? It is not essential to get the voltages under load, as you have the current readings.

I can just reach though the spinning fan blades...right?

No-load measurements should not be an issue.

Customer is in favor of the jbox; this is the 3rd motor in there. Also thinking he might want a fan-cooled one next time even though it's spinning a fan already.

 

[GoldDigger]

I can just reach though the spinning fan blades...right?

No-load measurements should not be an issue.

Customer is in favor of the jbox; this is the 3rd motor in there. Also thinking he might want a fan-cooled one next time even though it's spinning a fan already.

Just the odd question: Does he have any other similar motor and fan installations that are working just fine, or is this the only one?

If the motor is running on a reduced input voltage but being called upon to deliver full power, it certainly could overheat.

 

[Open Neutral]

Just the odd question: Does he have any other similar motor and fan installations that are working just fine, or is this the only one?

If the motor is running on a reduced input voltage but being called upon to deliver full power, it certainly could overheat.

The issue I can see is external thermal. This is a rent-a-box industrial building w/35' ceiling.....and a glassworks within....3 furnaces. The exhaust air stream through this fan must be toasty. The last motor has seized bearings.....

I'll check line voltages tomorrow. Want to find a good graphic of NEMA terminal #'s tonight....

 

[GoldDigger]

The issue I can see is external thermal. This is a rent-a-box industrial building w/35' ceiling.....and a glassworks within....3 furnaces. The exhaust air stream through this fan must be toasty. The last motor has seized bearings.....

I'll check line voltages tomorrow. Want to find a good graphic of NEMA terminal #'s tonight....

Something like the one in post #2, or something showing the corresponding internal arrangement of the windings? Or showing the physical arrangement of the terminals?

 

[Open Neutral]

BTW, I found the enclosed and it was the most coherent drawing I located.

 

[Open Neutral]

Here's a brane teaser. Suppose you get such a motor but with the markers gone.

You can use an ohmmeter and ascertain the three coils, and the hardwire wye;
but how would you determine their phasing?

I'd try something like:

Use three 60 watt lamps in series [i.e. current limiting] with the wye; then with a dual trace scope compare the outputs from each phase to the coils. But since you can't reach the center point.....

Is there an easy, no instrumentation beyond a meter, approach?

 

[GoldDigger]

Here's a brane teaser. Suppose you get such a motor but with the markers gone.

You can use an ohmmeter and ascertain the three coils, and the hardwire wye;
but how would you determine their phasing?

I'd try something like:

Use three 60 watt lamps in series [i.e. current limiting] with the wye; then with a dual trace scope compare the outputs from each phase to the coils. But since you can't reach the center point.....

Is there an easy, no instrumentation beyond a meter, approach?

Use an ohmmeter as a current source and a neon or other high impedance indicator as a voltage tester. Run current from the ohmmeter on low range from one of the terminals of the internally connected wye. As you make and break the circuit, check to see which of the other three windings (in the other set) does not show a flash. That is the one which is in the same phase position in the wye as the terminal you did not connect the ohmmeter to.
Take it from there to identify all three.

Determining the polarity of the detached windings will be a little more difficult though. One way to do that might be to connect the three windings into what you think is the series configuration and use the same test but with the neon across the same winding pair as the ohmmeter to see which of the four possible configurations of the two windings you are working with gives you the biggest flash (most inductance). The other three combinations will either give no flash (both coils wired in reverse phase) or half the flash (one of the two wired in reverse phase.)
There are probably better ways than that, but it is too late at night for my brain to work.

 

[kwired]

Here's a brane teaser. Suppose you get such a motor but with the markers gone.

You can use an ohmmeter and ascertain the three coils, and the hardwire wye;
but how would you determine their phasing?

I'd try something like:

Use three 60 watt lamps in series [i.e. current limiting] with the wye; then with a dual trace scope compare the outputs from each phase to the coils. But since you can't reach the center point.....

Is there an easy, no instrumentation beyond a meter, approach?

I don't remember all the details, but it involves applying the lower voltage rating to the permanently wired wye segment, running that part of the motor at low volts rating and measuring voltages on the other leads to determine which sets go where. Of course you have to have little or no load coupled to the motor when doing this.

 

[Open Neutral]

Update

Went up 35 ft on fork lift, pulled motor.

Undid HV strapping and tied 456 together.

Then powered up ONLY 789, bumped motor, saw CCW rotation

Carefully moved L1 to 1, L2 to 3, L3 to 3.

Bumped motor, went CW. !?!?!Wisconsin Tourist Federation?!?!? [You'd need to be a "Wait Wait" fan to follow that aside....]

First I was sure motor was mislabeled. Pondered if I could swap any 2 in 123 or if I needed the correct wrong ones [Discuss..]

THEN I noticed it was 1) running unsmoothly b) Hot. Suddenly, it smoothed out!

Much teeth knashing later, with intervening clamp-on readings, voltage measurements, and colorful metaphors.....

The Square-D breaker is *intermittent* on one phase. Sometimes it excites all three legs, other times only 2.....and then the third if/when it feels like it.

The clue was the voltages AT the breaker terminals in bad mode were [p-gnd] 123, 125, and 203.... the latter from xfmr action in the motor.

Yet the motor moved every time I bumped/ran it, even with 2 phases; just not always the same direction. While single-phasing the working legs drew 25A, yet the 15A breaker did not trip.

At good w/no load, it draws ~2A/leg, vice FLA spec of 4.6-4.8A. That sounded low to me, but....

Not sure we can find appropriate breaker locally. Grrr...

He says he wants a 2nd adjacent fan Real Soon Now. Not sure that is kosher off one breaker with no separate disconnects but not my problem today.

ps: Dayton 1234etc wire markings are already becoming illegible... Brady time.

 

[GoldDigger]

UpdateThe Square-D breaker is *intermittent* on one phase. Sometimes it excites all three legs, other times only 2.....and then the third if/when it feels like it.

Congratulations! Unfortunately you have left us (and the Wisconsin Tourist Federation) wondering why it ran any better or more consistently when set up for HV mode!!
There has to be a clue in there somewhere related to the (apparent) fact that it started consistently when bumped with either one of the LV winding sets connected, but sat and vibrated with both connected. I will not be able to sleep tonight till somebody figures that one out. (Or until my bedtime, whichever comes first.)

OK, here is a guess about a very small part of the scenario:

The physical geometry of the two windings is such that they do not line up exactly to the degree in terms of rotational angle, but have narrow fields which combine to form wider virtual poles. That means that in all three cases (winding 1, winding 2 and both) the motor will be running single phased. No starting torque worth mentioning and rotational direction uncertain.
But with only one set of windings, the chances are good that motor will stop far enough off the neutral point that it will see a little torque in one direction or the other when power is applied. With both sets connected the poles are wide enough that the motor will always end up stopped at a neutral point and there will be no initial kick to get it moving.

That still does not give me a clue why it worked consistently with the series setup, unless somehow the extra inductance or the lower current affected the behavior of the defective breaker itself.