DC motor used to power AC brushless motor

Thread Starter

Mr Dufresne

Joined Apr 23, 2022
2
Greetings all,
I have a few questions for anyone inclined to advise me. I would like to create a device to take rotational power from a small engine crankshaft (possibly using a DC motor as a generator), and use it to power a small AC brushless motor such as would be in a radio control car. I would also like to charge a battery (7.4V Li-Po or NiMH, or similar) in the circuit as a sort of buffer. So far I'm considering a 500 watt, 16.5V, 40A, 3000KV brushless motor, and about a 3,000mAh battery. Could anyone give me an idea what specifications I should look for in a generator or alternator? It will need to tolerate about 12,000 RPM, or be geared down. Also, what kind of circuitry is needed for the generator to charge the battery properly while still allowing current to go through a speed controller and then to the AC brushless motor. Also, if a DC motor is used as a generator to power another identical motor (to test efficiency or whatever), does the current stay DC, or does it need to be converted back to DC?
Thank you in advance for any advice
 

MaxHeadRoom

Joined Jul 18, 2013
25,206
You could use a automotive alternator as a DC generator, to acquire the 16.5VDC, you would need to build a simple field regulator using a LM311 comparitor IC, ( I have done this in the past).
In using a DC motor as a generator to drive an identical motor, the voltage remains DC.
 

Thread Starter

Mr Dufresne

Joined Apr 23, 2022
2
Thank you, I'll look into that. I would like to design a system that most efficiently takes off the crankshaft, DC to a battery (and charger), to an ESC, to a brushless motor. I think my main challenge will be the simultaneous charging/maintenance of the battery and supplying the ESC with consistent power.
 

MisterBill2

Joined Jan 23, 2018
11,879
That 3000MaH battery is only 3 Amp hours, not much at all in powering a 40 amp motor, which is 660 watts, by the way. An automotive alternator can supply the needed DC power, BUT they need to spin fast. Consider that the pulley on an alternator is usually half the diameter of the one on the engine crankshaft, so that at 3000 engine RPM the alternator is spinning 6000 RPM
 

MisterBill2

Joined Jan 23, 2018
11,879
The alternator starts delivering power at a lower RPM, but the rated power is at a higher RPM. So some experimenting with the engine speed will be required.
 

LowQCab

Joined Nov 6, 2012
2,045
This seems like a strange contraption being cobbled-up here ..........
What is the overall purpose of this project ?
Why a home-made generator as the power source ?
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MisterBill2

Joined Jan 23, 2018
11,879
It may be that the BLDC motor is a very convenient direct drive to a wheel, while the engine and generator can mount wherever is convenient. This may be some custom sort of vehicle. Also, the TS mentioned an RC car, so we really don't have even a hint as to the vehicle size.
 

Boggart

Joined Jan 31, 2022
13
This seems like a strange contraption being cobbled-up here ..........
What is the overall purpose of this project ?
Why a home-made generator as the power source ?
It certainly does, seems like a very inefficient system, more details as to the purpose of this thing would definitely be of use.
 

MisterBill2

Joined Jan 23, 2018
11,879
It is the same scheme as is used in the diesel locomotives for most of the railways, except that those drive motors are brush type motors, I believe. So the concept is not new, although the hardware would be more current.. Locomotives are done that way because starting a train with a clutch would be a serious challenge, and also shifting gears. Just imagine the size of that 12-SPEED TRANSMISSION for the hundred car freight train!
Also, it is much easier to get two electric drive locomotives to share the load.
So the fact is that sometimes the only way that works is not the most efficient possible way.
Consider all of those hydrostatic-drive mowers and delivery vehicles as examples. That may include some of the big cement trucks.
 

MisterBill2

Joined Jan 23, 2018
11,879
Railway locomotive motors have been AC variety for some many years now.
Each wheel set has a drive motor installed.
All electronic control.
OK, Max. My last discussion about the drive systems with an actual railroad engineer about engine drive systems was quite a while ago. Probably not current even then. So I am wondering if you can share those details with us? A direct drive AC motor scheme like that should be very interesting.
 

shortbus

Joined Sep 30, 2009
9,372
This is another one of those cases where 2 wrongs don't make a right.

Consider that the pulley on an alternator is usually half the diameter of the one on the engine crankshaft, so that at 3000 engine RPM the alternator is spinning 6000 RPM
Wrong. The pulleys are closer to 4 times different. Crank pulleys are 8 or 9 inch diameter, and the alternator is close to 2 1/2 inch diameter.

The alternator starts delivering power at a lower RPM, but the rated power is at a higher RPM. So some experimenting with the engine speed will be required.
Wrong. The alternator output is controlled by the rotor voltage. More engine RPM only lets less voltage be applied to get the needed current output.
 

MaxHeadRoom

Joined Jul 18, 2013
25,206
I did not get involved too heavily with the AC versions. a few years back however, I did get involved in converting two older retro DC locomotives that had seen duty on the flat lands of Ohio, they were purchased by CP Rail for passenger duty through the Rockies, and they did not have dynamic braking, so I was asked to do the conversion to bring them up to safety spec.
BTW, the DC versions were the same idea with a motor on each wheel set.

The end finished product after a visit to the paint shop. !
 

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MisterBill2

Joined Jan 23, 2018
11,879
This is another one of those cases where 2 wrongs don't make a right.



Wrong. The pulleys are closer to 4 times different. Crank pulleys are 8 or 9 inch diameter, and the alternator is close to 2 1/2 inch diameter.



Wrong. The alternator output is controlled by the rotor voltage. More engine RPM only lets less voltage be applied to get the needed current output.
An alternator's output POWER is LIMITED by the mechanical input power. The voltage is indeed controlled by the regulator system. But for standard passenger car alternator systems the rated amps are not available at lower RPMs.

On my vehicles the crank pulley is not that large a diameter, just a bit over twice the alternator pulley diameter. Not all car engines are the same.
 

MisterBill2

Joined Jan 23, 2018
11,879
Looking back at post #1 it does not seem like the vehicle is even big enough for an automotive sized alternator.
And we have not heard from the TS in a while.
And I am really wondering just what sort of vehicle it is going to be. That 16.5 volt 40 amp motor is serious, but the ""3000KV" has me confused. Or is that a typing error, and it should have been 3000KW? That would be around 5HP, it seems But 40 amps at 16.5 volts is 650 watts, not even one electrical HP. So the numbers get confusing now.
And it is mentioned that the alternator will need to stand 12,000 RPM, which is rather fast for an alternator. And that will not be a standard passenger care engine spinning 12,000 RPM.
So I am waiting for more information..
 
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