volts/amps question
 

If a motor is rated at 70amp max is that all it will draw from the battery? Will it draw the same amps(if bats have enough amps) on 2S? 3S? 4S and on up?

Also if it only can draw 70 amps what inside the motor limits the power? Thanks

The ratings you see (if they are noted in the specs, not always), is the maximum the motor will pull (peak) at its maximum rated voltage. I know Neu list the maximum amps and wattage achievable, so that makes choosing batts and escs easier. Generally though, you only see high current spikes under acceleration, or any time there is a heavy load o the motor- landing jump at WOT for example (bad idea).

As for what limits the power, its in the design of the motor. Smaller motors pull less current, and the number of turns/winds (kv rating ultimately) effects its power output; a higher kv motor will pull more amps than a lower kv motor of the same design/size.

To follow up on what suicideneil said - if you let it the motor will pull as many amps as needed for the demand and blow itself up in the process if you let it. There is nothing to "limit" the current.

Like he said for Neu - the max amps is the amount of current the motor can handle for 30 seconds and the max rated voltage.

For example (and this doesn't correlate to any specific motor):

Rating: 3000 watts max for 30 seconds, max voltage 30 volts - max amps = 3000/30 = 100 Amps for 30 seconds.

A few scenarios running with lipo packs:

4S = 14.8 volts; 3000 / 14.8 = 202.7 Amps @ 30 sec.
5S = 18.5 volts; 3000 / 18.5 = 162.2 Amps @ 30 sec.
6S = 22.2 volts; 3000 / 22.2 = 135.1 Amps @ 30 sec.

It's really just a measure of heat dissipation, power and the max RPMs of the motor (to calculate the max voltage a motor can handle)

OK thanks. I'm running a medusa 35-50-2200v2 and it says 700watts 75 amps and 22 max volts. Running the numbers 75amps at 22volts is waaaay more then 700watts. I was playing around with it today and ran a 3S for awhile and checked temps..MM was 110 and the motor was 98. Put in my 4S and and the temps rose to..MM 136 and motor 116. Figured since temps were running that low I might just run a 5S. I can see the motor raising temps because of higher rpms but I couldn't figure out why the MM would get hotter well really warmer.

Sorry still trying to get a handle on all this stuff. Thanks

Also the temps were without either of my fans running because I wanted a accurate reading.

Try a smaller pinion with 4 or 5S and see how your temps are.

Could be the 700 watt rating is a continuous power rating. I'm not really familiar with the Medusa motors.

Yeah, the 700watt rating is continuous power output. It must be a conservative rating they give them then, that's good to hear. It weights 230g.

Comparing that to a 1509 Neu motor that weights 215g, which has 900-1000 watts rating continuous output... so not far off of Neu's power density. So the medusa is roughly the equivalent to the Neu 1506 motors, which are very light at only 170g.

Your numbers aren't really true. The max power rating of the motor is based on a specific input voltage. The current does not increase with a lower voltage to achieve the same power as the motor is not a constant power device.

No, it's not a constant power device... but, that's what the motor can handle (before hitting specific temp) at different voltages... when running a lower voltage, it can handle more current passing through it's windings, this has to do with efficiency curves of motors at various voltages/rpms. It's kinda hard to explain, but you get it don't you?

I don't believe that's true either. Higher current in the motor will produce more heat, period, no matter the voltage applied to it. If it were designed to handle 100A for 30S, it's not going to be able to handle 200A for 30S just because you half the voltage. This has nothing to do with efficiency curves either, it has to do with I^2R losses in the motor windings.

this maybe a kinda thread jack, but its not too far off.
If the motor and gearing stays the same but the volts go up say from 4s to 6s Will the amp draw go up as well. Or will it stay the same because the gearing didnt change. I understand and correct be if im wrong but, if you gear down, the amp draw will drop as well correct? Or if you use a lower kv motor it will drop as well, but if the motor stays the same and just the voltage goes up. I know it will go faster(watts will go up) but will the amp draw go up as well, Or will the amp draw drop?

Wow looks like I started something.

Let's say for instance on my rustler with the medusa motor running 4S then putting in a 5S but gear it down too the same speed will I have the same run time given both bats are the same mah?

I just got done doing some speed runs geared 35/76 and it's fast but want to go more but I'm out of room for more gear unless I go milling out the slot on the motor plate.

Would like to get abit more run time but have enough power to gear up for some sick speed(with a 5S) and keep my motor nice and cool.

My MM with fan now was 121 with the 5 tooth larger gear. My motor was up to 136 but without the fan for over half the run(nasty end over end over end over end crash). Thanks

Yes, the current will go up when you go from 4S to 6S with the same gearing. Yes, gearing down will help reduce the current draw of the motor as you have reduced the torque load on the motor. At a given voltage, a lower kv motor of the same style will have a lower current draw. If you take that lower kv motor and apply a higher voltage, say 6S, it's possible it could still draw more current than a higher kv motor on 4S depending on the motor being used. However, the idea is that if a motor running on 6S draws the same current as a higher kv motor on 4S, you have essentially increased your power output by 50% while keeping power dissipation in the motor equal between both setups. Based on that, it's quite easy to see why a HV setup is preferable to lower voltage with high kv.

It's unlikely that scenario will happen. You'd have to make sure that the gear reduction itself reduces the current draw by roughly 20% with the 5S setup. I'm not sure that gearing for the same speed with both batts would do that. I think the only way you would really know how close you can match the current draw of the setups is to run them on an Eagletree or similar datalogger.