A deeper look into transient voltage peaks

[serendipitor]

Here is a document that shows a number of oscilloscope captures of the types of wave forms with which many of you are familiar, and some discussion. I make some recommendations as to how layouts could be optimized a bit to hopefully avoid destroying too many FETs. I welcome any constructive comments, and I will continue to edit the document for clarity. Check back here for the latest version.

This is mainly for the case of solid state or slowly rotating machines. There will be a part 2 in a while, as I put together a similar approach for rotating machines at higher speeds.

Serendipitor

[serendipitor]

Here is Part 2 of this document. I have looked at some traces of rotating pulse motors, and have floated a few ideas on how they can be approached. Many of you will have tuned into these details already, but maybe some can benefit from a reduced level of cut and try building. If there are any corrections to be made, let me know.

[serendipitor]

I updated the PDF for Part 2 to make it more clear that at high RPM, the back EMF that causes unwanted back torque is feeding through the body diode of the FET switch, in the opposite direction from the drive current. While this effect can be bypassed with the use of blocking diodes, there is still too little recovery current available to be useful, so RPM would need to be reduced. At least, that is how it looks to me so far.

[jettis]

serendipitor, thank you for putting together these two documents, I took the time to read them and am grateful I did.

Dave Wing

[thaelin]

Consider using an IGBT instead of the FET which has no body diode. Then use fast shotky diodes and steer it on both sides to a recovery. You will find it works a lot better. As for the reverse mag effect, have no idea. I have seen >90% recovery on some setups. Coil size and Cap size matter a lot.
thay

[serendipitor]

I noticed this document from another thread which gives a method to steer the back EMF, but as he says, you are still going to pay for that recovery with torque, so it may not necessarily be advantageous. I am amazed by the ability of some to proceed without using an oscilloscope. We need in these efforts more clear data than "it works". But he has come to some of the same results I am seeing.

I have not yet gone through the same process as above using a cap dump recovery, but there are indeed a wide range of options to try.

Simple Adams Motor Concepts.pdf

[serendipitor]

serendipitor, thank you for putting together these two documents, I took the time to read them and am grateful I did.

Dave Wing

What a difference the new digital scopes make, in terms of just pushing the button and saving a screen shot to memory. I remember when a screen shot involved a mounting for a Polaroid camera on the front of the instrument, and hoping the brightness was sufficient, especially with low rep rate signals.

[jettis]

What a difference the new digital scopes make, in terms of just pushing the button and saving a screen shot to memory. I remember when a screen shot involved a mounting for a Polaroid camera on the front of the instrument, and hoping the brightness was sufficient, especially with low rep rate signals.

When it comes to the money you can’t go wrong, even when it comes to cheapest digital units, true blessings indeed.

Dave Wing