Inverter (vfd)

[Lc85]

Hi,
I have been told that if the distance between the inverter and the motor is greater than 100m it's needs a choke and if its greater than 300m it needs a filter. Can someone explain these please?

Thanks

 

[Skogsgurra]

Those limits are VERY questionable. Lots of information available. Google PWM, reflected wave, transmission line, wave impedance and you will get more information than you want.

There are two types of information: Confusion and True Information. Sort out the 95% confused and you will be close to understanding.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ozmosis]

It also depends on the make of VFD. The manufacturer should be able to advise in their info the necessary data. Probably best to check motor cable type: screened or unscreened.
Also, do you have any EMI restrictions. This will also limit cable length depending on category you need to meet.

 

[X49]

The maximum distance will depend on many factors: VFD carrier frequency, motor/VFD size, and the system voltage relative to the motor's insulation rating. There is a good Allen Bradley white paper on the subject that should be floating around the internet.

You can analyze this to death with every drive / motor combination, or you can adopt useful rules of thumb. My personal rule of thumb is to always use a load reactor regardless of cable length (and an inverter duty motor). They are inexpensive and provide multiple benefits.

 

[Drivesrock]

There are some useful documents at Gambica. Here's one

http://www.gambica.org.uk/web_image...tion_Voltage_Stresses_Report1_3rd Edition.pdf

For the dV/dt effects it seems that the longer the cable, after approx 42 metres, the better. But then the voltage drops in the cable will certainly become a factor at 300m.

 

[luktek]

Often Engineers will use a rule of thumb in applying output filters, but it has more to than with just length. It also depends on the system voltage. For example, you should probably always have at least a load reactor on 600V vfd outputs. At 480V, the application of filters on the output of a VFD is a question of economics. A sine wave filter is expensive, but is the best solution for the life of the motor and cables. The two basic types, sine wave, and dv/dt, are essentially both LC filters. The difference is that a sine wave filter will actually shape the wave form so that you will not have the rapid rise and fall you see on a PWM wave, while the dv/dt filter will just clip the peak voltage of reflected waves at 1200V. A sine wave filter will also eliminate bearing currents and common-mode noise. Another factor to consider is the material your output cables are travelling through. If your cables are travelling through water, it is much more likely that you need a sine wave filter as the dielectric constant of water is 80 times that of air. If you have to feed loads such as non-inverter duty rated motors, step-up transformers etc.. then a sine wave filter is necessary. Consider also efficiency - sine wave filters can have significant losses associated with them - and you will also see a reduction in pull-out torque. You can also find VFDs that have a multi-level output, and this will reduce the requirement for output filtering. It is quite common for MV drives to offer multi-level output, and I am aware of at least one manufacturer that has a low voltage multi-level output VFD.