Solving 3ph Cable Issues with Inverter Driven Motor

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Discussion Overview

The discussion revolves around issues related to a 3-phase cable connected to an inverter-driven motor, specifically focusing on the cable hardening and breaking down at the motor end. Participants explore potential causes, including environmental factors and electrical characteristics, while considering the implications for the motor's operation and maintenance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the cable hardening and breaking down at the motor end, despite using the recommended screened cable and correct conductor size.
  • Another participant suggests that ozone might be a factor, referencing experiences with power cables in vacuum systems and proposing that high voltage from inductive flyback could be causing the rubber insulation to harden.
  • A different participant questions how changing the power factor could help with the issue, indicating a lack of understanding of the electrical concepts involved.
  • Several participants emphasize the importance of ensuring good cable connections with low resistance and inquire about torque specifications for connector bolts.
  • There is a discussion about the insulation material, with one participant clarifying that the cable is PVC covered, not rubber, and another noting that the insulation appears to be breaking down.
  • Some participants speculate about the effects of environmental conditions, such as dampness and temperature, on the cable's integrity, with one noting that the cable is in a protected area but could still experience dampness from cleaning processes.
  • One participant shares information about the properties of PVC insulation, including its temperature limits and potential degradation from UV radiation, while questioning the presence of absorbent materials that could affect the cable.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the causes of the cable issues, including ozone, voltage spikes, and environmental conditions. There is no consensus on the exact cause or solution, and multiple competing views remain throughout the discussion.

Contextual Notes

Participants mention the cable's insulation breaking down and the implications for motor operation, but there are unresolved questions regarding the specific failure mechanisms and the role of environmental factors. The discussion includes assumptions about the effects of temperature and humidity on cable performance.

wolram
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We have an inverter driven 3kw motor at work that is the main drive for
a piece of kit we have, it drives via a 300-1 or so gear box, you can turn
the whole thing with one finger via the fan on the motor, the problem is
the 3ph cable to the motor hardens an breaks down, it is the recomended
cable to use (screened) and correct conductor size, the motor hardly gets
warm in opperation, and the cable only hardens at the motor end, any one
have an idea? if you do i can supply free triffles, cheese cakes, for a year.
 
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Can you post a picture? Also, what voltage is the 3-phase power typically?
 
Ozone. Curing things is a bit trickier --- or, maybe just not possible. Had several vacuum systems that used to "eat" power cables, usually to the roughing pumps, and same power cable used for diffusion pump inches away was fine, occasionally vice versa. Went to "new cables every 6 mos." to avoid troubles --- didn't really "solve" the problem, just dealt with it. Picked up non-rubber insulation (vinyl? Just about universal on 10-20 amp power cords these days) and got much longer service.

"Ozone" was the "folk" explanation --- might actually be high voltage from inductive flyback at "unfavorable" load conditions devulcanizing (hardening) the rubber jackets. Check with your electrician(s) about hanging bypass capacitors in the circuit, or tweaking power factor.
 
WHOA, Bystander, i know that these things can deflect voltage back to
the inverter, giving twice the output voltage, the motor does live in a damp
environment, but i am not at your level of understanding, how can changing
the power factor help?
 
Tweak the power factor and cut the flyback voltage. Or, "soften" the spikes a bit with bypass capacitors.

This is assuming that the speculation about voltage spikes breaking down rubber insulation is even close to a mechanism for what happens.
 
You want to make sure the cable connections are very good i.e. as low resistance as possible. Is there a torque specification for the connector bolts?
 
Pictures. We all want pictures. We want to help! (and learn too...)
 
dlgoff said:
You want to make sure the cable connections are very good i.e. as low resistance as possible. Is there a torque specification for the connector bolts?
There may be a spec for nut torque but it is not in the paper work we have.
 
berkeman said:
Pictures. We all want pictures. We want to help! (and learn too...)
I doubt if a picture would help, the cable looks the same along its lenght
It is PVC covered cable by the way not rubber.
 
  • #10
berkeman said:
Can you post a picture? Also, what voltage is the 3-phase power typically?
It is 240v 3ph.
 
  • #11
When you say that the cable hardens and breaks down, what do you mean? Is the insulation or the metal conductors that harden and break down? What is the failure exactly?
 
  • #12
berkeman said:
When you say that the cable hardens and breaks down, what do you mean? Is the insulation or the metal conductors that harden and break down? What is the failure exactly?
On the inverter we get a motor over volt message, when we test the cable
( disconnected from motor) between conductors we only have a few meg
ohms resitance, it seems the insulation is breaking down, the first motor
had been on the machine for 5 years, we mistakenly sent it for a rewind
only to find it was a cable fault, that cable has been replaced, but after
only 8 months is showing the same fault.
 
  • #13
There is a second cable that goes to this motor, same junction box, that
used to opperate a clutch brake, it is not connected at the motor end as we no longer use clutch brake, this cable is still supple.
 
  • #14
Do you think the ozone suggestion might be valid? If so, just aim a fan from a clear air area at the end of the cable at the inverter. That should keep the ozone away from the insulator/cable.
 
  • #15
berkeman said:
Do you think the ozone suggestion might be valid? If so, just aim a fan from a clear air area at the end of the cable at the inverter. That should keep the ozone away from the insulator/cable.
I not sure about ozone, all i can say is the room is at a constant 10c and
is all ways damp, is this good conditions for ozone production?
 
  • #16
I think it's arcing that produces the ozone, but I don't know much about that. It sounds like a refrigeration application (10C and damp) -- is the cable getting wet too?
 
  • #17
berkeman said:
I think it's arcing that produces the ozone, but I don't know much about that. It sounds like a refrigeration application (10C and damp) -- is the cable getting wet too?
The cable is in the bowles of the machiene, so it is well protected from any
washing that takes place every night, but i guess it could get damp at times
from evaporation of cleaning water.
 
  • #18
I have found this,

At high temperatures the material becomes soft so that conductors which are pressing on the insulation (eg at bends) will 'migrate' through it, sometimes moving to the edge of the insulation. Because of this property the temperature of general purpose P.V.C. must not be allowed to exceed 70°C, although versions which will operate safely at temperatures up to 85°C are also available. If p.v.c. is exposed to sunlight it may be degraded by ultra-violet radiation. If it is in contact with absorbent materials, the plasticiser may be 'leached out' making the p.v.c. hard and brittle.

i do not know of any absorbent material ?? sun light, no chance.
 

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