This page created 04-10-2001

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Testing version 1 and 3

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Subject: [TCRG] Motor controller experiments
Date: Mon, 09 Apr 2001 17:11:09 -0500
From: Jeff Sampson
To: ""

I finally cleaned off my work bench so I could do some real testing.

I used my "one fet" driver and 2 - 24V 7Ah gelcells. I started by just running one of the 30V Ametek motors. I took some pictures of the oscilloscope. But the motor didn't draw much current. So I put my plywood RoboWar prototype on the bench so I had something to grab on to. It has a 12" diameter tire off my Earth Explorer. So I powered it up and took some more pictures. The final torque on the wheel is alot less than I expected when running at 24V.

Then I hooked up some power resistors. I have 3 - 0.33ohm power resistors. They look like they are about 300 watts each. So I wired them in series to get approximately 1 ohm. The waveforms were nice and square now. I ran it at about 10% duty cycle. Then ran it up to about 50%. Nothing was melting down so I went up to about 95%. I could smell my resistors and I didn't want to touch them. The heat sink was warm, but not hot. It was two or three times warmer than my 16 gauge wire.

I took some pictures of the gate waveform vs. the output wave form. And sure enough it is alot slower than it should be. (But I'm not surprised considering how I am driving it.) The turn on is about 3us and the turn off is about 150us. The FET is turning on and off faster than that.

But I noticed my 7Ah batteries aren't holding up as well as I hoped. So I now have my 18Ah batteries wired up. So I will go run some more experiments.

I will try to get some pictures on my web site before the night is over.

Jeff Sampson

Subject: [TCRG] First casualty
Date: Mon, 09 Apr 2001 17:39:30 -0500
From: Jeff Sampson
To: ""

Boy, MOSFETs sure stink when they go up in flames!!!

I left the driver running at about 90% PWM. I wanted to see if the heat sink would heat up if I left it running. I went back downstairs after 5 or 10 minutes and it was still running and the resistors smelled REAL hot. Just about then I heard the tell-tale PFFFFFT... And I had real pretty yellow flames coming out of my FET with the usual white smoke spewing out. About then I thought, "Hmmm... No fuses, I guess I better unhook the battery."

So I go it unhooked and checked the temperature of the heat sink. It was real hot. I guess I need to drive the gate a little harder. :-) And maybe that will taech me to not walk away from my power experiments.

PS. It really stinks down there now...

Jeff Sampson

Subject: Re: [TCRG] Motor controller experiments
Date: Tue, 10 Apr 2001 00:42:08 -0500
From: Jeff Sampson
To: ""

I have my version 3 MOSFET driver wired up. It uses a Telcom TC4427 dual MOSFET driver. I ran it without the battery supply hooked up. I am getting a rise and fall time on the gate of 200ns. Much better than 3us rise and 150us fall times.

So my next step is to connect the battery and attempt to blow up another FET. ;-)

Jeff Sampson

Subject: Re: [TCRG] Motor controller experiments
Date: Tue, 10 Apr 2001 03:26:50 -0500
From: Jeff Sampson
To: ""

I added a bunch of pictures regarding my latest experiments:

I haven't added all the pertinent descriptions yet.

I haven't decided if the first FET blew up because of sloppy gate drive (probably) or because it wasn't screwed down tightly or it just got too hot. I noticed the screw was loose when I was doing the post-mortem. But I also noticed the plastic shoulder washer had melted slightly. So maybe it was tight to start with and the plastic melted out from under the screw.

When I connected the battery for my new version the gate was ringing REAL bad. I had forgotten to add a series gate resistor so I added a 10 ohm resistor. That didn't help much. I ended up adding a 1000uf cap across the power input. (Which I should have had in the first place.) It fixed the problem as you can see in the photos.

My new one runs hotter than I think it should. Let's see, 25 amps and a .040 ohm FET. So 25 * 25 * .04 = 25 watts. The FET is quite warm to the touch and the heat sink isn't getting hot. So either I haven't run it long enough to heat up or the sil-pad is not transferring the heat. Right off hand I don't know if a sil-pad is better or worse at heat transfer than a mica insulator and heat sink compound.

But my R/C battery went dead and I had to stop experimenting. It gave me time to build the web page with pictures.

Jeff Sampson

Version 1 results

The setup for the testing.
The motor and wheel used as a
Motor waveform with approx.
10% duty cycle.
Motor at approx. 70% duty
cycle. The motor is running
at full speed.
Motor at approx. 70% duty
cycle. I'm slowing the motor
with my hand.
I switched to 1 ohm resistor
Approx. 70% duty cycle.
Approx. 95% duty cycle.
100% duty cycle. You can see
the motor commutation.
Gate rise time on bottom.
Motor voltage on top.
Gate fall time on bottom.
Motor voltage on top.
I switched from my 7ah
batteries to my 17ah
My FET after it blew up.
Another view of the bad FET.

Version 3 results

The setup for the testing.
The dual driver board.
The gate rise time. Battery
not connected.
The gate fall time. Battery
not connected.
The gate fall time. Battery
Close up of gate fall time.
Gate (bottom) and output
(top) after adding 1000uf
Gate rise time.
Gate fall time

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