Lowering Voltage, Upping Amps to Get Around Reduction Gears

[theyerb]

I think the title pretty much sums it up. Using an RC motor poses some problems with reducing the RPMs from the motor to the wheel. Is it possible to drop the motor voltage to something like 12 volts, but keep the amperage at something like 100amps? In my mind, I envision a motor that runs slow enough via a low voltage that it requires no gear reduction, but has so much power behind it that it could provide ample torque. I'm usually in the hub motor section and this RC motor stuff is pretty new to me.

Cheers,

G

 

[bzhwindtalker]

I think the problem would be:
heat dissipated power = R.I²
so efficiency will be lower...

exemple
if the motor's phase R at 100 mOhm (0.1Ohm),

At 120v and 10 A > 1200w
power loss = (0.1).(10)²=10w

at 12v and 100A >1200w
power loss=(0.1).(100)²=1000w


so i think it's not doable...


get fun

 

[theyerb]

Thanks for the input. I have lots of ideas, but I'm new to electrical theory! So what I understand is that amperage is the major form of energy loss, thus explaining why they transmit power using 400,000 volt lines. Makes sense.

Cheers!

 

[MitchJi]

Hi,

12v 1200watts?

I think its doable.

Here is a 24v/36v 2kw build. He switched back and forth between 36v and 24v:
http://endless-sphere.com/forums/viewtopic.php?f=6&t=5168
http://endless-sphere.com/forums/viewtopic.php?f=3&t=5156

Note the wire size!

If 24v 2kw plus is possible I think 12v 1.2kw is also possible. The question is is it a good idea? He was forced to use 24v instead of 36v because he couldn't get sufficient reduction. In other words he would have preferred a 36v system with greater reduction.

 

[fechter]

My old Zappy was running near 2kw with a 24v system. The motor wires need to be really fat and the motor needs to be designed for that amount of current (amps).

 

[oofnik]

In general it is easier to design a system to run on a higher voltage than it is to design it to run at higher currents. Energy loss is proportional to the square of the current, as bzhwindtalker pointed out. Since the motor's resistance remains relatively constant (ignoring effects of temperature), this basically means that you will be able to get more power out of a given system if you can design around a higher voltage.

Say for example I ran my setup on 12v instead of 40v. The motor would draw less peak current and reach a much lower top RPM, meaning my gearing would not have to be as high. But my peak power would be probably 1/10th what it is now. There exists a complex relationship between the electrical and mechanical components of such a system and I wish I had a way to explain it better.

 

[liveforphysics]

The motor laminations will saturate at some point, and all the extra current just becomes heat rather than useful magnetic field strength. If this didn't happen, a motor at stall would have infinate torque for any amount of input power.

High voltage and low current is the hot ticket. You can always re-wind your motor to use fine wire with a ton of wraps, and terminate it in Wye if you are looking to make a very low RPM torque monster with efficiency.'

 

[fechter]

There are some advantages to running lower voltages. Batteries are easier to deal with for one.

If a motor is designed to run on, say 12v, it will have fewer turns of heavier wire. Iron saturation is a function of the amp-turns (amps times number of turns), not just the amps, so that works out to be equal for a given motor size.

The big problem is resistance losses in the system, which as pointed out above, increase with the current squared. Still, you can largely design around this. Fat wire costs more than skinny wire, but you don't need very much. Again, the motor has to be wound properly, but there will still be a disadvantage to running a lower voltage in terms of winding resistance. The controller is where you have the biggest disadvantage. If the controller is disigned for a lower voltage, it's easier to find FETs that have really low on resistance. This may be hard to find in the marketplace. I've seen a lot of big pallet jacks that run on 24v though.

So, if you use fat wire, and have a motor with fat wire, you can be almost even with a higher voltage system, but the controller and motor heating will be limiting factors at higher power.