Medium Voltage VFD's and Output Filters

[Cooper Dog]

I am working on a project where existing motors are being retrofitted to new medium voltage VFD's (older VFD's are being replaced). The motors are 27 years old, 4000V, 2000hp, wound rotors, stators are not inverter duty rated. The motors will be converted such that they will become like squirrel cage induction motors, hence the new configuration will be that the stators will now be connected directly to the new VFD's where as before the wound rotors where driven by the old VFD's (slip power recovery type). Owner does NOT want to replace or rewind the existing motors and feels that they are in good condition to be operated with newer technology VFD's. This poses a challenging design since these motors are part of a critical process.

I have been dealing with various MV VFD manufactures and of course everyone one of them has their own version of what type of output filter to use to protect the older motors. One manufacture is adament about using sinewave filters which is the ultimate protection for the motors and provides a nearly perfect output sinewave, however, they are expensive ($43K) and are apparently intended for long cable lengths (greater than 1000FT). Most other manufactures recommend dv/dt filters ($13k) which are used to lower the fast rise time of the PWM waveform.

Can anyone tell me if they have dealt with a similar project such as mine and what their solution was for dealing with older medium voltage motors with new medium voltage variable frequency drives?

Regarding output fitlers, I am "on the fence" regarding which output filter to specify so any opinions and real life stories would be much appreciated!

Many Thanks

 

[weressl]

I am working on a project where existing motors are being retrofitted to new medium voltage VFD's (older VFD's are being replaced). The motors are 27 years old, 4000V, 2000hp, wound rotors, stators are not inverter duty rated. The motors will be converted such that they will become like squirrel cage induction motors, hence the new configuration will be that the stators will now be connected directly to the new VFD's where as before the wound rotors where driven by the old VFD's (slip power recovery type). Owner does NOT want to replace or rewind the existing motors and feels that they are in good condition to be operated with newer technology VFD's. This poses a challenging design since these motors are part of a critical process.

I have been dealing with various MV VFD manufactures and of course everyone one of them has their own version of what type of output filter to use to protect the older motors. One manufacture is adament about using sinewave filters which is the ultimate protection for the motors and provides a nearly perfect output sinewave, however, they are expensive ($43K) and are apparently intended for long cable lengths (greater than 1000FT). Most other manufactures recommend dv/dt filters ($13k) which are used to lower the fast rise time of the PWM waveform.

Can anyone tell me if they have dealt with a similar project such as mine and what their solution was for dealing with older medium voltage motors with new medium voltage variable frequency drives?

Regarding output fitlers, I am "on the fence" regarding which output filter to specify so any opinions and real life stories would be much appreciated!

Many Thanks

I have installed one 800HP drive on an older motor. The question of output filtering never came up as each manufacturer informed me that while it is a problem with low voltage motors it is NOT an issue for MV motors. Make sure you deal directly with an aplication engineer from the manufacturer, not with manufacturers' "representatives". Have the drives built for the specific characteristics of the motors, or characterized during startup.

Glad to see conversions of wound rotor motors. I have done one 200HP/460V about 20 years ago and the motor manufacturer was ADAMANT that it should not be done, without willing to offer any reasons that would stand the rigor of questioning. So I shorted out the slip-rings and let'er rip. I added an external thermocouple to monitor the temperature and could not get the motor to heat up under any operating conditions. They built those motors with much more slop in the design with lotsa copper and iron. It still works.

 

[beanland]

VFD Output Type

VFD Output Type

I have not dealt with large VFD but know that the type of output can impact the need for filtering. The problem is that high speed transients in the current feeding the motorwill lead to high voltage transients at the motor which can lead to winding insulation failure. An SCR-drive has large transients. But IGBT drives switch at very high frequencies. It is much easier to filter the high frequencies with a passive filter.

But, I agree with the other post, get connected with a VFD application engineer (not a salesman!) and discuss the issues.

 

[Cooper Dog]

Output Filters on MV Drives

Output Filters on MV Drives

Thanks for the response.

I have been talking directly with Application Engineers and NOT sales people so I understand what you are saying. Not too many applications such as the one I had mentioned so it has been a difficult project to get a any experiences or history. Most owners just have the motor replaced so they don't have to worry about unexpected downtimes during a critical process. But, this particular owner is more concerned about the dollar aspect versus the reliability of the system hence the reason to reuse the existing motors.