# [EVDL] Direct drive, cooling etc



## EVDL List (Jul 27, 2007)

Hi Joseph. Your questions are pretty basic, so I'm guessing that you aren't familiar with motor design. I'll try to keep my answers fairly basic. In fact, they will be a bit oversimplified; but may inspire you toward further study.

I'll assume you are thinking of a conventional series motor; stationary field in the stator, rotating armature and commutator in the rotor. This is not a common type for wheel motors, but it is possible.

From: [email protected]
> 1. Can anything be done in the design of a series motor to make it more
> efficient at low speeds for direct drive, like different winding size
> etc? I am trying to avoid a reduction gear if possible.

A normal series motor is 75-85% efficient over a range of about 1000-6000 RPM. It falls off gradually at the high end due to slowly increasing friction (bearings, brush drag), windage (wind resistance and internal fan), and core (magnetic) losses. It falls off more sharply at lower speeds due to higher currents causing higher I^2R (resistive), magnetic saturation, and from the relatively constant brush voltage drop becoming a larger percentage of the total voltage (because voltage goes down with RPM).

To improve low speed efficiency, you need to add to or improve the iron core to raise the volts per turn and raise the magnetic flux density achievable before saturation. This will lower speed and increase torque for a given applied voltage and current. If you can give up high RPM, you can use thicker laminations in the rotor (or perhaps even a *solid* cast rotor if max RPM is low enough.

If you're designing a new motor, then you would significantly increase its diameter and shorten its length. Again, this lowers RPM but increases torque for a given choice of materials.

Going further... a wheel motor is typically easier to make "inside out". That is, the center shaft is fixed, and the outside casing rotates with the wheel. You can make an inside-out series motor, but it is quite a bit different from a normal series motor!

I don't know if it would be worth pursuing such a motor. On the plus side, series motors are good at providing very high torque per amp, and the stator and rotor in this case could probably solid iron castings. The rotating outer casting could well be the brake drum or rotor itself. Materials and construction are fairly cheap and simple.

On the minus side, such a motor would tend to be heavier and have slightly lower efficiency than the PM motors more commonly used for wheel motors.

> 2. Is it possible to convert a regular air cooled motor to water cooled
> by say winding soft copper tube on the outside of the motor? It will
> cool the case, but not armature.

Yes, it's possible; though cooling the outside casing will not be effective. Most of the heat is produced in the armature. It is not connected to the outer case, so heat transfer is very poor.

To do this "right", you need a way to plumb coolant into the rotor. When it's been done, it was accomplished with a hollow motor shaft with rotating seales at each end so coolant could go in one end and out the other.

--
Those who say it cannot be done should not interrupt the one who is
doing it. -- Chinese proverb
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart-at-earthlink.net

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## EVDL List (Jul 27, 2007)

Hi Joseph,

I see Lee has responded. Good comments. He seems to
assume you intend direct drive to mean wheel motors. 
This definition of direct drive has been the subject
of debate here. I read your post and assumed you were
speaking about a direct drive meaning directly coupled
to the rear end in a conventional axle with a
differential including a final gear reduction. This
fits my concept of direct drive. Wheel motors are a
subclass of direct drive IMO. Anyway, a few comments
inserted......




> --- [email protected] wrote:
> 
> > 1, Can anything be done in the design of a series
> > motor to make it more
> ...


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## EVDL List (Jul 27, 2007)

I have a Softub Spa which has no heating element. The hot water is
supplied solely by a coil of tubing around the circulator pump. It
seems to transfer lots of heat.

-- 
http://www.austinev.org/evalbum/1059
http://stormselectric.blogspot.com/
Storm

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## EVDL List (Jul 27, 2007)

Thanks Lee and Jeff, but, I am afraid, I did not
make myself clear, let me expound.

1, I am not talking about a wheel
motor, but 4 series motors connected to four wheels directly using CV joints and
axles. The diameter of the wheels ensure that the max RPM need not go above 1200 or
so. I can even connect all these motors in series/parallel sequences to optimize
further. There is no differential or fixed reduction gear. Does this change your
response?

2, Regarding water jacket, how does then available AC induction
motors which are water cooled, work? None of them I assume have cooling of rotors
only case? Maybe in AC topology is heat less in rotors compared to dc series wound?


And Lee, even though the questions sound basic, dont hold yourself back
in sophisticated answers and links, as I can understand them, though I am not a
trained EE. The more info , the better.

Thanks

Joseph







> On Wed, April 9, 2008 1:08 am EDT, Lee


> Hart wrote:
> >
> >
> > Hi Joseph. Your questions are pretty
> ...


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## EVDL List (Jul 27, 2007)

> 2, Regarding water jacket, how does then available AC induction
> motors which are water cooled, work? None of them I assume have cooling of rotors
> only case? Maybe in AC topology is heat less in rotors compared to dc series wound?

A typical induction motor only sends current to the stator (outside).
There is current in the rotor, due to induced current, but my
understanding is that the primary current and heating is in the
stator, which is of course attached to the outside of the motor.

In a typical DC motor, unless built inside out, the main current is in
the rotor, and only the field coils are in the stator -- either low
current in shunt motors, or low resistance short coils for series
motors, so power dissapation in the stator is relatively low compared
to the rotor.

Z

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## EVDL List (Jul 27, 2007)

> --- [email protected] wrote:
> > Thanks Lee and Jeff, but, I am afraid, I did not
> > make myself clear, let me expound.
> >
> ...


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## EVDL List (Jul 27, 2007)

> 2, Regarding water jacket, how does then available AC induction
> motors which are water cooled, work? None of them I assume have cooling of
> rotors
> only case? Maybe in AC topology is heat less in rotors compared to dc
> series wound?

They are built inside out, compared to series wound motors. The rotor
generally has no direct connection to the electricity, i.e. it either uses
a permanent magnetic field or an induced field.

At any rate the main electrical coils on an AC motor are located on the
outside, so cooling the case works for them.

On a series motor, the field coils are on the outside, but the bulk of the
electrical power (and I2R losses) flows through the armature coils.
I.e. in a series wound motor, you might drop 2V across the field coils
(case) and 100-200+ volts across the armature coils (rotor).
Plus a series wound motor uses brushes which generate a lot of heat too.


>
>
> And Lee, even though the questions sound basic, dont hold yourself back
> in sophisticated answers and links, as I can understand them, though I am
> not a
> trained EE. The more info , the better.
>
> Thanks
>
> Joseph
>
>
>
>
>
>
>
>> On Wed, April 9, 2008 1:08 am EDT, Lee


> > Hart wrote:
> >>
> >>
> >> Hi Joseph. Your questions are pretty
> ...


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## EVDL List (Jul 27, 2007)

> Jeff Major wrote:
> > Hi Joseph,
> >
> > I see Lee has responded. Good comments. He seems to
> ...


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## EVDL List (Jul 27, 2007)

> storm connors wrote:
> > I have a Softub Spa which has no heating element. The hot water is
> > supplied solely by a coil of tubing around the circulator pump. It
> > seems to transfer lots of heat.
> ...


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## EVDL List (Jul 27, 2007)

> [email protected] wrote:
> > Thanks Lee and Jeff, but, I am afraid, I did not
> > make myself clear, let me expound.
> >
> ...


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## EVDL List (Jul 27, 2007)

I assume because each low-RPM motor is just a higher power (bigger) motor?

Is there a good way to evaluate motors for low RPM power? I am facing 
somewhat the same problem with my BMW motorcycle conversion. I want to 
direct couple to the driveshaft (which has a 3.36 ration to the rear 
axle). As in the case being discussed it means more low RPM torque is 
required and less max RPM. How does one look at potential motors with 
this in mind? I'm thinking lower voltages since a high RPM isn't needed. 
But how do you evaluate the critical start up and low speed power?

- SteveS



> Lee Hart wrote:
> > However, total motor weight
> > will be higher without gears. First, because each low-RPM motor will
> > weigh more than its high-RPM counterpart.
> ...


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## EVDL List (Jul 27, 2007)

You could keep the transmission and drive the crankshaft end of it? 
I've heard of this being done, but never seen it. (It involves a lot of 
cutting, but I think having the transmission has a lot of advantages.) 
Or, though it would make it less BMW-ey, but you could convert it to 
chain drive, then the world is your oyster w.r.t. gearing. I like 
chains. Thing is, a low RPM motor is always going to be heavier than a 
high RPM motor. (Same for gas engines, why do you think superbikes wind 
up to almost 20,000 rpm?) So gearing down a high-RPM motor just makes a 
lot of sense.



> SteveS wrote:
> > I assume because each low-RPM motor is just a higher power (bigger) motor?
> >
> > Is there a good way to evaluate motors for low RPM power? I am facing
> ...


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## EVDL List (Jul 27, 2007)

Thanks for the input Chuck. I don't want to hijack the thread but let 
me answer, and get back to low RPM motors.

Actually at the moment I do have a transmission, as the motor I had to 
play with was too weak for direct drive. But the tranny takes up a lot 
of space and chain drive would add a lot of noise. It is nice and quiet 
now.

Besides I really like the idea of the direct drive. And in my case it's 
not really 'direct' as I do have the 3.36 rear end. It would be like 
hooking up to the differential on a car. I don't mind a bigger, heavier 
motor (to a point), as it will take up less space and MAYBE less weight 
than a smaller motor and tranny. The question is how much bigger? Are 
there motors out there that are better at low RPMs?

The ultimate solution in my mind that would work for me (and possibly 
the original poster) would be some kind of in-line 2-speed gearbox with, 
say a 4:1 and 1:1 ratios, something like the old bicycle three speed 
hubs. The low gear would be perfect for start up and city driving, then 
switch to direct at speed. I think it could be small and simple.

- SteveS



> Chuck Homic wrote:
> > You could keep the transmission and drive the crankshaft end of it?
> > I've heard of this being done, but never seen it. (It involves a lot of
> > cutting, but I think having the transmission has a lot of advantages.)
> ...


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## EVDL List (Jul 27, 2007)

> I assume because each low-RPM motor is just a higher power (bigger) motor?
>

You're not paying attention 
To get THE SAME POWER at lower RPM requires a bigger motor, this is NOT
because it's a more powerful motor (it's the same power) it's because it
needs to produce more torque.

Power = RPM * Torque
If you want to keep power the same and reduce RPM, then you have to
increase torque.

I'm sure you remeber learning about levers in school. If you want to lift
a given weight, it takes less strength if you have a longer lever, or with
the same strength you can lift a heavier object.
You can think of torque as a shaft with a lever sticking out of it. To
get more torque you either have to press harder (more force) on the lever
or get a longer lever.
The force is related to the strength of the magnetic fields interacting,
stronger fields = more force.

At a given radius, you are limited to how strong a field you can get in
there by either how much copper windings you can fit or how powerful the
permanent magnets are.

So the easiest way to get more torque is to go with a larger radius
(longer lever) this gives you two advantages, longer lever (obviously)
plus more area to cram in copper/magnets (stronger field)

The down side is that now we have a larger radius. This requires more
steel for the case and the rotor. It also needs to be stronger because it
has more torque AND because larger diameters are affected more by
centrifugal force (the motor wants to tear itself appart when spinning).

Being larger doesn't automatically mean it's more powerful, because the
larger diameter limits the maximum RPM (because centrifugal force).

So you get the same power (RPM * Torque) but it's a bigger, heavier motor.

P.S. This is one of the reasons you can get model airplane motors that
produce 1hp-2hp and fit in the palm of your hand. They spin at
50,000-60,000 rpm

> Is there a good way to evaluate motors for low RPM power? I am facing
> somewhat the same problem with my BMW motorcycle conversion. I want to
> direct couple to the driveshaft (which has a 3.36 ration to the rear
> axle). As in the case being discussed it means more low RPM torque is
> required and less max RPM. How does one look at potential motors with
> this in mind? I'm thinking lower voltages since a high RPM isn't needed.

No, while a given motor will spin faster at a higher voltage, there is no
direct link between volts and rpms when talking about different motors.
You can have a 3V motor with a max of 12,000,000 RPM and a 600V motor with
a max of 1,000 rpm. The RPM vs volts relationship of a motor is variable
and depends on the design.

If you want a low RPM motor, look for a low RPM motor.
It's probably better to figure out how many batteries you can fit on the
bike and then add up what the voltage will be for those batteries, and
then shop for a motor that will work at that voltage.
> But how do you evaluate the critical start up and low speed power?
>
> - SteveS
>


> > Lee Hart wrote:
> >> However, total motor weight
> >> will be higher without gears. First, because each low-RPM motor will
> >> weigh more than its high-RPM counterpart.
> ...


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## EVDL List (Jul 27, 2007)

For gear reduction with direct drive:
The old VW bus had a gear reduction at the wheels. Monster trucks also
utilize a planetary transmission at each wheel.

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## EVDL List (Jul 27, 2007)

Replies below:



> Peter VanDerWal wrote:
> > So the easiest way to get more torque is to go with a larger radius
> > (longer lever) this gives you two advantages, longer lever (obviously)
> > plus more area to cram in copper/magnets (stronger field)
> ...


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## EVDL List (Jul 27, 2007)

Hey SteveS,

The way to do it is to determine the torque needed. 
Then find a motor that will produce that torque. The
higher the torque, the bigger the motor. Simple as
that.

Regards,

Jeff M



> --- SteveS <[email protected]> wrote:
> >
> > Is there a good way to evaluate motors for low RPM
> > power?
> ...


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## EVDL List (Jul 27, 2007)

> SteveS wrote:
> >> If you want a low RPM motor, look for a low RPM motor.
> > You mean look at the RPM at rated voltage, or hopefully a curve? But the
> > real trick is at or near stall. Even with a 3.36 ratio (and my tire
> ...


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## EVDL List (Jul 27, 2007)

Thanks Lee, that helps. I guess what bothers me is that most (all?) 
motor curves I look at don't take the curve down past 1000 RPM. If you 
extrapolate you get ridiculous torque near zero. Now I know a series 
motor has max torque at zero, but I am reminded of the saying: " In 
theory, there is no difference between theory and practice, but in 
practice there is". Anyway, so far I guess a bigger diameter motor is 
certainly in order. Well part of the reason I wanted to go with the 
shaft drive was to do something a bit different. Now I get to learn 
something.

- SteveS



> Lee Hart wrote:
> > SteveS wrote:
> >
> >>> If you want a low RPM motor, look for a low RPM motor.
> ...


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## EVDL List (Jul 27, 2007)

The current limit sets the max torque, not the RPMs.

>From 0 RPMs until some other speed (usually 1500-3000 RPMs) the
controller is in current limit. Torque is constant, and power
increases linearly with speed (as does voltage). Above that speed, the
torque and current decrease.

The motor has a "safe operating area". Take the torque curve. Now draw
a horizontal line at the max RPMs; you can't go above that line. Draw
in a vertical line at max current; even though the torque curve
extends past that line, you can't safely operate the motor there.

Of course, the "max current" isn't a hard and fast number; it varies
with how long you run it for, the ambient temperature and humidity,
and many other factors you can't control or even measure. But if you
extend the torque curve to zero RPMs, also extend the current-torque
curve out and you'll see that you'll fry the motor if you try to get
that much torque.

-Morgan LaMoore



> SteveS <[email protected]> wrote:
> > Thanks Lee, that helps. I guess what bothers me is that most (all?)
> > motor curves I look at don't take the curve down past 1000 RPM. If you
> > extrapolate you get ridiculous torque near zero. Now I know a series
> ...


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## EVDL List (Jul 27, 2007)

> SteveS wrote:
> > Thanks Lee, that helps. I guess what bothers me is that most (all?)
> > motor curves I look at don't take the curve down past 1000 RPM. If you
> > extrapolate you get ridiculous torque near zero.
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> > Are you willing to build your own motors? While perfection is very
> > difficult, with care it is possible to build a pretty good motor yourself.
> >
> ...


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## EVDL List (Jul 27, 2007)

Hey, this not true for series motors! They decrease in RPM when loaded and 
increase torque per amp.


----- Original Message ----- 
From: "Lee Hart" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, April 11, 2008 12:25 PM
Subject: Re: [EVDL] Direct drive, cooling etc

> What you want are the curves that plot torque versus current. This is
> basically independent of RPM and voltage. If the motor generates (say)
> 100 ft.lbs at 100 amps, then it will do it at 0 RPM or 1000 RPM.
>

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## EVDL List (Jul 27, 2007)

Yeah, I have also heard these motors can be easily destroyed. They're very 
expensive, too.


----- Original Message ----- 
From: "SteveS" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, April 11, 2008 1:22 PM
Subject: Re: [EVDL] Direct drive, cooling etc


>> I would look at the design of the Lynch/Lemco/Etek type motor. Cedric
>> Lynch invented it, and built his first ones by hand with tin snips and
>> sheet metal. Their basic design is well suited to high torque and low 
>> RPM.
>>
>
> I've thought about them. They have a bigger diameter which I assume
> would mean more torque. I've read reports though that they tend to
> overheat and self destruct.
>
> - SteveS
>
> _______________________________________________
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## EVDL List (Jul 27, 2007)

> Dmitri wrote:
> > Hey, this not true for series motors! They decrease in RPM when loaded and
> > increase torque per amp.
> 
> ...


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## EVDL List (Jul 27, 2007)

> Lee Hart wrote:
> >> Are you willing to build your own motors?
> 
> SteveS wrote:
> ...


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## EVDL List (Jul 27, 2007)

Thanks for the lively discussion! I still have a
doubt. In DC series motor, the field has more coil and hence should have more I2 R
heat, right? Then why is the rotor getting more heated up, thanks

Joseph




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## EVDL List (Jul 27, 2007)

> <[email protected]> wrote:
> >
> >
> >
> ...


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## EVDL List (Jul 27, 2007)

Lee Hart wrote
> Google is your friend. There are dozens, perhaps hundreds of websites on 
> building your own motor. Unfortunately, many of them are well meaning 
> amateurs with very little knowledge. It is easy to build bad motors, but a bit of an art to build good ones.
>
> 
I did google, but it was hard to find anything real. Lots of references 
to making a toy motor (which I used to do with local schools - we built 
hundreds of 'em). Do you know anyone that has built a viable motor and 
has info on the web? It sounds fascinating.

> But you can't do this on a Lynch motor. If it says 48v, then you're 
> likely to destroy it if you put 60v on it. Likewise, higher than rated 
> current will burn it up very quickly.
> 
That rules them out. I'd could easily burn one up with the low RPM 
current I'd probably need.

I appreciate all the feedback. I hope my experimentation comes up with 
something useful to feed back to the list.

- SteveS



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## EVDL List (Jul 27, 2007)

> SteveS wrote:
> > Lee Hart wrote
> >> Google is your friend. There are dozens, perhaps hundreds of websites on
> >> building your own motor. Unfortunately, many of them are well meaning
> ...


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## EVDL List (Jul 27, 2007)

> [email protected] wrote:
> > Thanks for the lively discussion! I still have a doubt. In DC series
> > motor, the field has more coil and hence should have more I2 R
> > heat, right? Then why is the rotor getting more heated up, thanks
> ...


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## EVDL List (Jul 27, 2007)

> SteveS wrote:
> > Do you know anyone that has built a viable motor and
> > has info on the web? It sounds fascinating.
> 
> ...


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## EVDL List (Jul 27, 2007)

I didn't mean these motors as toys! I was speaking of something like:

http://home.hiwaay.net/~palmer/motor.html

That's the kind of thing I kept seeing when I tried Googling things like 
'building your own motor' etc.

- SteveS



> Matt C wrote:
> > I wouldn't exactly call them toys...but these guys are into building
> > their own "outrunner" motors. Smaller outrunners are popular with RC
> > planes, but the have built one up to 2kW,
> ...


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## EVDL List (Jul 27, 2007)

> --- Lee Hart <[email protected]> wrote:
> 
> > [email protected] wrote:
> > > Thanks for the lively discussion! I still have a
> ...


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