| T O P I C R E V I E W |
| boron |
Posted - Apr 19 2009 : 7:07:40 PM
Hello all,
I have been trying to wire a 350 watt 24 volt dc electric motor with PWM in order to control the speed of a scooter. The PWM works very nicely on a smaller scale when the signal is fed into a MOSFET. When I then scale it up to the full test, with the scooter suspended meaning no load, the acceleration PWM and motor all work beautifully.
However, when you try to ride it we hit some trouble. The rider gets on and manually gets the scooter moving, then engages the motor. At minimum speed the motor is not turning fast enough to move the rider. So he accelerates with the PWM. But then at a certain point in the acceleration, as the duty cycle increases and the motor starts to get to the point where it starts pulling the rider(not very far into it) the mosfet blows, shorting and causing the motor to attain max speed until we turn it off.
My question is why does this keep occuring? I have tried this with two different mosfets: IRFZ46 and IRF540N both blow in the same way at the same time.
I know that as the load increases so does the current flowing into the motor and mosfet. Does ohms law give the maximum possible current if I use v=24 and r= motor winding resistance? If not how can I approximate the maximum current that can occur?
Also, I have been using about 5 volts to activate the gate of the mosfet. Is that enough and if not would that be causing this problem?
Any tips or help would be very greatly appreciated. I think its just something I'm accidentally overlooking.
Thanks in advance. |
| 15 L A T E S T R E P L I E S (Newest First) |
| boron |
Posted - Apr 29 2009 : 8:45:14 PM
Alright so the diode is still in perfect working order. (turns out I bought one capable of handling wayyy more then enough current) However I believe the culprit has been identified... we were using alligator clips for a test and the clip connecting the middle pin of the pot to the input of the op amp was creating a poor electrical connection, it was loose and conducting/not conducting with the slightest bump... hopefully that's the whole reason for this, I'll try it out again tomorrow... |
| CurrentOverflow |
Posted - Apr 28 2009 : 4:12:27 PM
And it must be able to switch as fast as the pwm frequency right? |
| audioguru |
Posted - Apr 28 2009 : 3:04:32 PM
The current rating of the diode must be the same as the max current of the motor. |
| boron |
Posted - Apr 28 2009 : 2:30:17 PM
Yep that's what I thought. I'll double check all the connections and mosfet's next time I get a hold of the scooter.
We may also think that potentially the diode across the motor may have blown which would cause the current to be diverted, reducing power, but the pwm would still function. I shall try to check it asap. If this is the case, what should be the minimum specs for this diode? |
| audioguru |
Posted - Apr 28 2009 : 09:50:15 AM
The old Mosfets worked but the new Mosfets are very weak. Then something is wrong with the new mosfets?
Or you have a poor electrical connection to both drains? |
| boron |
Posted - Apr 28 2009 : 08:03:34 AM
I'm afraid I may have done something else stupid this time but do not know what... 
After that successful test, in which the mosfets were held down to the same heatsink with an elastic band, we decided to make the mosfet assembly neater and more permanent. We took two new mosfets and drilled holes into the heatsink (its like a small cpu heatsink)we then took two screws and screwed the mosfets through the hole in the top of their casing into the heatsink (gently) so they would not move and be secured. We then placed thermal paste between the backs of the mosfets and the sink. We realizaed that the screws and sink would cause the backs of the mosfets to be linked together through the sink but since they are in parallel figured it didn't matter. We then soldered with the appropriated gauges of wire the two mosfets in parallel and tested them with a multimeter before mounting in the scooter.
Here is what happened: When the rider gets on and tries to accelerate, it seems as if he has lost all power, moving along barely with any speed. If the back wheel is lifted (no load) the pwm works very nicely and the wheel seems to spin fast. With the rider, the mosfets do not blow, they don't short or smoke or anything, but as I said there is almost no power and the rider moves at a pitiful walking speed, not even.
The only difference between the test that worked and all subsequent tests was the mosfet mounting... but even after undoing that the problem now seems to persist...
I am now very confused as to what I did to cause the power to drop off like that? I have checked wiring and circuit boards and all seems fine... and its not the batteries because they were recharged and can propel a rider no problem. Any suggestions you have will be greatly appreciated yet again.
Thanks. |
| audioguru |
Posted - Apr 26 2009 : 07:41:20 AM
I am glad you fixed it.
The transistors will allow the Mosfets to be a little cooler but might not be worth it. Try them to see. |
| boron |
Posted - Apr 25 2009 : 8:40:18 PM
Well we tried a test this afternoon in which we replaced the lm324 with the TL084 and tried it without the transistors, the output of the TL084 was directly fed (through a resistor this time) into the gate pins of two parallel IRF540N's.
The result was that the scooter accelerated under load when throttled up. The mosfets worked lovely and did not blow. The driver was able to accelerate and decelerate no problem. After the ride (which lasted maybe max 30 seconds) the mosfets were slightly warm (although contact with the heatsink wasnt perfect and has since been corrected). So now that it works very nicely, will adding the transistors bring any extra benefits? ie. will the transistors maybe keep the mosfets a little cooler?
Thanks a lot for all your help audioguru. I hate to say it but you were absolutly right about the lm324  |
| audioguru |
Posted - Apr 25 2009 : 4:39:13 PM
A TL084 and many other normal quad opamps have an output of 30V p-p at 100kHz.
The low power LM324 has an output of 28V p-p at about only 1kHz. It can't produce 28V p-p at a higher frequency because the output is slowly ramping instead of switching quickly.
A 2N4401 and a 2N4403 have a max allowed current of 600mA and would make a good Mosfet gate driver. |
| boron |
Posted - Apr 25 2009 : 10:38:05 AM
alright. so what would be a suggestion for a "normal" op amp? I can get a TL084 which has the same pin output. Would that be better? Also in that case, what would be good transistors for the job?
Thanks. |
| audioguru |
Posted - Apr 25 2009 : 10:16:55 AM
The lousy old LM324 is too slow to drive the gates of Mosfets. It takes all day and half the night for its output to switch. Its slow output is a ramping voltage where the Mosfet is half conducting and its chip is heating a lot.
The LM324 also does not have enough output current to quickly charge and discharge the high capacitance of the gate of a Mosfet.
If you use "normal" opamps that switch fast then two transistors can be used as complimentary (NPN and PNP) emitter-followers to drive high currents into the gates. |
| boron |
Posted - Apr 25 2009 : 09:49:45 AM
Ah... okay I'll make sure to include that resistor this time round. (However on the circuits page it said it wasn't a necessity... I guess since I modified the circuit it now is...)
As in the diagram I have two capacitors, 0.1uF and 220uF going accross the 12 volt supply into the PWM circuit. There is nothing accross the 24 volts powering the motor. (Its set up so that two 12 volt batteries are powering the whole thing. one of the batteries does the PWM while both do the 24 volt motor, sharing the ground) Are these the caps you were reffering to?
Also, is there a minimum amount of current that must be fed to the gate pin of the mosfet? If so, is the lm324 capable of delivering that much current?
That's a pretty wicked pdf. I don't know if I can read all 37 pages, but its really interesting anyway. Thanks. |
| Gus B |
Posted - Apr 24 2009 : 07:31:39 AM
Look at...
http://focus.ti.com/lit/ml/slup169/slup169.pdf
Everything you ever wanted to know. |
| audioguru |
Posted - Apr 23 2009 : 10:16:45 PM
Mosfets oscillate at a VHF frequency when the very important resistor is missing that is in series with the gate and mounted at the gate pin. Mosfets blow up when they oscillate at such a high frequency. Try 10 ohms.
Also the very important supply bypass capacitors are missing that should be a 0.1uF ceramic disc and a 220uF electrolytic capacitor mounted very close to the circuit. |
| boron |
Posted - Apr 23 2009 : 8:17:07 PM
I have another question. On the diagram there is a resistor between the output of the lm324 and the MOSFET gate pin. I haven't been putting this resistor in place during the tests. Could that be a cause of this problem as well?
I performed another test with two IRF540N wired in parallel and this time the same thing occured. You give the scooter a kick start and engage the motor, upon throttling up it blows and kicks to full power. This time however the mosfets were warm afterwards and smelt.
So is that resistor between the output and the gate pin crucial and potentially also causing this to fail? If not, what can be done to stop this from happening? Will using the IRFZ46's and better cooling stop this from occuring?
Any suggestions would be appreciated. |
