Why we need separate PTC device or module for VFD driven Ex Motor (LV)? 1

[ElecEE8]

Dear All,
Can anyone explain why we need a separate PTC device or module for VFD driven Ex Motor (zone 1& 2 classified area) for the thermal protection?
I received this requirement from my client. Currently our design is without this device means the PTC signal from Ex Motor going into the VFD directly.

Thanks & Regards

 

[waross]

As a WAG, the device is probably certified for Ex protection, even if the device or module itself is not inside the Ex zone.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[ElecEE8]

Thanks Bill!
Do we have any standards on this requirement? Please share if any.
To me the PTC device acts like barrier which we normally designed for intrinsically safe circuit which is most of the case is applicable for zone 0! But the EX motor is normally installed in zone 1&2 only. Correct me if i am wrong!

Thanks & Regards

 

[waross]

Sorry. My guess was based on general information.
However, when you mention "intrinsically safe", while you may have a VFD that has a PTC input, I doubt that you will find a VFD with an certified intrinsically safe PTC input. Hence a certified intrinsically safe PTC module is interposed.
Apologies if this is not to your point.
Another WAG, the applicable standard may be the clients internal engineering standards. Good luck challenging those.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Im confused - I looked up PTC device and found "Positive Temperature Coefficient (PTC) Thermistors" which are relatively low power devices. Either you have a low power application, or I misunderstand the question.

I only experienced somthing similar with a KSB sewage pump. In that case the "Pump Protection Module" provided by KSB with the motor, brings motor winding over temperature contact (and wet motor) out. The customer must connect the output into the run permissive control.

In the case of submersible sewage pumps the Ex / NEMA 7 type listing includes both the motor and the cord. The protection module will trip the motor long before the housing gets to the maximum temperature allowed by the type listing.

There can't be intrinsic safety in the VFD/motor power leads or the motor winding temperature circuits, the power levels are just too high.

 

[waross]

While the motor cannot be intrinsically safe, the PTC circuit may be.
The wiring methods for an intrinsically safe circuit are often less demanding and less expensive that wiring methods for Ex rated circuits.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Please carefully examine the NEC requirements for intrinsically safe circuits. It is more than just dropping a barrier into the circuit. Perhaps I am being too picky, an intrinsic safe barrier RTD repeater diagram looks like this.

It may contain a PTC, but a PTC alone doesn't not make a barrier.

Please verify if the PTC is a barrier or not. Then if the device is a barrier, see Articles 500, 503, 504, 505 and 506 for the requirements.

Fred

 

[waross]

Can anyone explain why we need a separate PTC device or module for VFD driven Ex Motor

The most important reason.

I received this requirement from my client.

And the customer should be willing to pay for it.
Call it an extra cost option and bill him for it.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

Here is what I think is going on.

Rules outside of North America for Explosion Proof motor applications where VFDs are used are different, requiring thermal devices inside of the motors as a supplement to the normal Overload Protection. SOME brands and models of VFD have an input for a PTC thermistor built into the drive control terminals, but some do not. The motor or equipment mfr has no control over what brand of VFD you use, so they require that a PTC relay be used, that way the system design is brand / model agnostic.

I have never seen one that requires intrinsic safety on the PTC thermistor circuit, but that may be a separate issue regardless of whether the thermistor sensing device is the VFD itself or a separate Thermistor Trip Relay.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Gr8blu]

Pretty sure we have a combination of <waross> and <jraef> as the final answer.

Customer specifies it as a requirement - end of story.
Reason for specifying? Either an attempt to make things interchangeable (any drive with any motor, as jraef points out), or even a simple "hard-wired back up" to the calculation being done internal to the drive. Kind of like having multiple sensors in the first place - if one is a dud, but the rest look good, then things are not (likely) in as much trouble as one might assume when seeing only the "dud" input and taking that as gospel.

Personally, I'm old-school enough to specify a hard-wired device as a measurement method, myself. Too easy to miss a linkup between a couple of lines of code somewhere and end up with a smoking ruin at 0200h.