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I have an old treadmill motor:

Motor Specs:

Permanent Magnet DC Motor

Electrical Rating: @130vdc 2.5hp 6700rpm 18amps
Continuous Duty @95vdc 1.5 hp

I need a speed controlled drive circuit for it. I didn't want to spend $60-$100 on what seemed like a pretty trivial task. Create a PWM signal, run it through a MOSFET driver, drive my higher voltage with this MOSFET and thus PWM ~90vdc to my motor and control it's speed via a POT.

Well in theory it all worked. I polled a POT from my Netduino, giving me control of a PWM output signal. I fed this signal to a MOSFET Driver IC (tc4452), which would in turn drive my MOSFET(s). The MOSFET(s) would thus be switching the higher voltage.

enter image description here

FETs- http://pdf1.alldatasheet.com/datasheet-pdf/view/43633/SHINDENGEN/2SK2196.html Driver - http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en025917

Tested it out with a lamp(no scope at home) so I could see if I was on the right track, and it dimmed and brightened (thus modulated the 90vdc) just as expected. However, it did not run my motor. I hooked it all up, everything was cool until I started moving the POT higher and the motor started reacting and then FIREWORKS! My MOSFET was toast. I tried placing 2 parallel thinking it could possibly be too much current and this might divide it, but no dice. More fireworks! :) I always wear my safety gear when toying with this kind of energy though, so no worries. Just frustrating.

So rather than wasting anymore components I would like to ask some expert opinions on the matter. Are my FETs just not beefy enough for this? or is there a possibility that the FET driver isn't actually saturating the FET properly? I would like to hear any opinions really. I just don't wish to waste money on beefier FETS if I'm just not driving the type I have with enough juice, or if I'm just overlooking something really stupid.

Thanks a ton!

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  • \$\begingroup\$ Post your circuit layout. Its critical when you are designing a circuit running this much amperage. Tweeds answer is mostly on track, but you have to probe the gate to see what kind of signal its getting. It has to be very clean and fast with this level of amperage. \$\endgroup\$ – Erik Friesen Nov 16 '13 at 13:16
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4700Ω on the gates of the MOSFETs is far too high. These should be no more than about 100Ω.

With such a high resistance, the MOSFETs are switching far too slowly and dissipating way too much power, especially when the motor isn't actually running yet and passing its "stall" current.

Also, what is C2 in there for? Probably should be deleted.

One more thing: Although the body diodes on the MOSFETs will protect them somewhat, it would probably be better to add a beefy freewheel diode directly across the motor.

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  • \$\begingroup\$ Thanks for the speedy reply! The resistance thing makes a ton of sense, and if I can scrounge up some more FETs I'm going to test this ASAP. C2 however was on the documentation for the 4452. I was confused as well but it worked fine with the lamp so I figured that's probably not that case? The diode was an oversight actually, i meant to add this from the start and completely got sidetracked with this PWM stuff. Thanks for mentioning that! \$\endgroup\$ – While-E Sep 29 '13 at 22:36
  • \$\begingroup\$ Regarding C2: I think you misinterpreted the "Test Circuits" diagram -- the capacitor shown there represents the gate capacitance of a typical MOSFET, not a required additional component! Delete it -- it's just slowing things down. \$\endgroup\$ – Dave Tweed Sep 30 '13 at 0:11
  • \$\begingroup\$ Ty will do when I get back home! \$\endgroup\$ – While-E Sep 30 '13 at 14:04
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    \$\begingroup\$ 100Ω may still be too high. This really needs to be tried at low current levels with a scope hooked up to the gate, then the output, and adjusted until the output looks right. \$\endgroup\$ – Erik Friesen Nov 16 '13 at 13:19

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