# Direct Drive Motor



## Nathan219 (May 18, 2010)

Yasa Motors Has proven an efficient direct drive motor is possible, now how can we make it affordable? Lets forget the wheel motor concept on any vehicle not equipped with a axle through wheel design. 
The industry needs a point to design to, for a nearly maintenance free vehicle. 
My specifications for a direct drive system are as follows

40Kw max power
10Kw continuous
2000 max RPM
640-800 RPM 45nm torque most efficient RPM
~$2000.00 (motor + controller)
Controller should be designed to be used with other controllers like it's self. Here are a few manufactures making torque motors. 
Etel 
Siemens
Hiwin
With a drive train like one of these you would need a replaceable vehicle exterior, and interior.


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## McRat (Jul 10, 2012)

Opinion Piece:

As a long time racer, Torque is not your buddy.

If I make a driveline for 50HP and the engine spins to 14,000 RPM, it weighs >40lb including clutch and transmission.

If I make a driveline for a 50HP tractor that spins to 2,000 rpm, it weighs over 500lb.

This applies to all kinds of things. Torque requires more metal to hold things together, if you can spin it faster, you can reduce the weight.

Think of Hayabusa engine. 170HP. The whole engine/trans weighs less than half what the 140HP Volkwagen TDI (newest and greatest) engine weighs, and that's without the transmission.

If you need to move something REALLY heavy constantly, then a big torque motor is the solution. If weight is important, you need high RPM.


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## ruckus (Apr 15, 2009)

Nathan219 said:


> My specifications for a direct drive system are as follows:
> 40Kw max power
> 10Kw continuous
> 2000 max RPM
> ...


These specs seem a bit off to me. Is this just for one side/wheel? 10kw is hardly adequate to drive at highway speeds. 




McRat said:


> Opinion Piece:
> 
> As a long time racer, Torque is not your buddy.
> 
> ...


Overall, your thinking is correct. The reason tractors moved from 540pto to 1000 pto is they couldn't push any more torque through the system so they doubled the power by doubling the rpm.

However, when you are talking about an axle shaft it is already built for full torque and low rpm. So when building a direct-wheel drive (through axle shafts) the motor can be a little heavier and still come out ahead because you are ditching the differential, drive-line, transmission, flywheel, clutch, bell-housing, coupler, etc... If you bother to weigh all that stuff it comes out to a LOT (several hundred pounds).

Some of this depends on the vehicle being converted. A rwd vehicle has a lot of driveline/differential/transmission weight compared to a Tercel.

Industry is already using direct-drive high-torque low-rpm brushless motors in many applications. Since they seem to be 50 years ahead of what goes into the average EV (they ditched series dc decades ago), the average DIY conversion will be using 'old' high-torque direct-drive motors around 2060.


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## PStechPaul (May 1, 2012)

I am rather skeptical about the YASA motor. It seems quite a stretch that a 25 kG (55 lb) motor 372mm (14.6") diameter and 74mm (2.9") thick, can produce 40kW (53HP) at only 2000 RPM at 93% efficiency (or 95% as it says elsewhere). So I looked elsewhere for information on the axial flux motor. 

http://www.infolytica.com/en/applications/ex0072/
This is actually a demonstration of the use of Infolytica MagNet software, and it has some cool illustrations and animated demos. The motor was modeled from M G Say and E O Taylor's "DC Machines". According to the information, it is a brushed DC motor with rather short thick U-shaped conductors. Judging from the size of the conductors, this would seem to be a rather low voltage motor (maybe 12-24V), while the YASA page says it is designed for 400VDC. I'm not a magnetics expert, but this indicates that it must produce a back EMF of 400 volts with a winding diameter of maybe 12 inches, and with 21 loops and 2000 RPM this means about 700 field reversals per second. Judging from a 2kVA toroidal core which is probably about the same weight as the iron in this motor, it has roughly 1 volt per turn at 60 Hz, which is equivalent to 3.4 V/t at 700 Hz. To get 300 V/t would require something like 5.4 MHz. But I might be wrong. Magnetics is not my strong suit. 

http://www.imtmotors.com/technology.htm
This describes a brushless pancake motor designed for e-bikes, with 87% peak efficiency, which seems more believable. This is a 200W motor which appears to be about 8" dia and 4" thick. I'm a believer on this one! 

http://www.axcomotors.com/axial-flux_technology.html
Has some nice illustrations and examples. It says the design is based on that of Michael Faraday over 100 years ago, and says the design is suited to low-torque, high speed applications. 

http://med.ee.nd.edu/MED10/pdf/257.pdf
This is a highly technical discussion of an axial flux BLDC motor for EVs, but it runs on 15V and has maximum power of 3 kW and 430 RPM. It weighs 10 kG so I might believe a 25 kG version with 10 kW peak, but not 40 kW nominal. 

Any other opinions? It would be great to have a motor with awesome specs like the YASA, but I think it must be "alien technology"!


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## butterflygirl (Jul 24, 2012)

m really primarily interested in power, specifically, seeing the whole power curve, not just peak output. I don't see why car people talk so much about torque. Am I wrong?


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## Jordysport (Mar 22, 2009)

butterflygirl said:


> m really primarily interested in power, specifically, seeing the whole power curve, not just peak output. I don't see why car people talk so much about torque. Am I wrong?


put a hayabusa engine in a truck then you will understand why torque is so important. you don't measure power on an engine dyno you measure the torque and then the power is ESTIMATED from the rpm, efficiencies etc. 

my last car an Audi S3 Quattro was reasonably fast, but my Clio 200 cup is just as powerful but no where near as fast, for the reason of torque. 

When i am looking at electric motors i am only looking at the torque across the range of the motor rpm, power has no influence on my calculations for reasons discussed above.


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## PStechPaul (May 1, 2012)

It took me a while to really understand the concepts. What you feel when driving a car is actually force, which can be from gravity or the linear acceleration of the vehicle. The ability to take off from a standstill is all torque, because without speed there is no power of motion, or usable power. Of course the motor may be using energy and creating power in the form of heat, but what you need to move and accelerate to go faster is torque. In order to keep accelerating you need power, and you also need power to overcome rolling and wind resistance. It may help to look at the equation:

HP=RPM*Lb-Ft/5252. So at zero speed or zero torque there is no power.

Lb-Ft=HP*5252/RPM. So torque is undefined if RPM is zero, and as RPM increases, you need more HP to maintain the same torque. Eventually you run out of power and can't go any faster, but that's because you can't generate the torque needed to accelerate.


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## McRat (Jul 10, 2012)

Gearing is how you convert big torque numbers into in high RPM HP, and how you convert a Hayabusa to run an S10 truck for racing (really, there is a guy running one).

But at the engine end of it, the more torque the engine is capable of, the beefier the motor design and mounts, clutch, and primary trans pieces must be.


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## ruckus (Apr 15, 2009)

butterflygirl said:


> m really primarily interested in power, specifically, seeing the whole power curve, not just peak output. I don't see why car people talk so much about torque. Am I wrong?


It took me a looong time to figure out how to explain this. I will try.

Torque is All-important because it DOES everything. Torque is real. Torque is what you 'feel' when you press on the go-pedal.

Horsepower is merely a calculated figure derived from knowing torque at rpm. At the end of the day when torque is finished with it's work, you can then calculate just how much work it did. This is horsepower. Dynomometers measure torque and then calculate hp. 

Here is the perfect example: Climbing grades. I think most have had the experience where you are unable to hold 5th gear on a hill. This means you have insufficient torque. You down-shift into 4th and are able to barely hold your speed steady, but not gain. This means with the reduced gearing you have increased your torque to equal that required by the hill. This is why some tractor companies call a transmission a 'torque multiplier'. Please note that down-shifting does not increase your horsepower and a transmission is never called a horsepower multiplier. So your hp was the same in each gear. It was torque that was needed to actually do the work which is then calculated in hp.

Hp is a by-product of successfully applied torque.

The hp vs tq thing is often confused in combustion engines since they are very correlated. If you build an engine with more hp it also has more torque. So it 'feels' more powerful. You are actually 'feeling' the additional torque, which produces, through calculation, power. 

In diesels and electric motors these two measurements are almost completely de-coupled. For example my Cummins diesel truck 'feels' like it has 400hp even though it only has 160hp. This is because it has 400ftlbs of torque and that is what creates the FEELING of 'power'. 

Likewise, if an electric motor has 500ftlbs of torque it will 'feel' like a 500hp gas engine under the hood even if the continuous rating is only 70hp.

Just to satisfy the horsepowerians, steady top speed is where hp means something. So running steady at 70hp in a given car will equate to a certain cruising speed. Notice that you 'feel' nothing when using hp. You are just buzzing along. If you buzz too fast then you overheat. That threshold of overheating is the continuous hp rating.

Ok, that was clear as mud. Maybe next time..


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## major (Apr 4, 2008)

ruckus said:


> Torque is All-important because it DOES everything. Torque is real.


This subject was discussed at length a few years ago. You might find some interesting examples or opinions here http://www.diyelectriccar.com/forums/showthread.php?t=36904


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## ruckus (Apr 15, 2009)

Hi Major, glad we agree. I'll add one more since I'm out doing hay..

It requires a large amount of force to compress the hay in the baler chamber with the plunger. It works just like a giant ICE piston. 

With insufficient torque the plunger cannot complete it's stroke and you aren't making hay at all. Once you have manifested enough torque to run the machine, the amount of input power determines how FAST you can bale hay.

At the end of the day a 20hp tractor will have made fewer bales than a 30hp tractor cause it had to use a lower gear to manifest sufficient torque to run the baler.

So power matters, but can only be measured as a result of torque doing it's job.


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## Arlo (Dec 27, 2009)

I always love the torque vs hp debates!  
My 2c if you make the power at lower rpm you have a more efficient motor and there is less need for gear reduction which will save you weight. The CV shaft will need to be just as strong either way because with a higher revving motor you need to gear it down which multiplies torque. So at the end of the day I choose a lower revving motor with the same or more HP at low rpm (which is more torque) because that will reduce the need for a tranny which makes things more simple and reliable!

Another simple topic is to use a tranny or no tranny with some other motors. IN the end its a wash because you can add tranny weight or motor weight to get the same end result of the same power to weight for the whole vehicle but when you add motor weight instead of tranny weight you have less moving parts to need maintenance and possibly fail!


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