LCR meters 5

[metaljohn]

Hello Everyone,

First time here and very impressed how much one can learn here. I have a question concerning 5,000-volt vacuum breakers & 480-volt motor controllers. The coils are 120-volt AC and occasionally a coil will become defective and will replace the coil according to manufactures instructions (proper spacing, torque, etc.). But still there is an occasional coil going bad and curious if a LCR meter can be used to inspect a coil before installation at a certain temperature (usually 20C). We don't have voltage issues such as the coils being over or under voltage and a constant 120-volt AC is maintained.
As the coils age and the insulation begins to breakdown, can a LCR meter detect this?
Recently we had a 30-hp, 3 phase AC motor tripped its circuit breaker and after checking ohms & megging with a Fluke meter, the stator appeared good. But with my LCR meter I detected an ohm imbalance and inductance imbalance & after replacing the motor, the VFD & motor are working correctly. If a LCR meter can be used for checking coils mentioned above, is a particular meter recommended?
Thank you

 

[VEBill]

"...As the coils age and the insulation begins to breakdown..."

Perhaps a 'Hipot' test? Megger is one brand within that general approach.

 

[itsmoked]

I do not see how an LCR meter can help here. If something has 'aged' to the point of changing any aspect of inductance or resistance or capacitance you're going to have insulation issues and that's what the megger is supposed to be seeing. You said you meggered the motor but did you megger it correctly with the correct voltage setting with a known good megger?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

It might be capable of detecting an inter-turn fault which a megger would miss. A decent 10A micro-ohmmeter (e.g.) Megger's DLRO 10X or Cropico's DO7 should be able to resolve a winding with a shorted turn from a sound winding.

 

[metaljohn]

Thank you VE & Scotty,

The testing of the 30 hp motor was done with three different Fluke 87's with megging capabilities with the same results (no issues). When touching the leads together the meter displays approximately 0.3 to 0.4 ohms but the actual ohm reading of the motor is less than that so I could not detect an issue with the stator windings. But with my Extech LCR meter that can measure resistance in tenths of an ohm I read a stator imbalance. When the motor was apart it found to be defective and so my LCR meter did a better job than my Fluke.
So I'm curious if I can obtain the manufacturers data sheet concerning this particular coil if I can monitor the inductance of the coil with my LCR meter. My LCR meter is great when testing capacitors and is more accurate than the capacitor feature on digital meters.
What part of England are you located Scotty? I have read several books about the UK.

 

[iop95]

May check new coils and do average values for R and L; after, monitor time to time R and L and note modifications.
In time, may see a patern that show when a coil is near to became defective.

 

[Skogsgurra]

The number of coils and how often they fail can indicate what action you need to take.

If you have hundreds of them and the failure rate (mean life) is in a few years range, then you have a problem that you need to find a solution to. It can be temperature, dust, too frequent operation, vibrations, over/under-voltage, DC components or just about anything.

If everything checks normal and life is in the ten to twenty years range then include them in your standard service plan and change them every tenth year. Or whatever time the manufacturer specifies for MTBF.

Trying to predict end of life is next to impossible with components like these and does not seem to be a good idea.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ScottyUK]

I'm in the north-east John, an area once famous for engineering, shipbuilding, mining and chemicals.

 

[VEBill]

"...change them... ...MTBF."

That approach can help, but some significant fraction would still fail before the Mean Time Before Failure (MTBF) has elapsed.

In the extreme example, if the failure curve is skewed by a majority essentially never failing, then those minority that do fail can all be much quicker than the MTBF.

Would it be worthwhile to modify the design or construction of the coils so that the most common failure mechanisms are eliminated? It wouldn't normally be very difficult to design a coil that can take extreme abuse and yet rarely fail. There are various compounds that can be used to insulate wires, some better than others. Coils can be vacuum potted to eliminate relative motion. Etc.

 

[ScottyUK]

Over-frequent replacement leaves the user exposed to the early failures on the initial downward slope of the 'bath-tub curve'. A former employer's QA dept referred to these early failures as 'infant mortality' - an awful phrase but it has stayed with me. After that initial failures the failure rate drops to a relatively low level until at some point failures once again rise due to old age.

 

[waross]

"When touching the leads together the meter displays approximately 0.3 to 0.4 ohms but the actual ohm reading of the motor is less than that so I could not detect an issue with the stator windings."
When testing a good motor with a megger, you should be reading over 100,000 Ohms and possibly in the high meg-Ohm range.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

I read that as the OP trying to measure winding resistance using an insulation tester. If so then it's the wrong instrument for the job.

 

[zeusfaber]

If so then it's the wrong instrument for the job.

I think that's the OP's conclusion too - and he's interested in whether the alternative he's identified is suitable.

A.

 

[waross]

Agreed Scotty.
three different Fluke 87's with megging capabilities
It may be time to RTFI on the Fluke in regards to meggering.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

I made a couple of suggestions of instruments perfectly suited to the job earlier. Most LCR instruments, other than bridge types such as the excellent bench instruments made by GenRad, don't have great performance at very low resistances. A GenRad bridge is an expensive bit of kit though, and even the older less capable ones still command a decent price on the surplus market. I have no idea what a new one sells for, but well into five figures.

 

[waross]

I agree that the meter will probably detect the change in inductance of a shorted turn. It may not see a difference before the short actually develops and shorted turns have a way of quickly leading to complete failure in many cases.
It may bear investigating as to whether the capacitive constant of the insulation changes as the insulation ages. This may be detected by an LCR meter.
This is speculation. If anyone has direct information or a link to information concerning a possible change in capacitive constant as insulation ages, please share with us.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Hi Bill,

On HV machines the tan delta test is used to measure the losses in the built-up insulation (dielectric) of form-wound bars. For magnet wire used in LV motors I'm not aware of any equivalent test.

 

[waross]

Hi Scotty;

The coils are 120-volt AC and occasionally a coil will become defective and will replace the coil ...
But still there is an occasional coil going bad and curious if a LCR meter can be used to inspect a coil before installation ...
As the coils age and the insulation begins to breakdown, can a LCR meter detect this?

My point is that the C function may be more useful than the L function of the meter.
Unlike motors, coils are seldom subject to overloading. On AC, a mechanical issue that prevents the solenoid from seating often leads to coil failure. However the chances of anticipating mechanical issues by testing the coil are nil.
Another issue with testing the inductance or impedance of a solenoid coil is the ratio of inductance of a solenoid coil when de-energized and when energized.
That is why I suggested that capacitance changes may be a more effective test.
Realistically, testing may involve disconnecting and re-connecting leads.
Eventually a mistake will be made.
If the leads are never disconnected there is no chance for a mistake in connecting. (Not just wrong connections but loose connections and possible stripped threads.)
Given the rarity of coil failure due to ageing, I suspect that a testing schedule of all coils in a plant may lead to more failures as a result of testing then failures due to ageing.
Scotty; I understand that a tan delta test is the same as a Power Factor test, with the raw data arranged slightly differently.
Something like altitude over base versus altitude over hypotenuse when the base and hypotenuse are almost equal.
Is this correct?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Tan delta test measures how lossy the dielectric has become, with a 'perfect' dielectric having no losses.

I don't think the random-wound windings of a typical LV machine would ever give predictable capacitance, or at least not with great enough accuracy. Perhaps some change could be detected if the motor had a 'fingerprint' taken during manufacture and this was used as reference for the specific motor in question. That said, I doubt that regular magnet wire changes its dielectric properties much unless it is as a result of burning - the polyimide coating is very stable and doesn't degrade in the way that built up HV insulation systems can.

 

[waross]

The original question was concerning 120 VAC operating coils on vacuum breakers and motor controllers.
The coil windings may be more regular than random wound motor windings. However there may not be a suitable ground plane to get an accurate indication of insulation quality by capacitive measurement.

This is the power factor test that I was referring to.
From a Doble Document:
POWER FACTOR TESTING
Insulation power factor tests are used to measure dielectric losses, which relate the wetness, dryness or
deterioration of transformer insulation. Both factory and field testing are performed to verify the insulation
integrity of substation transformers.
Power factor testing a two-winding transformer is conducted by energizing the winding at a known ac
voltage (typically 10 kV for windings rated greater than 10 kV) with the common winding bushings shorted together.
The results of overall power factor tests on power transformers reflect the insulation condition of the
windings, barriers, tap changers, bushings and oil. Modern oil-filled power transformers should have power
factors of 0.5% or less, corrected to 20°C (68°F), for individual windings to ground (CH and CL) and interwinding insulations (CHL).
The National Grid transformer specification states that the power factor of the insulation system shall not exceed 0.5% at 20°C.

http://www.doble.com/wordpress/wp-content/uploads/Power_Factor_Testing_Nov2005.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter