VFD - Motor Distance

[cesarm]

Hi, everybody,

Can you tell me What is the longest distance between a VFD and the motor controlled?. Voltage atenuation appart, what anything else could be happen?

 

[dpc]

At longer feeder lengths, standing waves on the feeder due to higher order harmonics in the inverter PWM output can create high voltage across the motor windings.

The exact distance will vary depending on the drive, carrier frequency, etc. The drive supplier can give you some maximum length guidelines.

A filter on the drive output will attenuate the higher frequency components and reduce this effect.

 

[Marke]

Another problem is related to the capacitance of the cable. As the length of the cable is increased, the capacitance is increased and this can cause issues with the chraging current at each transition of the output PWM waveform. Screened cables must be shorter than non screened cables.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Not to mention good old fashioned Voltage Drop, the effect of which is often masked with VFDs because of the natural tendancy to use them at reduced speed anyway. Wire sizing of course can mitigate it, but don't forget about it just because you have a VFD.

RF emmissions from the cables can get really nasty if not properly shielded. This happens at any legnth, but gets exponentially worse with cable length. If it's in metal (ferrous) conduit don't worry about it, but PVC conduit or direct burial unshielded cables can be a headache (I know from experience).

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[DickDV]

There is no precise answer but it can be well over 1000 ft. To do that, you would need to install a sine filter between the drive and the motor so the leads see very nearly a sine wave but at varying voltage and frequency.

A sine filter is not cheap or small but I've used them on motor leads of 850 feet without any problems.

 

[ozmosis]

It also depends where in the world you are: if you are in Europe/Australia/NZ where the EMC Directive CE/C-Tick is law then you will have restrictions imposed by the drive manufacturer depending on the Classification of EMC environment. If you have to meet the stringent levels of emissions for C2 (or even C1 but this is for quite specialised applications) then the manf will provide compliance as long as the drive is installed correctly and within the maximum stated motor cable lengths (with shielded cable). So whilst you can 'technically' run longer as mentioned previously, the lawful limit will take priority.