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Old 10th August 2010, 09:30 PM   #1
dareon is offline dareon  United States
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Default Three Phase Brushless Motor Amplifier

I am working on designing a simple linear amplifier (current feedback) used for driving three phase brushless dc motors (for work). Currently each phase is controlled via a current cmd signal from an Aerotech controller. The DC motor windings are connected in a wye configuration. The amplifier works great if the center is connected to ground as well. However if the phases are just connected with no reference to ground the voltage outputs just end up controlling straight to the rail and getting stuck there and confusing the amplifier.

In the future I will be able to make the ground connection if need be but for testing at this point it will be rather difficult (not impossible). It would also be nice to have the added flexibility if it doesn't degrade the performance of the amplifier. I have attached a simplified schematic.

http://img820.imageshack.us/img820/9145/singlephase.jpg

I know its off topic but any suggestions would be greatly appreciated.

Thanks

Russell
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Old 10th August 2010, 10:21 PM   #2
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Are you using linear for simplicity, or for some other reason such as power factor or EMC?
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Old 10th August 2010, 11:12 PM   #3
dareon is offline dareon  United States
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EMC we work with precision measuring equipment and we can't tolerate the switching.
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Old 10th August 2010, 11:32 PM   #4
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Trace the current path. If you have no neutral connection, the current through phase A must return through phases B and C. So your driving circuitry has to allow for that.
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Old 10th August 2010, 11:35 PM   #5
benb is offline benb  United States
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I'm thinking add a "servo" feedback loop. Either take the Y center connection, or perhaps(!) equivalently, three equal resistors from the three phases to a common point, and have that go to an integrator with a time constant of several seconds (or lower than the lowest frequency this thing uses, but then if it's real 3-phase AC, the average should always be zero and it shouldn't matter). This output would then be summed with the CurrentCMD signal of each of the three phases. An op-amp integrator inverts, so this will give negative feedback, keeping the Y center connection at zero volts.

And of course this solution should be robustly tested to insure it never goes into oscillation, etc.
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Old 11th August 2010, 12:22 PM   #6
dareon is offline dareon  United States
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benb,

That sounds like it could work. I will give it try this morning and let you know.
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Old 12th August 2010, 08:17 PM   #7
dareon is offline dareon  United States
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The suggestion didn't work. I have tried on the motor and on just resistor in a wye network with basically the same results.

http://img217.imageshack.us/img217/3...yefeedback.jpg

Basically when the integrator figures out that the center of the psudo resistor WYE network has gone from zero it puts some output in an attempt to correct. But since all three channels get this signal summed at their current command input they all try to change their outputs to attempt to put out this new current. However since they are all changing in the same direction/rate the amplifier just rails because it is never able to make them put out any current.

Perhaps the problem is where the integrator feedback is going. Instead of summing it with the current command I could use U4 as the current feedback amplifier and then sum its output with the integrator out using U2. How does this sound?

http://img683.imageshack.us/img683/9...evoltagefe.jpg

This is the schematic for the updated feedback.

Thanks for the comments
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Old 12th August 2010, 10:57 PM   #8
benb is offline benb  United States
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Now I see my mistake - I set the integrator to sum with the CURRENT control signal, where it actually needs to control the VOLTAGE. Your second schematic puts the integrater's feedback inside the current feedback loop, at the point where the integrator DOES control the voltage of the output. I think that ought to work.
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Old 13th August 2010, 03:40 PM   #9
dareon is offline dareon  United States
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I hooked it up like that with no real luck. It kept going randomly unstable and I could not figure out why. I'm gonna have to call it good enough for now and just open up the drives and ground the common connection.
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Old 13th August 2010, 06:58 PM   #10
benb is offline benb  United States
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Hmm, sorry to hear it doesn't work. I'd think this should be doable, but I'm completely out of ideas.
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