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3 Phase, 400hz Soft Starters

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#1 MSDav


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Posted 21 July 2009 - 09:01 PM

This is a totally new area for me so I'm on the front end of a huge learning curve. I guess if there is cause for hope it's that my experience is in embedded processor designs as well as software design. But I'm virtually clueless when it comes to power electronics and controlling scrs and soft start applications.

My task is to design a soft start for a 7KW, 200V, 3 phase, induction motor we are using. The power input is 200V, 400Hz. The frequency will vary +/- 30 Hz. I really have no requirements except to control the initial startup current.

I planned to use reverse parallel SCR, on each phase (6 SCR total), to control the starting current with some feedback to sense the load current. And I was going to use a micro board to control the firing of the SCRs. Seems pretty straight forward, in terms of blocking out the system. But it's the control and synchronizing that has me stumped.... so far. This is not going to be a VFD device. I simply want to control the inrush current to the motor.

However, I'm really at a loss of what I'm up against when it comes to 400Hz, 3 phase power. I mean I understand the 3 phase power and motor part but am stumped on how I'm going to control the synchronizing the firing pulses with 400Hz frequencies (or 60Hz for that matter). But my application is 400Hz.

So, I'll start with some simple questions:

1) Is there a good book (or web site) that can help me get up to speed on this?

2) I understand one method would be to control the firing angle of the SCRs. But that is easier said than understood. Is this something that can be done via a micro (i.e. PIC or DSP) or does it require external circuits.

3) Another method I have seen is referred to as picket fence gating. Which method #2 or this one, for controlling inrush, if that's really all that is required? Which is simpler to implement using a micro? Is a micro even the best method of control?

4) With regards to 2 and 3, I have seen graphics showing the pulses and when the current flows using these methods, but nothing showing how the control is actually implemented. Can someone explain or point me in a direction that will give a resource to study, to understand the synchronizing of pulses to phase?

5) I have found quite a few soft starters that are for 3 phase, 50/60 Hz but nothing for 400Hz. Does that mean that 400Hz is a more difficult problem to solve?

6) I'm more interested in learning how this is done, than just purchasing an off the shelf solution but if there are soft starters that could be purchased for my type of application that will work with 400Hz, 3 phase power, that is one option. But I'd rather learn how to do it wink.gif

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