# N00b question about amp draw



## major (Apr 4, 2008)

Dougnutz said:


> Is this just logic built into the controller?


Hi Doug,

You might say that  The controller doesn't know what gear you're in and therefore behave differently. It behaves the same, but because you're in a different gear, you perceive the response differently.

You have a different RPM/mph ratio 1st to 2nd. Also a different torque multiplication, motor shaft to wheel. And less reflected inertia at the motor shaft in first. And then throw in another variable of throttle position. All this determines the acceleration, or rate of mph increase, and the RPM rate of increase. What you describe sounds normal to me. It could be better explained if you had the data recorded for the events including throttle and mph.

Regards,

major


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## Dougnutz (Aug 22, 2011)

Major,
Thanks for the reply. But I think I may not have been really clear in my original post. The part that seems odd to me is that the controller won't deliver high current (amps) at low rpm. 

It's not really noticable in first since the rpms climb very quickly, But if I start off in second or third I can floor it and only see 100 amps initially which will slowly climb as the rpms do.

I guess I would expect to see amps increase with throttle not rpm.


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## Yabert (Feb 7, 2010)

Dougnutz said:


> I guess I would expect to see amps increase with throttle not rpm.


It's true with motor amps, but if you monitor the battery amp, that seem normal.


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## Dougnutz (Aug 22, 2011)

really? Motor amps don't correlate to battery amps? That's fascinating, how does that work?


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## major (Apr 4, 2008)

Dougnutz said:


> really? Motor amps don't correlate to battery amps? That's fascinating, how does that work?


Hi Doug,

We've gone over this time after time. But I guess it might be difficult to search for. Motor current is always higher than battery current. At or above base RPM where the controller is at 100%, motor current is equal to battery current. The ratio of (motor amps)/(battery amps) = (battery volts)/(motor volts). 

So when starting from a low motor RPM or standstill, motor voltage is low and battery amps are low even though motor current is high (current limit if you have it floored).

Regards,

major


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## Dougnutz (Aug 22, 2011)

Thanks Major,

I did try searching but mainly found a lot of dicusssions about dyno results and motor models. Alot of which I don't really understand. I'm used to looking at ice dyno charts 

What your saying about (motor amps)/(battery amps) = (battery volts)/(motor volts) makes some sense to me.

let me see if I understand this: maxed out the controller should produce 500 amps so the equation would be something like 500/100=144/mv meaning the motor voltage is about 28.8? 

What causes the low motor voltage? Is it a result of the pwm of the controller limiting the amps?


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## dladd (Jun 1, 2011)

I'm a newbie, lets see if i get it... think in terms of power. (i keep hearing people say that  ) Say you have a controller capable of 500 amps on the motor or battery side. Also say you have a 120v battery pack capable of at least 500amps. At low RPM, there is low voltage going to the motor (motor RPM and voltage are more or less linearly related). But, power in = power out, so you will have high amps. Limited by the controller at 500. So, while the battery pack is capable of putting out 500a at 120v, the motor is only at, say, 50v at low rpm, and the controller is limiting the motor to 500a. So, on the motor side you can only draw 50v x 500a of power, which is right about equal to 120v x 200a at the battery. So, a shunt between the controller and the motor will see 500a, but a shunt between the battery and the controller will only see 200a. Power is the same.

obviously ignoring losses, voltage sag, etc. Am I close? Still learning all this stuff myself!


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## major (Apr 4, 2008)

Hi dl and Doug,

I think you guys are grasping the concept. If you want to understand more and can't find posts here by the likes of Tesseract, I suggest you do a google on buck converter. The basic DC motor PWM controller is a buck converter. Just four main components, Capacitor, Switch (fet or igbt), Diode and Inductor. It is actually even simpler for a motor controller because the motor acts like the inductor  Of course there's the control logic, sensing and protection. I have seen many a good web site describing the fundamental operation and theory.

Regards,

major



Dougnutz said:


> What causes the low motor voltage? Is it a result of the pwm of the controller limiting the amps?


Low motor voltage is required at low motor RPM. Otherwise motor current would be excessive.


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