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| picclock:
Hi Bernd Something doesn't sound quite right if they are those motors :scratch:, and the size you say sounds about right. 20 Amps is really too much for a motor of that type - I would think even 10 Amps is a lot for a single motor. Looking at the ride on car ads they seem to indicate motor powers of <40W which equates to <4 Amps. I am pretty sure the one my kids had was more than that (possibility of corrupt memory here) so I may be wrong. If you look at the ride on toy gearing you can relate that to the correct motor speed assuming a speed of 5mph for the toy. If I remember correctly the motor drive to the wheels on my kids one was with a toothed belt and a reduction gear - but it was a few decades ago. I would estimate that at around 30:1 which would only give a motor speed of around 2-3000 rpm. Stupid question but - when you measured the current were the meter leads plugged into the high current sockets on the meter ? (forgive my impertinence for assuming that you wouldn't do it that way). The other question is do you know what the off load (no prop) motor current is ? My guess is it should be < 1 Amp. When you make your PWM controller two things to look out for. Before the motor starts turning the current is only limited by the winding/brush resistance and the pulse width/inductance rise of current. So either large current FET's or ones that have a built in current/thermal limit. Before rotation the back emf of the motor winding causes a large current to flow through the flyback or commutation diode which should be rated to handle it. Failure of either of these components will likely cause smoke and flame if your batteries are up to it :zap: . Looking forward to seeing the project completed. Best Regards picclock |
| Bernd:
--- Quote from: picclock on January 05, 2011, 07:00:12 PM ---Hi Bernd Something doesn't sound quite right if they are those motors :scratch:, and the size you say sounds about right. 20 Amps is really too much for a motor of that type - I would think even 10 Amps is a lot for a single motor. Looking at the ride on car ads they seem to indicate motor powers of <40W which equates to <4 Amps. I am pretty sure the one my kids had was more than that (possibility of corrupt memory here) so I may be wrong. --- End quote --- I guess I better fess up and tell you what I'm doing. Both motors are from one of those toys. Can't tell you right now what it came out of but is was a bigger toy. There is an in-line fuse with a rating of 35AMPS, It's one of those car fuses. The motor's are actually run on a 6 volt battery, two of them to be precises. The motor's were set up for two speeds, fast and slow. I think the fast gave 8 volts at the motors. I'm going to run them on 12 volts since I don't have the friction I would if they were in the toy hauling a kid around. I've also run them for some time on this high voltage and they didn't get very warm. As I said before, this is an experiment in fun. If it works great, if not I had fun building it and designing it. --- Quote ---Stupid question but - when you measured the current were the meter leads plugged into the high current sockets on the meter ? (forgive my impertinence for assuming that you wouldn't do it that way). The other question is do you know what the off load (no prop) motor current is ? My guess is it should be < 1 Amp. --- End quote --- Not at all. The meter will only read up to 10amps. At 12 volts with the prop attached the meter flashed 18amps. So I'm guestimating. Just checked it with no prop. Motor pulls about 6amps at 12 volts, so we're looking at 72watts free wheeling. Here's some motor dimensions. 65mm long, 45mm dia. permanet magnets. The FET's I've ordered are IRF540 rated at 27 amps. If it burns those out I'll have to get one's with a higher amperage rating. Looking at using a 25 amp fuse --- Quote ---Looking forward to seeing the project completed. Best Regards picclock --- End quote --- Electrical parts are on their way here from Kallyforina. Should be here next Tuesady. I think I could have driven out there, picked them up and got back before then. Oh well that's what happens when you don't get next day air. :lol: Bernd |
| picclock:
Hi Bernd That makes more sense. "Just checked it with no prop. Motor pulls about 6amps at 12 volts, so we're looking at 72watts free wheeling. Here's some motor dimensions. 65mm long, 45mm dia. permanet magnets." With 72 Watts in the motor under no load it sounds like the motor core is saturating (too many volts). With all due respect, I can't see a motor of that size lasting long under those conditions. The size and description of the motor sound the same as the one for my kids ride on Quad bike - I could ride on it and often both my kids and a neighbours child would all clamber on it and ride around the garden. A very good toy. However I'm pretty certain the motors on mine were 12V (2x17Ah 6v red batteries if I remember correctly - would run for about 1 1/2 hours in continuous use). With a 35A fuse I'm guessing around 100 Watts per motor in original use. If you want them to last long enough to retrieve your boat run them off 6 volts and up gear the prop (and/or 3-4 blade big as you can get prop). If you can get the motor to use 10 - 15A at 6V with gearing/prop sizing this will allow the motor to run in its sweet spot allowing most of the battery power to be used for propulsive force, as opposed to heating and knackering the motor. Good luck and best wishes for the project. picclock |
| Bernd:
--- Quote from: picclock on January 06, 2011, 03:49:15 AM ---With 72 Watts in the motor under no load it sounds like the motor core is saturating (too many volts). With all due respect, I can't see a motor of that size lasting long under those conditions. The size and description of the motor sound the same as the one for my kids ride on Quad bike - I could ride on it and often both my kids and a neighbours child would all clamber on it and ride around the garden. A very good toy. However I'm pretty certain the motors on mine were 12V (2x17Ah 6v red batteries if I remember correctly - would run for about 1 1/2 hours in continuous use). With a 35A fuse I'm guessing around 100 Watts per motor in original use. If you want them to last long enough to retrieve your boat run them off 6 volts and up gear the prop (and/or 3-4 blade big as you can get prop). If you can get the motor to use 10 - 15A at 6V with gearing/prop sizing this will allow the motor to run in its sweet spot allowing most of the battery power to be used for propulsive force, as opposed to heating and knackering the motor. picclock --- End quote --- This makes more sense. I'll see what I can do. The only reason I was going for 12 volts was that the diagram I' working off of is for 12 volts. I'll try her at 6 volts. I think the props will push the boat. Only time will tell. Thanks for hashing this out with me. Bernd |
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