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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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Old 06-14-2009, 12:24 AM
themotorman themotorman is offline
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Default FET and Power problems

Originally Posted by Farringa View Post
My friend and I have been working on our Senior Project for about six months now, and are just about out of ideas. This is our project:

We are making a model Power Transmission system, to show what the power company needs to do to be able to keep a constant 120VAC at your home. To do this, we are using a 24V, 22A, 2200 RPM, DC Motor to belt drive an alternator we removed from a Grand Prix. The alternator normally puts out ~14V to power your car battery, but we removed the diode pack, and are able to get a three phase voltage on the output (roughly 30VAC phase to phase, once a field current has been applied). When the load on the alternator varries (shown by turning on and off lightbulbs), we can adjust the voltage on the DC Motor to keep the voltage at the homes constant. We know this is a rough model, but this is the general idea.

Using this principle, we decided to automate the process. We thought this would be a difficult circuit to make, but once we sat down and designed the circuit, it ended up being far simpiler than we could have imagined. The circuit can be found here:

The Control Circuit works. We are able to turn on and off the motor using the potentiometer, and we are able to adjust the AC voltage, and turn on and off the motor. The trouble comes when we connect the motor to the alternator... it seems to be a current issue. But we have tried the following components in place of the IRF250:


They all are able to control the motor, and all of them fail when we begin to drive the alternator with the motor. Here are the things we have tried.

HEAT - At first we thought it was a heat issue. And it was, because when we were driving the motor alone, they did get hot and fail. We have now mounted the transistor on a heatsink, with a 60 CM fan blowing directly on it. We do not believe this is the issue any more.

PULSE CURRENT - This was the next thing we thought might the problem. It seemed that they were failing when we shut off the motor. At low voltages (less than 6 volts), we were able to turn the motor without issues. But once the voltage got too high, and we tried shutting it off, the motor would start to slow down, and then speed back up and stay on, leaving us with no control. We felt this was due to the motor acting as an inductor, and forcing current through the transistor's internal diode, and destroying it. We then put in a flyback diode in parallel with the motor, and this took care of the problem. At this point, we could continuously turn on and off the motor for long periods of time, as well as leave it on and turn it off. But once we started to drive the alternator with the motor, the transistors began failing again.

So, we started measuring the current and pulse current. When driving the laternator with the motor, we were drawing roughly 14A continuous, and roughly 72A pulse. These values are under what our equipment was rated for. We are not sure why they are failing... We figured it was still a pulse current issue, so we ordered the FDH44N50... Today we tried this, and fried two of them, though it appears to be a seperate issue. They smell hot, and the motor runs slowly, before speeding up. Once it speeds up, we know that the MOSFET has been destroyed. Again, it controls the motor just fine, but when we drive the alternator with the motor, it fails...

Trying to limit the pulse current, we placed a 470 uF capacitor in parallel with the 24V source. When did this, we actually had some success. We had have one of the NTE2389 working for roughly 20 minutes, driving the alternator. We were able to turn it on and off repeatedly, and leave it running for 4 - 5 minutes, and still have complete control. At this point, we started to apply a field current to the alternator. Encountering a different problem (now resolved), we stopped everything to ask one of our professors. We decided to show him the working circuit, and the first time we turned it on, we lost control of the transistor again.

So, after this long and drawn out post, I must apologize for boring you all. If you managed to get all the way through it, thank you. It is a difficult problem to explain, and I have a hard time puting down all of the troubleshooting steps. Please post questions, and I will answer them the best that I can. I appreciate any ideas and/or direction at all.

Thanks in advance for the help.

EDIT: See post #21 for pictures of the project.
First I'd go with a PWM system , However the mosfet failuer mode might be simple. The DC brushed motor puts out a lot of high voltage transients and they increase with the power and current. Sinc eyou have no easy way to stop them you can abosorb them either by using transient suppressors across the motor or sometimes a capacitor and resistor combo. Both of thes eideas will reduce the sparking.. and HV problems.
For a check try isolating the motor with a small inductor on each side of the motor as close to the brushes as you can and put a capacitor 10 nF seems about right and also put transorbs ( voltage limiting energy absorbers) across the brushes.
If this works and your mosfets are safe then the cooling issue will come up .. if you insist on a linear operation then you will have to use enough mosftes and heat sinks to handle the power.
Nice ideas to play with.
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Old 07-01-2009, 09:32 PM
themotorman themotorman is offline
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I have similar problems with DC motors driven by PWM . I think the failure is due to a gate failure although I have tried diodes across the gate-source and also a transorb ( non-linear semicoinductor) and a zener. I find that i have very high spikes with severe ringing at each switching time even though I have ultrafast diodes acorss the motor , ferrite and cap suppression for sparking of brushes. The probelm is one of high currents, low resistances and high voltages.. a difficult mix!
I do not think it is the brushes as I can get a failure with the rotor locked no sparking then. . In th e ideal world I could keep all wires very short but in an EV it is hard to keep battery leads, and or motor leads so short that they do not present a high impedance to the very high instantaneous currents. Putting a large cap across the line seems a possible idea but the physical size makes this impractical. I have tried slowing down the MOSFET drive but this will increase the switching losses, and gets rid of the advantages of a MOSFET, and still had failures.
I kknow it can be done as there are several commercially available controllers that claim to be bullet proof.
I would like some ideas to try as I am building up a supply of blown MOSFETS... BTW if the gate is shorted to the drain after failure then the failure is usually due to gate oxide breakdown. This can occur in nanoseconds so difficult to see happening .
any one with a bullet proof circuit for 50 amps 200 volts system??
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Old 07-02-2009, 12:00 AM
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beenthere beenthere is offline
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Welcome to AAC.

I am not sure how well related your two posts are, but, since they both hijack this thread - - I though I would keep them together.

Possibly you could post up a schematic so we can comment on the hardware?
Perhaps you see how difficult it becomes to reply to questions when they are posted in another's thread?
First comes the hardware, then the software.
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Old 07-02-2009, 04:44 AM
DC_Kid DC_Kid is offline
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Posts: 504

a few suggestions.

1. use avalanched rated fets
2. use a zener to ground to help protect the fet
3. what henries is the motor?
4. ringing on the drain after fet stops can be squashed with a ceramic and resistor to ground (snubber)
5. what fets were you using?

50A @ 200V doesnt sound like much.
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