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

 

[ScottyUK]

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!

 

[cbarn24050]

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.

 

[buzzp]

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

http://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).

 

[aolalde]

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.

 

[jraef]

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/StdDrive...g/Documentation/Documents/ACS550-02-US-04.pdf

"Venditori de oleum-vipera non vigere excordis populi"

 

[ScottyUK]

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!

 

[EEIre]

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

 

[Skogsgurra]

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.

 

[GordS]

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.

 

[buzzp]

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.

 

[ozmosis]

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.