Will coiling 240v 3 ph motor leads cause damage 4

[silverfox8028]

I have a 20 ft deep wetwell with 2- 20 hp 240v, 3 ph motors fed from surface starters. The thhn motor feeds are sized correctly, not in conduit in the wetwell and lashed together with plastic ties. Inside the top of the well the leads are coiled together (2-3 times) like a lasso on a saddle before dropping to the motors. We keep losing motors and/or the control circuit a few times per year.
Is coiling the leads the problem?

 

[Skogsgurra]

No.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rbulsara]

Voltage sags or momentary power outage would be more probable culprits.

Rafiq Bulsara

http://www.srengineersct.com

 

[electricpete]

Silverfox – I think you are envisioning some kind of coil effect resulting in inductor that creates voltage drop. But the net flux from three phases together is zero, so if you wrap take those three phases and wrap them several times you are rouhgly linking zero to zero… would not create large inductive effects beyond the same length of the wire in a straight configuration. On the other hand, the you might think about the temperature effects of coiling those wires..

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(2B)+(2B)' ?

 

[electricpete]

On the other hand, the you might think about the temperature effects of coiling those wires..

...not as a cause of your trips. It was just something to think about in evaluating the general acceptability of that configuration.

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(2B)+(2B)' ?

 

[silverfox8028]

The only reason I thought of an inducter affect is because each lead is individual and not part of an SJO 3- conductor cable, for example. I was concerned that the leads may not cancel each other out because they may not be wrapped perfectly. I'll assume the coiling is not the problem.

Appreciate everyone's help.
Thanks

 

[Skogsgurra]

Even if the inductivity of the three coils weren't canceled at all, you would have less than 100 microhenries from that coiling. With the trifilar configuration, inductivity will be less than 10 uH.

And, inductivity in series with a motor is not a big problem. Not as long as the voltage drop across it is within a volt or two, which it is.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

I would think that the extra length of wire will have more of an effect on voltage drop than 3-loop coils. I have seen looped excess conductors cause problems, but they were long big conductors left on a steel reel (because the user planned on returning it to the supplier when a temporary job was completed) connected to a 500HP motor. It dropped the voltage so much that it wouldn't accelerate!

Biggest culprit on 3 phase 230V well pumps in the US, 230V 3 phase comes from a "red leg" transformer that allows a 120V tap off to neutral from 2 of the legs, saving them from having another transformer for lighting etc. People often tap off too much 120V power, leading to a voltage imbalance. A small voltage imbalance translates to a lot of motor heating and that is often critical on submersibles because the motors are often designed with little or no "fudge factor". Some submersible mfrs, such as Franklin, will void the motor warranty with any more than a 5% imbalance.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[2dye4]

If you mean the motors are failing and need replacing.

1 Overloaded, pushing water too far.

2 Undervoltage, too much drop in the lines to maintain motor voltage.

 

[edison123]

Looping the single cables will overheat the cables in a short time. In my shop, I found this long time ago when we tested motors. So now we zigzag the extra length like hairpins.

Muthu

http://www.edison.co.in

 

[electricpete]

Wow. What the heck happened here? Lots of stuff gone ;-(

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(2B)+(2B)' ?

 

[electricpete]

I have recreated my comments below. If anyone thinks they are in some way inappropriate or wants to comment why they were removed I'd be interested to hear.

I explored this question with the free magnetic F.E. software "FEMM".
Attached slide show has the results. Summary of slides is as follows:
SLIDE 1: Analytical calculation of single loop in air. Loop radius = 0.03m, Wire radius = 0.005m. L=7.05E-8 H
SLIDE 2: Same Geometry, shown in "axisymmetric" view. i.e. (r,z) plane where radius measured horizontally and height vertically. The center axis of the loop is the left side of this figure
SLIDE 3: Same Geometry (simple loop) FE solution. L = 7.86E-8 H.
SLIDE 4: Add Iron Slug. L = 4.47E-7 H
SLIDE 5: Same-polarity loop pair (no iron). L=1.03E-7H for our loop
SLIDE 6: Add iron Slug to same-polarity loop pair. L=8.2E-7 H
SLIDE 7: Opposite-polarity loop pair (no iron). L=5.4E-8 H
SLIDE 8: Add iron Slug to opposite polarity loop pair. L = 6.8E-8
SLIDE 9: 6 opposite-polarity loop pairs arranged symmetrically, no iron. L = 5.5E-8 H
SLIDE 10: Add iron Slug to 6 opposite-polarity loop pairs, L= 6.6E-8 H
SLIDE 11: 6 opposite-polarity loop pairs arranged irregularly (no iron). L=6.75E-8 H
SLIDE 12: Add iron Slug to 6 opposite-polarity loop pairs. L=1.36E-7H Significant increase.
SLIDE 13: Change iron slug from slide 12 to iron enclosure (enclosing the current.) L = 1.32E-7 H
SLIDE 14: Change iron slug from slide 4 to iron enclosure (enclosing the current.) L=4.4E-6 H

All of these are actually dc simulations. Neglecting eddy effects, they are also representative of single-phase 60hz circuits. Certain geometries are claimed to act similarly to 3-phase circuits as will be elaborated in the discussion below (under slide 7)

Slide 1 is simple analytical solution of a loop as a test case.
Slide 2 shows an overview of the axisymmetric geometry used for the FE solution. The geometry is very similar to what is used in this link, including boundary conditions described in section 2.2:

http://www.femm.info/wiki/CoilGun

Slide 3 shows FE of the same geometry as slide 1 and the inductance is calculated... results agree reasonably well.

In slide 4 we add iron slug which causes a dramatic increase in inductance (factor of 6 increase) for this single loop, as expected.

In slide 5 we add another identical loop with current flowing in the same direction. The inductance is reported as 1.03E-7H (compared to 7.8E-8H for single loop inof slide 3). This does NOT represent the inductance of the series combination of the 2 loops (that would be at least twice slide 3). What L represents in this slide and all remaining slides is the total flux linking one specific loop (the one with the red arrow) divided by the current in that loop. So we can think of it as inductance per loop or inductance per length when we compare. The effective inductance of our loop goes up by roughly 20% from slide 3 to slide 5 when we added another loop of same polarity nearby.

In slide 6 we add an iron core to the geometry of slide 5 which results in dramatic increase in inductance (factor of 8) as expected.

In slide 7 we examine an opposite-polarity loop pair in air. * The single-phase opposite-polarity loop pair is interesting to us because the current and flux from the pair sums to zero, so we can think that it acts somewhat similar to the sum of 3-conductors in a 3-phase system (except the opposite-polarity single phase loop pair is much easier to set up, which is why I used it). The inductance (per loop) is now 5.4E-8. It is around 20% lower than our single loop of slide 3. That is similar to a set of 3-phase conductors... their effective inductance decreases when we put them close to each other.

In slide 8 we add an iron core. The increase in flux due to adding core (from slide 8 to 9) is very modest at approx 20%. This is much much lower than all previous configuraitons where we saw 600%-800% increase from adding iron. When we have our conductors arranged in groups that sum to zero (like our opposite-polarity pair or a set of 3-phase), the iron tends to have much much less effect.

Slides 9 and 10 resemble slides 8 and 9 except we have used 6 symmetrically-arranged opposite-polarity loop pairs (resembling 6 symmetrically arranged 3-conductor cables). There is very little change in inductance from adding additional pairs if the symmetry is maintained.

Slides 11 and 12 resemble slides 9 and 10 except the pairs are not symmetric, they are placed in a haphazard arrangement. In absence of iron (slide 9), there is not much change (about 20% increase from slide 9 to 11). With iron in the picture, the asymmetry is more important (about 200% increase from slide 10 to 12).

Slides 13 and 14 resembles slides 12 and 8 respectively, except that the iron slug (going through center axis of the conductor loops) is changed to an iron enclosure which encircles the conductor loops. The intent was to explore whether enclosing had a different effect than the iron slug For slide 14 (single loop/opposite direction), there is a huge difference.. the inductance is around 4.4E-6, which is more than 6 times as high at the same geometry with slug (slide 8). For slide 13 (6 opposite-polarity loop pairs irregularly arranged), the inductance is 1.32E-7 which is roughly the same as the same geometry with slug (slide 12). We conclude that closing of the iron loop makes a huge difference when the enclosed electrical circuit has non-zero net current, but not much difference when the electrical circuit has zero net current (like a 3-phase circuit).

Interesting to note that iron alone (slide 10) and assymetry alone (slide 11) did not have much effect, but with both present there was a more dramatic increase as in slide 12(sort of a synergistic effect). If we consider slide 7 as represntative of straight run of cable (including 2 opposite single phases or 3 3-phase wires), then the "worst case" that we created by looping around iron and adding assymetry (slide 12) is slightly more than twice as much. For other patterns of assymetry it certainly may be more than that factor, but my gut feel is as others said that it is not a tremendous problem unless you were very marginal to begin with.

You were right to ask about asymmetry though. That turns out to be an important factor.

Other miscellaneous notes:
All conductors are the same size and the target coil is always the same radius = same distance from the axis on the left... if they appear different it is just because I was sloppy in zooming to different levels for the screen shot.


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(2B)+(2B)' ?

 

[electricpete]

By the way, I'd say Gunnar's approach is more straightforward than mine... just make some worst case assumptions and you can bound the inductance and the voltage drop and for 3 turns it will most likely be negligible.

Let's say there is a complete closed loop path in iron... we can analyze it using a magnetic circuit:
Rel = Len /(mu*A) where Rel = Reluctance and A = area of iron path and Len = mean length of flux through the iron loop
Phi = N*i/ Rel
L = N*Phi / i = N^2 /Rel = N^2 * A*mu /Len
I don't know much about your geometry, but let's say N=3, A = 0.01 m^2, Len = 5 m, mur = 1000, mu = 1000*4*piE-7 H/m

That gives:
L = 3^2 * 0.01 * 1000*4*piE-7/ 5 = 2.2e-5H = 22 microHenries
XL = 2*pi*60*L*j = 0.0085j ohms
With 100A flowing you could have 0.8vac drop.

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(2B)+(2B)' ?

 

[electricpete]

... and that was considering 3 loops all of same polarity with NO flux cancellation.... actual in a 3-phase circuit with flux cancellation will be much lower as mentioned.

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(2B)+(2B)' ?

 

[silverfox8028]

This was the first time I used this site. I never could find how to thank everyone for their help...then it was obvious...today.
So thanks again.

I'm going to try again with another question about "Solar Farms."
Please help if you can.