# which motor has the highest torque ability at low RPM?



## Woodsmith (Jun 5, 2008)

What maximum speed do you need?

Most large forklift motors are used to moving that sort of weight about but usually have more gear reduction. 

My 11" motor came from an 80V, 4 ton capacity, forklift that had a 14:1 axle ratio.

Even the old electric milk floats were that weight but their axles were double reduction with 9:1 through to 15:1.
I have a 12" motor from a Morrisons milk float that is more then capable of moving that weight at 96V.


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## miernik (Nov 28, 2009)

Woodsmith said:


> What maximum speed do you need?
> 
> Most large forklift motors are used to moving that sort of weight about but usually have more gear reduction.


Maximum speed? Something like up to 50 km/h, 30 mph. But 40 km/h = 25 mph would be OK too.

The forklift motors capable of heavy weight you are talking about, which kind of motors they are:

DC series wound
DC shunt wound
DC permanent magnet brushed
DC permanent magnet brushless
AC 3-phase induction
AC 3-phase permanent magnet synchronous

I'd like to have my battery pack 48V (simplifies a lot of things).

Forklift motors sound a good idea, as this is low-budget, and there are lots of used forklifts for close-to-zero money on eBay.

Do these motors have a double ended shaft (I'd like to mount it in the drive-shaft)?


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## Woodsmith (Jun 5, 2008)

Older forklifts are series wound DC but some of the new ones are AC. It is more likely that you will be looking at a series DC motor.

They vary quite a bit depending on the forklift and you will need to look at the donor forklift to see what layout the motor is.

This is how my 11" came. It was, as Jimdear2 described, a giant golf buggy motor.









The motor is a part of the whole axle assembly.
I removed the motor and it looked like this.









I had to make a new DE cap for it.
Here's video.


This is the one jackbauer had from the same chap. Under the cables is a 12" motor but it has a drive flange for a propshaft.









It is similar to my 12" from the milk float. More video.


Whether it has a tailshaft or not depends on the set up. Larger trucks generally use conventional drum brakes on the axle hubs whereas the smaller ones often use a brake on the motor tailshaft like this 9" motor I have.
More video.


Are you planning on using the motor to drive the drive shaft to the drive axle? You only need the DE shaft to do that.

If you want a shaft both ends then that sounds like you are putting it between the transmission and the axle and keeping the ICE?
You can then have the problem that the ICE will spin the motor too fast and explode it.

You can do this with 48V but the battery pack could be all your payload capacity depending on the range you need. Also the current draw would be huge to get the torque at low speed. You might be looking at finding a 48V forklift and taking the controller from it too. But the bigger forklifts tend to have a higher voltage too, to reduce the current.

Ultimately it will depend on what fork lift you can find to get the bits from adn what you want to do with the truck.
Is it highway use? Off highway use? Limited range with large payload?


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## miernik (Nov 28, 2009)

Woodsmith said:


> If you want a shaft both ends then that sounds like you are putting it between the transmission and the axle and keeping the ICE?
> You can then have the problem that the ICE will spin the motor too fast and explode it.


Yes, exactly.
Well, I need a motor that will survive spinning it with ICE. Especially as I want to charge batteries this way - the idea is - not so large batteries, drive a bit on electric - when battery empty switch to ICE - charge batteries while driving, when batteries full switch back to electric.



Woodsmith said:


> You can do this with 48V but the battery pack could be all your payload capacity depending on the range you need. Also the current draw would be huge to get the torque at low speed.


But why would the battery need to be larger for a lower voltage then for a higher voltage? To me it seems the size of the battery would be exactly the same - only cells wired mostly in parallel instead of in series. 100 Ah x 48 V is the same size of battery as 25 Ah x 192V, yes? And the first one would provide 4 times as current as the latter one, yes? So why do you claim that it would need to be larger with a lower voltage?



Woodsmith said:


> Ultimately it will depend on what fork lift you can find to get the bits from adn what you want to do with the truck.
> Is it highway use? Off highway use? Limited range with large payload?


Off highway, city use only, and small countryside roads for max 50 km/h, 30 mph driving. Small range, something like 20 km = 12 miles.


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## Woodsmith (Jun 5, 2008)

If you want to regen then maybe not series wound. They are not easy to regen and probably not worth trying.

The main problem is going to be that a big motor to give you big torque will not spin fast. A fast spining motor will be smaller in diameter and produce less torque.

Low voltage requires more current and so you will need batteries that can supply a bigger current. Although the energy capacity will be the same the difference will be the amount of current you can pull from the pack, the C rating of the cells.
Higher currents also mean greater I²R losses and so you will lose energy as heat in every element of the system.

For such short range you can do without the ICE and use the weight saving for the battery pack.


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## miernik (Nov 28, 2009)

Woodsmith said:


> If you want to regen then maybe not series wound. They are not easy to regen and probably not worth trying.
> 
> The main problem is going to be that a big motor to give you big torque will not spin fast. A fast spining motor will be smaller in diameter and produce less torque.
> 
> ...


No, you don't understand, I meant the range on one battery charge. When I run out of battery, I just switch on the ICE.

The weight saving doesn't matter so much, hey, its a 3500 kg van, a few hundred kg of batteries add or subtract is not gonna change much in percentage of the total weight.

Its the huge cost of batteries that is the limiting factor, not their weight. If not the cost of batteries required to have a sufficient range (300 km / 200 miles - tha twould be at least 20000 EUR = 30000 USD!), I could kick out the ICE. I can only afford 20 km worth of batteries. A123 LiFePO4, so current will be sufficient.

I plan about 5 kWh of A123 batteries: 100Ah x 48V. At 20C that would give me 2000 A current, even at moderate 5C that is 500 A.


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## Woodsmith (Jun 5, 2008)

Ahhh, I see. You are looking at a hybrid set up.

With the method you are thinking of I think you are still going to have the conflicting interests of high torque at low speed, high motor speed and regen.

Probably an AC motor will be better as it would allow for regen much more easily and might even give you a higher safe speed as there is no comm bars or armature windings to worry about. Not sure about the torque though.

I guess someone more knowledgable about AC and batteries might chime in.

That's the extent of my knowledge used up I think!

Anyway, time for bed.


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