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AC motors and VFDs

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jonescg View Drop Down
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    Posted: 04 June 2015 at 9:00am

This link shows all the basics for how a three phase AC motor is controlled using a Variable Frequency Drive (inverter). It highlights a few important features too - like the need for a choke on the DC bus and precharge for the bus capacitors.

Well worth a read!
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Richo View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richo Quote  Post ReplyReply Direct Link To This Post Posted: 04 June 2015 at 12:50pm
A good place to start for some info.
Just remember this is for connection to mains not an ev.
The choke is to reduce the mains ripple / harmonics.
Smaller VFD's dont have precharge either.
Precharge only becomes an issue if it exceeds the current rating of the caps or the fuse.(poor design)

For the most part ev VFD's dont care about DC ripple and harmonics as much so do away with the choke.
And the DC bus caps are usually on the smaller side for the same reason which reduces the surge current and reducing the need for precharge.

Some people had issues with the surge current popping the traction fuse.
Mainly due to the poorly spec'd overpriced batteries in their system and tight fuse rating to protect their investment.
A cheap precharge helped this situation.
Seems to have become the norm now tho...

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