Motor Thermistor wiring 2

[tonerlow]

Can someone advise me as to whether it is OK to include the Thermistor protection wiring in with a flexible armoured multicore cable also carrying the 400Vac power to a 3 phase motor. multicore would be 6 core 2.5mm.
in thei case the motor is a 7.5kw being controlled by a Variable speed drive module, (Eurotherm 650).

Tonerlow

 

[buzzp]

So you want to use some of this multicore cable conductors for your thermistor? Or will the thermistor wiring just be in a conduit or raceway with this 6 conductor armored cable?
I would avoid using the cable to carry your thermistor signals because of high current and switching signals will cause problems with your thermistor signal for sure.
If the wiring is seperate from motor cable and your just going to run in the same conduit or raceway then you should be fairly safe since it is armored cable, although not ideal.

 

[dpc]

In the US, this would not be a Code violation, but you may have noise problems as buzzp indicates.

Our practice has been to put thermistor electronics right at the motor and take just a dry contact back to the motor starter. In this case, you have much less problem with noise.

 

[ScottyUK]

In the UK it would considered bad practice at the very least. I haven't got my copy of the regs at home - I'm not that dedicated - so I'm not certain it is a violation of British legislation. You will have all manner of pickup in the sensing lines as indicated by BuzzP, giving rise to false readings.

I'd strongly recommend using a separate screened cable for the thermistors to bring the signal back to the drive.



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If we learn from our mistakes,
I'm getting a great education!

 

[cbarn24050]

Hi, yes no problem doing that. I've never seen a motor that size that had a seperate inlets. Drives are made to deal with any noise that may get picked up.

 

[ScottyUK]

Hi CBarn,

That might work, but it is still bad practice. The fact that the drive manufacturer does their best to make their product resistant to bad design does not mean that bad design is acceptable!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[cbarn24050]

Well scotty it may think it's bad but it's the only way it's ever done. I guess you think that burying the thermister in the motor windings is also bad, fortunaltly every motor manufacturer disagrees with you.

 

[jraef]

Hold on, I thoughttThermistors were just resistive devices, and the relays are just using an adjustable threshold trip level. If that were the case, once installed and adjusted to trip at a particular level, that would not typically be detrimentally affected by induced voltages would it? We rarely see thermistors in motor circuits, RTDs are what we prefer so I have little direct experience. However I have studied Thermistor Trip Relays for our newer systems that will be going overseas and see nothing in their design that would be altered by noise picked up in the leads. Am I wrong?

"Venditori de oleum-vipera non vigere excordis populi"

 

[buzzp]

First off, the noise on the thermistor wires will be from the drive AND from the current carrying conductors from the motor leads. I am sure if the drive is decent it has filtering to some degree. However, given that noise should be suppressed as close to the origination as possible to shunt it before it reaches larger levels (tends to happen with noise as it propagates along a wire), relying on the design of the drive to handle this is simply not a good idea. Many cheap drives are made these days and some will have no filtering. Heck, I even seen a drive that measured the current using an averaging scheme and a shunt internal to the drive. What were they thinking when they did this? The numbers they displayed for current where on average 15% off. So do not rely on the drive.
If the cable is armored this will help reduce coupling between the thermistor wire and the motor leads. Ideally this would be avoided although it is commonly done. If the motor leads were not armored I would avoid it for sure.

No matter if your using thermistors, RTD's, thermocouples or whatever, they are all generally very low voltage signals (depending on the driving voltage) so any noise affects these signals. The thermistor still needs to have a current through it to get a measurement of resistance to some measuring device.
I am sure this would not be a violation of the NEC since it is technically used by the same piece of equipment/circuit. But then the NEC is a safety code not a best practices code.

 

[DickDV]

Now wait a minute, guys. Thermistors are simply thermostatically operated switches and are not necessarily low voltages devices. Most drive control terminals are looking for either 12VDC or 24VDC so noise is not so much a problem.

Having said that, I still would not like to see thermistor leads in the same cable as the motor leads. Whether it works or not, it just seems like bad practice to me. Not so much because of electrical noise (the switches are buried in the stator winding of the motor, after all) but because I hate to see low and high voltage signals mixed together like that.

Now, sometimes I see the thermistor circuit (motor "p" leads) powered by 120VAC and passed thru a relay at the drive. In that case, running all of them in the same cable would seem ok.

 

[blacksea]

DickDV: it seems (!) that you don't know that "thermistor" is temperature sensing resistor with negative temperature coefficient usually and operate by low voltage as wrote above. The termistor is'not "termostat" switch.

 

[jraef]

It appears to me that DickDV has the very common unfamiliarity with thermistors that I have observed here in the US, which I surmise comes from the fact that we generally do not use them. If you want a motor with thermistors imbedded in it here, you must remember to ask for it up front and generally wait 2-3 weeks longer to have it specially made. Then good luck finding a Thermistor Trip Relay off-the-shelf somewhere. It is true that because of world market demands, most VFD and now even Soft Starter manufacturers are including PTC thermistor input circuits on board. However, I have yet to find an electrician in the US who even knows what they are for until some manufacturers technician explains it to them!

Back to the point at hand though, I have seen countless submersible pump motor systems from EU manufacturers where the thermistor leads are part of the special submersible pump cable assembly, ITT/Flygt pumps being a good case in point. There is no practical way to separate them because there is only one gland seal where they enter the pump housing. Yet VFDs are applied successfully to these pumps on a mass scale. I have never heard of problems with the thermistor circuits in these applications. Comments?

"Venditori de oleum-vipera non vigere excordis populi"

 

[DickDV]

Aaaack!! My mistake! Sorry! In my circles, thermostatic switches are called "Klixons" which is a brand name, not a generic name.

My comments above are for thermostatic switches, not thermistors. Thanks for the midcourse correction!

 

[buzzp]

I think the bottom line here is it is NOT ideal to run these wires together if it can be avoided. It does not mean there WILL be problems but it MAY cause problems. And it probably depends heavily on the drive manufacturers input circuitry.

 

[cbarn24050]

So what your saying then Buzz is. 1 Op removes the motors terminal box and sends it out to an enigineering company to have an extra outlet hole machined. 2 Sends the motor to the rewind shop to have the thermister unburied from the windings and put in an electricaly quieter place. 3 He runs seperate armoured cable runs between motor and drive. I don't think so.

 

[buzzp]

Be real Cbarn.
The motor windings have a larger inductance than does the wires leading up to the windings. Therefore, I contend that the noise is less within the motor windings due to the smoothing affect had from the inductance of the motor windings.
Since your analogies are so far out there, let me show you the far end of your argument; what your saying is you could take an infinitely long cable (essentially what your saying for the sake of this argument) carrying 80A (or whatever it is) switched with a PWM drive and run a low voltage wire (signal) (bet it is no more 24VDC or 24VAC) next to it and not experience any problems. I don't think so, not without other considerations. Did you ever wonder why they make motors and cables especially for VFD applications now? This is due to the transients (noise) generated from the PWM switching was prematurely breaking down the insulation. Obviously, this is generating noise. The longer the cable, the higher the voltage spikes seen within the motor and cable.

Let me address your summary:
"1 Op removes the motors terminal box and sends it out to an enigineering company to have an extra outlet hole machined." Not necessary to do. They can come out the same hole but I would run them seperately back to the drive unless one of the situations I stated above are true.

"2 Sends the motor to the rewind shop to have the thermister unburied from the windings and put in an electricaly quieter place." Again, not necessary. I will let you call and ask them to take it out.

"3 He runs seperate armoured cable runs between motor and drive. I don't think so." Who said the thermistor wire had to be armored? I would use shielded twisted pair unless this a submersible pump application.

 

[cbarn24050]

Hi Buzzp, hit a raw nerve there. The harsh reality is that this is not your field. Maybe a little research on your part would help you out. If your suggestions were adopted by the op, not only would it cause him added costs but would also fail the regulations in this country (UK). You might also note that over here we have laws that prevent a drive maker from selling unfit products.

 

[ScottyUK]

Cbarn,

Please try to keep your posts civil.

I refer you to the manual published by Eurotherm for their Model 650 drive, specifically to page 3-3, the eleventh page in the PDF document.

http://www.ssddrives.com/usa/doc/HA464828U002.pdf

It shows in absolute clarity that the three-phase output wiring of the drive is within a cable provided with an electrostatic screen. The thermistor wiring is shown separately. Unless a custom cable is used, it seems impractical to screen the power conductors separately from the conductors you would assign to the thermistor wiring. Your proposal is therefore in contradiction of the drive manufacturer's recommendation, and in spite of the fact that it may work it is still bad design.





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If we learn from our mistakes,
I'm getting a great education!

 

[cbarn24050]

Trust me Scotty I was being civil. The document was too big for me to be bothered downloading but if you look on page 3 you will see that thermister wiring is regarded as a power connection. I know of no driver maker who recomends splitting the wiring as has been sugested.

 

[buzzp]

I guess you can not read or see then Cbarn. Better look a little closer.
I dont see why he would have issues with regulations in the UK but I am only familiar with product standards not safety standards of Europe. It seems with CE marking and the major added cost of this that the drive mfg would not show an installation that violated your safety codes.
There is no need to get defensive when you know you are wrong and there is no need to bring the US - UK comparison into the picture.

"Hi Buzzp, hit a raw nerve there. The harsh reality is that this is not your field."
I am glad you know me so well. I must be famous because I have no idea who you are but apparently you know me.

"Maybe a little research on your part would help you out."
No research will not help much here, it is called experience with noisy environments and precautions you can take to limit the affect of the noise while spending no money. The fundamentals are the same.
"If your suggestions were adopted by the op, not only would it cause him added costs but would also fail the regulations in this country (UK)."
How will it fail regulations? Also, I only pointed out the preferred way. It is up to him to evaluate costs vs. interruptions in operations.
"You might also note that over here we have laws that prevent a drive maker from selling unfit products."
You have regulations to make a safe product. I do not know of any REQUIRED regulations in the UK that you need to pass that state the quality or functionality of the product. There are marks for this, but to my knowledge these quality marks are not required. So your perception of what the CE mark is doing for you is very wrong. I have designed several products for use in Europe and your CE mark is equivalent to our UL/FCC mark. Both are SAFETY standards and neither care less whether or not the product works as stated.

I don't see the need to reply to any more of your comments given your defensiveness and attempts to belittle the US, for whatever reason. Your the type that will argue your point and never admit that anything you said was wrong or admit that what the other guy is saying is true. Good luck in your career, you will need it.