Should they be installed as close to the motors as possible or does it matter on the distance the motor is. Some of our motors are like 200'-250' away from the Control box. Would it be better to run the line that feds the power to the VFD to cover half the distance or run the power feed line all the way to the motor and then install the VFD???????
I was just looking at NORD VFD's they have a Software to program frequency inverters does all the VFD's have software like that.
As mentioned, over 200, maybe 250 feet, you add load reactors (5% or 3% of full load current, depending on who you talk to). This stretches out the pulses, rounds out the corners sort of.
Over 250 you can use a dv/dt filter up to around 1000 feet. This further stretches the voltage changes.
Over 1000 feet you are best to use a sine wave filter. Output of the sine wave filter only looks like a sine wave at rated current, frequency, and voltage.
There are youtube videos that explain reflected waves much better than I can. To summarize:
- VFD outputs voltage pulses that travel down the motor cable
- when the pulses hit the change of impedance at the motor terminals, they partially reflect
- this is a feedback loop .. so at specific frequencies it can build up to about 2X supply voltage .. Kind of like the microphone squeal when the microphone gets too close to the speaker... depending on the details of motor impedance and motor cable impedance.
IGBTs are regularly rated at 2X to 3X supply voltage for this reason (and others). The capacitors see some of this ripple as well .. in some cases. The capacitors I have seen are rated 20% - 30% more than system voltage.