Amps, power, torque? - DIY Electric Car Forums

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#1
03-11-2010, 02:49 AM
 Woodsmith Admin: 'one of many' Join Date: Jun 2008 Location: Manchester UK Posts: 4,838
Amps, power, torque?

I am using a 7" Club Car motor, in my compact tractor project, like this one but it is rated at 3.1hp at 48v.

Now from my calculations of Power = volts x amps that equates to:
3.1hp = 2,312W
2,312W / 48v = 48.2A.

In my thread in Batteries it was suggested that my 350A Curtis controller might be a bit small for my tractor.
That implies that I could be drawing more then 350A.

So, 350A x 48v = 16,800W
16,800W = 22.5hp.

So my thoughts go along the lines of:
Am I missing something or is this about right?
Will I fry my motor or snap my drive chain?
Are all those amps going to be consummed to 'make' the motor spin at 4200rpm under load or will it be at stall speed?

My assumption is that series motors produce torque, and that power is torque x rpm.
Therefore the motor can/will spin at up to 4200rpm up to the point where the load requires more then the 2.3kW power that the motor can develop.
The motor speed then reduces but draws more current to produce more torque.
The speed continues to reduce as the current increases so that the motor can draw 350A, producing lots of torque but little power as it is near stall.

I know this has been explained before on other threads but if I got it the first time I wouldn't need to ask so bear with me please.
Machinery and industrial ac motor installations working at their rated power are so much easier to understand.

Thank you.
#2
03-11-2010, 03:53 AM
 Bowser330 Senior Member Join Date: Jun 2008 Posts: 1,683
Re: Amps, power, torque?

Quote:
 Originally Posted by Woodsmith I am using a 7" Club Car motor, in my compact tractor project, like this one but it is rated at 3.1hp at 48v. Now from my calculations of Power = volts x amps that equates to: 3.1hp = 2,312W 2,312W / 48v = 48.2A. In my thread in Batteries it was suggested that my 350A Curtis controller might be a bit small for my tractor. That implies that I could be drawing more then 350A. So, 350A x 48v = 16,800W 16,800W = 22.5hp. So my thoughts go along the lines of: Am I missing something or is this about right? Will I fry my motor or snap my drive chain? Are all those amps going to be consummed to 'make' the motor spin at 4200rpm under load or will it be at stall speed? My assumption is that series motors produce torque, and that power is torque x rpm. Therefore the motor can/will spin at up to 4200rpm up to the point where the load requires more then the 2.3kW power that the motor can develop. The motor speed then reduces but draws more current to produce more torque. The speed continues to reduce as the current increases so that the motor can draw 350A, producing lots of torque but little power as it is near stall. Is this about right? I know this has been explained before on other threads but if I got it the first time I wouldn't need to ask so bear with me please. Machinery and industrial ac motor installations working at their rated power are so much easier to understand. Thank you.
Maybe whoever mentioned that 350A wasn't enough did not know or was not thinking about what motor you were going planning to use...and applying more Amps than your motor can handle will fry/melt/destroy it..

3.1HP is not a lot but for a little go-kart (tractor? pics?) I guess it works.
#3
03-11-2010, 06:00 AM
 Woodsmith Admin: 'one of many' Join Date: Jun 2008 Location: Manchester UK Posts: 4,838
Re: Amps, power, torque?

Quote:
 Originally Posted by Bowser330 Maybe whoever mentioned that 350A wasn't enough did not know or was not thinking about what motor you were going planning to use...and applying more Amps than your motor can handle will fry/melt/destroy it.. 3.1HP is not a lot but for a little go-kart (tractor? pics?) I guess it works.
Thanks.

Jim mentions 350A being a little weak and he knows my project really well, JRP mentions pulling more amps due to the weight. The thread will put it in context.

I am hoping not to destroy the motor, at least not before I have a bigger one.

So should I limit the current, somehow, so something more like the 48A that it should pull at 3.1hp?

I am not yet fussed about how powerful the tractor will be, more I want to work out what happens and why as loads change and I get readings on the meters. It will be good if it would be able to drag more then its own weight around though.

The build thread in in my sig. If you start from the end and work backwards you should find some video with the images.

Last edited by Woodsmith; 03-11-2010 at 06:02 AM.
#4
03-11-2010, 06:27 AM
 Jimdear2 Senior Member Join Date: Oct 2008 Location: Richmond a Small town in Southeast Michigan Posts: 1,199
Re: Amps, power, torque?

Quote:
 Originally Posted by Bowser330 Maybe whoever mentioned that 350A wasn't enough did not know or was not thinking about what motor you were going planning to use...and applying more Amps than your motor can handle will fry/melt/destroy it.. 3.1HP is not a lot but for a little go-kart (tractor? pics?) I guess it works.
Woody,

In another thread I mentioned that I thought the controller was a weak link. Not because it couldn't supply sufficient current, but that it couldn't maintain the current you would be drawing when under big loads, that it would go into thermal limiting and shut down/back off, so I suggested additional heat sinking/cooling. I hinted that a bigger controller would be nice just because a bigger controller would stay cooler longer.

I'm fully aware that the motor is a bit undersized, but as long as you can keep it cool enough it will do the job SLOWLY

There will be times, traction dependant, when you will load the motor up to near stall, when the voltage sags the current requirements are going to go up. You will just have to be cautious and watch your temperatures.

You have lots of gear reduction so for general use, as I said before, it will be slow. But as long as you have traction you should get usable work out of it.

Sorry if I caused confusion.
__________________

In memory of my wife and her wheelie tractor,

Jim
#5
03-11-2010, 06:40 AM
 Woodsmith Admin: 'one of many' Join Date: Jun 2008 Location: Manchester UK Posts: 4,838
Re: Amps, power, torque?

Slow I don't mind, Jim.

Smoking the motor under seat I'd rather not. Or worse, snapping the chain at 4200rpm under my left butt cheek!

I see what you mean now, not that 350A isn't enough but that whatever the motor will draw under load may over heat the controller as it is not rated very highly.
That clarifies things.
#6
03-11-2010, 06:50 AM
 major Senior Member Join Date: Apr 2008 Location: Ohio, USA Posts: 7,251
Re: Amps, power, torque?

Quote:
 Originally Posted by Woodsmith I am using a 7" Club Car motor, in my compact tractor project, like this one but it is rated at 3.1hp at 48v.
Hi Wood,

Looks like a GE made golf cart motor. Not a bad thing. A little weak on the comm and brush size. Anyway, it is typically used with controllers of the 225 to 275 amp limit. The 350 you have is o.k. I suspect the motor will do fine and handle all the controller can deliver.

I have an out-of-town meeting to get off to, so can't go into detail now. One thing, I'd be sure to incorporate a speed sensor and overspeed cut out circuit. If you break the chain, the motor will spin to death.

Regards,

major
#7
03-11-2010, 07:08 AM
 Jimdear2 Senior Member Join Date: Oct 2008 Location: Richmond a Small town in Southeast Michigan Posts: 1,199
Re: Amps, power, torque?

Quote:
 Originally Posted by major Hi Wood, Looks like a GE made golf cart motor. Not a bad thing. A little weak on the comm and brush size. Anyway, it is typically used with controllers of the 225 to 275 amp limit. The 350 you have is o.k. I suspect the motor will do fine and handle all the controller can deliver.
Woody,

I feel much better now that Major has dropped in. Looks like my inexperiance has made me too cautious.

Keep going
Jim
__________________

In memory of my wife and her wheelie tractor,

Jim
#8
03-11-2010, 08:30 AM
 Woodsmith Admin: 'one of many' Join Date: Jun 2008 Location: Manchester UK Posts: 4,838
Re: Amps, power, torque?

Thanks Major, that is good to know.

Quote:
 Originally Posted by major Hi Wood, I'd be sure to incorporate a speed sensor and overspeed cut out circuit. If you break the chain, the motor will spin to death. Regards, major
That sounds like a really good reason to have a chain tensioner. The moment the tensioner springs open the controller is shut down.

Maybe interconnect the tensioner to a motor brake too.
#9
03-11-2010, 08:38 AM
 dimitri Senior Member Join Date: May 2008 Location: Sacramento CA Posts: 2,082
Re: Amps, power, torque?

Quote:
 Originally Posted by Woodsmith Now from my calculations of Power = volts x amps that equates to: 3.1hp = 2,312W 2,312W / 48v = 48.2A. In my thread in Batteries it was suggested that my 350A Curtis controller might be a bit small for my tractor. That implies that I could be drawing more then 350A. So, 350A x 48v = 16,800W 16,800W = 22.5hp. So my thoughts go along the lines of: Am I missing something or is this about right?
You are missing something important here. You are using max voltage in your calculations, but max voltage across the motor is only at max RPM, at which point back EMF will not allow max amps, i.e. you never have max voltage and max amps at the same time. You have min amps at max voltage and max amps at min voltage. That is why at stall you will have very low motor voltage, in order of magnitude less than max, which will drive the max current up to a magnitude higher, somewhere towards the controller's limit, which is why Jim suggested extra cooling for the controller. As long as you minimize the time spent at stall , RPMs will rise and current will fall.

Hope this makes sense.
#10
03-11-2010, 11:41 AM
 Sunking Senior Member Join Date: Aug 2009 Location: DFW Posts: 1,633
Re: Amps, power, torque?

I will chime in here with my 2-cents.

I made the same wrong assumptions you made with respect to voltage, current, and power. DC series wound motors do not follow common logic.

A Series DC motor develops it maximum torque and current at 0 RPM and is porptional to the available current supplied by the controller. As RPM increases the back EMF decreases current and thus torque. If you look at the graph below is very typical of all series DC motors. What you will conclude max HP is delivered at mid RPM and voltage. Note that once the motor has the full voltage potential applied at max rpm; power, torque, and current is almost 0.

It is very common practice, especially in golf carts to use very large oversize controllers. It does not increase the power or RPM's it only increases low end torque. Normally this is not a problem for the motor as it can take short burst of excessive current at low RPM's. Where it can get you in trouble is situations like climbing a steep grades or pulling heavy loads for extended period of time that exceeds the duty cycle of the motor.

__________________
Dereck BC, PE MSEE

Last edited by Sunking; 03-11-2010 at 11:47 AM.

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