×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Motor Thermistor wiring
2

Motor Thermistor wiring

Motor Thermistor wiring

(OP)
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

RE: Motor Thermistor wiring

2
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.

RE: Motor Thermistor wiring

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.  

RE: Motor Thermistor wiring

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.

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

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

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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!

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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"


RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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"


RE: Motor Thermistor wiring

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!

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.



RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.



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

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

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

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.

RE: Motor Thermistor wiring

Cbarn,

You will surely admit that the page of the Eurotherm manual that I referenced earlier clearly shows a SEPARATE cable for the thermistor wiring. Perhaps you know more about this subject than the designers of the drive in question? But I doubt it.

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

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

RE: Motor Thermistor wiring

Hi Scotty as I said i was not able to download the whole document, is there a written recomendation in the test somewhere?

Buzzp, if you know of a drive document that makes this specific recommendation then kindly give me the reference. The CE mark is not just a safety mark, we all wish it was. You stated that the extra cable need not be armoured, that is a requirement over here, I'm guessing it is over there as well.

RE: Motor Thermistor wiring

It is the drive in question. Follow the link provided (http://www.ssddrives.com/usa/doc/HA464828U002.pdf) to see clearly that they are saying the thermistor wire is seperate from the motor leads. If the link does not work then only put in www.ssddrives.com in your browser, then add the rest after you get to the page to get to the pdf file.
Also, the cable for the thermistor should be shielded not armored.  When I say armored, I mean protection from mechanical damage not necessarily electrical noise. A shield over here is braided wire or foil wrapped under the insulation, around the internal wires, to provide some immunity to electrical noise. And usually only grounded on one end. This thermistor wire is not required to be armored in the US. Heck, it does not even have to be shielded. Why would you put armor (as I define it) on a thermistor wire? Thats why they make conduit. We have real armored cable over hear that is often used in submersible applications for mechanical protection as the casing/pump/motor is lowered into the hole.
The CE mark includes EMC issues like the FCC over here. There are guidelines to be followed by the installer in order to assure conformity with EMC directives. Apparently this seperate wire requirement is one of them, although there is no mention of this in section 3-3 (might be mentioned at the beginning of the document).  

RE: Motor Thermistor wiring

Just a clarifiying note.
THERMISTOR = Thermal + Resistor

It is a semiconductor that has an abrupt resistance change when the triger temperature is reached.
It could be with positive temperature coeficient PTC, or negative temperature coeficient NTC.

The thermistors are normally an extension of the electronic circuit relay that protects the motor for overtemperature and regularly work with a few milliamperes.

RE: Motor Thermistor wiring

Gentlemen (and please let's keep it that way),

I at first was on the questioning side of this issue because of my experience with submersible pump wiring not being separate. As a result of this thread, I decided to do some informal research on VFD's that I am familiar with, approximately 10 manufacturers that I have quick access to installation manuals.

To a one, they side with Buzzp on this issue. They all very clearly indicate that the PTC Thermistor inputs are considered as any other control signal and should be spearately run and shielded (screened) from the power conductors. I found less cohesive information as to whether the power conductors needed to be shielded than I did on this issue. The likely reason is that in almost all VFDs that have PTC input capability, the +24VDC power source for it is the same common power source for all of the inputs, analog and digital. So any noise coming in on the PTC wires would be affecting other signals as well. None of them even mentioned having filtering of any sort on these inputs, and most in fact instructed the users to utilize one of the Analog Inputs if they wanted to hook up a PTC Thermistor.

ABB has some additional info that I did not see mentioned elsewhere. Look at page 17 of this pdf file. It is rather large, so for those who dont want to download it here is the text.

"WARNING! IEC 664 requires double or reinforced insulation between live parts and the surface of accessible parts of electrical equipment which are either nonconductive or conductive but not connected to the protective earth.
To fulfil this requirement, the connection of a thermistor (and other similar components) to the digital inputs of the drive can be implemented in three alternate ways:
1. There is double or reinforced insulation between the thermistor and live parts of the motor.
2. Circuits connected to all digital and analogue inputs of the drive are protected against contact and insulated with basic insulation (the same voltage level as the drive main circuit) from other low voltage circuits.
3. An external thermistor relay is used. The insulation of the relay from measuring circuit to output contact must be rated for the same voltage level as the main circuit of the drive."

https://www.abb-drives.com/StdDrives/RestrictedPages/Marketing/Documentation/Documents/ACS550-02-US-04.pdf

"Venditori de oleum-vipera non vigere excordis populi"


RE: Motor Thermistor wiring

If we have finished our internal argument, perhaps we should try to answer Tonerlow's original question:

The recommendation of the drive manufacturer is to use a separate cable for the connection of the thermistor to the drive. There is practical experience that indicates that combining the signal and power wiring may work. If you do this, you are not following the installation method prescribed by the manufacturer of the drive, so if you run in to difficulty or the drive malfunctions then you are pretty much on your own. If this is a commercial product, consider the legal aspects of a malfunction being traced back to improper application of the drive.

I'm not sure how CE marking of a system is affected when the system employs a CE-marked component in a manner not intended by the manufacturer - I would guess the CE mark would be invalidated. Someone from a manufacturing background might be able to answer this?

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

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

RE: Motor Thermistor wiring

In the bulk pharma, oil/gas and semiconductor industries in which I have worked the standard is to use a seperate cable for the thermiser cable. I also seem to recall that this is recommended by most manufacturers I have used.

This is not a CE issue; as I understand it a CE mark shows that a product conforms to the relevant EU directives and may be sold in the EU. It does not define how a product may subsequently used or installed. This is my understanding of CE and if someone else knows differently please post below

Regards

Sean

RE: Motor Thermistor wiring

Now that the dust has settled. I think that a short overview of protective elements is in place.

Thermal switches (Klixon and othe brand names) are bimetal strip switches with snap action. They can handle rather high currents and voltages. Sometimes used directly in series with motor winding. Reset themselves when temperature gets below trip point minus hysterisis. Usually input to DI on the drive.

PTC thermistors are "semi-switching". They have a cold resistance (tens of ohms) and a hot resistance (tens of kiloohms). The transition is at a well defined temperature and the action is similar to that of a thermal switch. This could be the reason for some of the arguments in this thread. Reset themselves when temperature gets below trip point, no hysterisis. Usually input to DI on the drive.

NTC thermistors are continously variable temperature dependent resistors. They can be used to measure the temperature although their non-linear temperature dependency needs linearisation. They have no switching action and need to be connected to an AI on the drive. Some drives have dedicated inputs for the NTCs and the evaluation is done in the drive program.

Semiconductor temperature sensing devices like the KTY 84 series have a positive temperature coefficient and should be called PTC devices. But that was already taken and has gained acceptance to such a degree that it is not used for these devices - although there is a logical reason to use it. They are more linear then PTC and better suited to measure temperature.

All these devices are not rectifying and can take a lot of noise without disturbing the "pay-signal". The inputs can be heavily filtered and it is usually possible to put the signal wires close to (or in) the motor cable. But, it is usually not recommended.

RE: Motor Thermistor wiring

The main thrust of the responses in this thread have centered around noise and whether the thermistor ciruit will work. The other aspect ( a lower probability event) is what happens if the power ciruit faults to the thermistor wiring anywhere along the ciruit? I am quite familiar with jraef's excerpt from the ABB manual since I am looking at a dead ABB drive fried because I had a 575V motor fault internally to my thermostat wiring. (We are going to isolate the thermostat wiring from the drive with a relay).

On a related issue, the power current will want to induce a voltage in the loop of wire comprising the thermistor ciruit. The resulting voltage needs to be compared to the withstand voltage of the drive input point.

RE: Motor Thermistor wiring

On a CE note, some manufacturers may have explicit installation instructions which need to be followed in order to maintain CE marking. I remember this in my CE classes (used to design low cost relays for Europe). However, I do not recall anything in our relay installation that had to be followed in order for the mark to be valid. I have not seen any mfg with this type of installation issues. Just know that it is possible to require certain installation procedures be followed in order to maintain the CE marking (such as front end filters or similar)(maybe this has changed in the last five years). In the US, this is the difference between UL listed and UL recognized. A listed product has no other requirements whereas a recognized product may have additional requirements (such as fusing - even though it may require fusing from the NEC).
As I said before, the bottom line is to avoid running these together if possible based on cost, application, etc.
But, as a general rule, it is not good to run higher current carrying conductors (such as motor conductors) next to LV signals such as thermistors, RTD, thermocouples, etc. Enough said.   

RE: Motor Thermistor wiring

Tonerlow
The final line from buzzp is correct.

You will not find the answer reading any CE standards, you have to consult the individual drive manufacturers installation guides/recommendations.
The basic problem is that high frequency noise (RFI) radiated by the motor cables could possibly migrate into the thermistor cable if it is in close proximity, and if these cables are within the same conduit then there is a high chance of this. The connection of the thermistor cables into the control board of the VSD can bring all this RFI straight back into the heart of the drive and potentially cause problems with functionality of the drive etc. The RFI can also pass into other control cables and then travel to other parts of the system. If you have a BMS or PLC connected a distance away, then you can potentially be putting HF noise into other systems, and so on. You want your HF noise to go to a good earth not pass into other cables, this is why bonding of the motor cables is a key issue.
It doesn't really matter whether it is a thermistor, RTD etc, it is the fact you have a control cable next to your motor cables.
I have seen myself, and taken scope shots of the noise generated by thermistor cables within the same conduit as motor cables and it is not very nice, especially if the cables are long.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources