IGBT effects

[dinkelja]

Another engineer has told me that I need to locate my MCC closer to my motor loads " to limit IGBT effects on the system". I have a few medium loads between 30 and 60 hp. Could anyone provide a brief explanation about the determental effects of lengthy cables and how this is related to transients from IGBT switching?

 

[Skogsgurra]

He probably means that the high-frequency components in the motor voltage can be viewed as a voltage that is short-circuited in the cable capacitance so that the thermal effects of the extra current may influence the tripping of the MCC.

Usually, there is no MCC between VFD and motor. It is only when you have several parallel motors on one VFD that you need to protect the motor using an MCC. In situations where you have one VFD and one motor, the protection is built into the VFD.

Gunnar Englund

http://www.gke.org

 

[CJCPE]

dinkelja: I think you must be using USA terminology and talking about the distance from a motor control center (MCC) to the motors. If you are talking about IGBTs, the motors must be connected to variable frequency drives (VFDs) in or near the MCC.

With IGBT switching in VFDs, there is a transmission line voltage reflection effect that creates high voltage spikes from the square voltage pulses. There is a critical cable length that plays a role in that effect. Anything more than 25 feet is likely to be a problem. If you can put the VFDs really close to the motors, the voltage spikes might be eliminated.

For withstanding voltage spikes and for other reasons, it is also important to use only motors that are rated and nameplated for use with VFDs. There are also VFD output filter units (output reactors) that are designed to reduce voltage spikes. With VFD duty motors, perhaps with filters, the motor cable length won't matter so much.

 

[dinkelja]

Yes, to clarify I planned on mounting VFD's in (fed from) the MCC's (in a seperate area) because all motors are in washdown / GMP areas at least 100 feet away. It might be better to use NEMA 4X wall mounted enclosures along the production line then for the VFD's? I still need to find some appropriate shieded cable to feed the motors. Off to consult the controls guys about their equipment. Thanks, if anyone has a good printed resource on IGBTransistors in VFD's let me know.

 

[LionelHutz]

What voltage are the motors?

I'd say you should be safe by using the VFD's combined with output reactors in your remote location. There are also DV/DT filters (motor guarding filters) that do a better job than just a reactor. I definately would not mount your VFD's in a washdown area.

If you google something like "vfd reflected wave" you will find tons of info on the effect. However, every VFD, cable and motor combination behaves differently so it's very unlikely you'll find any useful hard data.

 

[Skogsgurra]

MCC is where the VFD is located? OK. I thought that it was a Moulded Case Circuit breaker with thermal protection. So, if it is about distance between VFD and motor, then the reflected wave answers are the right ones. We Europeans are not quite at home with the American terms.

Gunnar Englund

http://www.gke.org

 

[davidbeach]

MCC = Motor Control Center
MCCB = Molded Case Circuit Breaker

 

[Skogsgurra]

Thanks! Never get that right.


Gunnar Englund

http://www.gke.org

 

[jraef]

I'll chime in on NOT mounting the VFDs in the washdown area. NEMA 4 rated or not, they will eventually leak. I have not yet come across one that didn't over time.

As mentioned there are plenty of solutions out there for the kind of distance you are looking at. Seeing that you used HP instead of kW it appears you are in N. America. Look at MTE, TCI and Hammond for info on load reactors and filters.

Suggestions for cables: Belden (website works on MSIE only), Alpha, and Lapp

http:/

http://www.jraef.com

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

 

[2571]

as far as cabling is concerned try TPC wire and cable speack with there Engineers and they will assit you is your cable selection. line reactors try Allied electronics or Newark electronics

 

[2571]

are you dealing with a wye or delta system?

 

[2571]

correct me if I'm wrong here but I had heard of putting resistor to ground to cancel harmonic?

 

[Skogsgurra]

You are probably very wrong.

I am actually very sure that you are wrong.

Where did you get that information?

Gunnar Englund

http://www.gke.org

 

[Skogsgurra]

Afterthought: You may have heard about snubbers (resistor and capacitor).

They reduce HF components, but not harmonics.

Gunnar Englund

http://www.gke.org

 

[2571]

That's why I had asked for the correction, and this is why this forum is very important to me. I spend a great deal of time just reading and learning from everyones questions and responces. I'm very new to the electrical feild and have focus my career/life on being as vaulable as I possiablly can.

2571