Article 430.126

Status
Not open for further replies.
L

lutherhd

Member
How do the changes in motor "overtemperature" protection required in this paragraph pertain to retrofit applications when the customer is just installing a VFD to replace existing motor control but wants to retain his existing motor?

Does the motor need to be replaced or re-worked to install thermal protection?

Lutherhd
 
Most VFD's provide overload protection. Its programmed into the VFD: motor FLA, SF, voltage. The drive program limits the motor current regardless of the load.
 
Tom, I'm concerned about protection from "over-temperature" not over- current. All VFD's include electronic means for limiting current to the motors which is intended to mimic traditional overloads, but with VFD's the condition exists where you can operate the motor at full load, but at a greatly reduced speed. Under those conditions, the motors will overheat due to lack of cooling air flow. I think that is what this new area of the code was intended to protect against.

Question is, what about "retro-fits" where the starters are replaced with VFD's, but the motors remain the same. Do they need to be upgraded, etc?

Lutherhd
 
I think Tom is still right. Look at 430.126(A)(2). I think that means the VFD keeps track of the power delivered to the motor, and it also keeps track of the motor speed. Using both those together the VFD should be able to approximate the motor temperature.

With standard overload protection, the VFD speed would not be a factor in the equation.

Steve
 
Yes you are correct on overheating. Most standard motors can run down to 30 hz on a VFD without overheating (cooling air is from a fan on the stator). If you want to run at a lower speed then you need a special motor.
Just make sure you understand that older motors may not last long on a VFD, due to the high voltage ringwave the VFD creates. Most polyphase induction motors have "invertor duty spike resistant windings".
NEMA has good information on these issues at their website.
 
Thanks for the replies. I'll check with more VFD suppliers then, as so far none of them will confirm that they can perform these calculations, and have the thermal memory retention on power loss or shutdown as required. That sure would make life easier.

Since I'm familiar with motor designs, I also couldn't figure out how the VFD would know if the motor was NEMA design B, C, D, E, TEFC, Open drip-proof, TENV, what the ambient temperature was, etc. All have a dramatic effect on heating and cooling times. That's why I was leaning heavily towards using thermals in the motors as the only logical solution, if required.


Lutherhd
 
I do not believe that the thermal overload requirement would apply unless the motor unit itself were to be changed. The electronic overload in the VFD software will NOT provide the same protection as a thermal unit in (for example) the motor windings visible from the j-box. Most all the VFDs I've seen have LV inputs which can be programmed to act as external fault signals, in this case supervised NC, of course. It's not such a big job to add this into a VFD upgrade, and makes sense.

The motor duty is programmed into the unit and at this point some common sense is required. The latest VFDs are sensorless vector drives which you just tell the voltage and number of poles, then they actually 'read' what's at the end of the wires, memorize it, and then all you do is set up the speeds, etc., which you expect. If this installation is running now, there's no reason to expect that a properly programmed VFD will create any more heat than running the unit from a starter. Of course, if you constantly reverse a fan's direction with 2 second accel/decel times, ...
 
Wow, I just reviewed the proposed changes for 430-126(A)(1) thru (4) for the 2008 edition and think it's gone from bad to worse!

I think we'll still recommend installing thermals in the motors as the only logical approach to compliance for this one.

Lutherhd
 
Status
Not open for further replies.
Top