# Amps, power, torque?



## Woodsmith (Jun 5, 2008)

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.


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## Bowser330 (Jun 15, 2008)

Woodsmith said:


> 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
> ...


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.


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## Woodsmith (Jun 5, 2008)

Bowser330 said:


> 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.

The battery thread is here.
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.


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## Jimdear2 (Oct 12, 2008)

Bowser330 said:


> 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.


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## Woodsmith (Jun 5, 2008)

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.


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## major (Apr 4, 2008)

Woodsmith said:


> 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


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## Jimdear2 (Oct 12, 2008)

major said:


> 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


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## Woodsmith (Jun 5, 2008)

Thanks Major, that is good to know.



major said:


> 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,
> ...


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.


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## dimitri (May 16, 2008)

Woodsmith said:


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


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.


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## Sunking (Aug 10, 2009)

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.


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## Woodsmith (Jun 5, 2008)

OK, I think I get that.

If I use the tractor to pull a load then the motor speed will reduce due to the load and the current will rise to try and keep the motor from stalling.

Do that for too long and the high current will overheat the motor and/or the controller.

So I can have fun whizzing around at 10mph (maybe) without too much current but if I am pulling a log at 2mph let it rest a bit and cool down as the current will be highest then. Maybe up to the 350A of the controller or maybe the motor will have burnt out by then.

Now, I can put a fan to blow on the controller, or draw hot air away from it. The motor, on the other hand, foesn't have a fan nor does it have any holes or vents at the com end of the case or in the end cap.
Should I add some holes at some point and then add a fan there too?


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## Jimdear2 (Oct 12, 2008)

Woody,

I would get some real world data first before drilling holes in the motor. 

In the long run it might be better to go with a different motor later on when you can.

Heat sinking and fan cooling the controller on the other hand is a fairly easy fix. Plus the stuff used can probaply be moved to another project if it turns out to be unneeded. As I understand it you can never have too much.


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## Sunking (Aug 10, 2009)

Woodsmith said:


> If I use the tractor to pull a load then the motor speed will reduce due to the load and the current will rise to try and keep the motor from stalling.
> 
> Do that for too long and the high current will overheat the motor and/or the controller.


Correct!

I bought a 36 volt EZ-GO Golf Cart and my first upgrade was changing out the factory controller of 175 amps to a 450 amp controller. By doing this I did not gain any top end speed to speak of. However with the upgrade, I can climb a tree from the added torque (current) the controller supplies.

FWIW I upgraded the motor after the controller upgrade to get the added speed I was looking for to start with.


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## major (Apr 4, 2008)

Jimdear2 said:


> I would get some real world data first before drilling holes in the motor.


Here ya go,

I have essentially the same motor on a tractor of mine. Maybe I can snap a pic tomorrow. Anyway, it is a SepEx version, same size GE. With a Curtis SepEx contoller. I think the controller is rated at 500 amps max on the armature. I've been beating the sh#t of it for like 5 or 6 years. No problem. Just chill out about the motor 

Been on the road all day to talk with lawyers. How bad is that? I'll see if I can do a better post tomorrow.

major


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## Woodsmith (Jun 5, 2008)

Cheers major.

I will get it all running and see.

I suppose that in its original use it had to haul around a big golf cart with maybe a couple of big men and their golf clubs at more top speed then I am geared for.

Perhaps I should be thinking more along the lines of up grading to a duplex chain before I need to worry about the motor or controller!


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## major (Apr 4, 2008)

major said:


> Maybe I can snap a pic tomorrow. Anyway, it is a SepEx version, same size GE.


Here is the bugger


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## Jimdear2 (Oct 12, 2008)

major said:


> Here is the bugger


Major, 

That's a pretty unique vehicle, at least to me. You can't just leave us all hanging, how about some history and description.

As an aside, thanks for being here with all of the information and advice, I have learned a lot.

Jim


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## major (Apr 4, 2008)

Jimdear2 said:


> You can't just leave us all hanging


Built about 8 years ago. 4 of us did it. Ground up. Front tractor section with the drive. Trailer with batteries, 4 twelves and chargers, 2 twins. Kinda like a chopped golf cart axle. 4 wheel independent suspension, coil over shocks front, torsion bar rear. Disk brakes front, drums rear. Articulation steering. Full SepEx features, 2 spd settings, reverse with reduced spd, regen. Top speed maybe near 30 mph. Full 12 volt system, lights, horn, strobe and back-up alarm. No stereo


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## Jimdear2 (Oct 12, 2008)

major said:


> Built about 8 years ago. 4 of us did it. Ground up. Front tractor section with the drive. Trailer with batteries, 4 twelves and chargers, 2 twins. Kinda like a chopped golf cart axle. 4 wheel independent suspension, coil over shocks front, torsion bar rear. Disk brakes front, drums rear. Articulation steering. Full SepEx features, 2 spd settings, reverse with reduced spd, regen. Top speed maybe near 30 mph. Full 12 volt system, lights, horn, strobe and back-up alarm. No stereo


Major,

No Stereo 

Get crackin 

I have always loved the odball stuff. This really qualifies. It has GOT to be a ball to drive. Do you know what it's original function was?

Woody obviously will love it too. We, will see what his reaction is when he gets back from London.I'm glad that you can verfiy that the motor/controller/battery combo that he has won't be an instant disaster. I'm just starting to learn and at my age it harder to stuff new knowledge into my head. 

A small hijack of Woody's thread, but later when I get the windows 7 and XP problems fixed I will ask you some questions on my/our latest odball. A 13 inch GE, direct drive to a narrowed car axle on around 300 volts and about 200 amps to be used as a pulling tractor.

Be well


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## Bowser330 (Jun 15, 2008)

Why only 200A of torque?


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## Woodsmith (Jun 5, 2008)

major said:


> Here is the bugger


Nice!

For a short while I was bidding on Ebay for a Ransomes (I think) mower that was a similar contruction with articulated steering and no engine. I lost out on that one but it would have been very similar to your beasty.


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