Motor PTC Thermistors

[nightfox1925]

Our contract requirements requires even a 5.5kW LV Induction Motor to have PTC Thermistors in a separate terminal box. One of our motor vendors applies for waiver because their PTC Thermistor connection are inside the motor main terminal box (with the motor main terminals).

Is there any motor vendor who could provide such contractual requirement for 5.5kW LV Motor? Is there any set backs for making both motor terminal and the PTC Thermistors in one common Terminal Box?

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[arcflash99]

They probably want to bring out the thermistor wires with the motor leads because on LV motors there usually isn't much room to machine another hole in the frame. As long as the rating of the insulation of the PTC thermistor cables are rated for the motor's operating voltage I wouldn't see a problem having both sets of wires in the same box.

My 2 cents.

 

[powerjunx]

Thermistor wires on separate terminal box is uncommon, particularly on general purpose machine. Except, on large and custom built machines.

Nightfox, what exact purpose upon segragating such wire?

 

[nightfox1925]

I also agree with you especially for LV motors since the insulation voltage levels are the same (unlike HV motors which requires voltage segregation). It is just because it is embedded as a written standard requirement by our client which I am sad to say...needs some improvements.

I will inform the vendor to apply for a waiver and supply me of a technical justification (i.e. motor frame size is too small, not a manufacturing standard, etc.)

Thanks, your inputs provided me confidence...

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[ScottyUK]

It is fairly common for motors for use in hazardous locations to be equipped with thermistors. In some jurisdictions it may be a code requirement.


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Sometimes I only open my mouth to swap feet...

 

[ozmosis]

If the motor is controlled from a variable frequency drive and the thermistors are connected into the VFD as well, then it is good practice (not necessarily common practice) to separate the PTC cables from the motor cables due to the high level of EMI (Electromagnetic Interference) that occurs on the motor cables. If the ptc's were in the same conduit as the motor cables and then the ptc is connected directly onto the control board of the VFD, the high level of intereference can be quite damaging to VFD's.

 

[powerjunx]

sed, you're right as pointing out the reason for the drawback when both wires contain in one conduit but the issue is to separate PTC terminal from the motor terminal box.
However, it is impractical to make a terminal box for PTC itself. The motor terminal box is enough to confine such tiny PTC terminal block. Consequently, in order to avoid such drawback as Sed pointed out; motor terminal box has blinded hole to cater for the installation of the conduit for PTC terminal only, as long as the motor you purchase has PTC.

 

[EEBubba]

In this case I believe that the PTC thermistor lead could be brought out in the same box as the main power leads. The operation of the PTC device is a very non-linear resistance response to temerature. I am unaware of any noise issues with PTC sensors.

What might be suggested is to put a daughter box on the side of the main terminal box.

PTC operate much like solid state thermostats, and thier trigger is a steep resistance change. If you are using a sensor that uses an small voltage, or very small current differential (Thermocouples and RTDs) you may want to pay close attention to induced noise.

Hope this helps,