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Discussion Starter #1
OK math wizards, is there something here, or am I off base. This looks like
calculus, at which I am impaired, too ADHD.

What happens if you couple a smaller motor to the back of a bigger motor and
parallel them (electrically).

I'm thinking the big motor might suck up most the current and produce
tremendous torque at low RPM, then when the big motor's back EMF is taking
it down as RPM rises, the little motor is getting into it's power band?
Might this combination make for a broader power band and reduce the need for
a transmission or series parallel switching? Another advantage might be that
the output shaft size of the small motor may be the same size as the small
shaft end of the big motor, and couplers may survive better as the big end
of the big motor isn't being asked to handle twice the torque it was
designed to.

I'm thinking ADC 11 and ADC 8 for example.

Comments?

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Discussion Starter #2
The problem I can think of right away is that once the 8" motor gets up into its "power band" as you put it you could be well past the redline of the bigger motor. It would probably be best for both motors that they share load equally at all power and RPM levels. If you want the torque of an 8" and 11" in combination why not go with two 9" motors. In my rough figuring two 6.7"s equal a 9", two 8"s equal an 11" and two 9"s equal a13" for torque. The benefit for the two smaller motors is the higher RPM limits and the ability to affect a series/parallel shift point.

My two motors worth,

Mike
Anchorage, Ak.

----- Original Message -----
From: Marty Hewes <[email protected]>
Date: Wednesday, August 15, 2007 7:46 am
Subject: [EVDL] twin, different motors?
To: Electric Vehicle Discussion List <[email protected]>

> OK math wizards, is there something here, or am I off base. This
> looks like
> calculus, at which I am impaired, too ADHD.
>
> What happens if you couple a smaller motor to the back of a bigger
> motor and
> parallel them (electrically).
>
> I'm thinking the big motor might suck up most the current and
> produce
> tremendous torque at low RPM, then when the big motor's back EMF is
> taking
> it down as RPM rises, the little motor is getting into it's power
> band?
> Might this combination make for a broader power band and reduce the
> need for
> a transmission or series parallel switching? Another advantage
> might be that
> the output shaft size of the small motor may be the same size as
> the small
> shaft end of the big motor, and couplers may survive better as the
> big end
> of the big motor isn't being asked to handle twice the torque it
> was
> designed to.
>
> I'm thinking ADC 11 and ADC 8 for example.
>
> Comments?
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

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Discussion Starter #3
My thought was to better emulate the wide power band of an AC motor. A
large DC motor pulls well at low RPM, but has too much back EMF at higher
RPM to draw much current and generate HP unless a high voltage pack is used.
The smaller motor isn't as torquey at low RPM, but comes on stronger at high
RPM due to less back EMF. I'm thinking a 6.7" coupled to a 9" might produce
the same power as twin 8's, but with a broader power band and less stress on
the small shaft end of the bigger motor. Might be cheaper also, I seem to
remember 8" and 9" being similar in price, but the 6.7" is significantly
cheaper. I also wonder about the red line on the bigger motor though.
Guess I'll have to get out the curves and graph paper.

Marty

----- Original Message -----
From: "MIKE WILLMON" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Wednesday, August 15, 2007 12:28 PM
Subject: Re: [EVDL] twin, different motors?


> The problem I can think of right away is that once the 8" motor gets up
> into its "power band" as you put it you could be well past the redline of
> the bigger motor. It would probably be best for both motors that they
> share load equally at all power and RPM levels. If you want the torque of
> an 8" and 11" in combination why not go with two 9" motors. In my rough
> figuring two 6.7"s equal a 9", two 8"s equal an 11" and two 9"s equal
> a13" for torque. The benefit for the two smaller motors is the higher RPM
> limits and the ability to affect a series/parallel shift point.
>
> My two motors worth,
>
> Mike
> Anchorage, Ak.
>
> ----- Original Message -----
> From: Marty Hewes <[email protected]>
> Date: Wednesday, August 15, 2007 7:46 am
> Subject: [EVDL] twin, different motors?
> To: Electric Vehicle Discussion List <[email protected]>
>
>> OK math wizards, is there something here, or am I off base. This
>> looks like
>> calculus, at which I am impaired, too ADHD.
>>
>> What happens if you couple a smaller motor to the back of a bigger
>> motor and
>> parallel them (electrically).
>>
>> I'm thinking the big motor might suck up most the current and
>> produce
>> tremendous torque at low RPM, then when the big motor's back EMF is
>> taking
>> it down as RPM rises, the little motor is getting into it's power
>> band?
>> Might this combination make for a broader power band and reduce the
>> need for
>> a transmission or series parallel switching? Another advantage
>> might be that
>> the output shaft size of the small motor may be the same size as
>> the small
>> shaft end of the big motor, and couplers may survive better as the
>> big end
>> of the big motor isn't being asked to handle twice the torque it
>> was
>> designed to.
>>
>> I'm thinking ADC 11 and ADC 8 for example.
>>
>> Comments?
>>
>> _______________________________________________
>> For subscription options, see
>> http://lists.sjsu.edu/mailman/listinfo/ev
>>
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>
>


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Discussion Starter #4
--- Marty Hewes <[email protected].xxx> wrote:

> My thought was to better emulate the wide power band
> of an AC motor. A
> large DC motor pulls well at low RPM, but has too
> much back EMF at higher
> RPM to draw much current and generate HP unless a
> high voltage pack is used.
> The smaller motor isn't as torquey at low RPM, but
> comes on stronger at high
> RPM due to less back EMF. I'm thinking a 6.7"
> coupled to a 9" might produce
> the same power as twin 8's, but with a broader power
> band and less stress on
> the small shaft end of the bigger motor.

Hey Marty

I'd advise against it. If you try to run both motors
at the same time they will probably fight each other.
On Waylands 8" motors I paid careful attention to
getting the RPM's and voltage as close as possible so
they pulled equal. Even on dual forklift motor drives
I always hate when they send just one. That one gets
back up to par and then the one they didn't have done
isn't running as well and it puts the new motor under
a harder strain.

As Mike stated you're now limited by the larger motors
RPM limits (whether it's commutating or not) which
kind of defeats the purpose.

In short, unless I was dealing with some free / cheap
motors I'd stay with "like piars" and stay on a more
tried and true set up. I sure wouldn't go out and buy
a new FB1 and L91 for example where in reality twin
8's would cost pretty close to the same and be a
reliabe system.

In as much as this might get you your desired power
outputs it says nothing as to whether the motors will
be happy about it. Just something to chew on.

I missed the first parts of this I believe like car
weight, voltage, and controller

Anyway, hope this helps.

Jim Husted
Hi-Torque Electric





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Discussion Starter #5
What Jim said :)

Well, except that if you weren't worried about money, you could setup
a dual differential of some kind that was connected to the controller
via a shaft RPM sensor at both motors. That way neither motor could be
on at the same time, but wouldn't drag each other much...the big one
would start you off and the little one would take over at X rpm.

Heehee.

Who wants to do the boring thing with a transmission when you can make
slipping, electronically controlled differentials!!

*ahem*

--T

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Discussion Starter #6
You are asking two questions:

1. Electrical compatibility
2. Mechanically compatibility

I assume you are considering series wound motors on a common controller.

1. Hooking two motors in parallel is similar to hooking two resistors in
parallel. The current is split between the motors inversely proportional to
their resistance's. If you use series motors, the resistance of the motors
can be described as R = A + B * RPM where A is the copper resistance and B
is the back EMF component of each motor.

2. Since the resistance of each motor goes up linearly with RPM, the torque
produced by each motor will be the current in each motor times the ft*# per
amp of each motor. These will probably be close to linear, therefore the
proportion of the torque supplied by each motor will be nearly constant.

Basically the two motors will act just like a single motor.

If you use a series/parallel switching network of contactors, you can
effectively have four electrical gears with two similar (not same) motors
and a common controller.

For instance, two 8 inch motors with different armatures (I forget the two
different commutator bar counts) will have different A and B constants. If
you chose an appropriate number of turns on the fields, you may be able to
get one motor with .2 milliohms per 1000 RPM and the other with .3 milliohms
per 1000 RPM ,then if you hooked them in parallel, they would be .12
milliohms per 1000 RPM and in series they would be .5 milliohms per 1000
RPM.

At 500 RPM and the motors in series, the motors would draw 480 amps.
At 1000 RPM, the current would drop to 240 amps and the user would shift to
Motor B only where the motor would draw 400 amps.
At 2000 RPM the current would drop to 200 amps and the user would shift to
Motor A only where the motor would draw 300 amps.
At 3000 RPM the current would drop to 200 amps and the user would shift to
Parallel only where the two motors would draw 333 amps.
At 4000 RPM the current would drop to 222 amps.
At 5000 RPM the current would drop to 200 amps.
At 6000 RPM the current would drop to 167 amps.

If you alter the Battery Voltage, Gearing and Back EMF numbers, you should
be able to set the system to draw an appropriate current for the motors and
controller you choose.

If you use two controllers, the system would operate as you described.

Joe Smalley
Rural Kitsap County WA
Former owner of 48 Volt Fiesta
NEDRA 48 volt street conversion record holder
[email protected]


----- Original Message -----
From: "Marty Hewes" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Wednesday, August 15, 2007 8:39 AM
Subject: [EVDL] twin, different motors?


> OK math wizards, is there something here, or am I off base. This looks
like
> calculus, at which I am impaired, too ADHD.
>
> What happens if you couple a smaller motor to the back of a bigger motor
and
> parallel them (electrically).
>
> I'm thinking the big motor might suck up most the current and produce
> tremendous torque at low RPM, then when the big motor's back EMF is taking
> it down as RPM rises, the little motor is getting into it's power band?
> Might this combination make for a broader power band and reduce the need
for
> a transmission or series parallel switching? Another advantage might be
that
> the output shaft size of the small motor may be the same size as the small
> shaft end of the big motor, and couplers may survive better as the big end
> of the big motor isn't being asked to handle twice the torque it was
> designed to.
>
> I'm thinking ADC 11 and ADC 8 for example.
>
> Comments?
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev

_______________________________________________
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Discussion Starter #7
Hey Jeff

I couldn't have said it better 8^) BTW I'm not
scrawny I consider it more like being bulk challenged
8^o

I will say some of those here are not overly concerned
about having to warranty a motor 8^P

Anyway I feel the wheelbarrow stories help people to
see how things work duty cycle wise. Maybe we should
write a wheelbarrows guide to motor usage, LMAO.

Had fun
Cya
Jim Husted
Hi-Torque Electric
--- Jeff Major <[email protected]> wrote:

> Hey Marty,
>
> Instead of talking volts and amps, I'll try it like
> some other guys explain stuff.
>
> Say Dad has a big pile of dirt in the yard he needs
> moved. He gets his twin sons, Bill and Ted, out
> there
> to move half of it. But he has just one
> wheelbarrow.
> He puts Ted on one handle and Bill on the other.
> Dad
> fills the wheelbarrow with dirt, he has a
> skidloader,
> he's not stupid. Then Bill and Ted push the loaded
> wheelbarrow over to the other side of the yard.
> They
> both go at the same speed, obviously. Bill and Ted
> are the same size boys and do an equal amount of
> work
> and get equally exhausted.
>
> Now, half the pile is moved and Bill and Ted go off
> down the street to tease the girls. Dad then calls
> out his other two sons, Jim and Marty, who are not
> twins. Jim is a little scrawny boy and Marty is a
> husky dude. Just like with the twins, Jim takes one
> handle and Marty the other. Now, when they start
> pushing, Marty really leans into it and Jim does all
> he can. Still going at equal speeds, they get it up
> to pace and continue across the yard. But Jim, the
> scrawny kid, is really working hard now, while Marty
> is just cruising. They get the job done. Marty
> goes
> off to the gym for his workout. But poor Jim is
> lying
> on the ground moaning. Dad takes Jim to the
> hospital
> and checks him in for a rewind.
>
> Get my meaning?
>
> Jeff M
>
>
>
>
>
>
>
>
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Discussion Starter #8
I am yet unconvinced. NetGain lists red line for all their motors at the
same RPM, so I'm not worried about overrevving the bigger motor. It's not
about total torque produced over a narrow band, it's about having good
torque over a wider RPM range, like an AC motor with lower cost. The curves
show that at low RPM, the big motor dominates, but as RPM goes up, a smaller
motor produces more torque than a big one unless the pack voltage is pretty
high. Just look at the torque and horsepower curves for the motors for
voltages of 144 or less. Big motors (11" or bigger) crap out below 3000
RPM, small ones then out pull them to 5500. The sum of the two might be
interesting. I need to draw some composite curves before I rule the idea
out. The combination wouldn't produce more peak power at the point the
controller is just coming out of current limiting, because that's determined
mostly by the battery and controller capabilities anyway, not the motor, but
I really think it might provide decent power over a wider RPM range,
counteracting a big motor's tendency to go flat as RPM rises because of back
EMF limiting current, which might benefit eliminating the trans. Our
problem with running without a trans is that the controller current limit
limits power at low RPM when current is high, and the pack voltage limits
power at higher RPM. I think that with two different size motors in
parallel, the big one would automatically dominate at low RPM, and as RPM
rises, the smaller one would take over. I suspect it would act a little
like a CVT in terms of matching battery capability to road speed. The
motors would have to stay electrically in parallel, or the power would
always go to the less capable motor instead.

Jim is absolutely correct on one point, that the smaller motor could easily
be overworked. As RPM rises, the current, and heat, migrates to the small
motor because of lower back EMF, so the small motor would have to have
enough capacity to do most of the work at higher RPM. This wouldn't be a
setup for Bonneville speed record attempts or highway speed mountain driving
without a trans. Here in Illinois? Maybe.

----- Original Message -----
From: "Jim Husted" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Thursday, August 16, 2007 10:13 AM
Subject: Re: [EVDL] twin, different motors?


> Hey Jeff
>
> I couldn't have said it better 8^) BTW I'm not
> scrawny I consider it more like being bulk challenged
> 8^o
>
> I will say some of those here are not overly concerned
> about having to warranty a motor 8^P
>
> Anyway I feel the wheelbarrow stories help people to
> see how things work duty cycle wise. Maybe we should
> write a wheelbarrows guide to motor usage, LMAO.
>
> Had fun
> Cya
> Jim Husted
> Hi-Torque Electric
> --- Jeff Major <[email protected]> wrote:
>
>> Hey Marty,
>>
>> Instead of talking volts and amps, I'll try it like
>> some other guys explain stuff.
>>
>> Say Dad has a big pile of dirt in the yard he needs
>> moved. He gets his twin sons, Bill and Ted, out
>> there
>> to move half of it. But he has just one
>> wheelbarrow.
>> He puts Ted on one handle and Bill on the other.
>> Dad
>> fills the wheelbarrow with dirt, he has a
>> skidloader,
>> he's not stupid. Then Bill and Ted push the loaded
>> wheelbarrow over to the other side of the yard.
>> They
>> both go at the same speed, obviously. Bill and Ted
>> are the same size boys and do an equal amount of
>> work
>> and get equally exhausted.
>>
>> Now, half the pile is moved and Bill and Ted go off
>> down the street to tease the girls. Dad then calls
>> out his other two sons, Jim and Marty, who are not
>> twins. Jim is a little scrawny boy and Marty is a
>> husky dude. Just like with the twins, Jim takes one
>> handle and Marty the other. Now, when they start
>> pushing, Marty really leans into it and Jim does all
>> he can. Still going at equal speeds, they get it up
>> to pace and continue across the yard. But Jim, the
>> scrawny kid, is really working hard now, while Marty
>> is just cruising. They get the job done. Marty
>> goes
>> off to the gym for his workout. But poor Jim is
>> lying
>> on the ground moaning. Dad takes Jim to the
>> hospital
>> and checks him in for a rewind.
>>
>> Get my meaning?
>>
>> Jeff M
>>
>>
>>
>>
>>
>>
>>
>>
> ____________________________________________________________________________________
>> Yahoo! oneSearch: Finally, mobile search
>> that gives answers, not web links.
>>
> http://mobile.yahoo.com/mobileweb/onesearch?refer=1ONXIC
>>
>> _______________________________________________
>> For subscription options, see
>> http://lists.sjsu.edu/mailman/listinfo/ev
>>
>
>
>
>
> ____________________________________________________________________________________
> Yahoo! oneSearch: Finally, mobile search
> that gives answers, not web links.
> http://mobile.yahoo.com/mobileweb/onesearch?refer=1ONXIC
>
> _______________________________________________
> For subscription options, see
> http://lists.sjsu.edu/mailman/listinfo/ev
>

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Discussion Starter #9
Hey Marty,

Pushing the wheelbarrow over to the side, I'll give
you a couple of my impressions.

First off I do not have the motor curves you refer to.
Please send them and I'll look them over. But as
for now, there are many different designs possible for
the two different motors. Just how they would
interact depends a lot on the particular design of
each.

You mention "having a good torque over wider RPM
range, like an AC motor." I have done both, DC and
AC, motors. When you push an AC motor to its limits,
the resultant speed/torque map shape closely resembles
that of a series wound DC motor. At the higher
frequencies (RPM) with the AC motor, you end up field
weakening (or reducing the volts/hertz), so you lose
torque. On the other end, the AC motor (induction)
will have a torque limit (breakdown) and the DC motor
a controller current limit. The shape of the
speed/torque maps are about the same, just that the AC
motor is higher speed/less torque that the DC. But
with proper gearing, they become nearly equal, some
torque advantage to the DC.

So, I guess I have to question your motivation here.

What you have proposed, in my opinion, will work.
But, again in my opinion, is likely to less effective
than two equal motors. That is for a given total mass
of iron and copper, two equal motors will give you
better performance and thermal suitability than two
different sized motors.

I realize that you have to work with what is
available, but if your goal is to shape the
speed/torque curve to suit a particular need, this can
be done by altering design in a single motor. Or by
the control method of the motor. A simple method
comes to mind when you say "counteracting a big
motor's tendency to go flat as RPM rises because of
back EMF limiting current". Switch in a field
weakening resistor.

Another point, you say "As RPM rises, the current, and
heat, migrates to the small motor because of lower
back EMF". Now, both motors have the same RPM. Both
have the same applied voltage. Why do you think the
Eg is less in the small motor? That is saying that
the equivalent motor resistance times the current in
the small motor is larger than in the big motor. It
might be. Might not be. The motor currents are not
necessarily equal. And their resistance is unlikely
equal.

But, I encourage you to pursue this. You might come
up with something to work well for your application.
Certainly, plotting out the motor curves and summing
the torques vs RPM will give you a good look.

I guess I just worry about the little guy going under
the knife for a rewind.

Good luck,

Jeff


--- Marty Hewes <[email protected]> wrote:

> I am yet unconvinced. NetGain lists red line for
> all their motors at the
> same RPM, so I'm not worried about overrevving the
> bigger motor. It's not
> about total torque produced over a narrow band, it's
> about having good
> torque over a wider RPM range, like an AC motor with
> lower cost. The curves
> show that at low RPM, the big motor dominates, but
> as RPM goes up, a smaller
> motor produces more torque than a big one unless the
> pack voltage is pretty
> high. Just look at the torque and horsepower curves
> for the motors for
> voltages of 144 or less. Big motors (11" or bigger)
> crap out below 3000
> RPM, small ones then out pull them to 5500. The sum
> of the two might be
> interesting. I need to draw some composite curves
> before I rule the idea
> out. The combination wouldn't produce more peak
> power at the point the
> controller is just coming out of current limiting,
> because that's determined
> mostly by the battery and controller capabilities
> anyway, not the motor, but
> I really think it might provide decent power over a
> wider RPM range,
> counteracting a big motor's tendency to go flat as
> RPM rises because of back
> EMF limiting current, which might benefit
> eliminating the trans. Our
> problem with running without a trans is that the
> controller current limit
> limits power at low RPM when current is high, and
> the pack voltage limits
> power at higher RPM. I think that with two
> different size motors in
> parallel, the big one would automatically dominate
> at low RPM, and as RPM
> rises, the smaller one would take over. I suspect
> it would act a little
> like a CVT in terms of matching battery capability
> to road speed. The
> motors would have to stay electrically in parallel,
> or the power would
> always go to the less capable motor instead.
>
> Jim is absolutely correct on one point, that the
> smaller motor could easily
> be overworked. As RPM rises, the current, and heat,
> migrates to the small
> motor because of lower back EMF, so the small motor
> would have to have
> enough capacity to do most of the work at higher
> RPM. This wouldn't be a
> setup for Bonneville speed record attempts or
> highway speed mountain driving
> without a trans. Here in Illinois? Maybe.
>
> ----- Original Message -----
> From: "Jim Husted" <[email protected]>
> To: "Electric Vehicle Discussion List"
> <[email protected]>
> Sent: Thursday, August 16, 2007 10:13 AM
> Subject: Re: [EVDL] twin, different motors?
>
>
> > Hey Jeff
> >
> > I couldn't have said it better 8^) BTW I'm not
> > scrawny I consider it more like being bulk
> challenged
> > 8^o
> >
> > I will say some of those here are not overly
> concerned
> > about having to warranty a motor 8^P
> >
> > Anyway I feel the wheelbarrow stories help people
> to
> > see how things work duty cycle wise. Maybe we
> should
> > write a wheelbarrows guide to motor usage, LMAO.
> >
> > Had fun
> > Cya
> > Jim Husted
> > Hi-Torque Electric
> > --- Jeff Major <[email protected]> wrote:
> >
> >> Hey Marty,
> >>
> >> Instead of talking volts and amps, I'll try it
> like
> >> some other guys explain stuff.
> >>
> >> Say Dad has a big pile of dirt in the yard he
> needs
> >> moved. He gets his twin sons, Bill and Ted, out
> >> there
> >> to move half of it. But he has just one
> >> wheelbarrow.
> >> He puts Ted on one handle and Bill on the other.
> >> Dad
> >> fills the wheelbarrow with dirt, he has a
> >> skidloader,
> >> he's not stupid. Then Bill and Ted push the
> loaded
> >> wheelbarrow over to the other side of the yard.
> >> They
> >> both go at the same speed, obviously. Bill and
> Ted
> >> are the same size boys and do an equal amount of
> >> work
> >> and get equally exhausted.
> >>
> >> Now, half the pile is moved and Bill and Ted go
> off
> >> down the street to tease the girls. Dad then
> calls
> >> out his other two sons, Jim and Marty, who are
> not
> >> twins. Jim is a little scrawny boy and Marty is
> a
> >> husky dude. Just like with the twins, Jim takes
> one
> >> handle and Marty the other. Now, when they start
> >> pushing, Marty really leans into it and Jim does
> all
> >> he can. Still going at equal speeds, they get it
> up
> >> to pace and continue across the yard. But Jim,
> the
> >> scrawny kid, is really working hard now, while
> Marty
> >> is just cruising. They get the job done. Marty
> >> goes
> >> off to the gym for his workout. But poor Jim is
> >> lying
> >> on the ground moaning. Dad takes Jim to the
> >> hospital
> >> and checks him in for a rewind.
> >>
> >> Get my meaning?
> >>
> >> Jeff M
> >>



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Discussion Starter #10
Hey Jeff

Here's the link to the NetGain motors.

http://www.go-ev.com/motors-warp.html

FWIW I did just read a news letter they put out
suggesting that they don't recommend using dual 9's
direct drive for dailies as the motors like the 2500
to 3500 RPM range where the fan can keep temps in
check. To much stop and go city driving may overheat
them. If you can keep the motors above 2000 RPM's
most the time you're golden. If you like to creep and
squeeze EVery last amp you might find you're actually
frying your motors. Just wanted to get that out to
those who might consider going direct drive for daily
driving. Blower cooling can increase the duty cycle,
to what exstent I'm sure varies.
BTW you had "me" at wheelbarrow, LMAO! 8^)

Hope this helps
Jim Husted
Hi-Torque Electric


--- Jeff Major <[email protected]> wrote:

>
> Hey Marty,
>
> Pushing the wheelbarrow over to the side, I'll give
> you a couple of my impressions.
>
> First off I do not have the motor curves you refer
> to.
> Please send them and I'll look them over. But as
> for now, there are many different designs possible
> for
> the two different motors. Just how they would
> interact depends a lot on the particular design of
> each.
>
> You mention "having a good torque over wider RPM
> range, like an AC motor." I have done both, DC and
> AC, motors. When you push an AC motor to its
> limits,
> the resultant speed/torque map shape closely
> resembles
> that of a series wound DC motor. At the higher
> frequencies (RPM) with the AC motor, you end up
> field
> weakening (or reducing the volts/hertz), so you lose
> torque. On the other end, the AC motor (induction)
> will have a torque limit (breakdown) and the DC
> motor
> a controller current limit. The shape of the
> speed/torque maps are about the same, just that the
> AC
> motor is higher speed/less torque that the DC. But
> with proper gearing, they become nearly equal, some
> torque advantage to the DC.
>
> So, I guess I have to question your motivation here.
>
> What you have proposed, in my opinion, will work.
> But, again in my opinion, is likely to less
> effective
> than two equal motors. That is for a given total
> mass
> of iron and copper, two equal motors will give you
> better performance and thermal suitability than two
> different sized motors.
>
> I realize that you have to work with what is
> available, but if your goal is to shape the
> speed/torque curve to suit a particular need, this
> can
> be done by altering design in a single motor. Or by
> the control method of the motor. A simple method
> comes to mind when you say "counteracting a big
> motor's tendency to go flat as RPM rises because of
> back EMF limiting current". Switch in a field
> weakening resistor.
>
> Another point, you say "As RPM rises, the current,
> and
> heat, migrates to the small motor because of lower
> back EMF". Now, both motors have the same RPM.
> Both
> have the same applied voltage. Why do you think the
> Eg is less in the small motor? That is saying that
> the equivalent motor resistance times the current in
> the small motor is larger than in the big motor. It
> might be. Might not be. The motor currents are not
> necessarily equal. And their resistance is unlikely
> equal.
>
> But, I encourage you to pursue this. You might come
> up with something to work well for your application.
>
> Certainly, plotting out the motor curves and summing
> the torques vs RPM will give you a good look.
>
> I guess I just worry about the little guy going
> under
> the knife for a rewind.
>
> Good luck,
>
> Jeff
>
>
> --- Marty Hewes <[email protected]> wrote:
>
> > I am yet unconvinced. NetGain lists red line for
> > all their motors at the
> > same RPM, so I'm not worried about overrevving the
> > bigger motor. It's not
> > about total torque produced over a narrow band,
> it's
> > about having good
> > torque over a wider RPM range, like an AC motor
> with
> > lower cost. The curves
> > show that at low RPM, the big motor dominates, but
> > as RPM goes up, a smaller
> > motor produces more torque than a big one unless
> the
> > pack voltage is pretty
> > high. Just look at the torque and horsepower
> curves
> > for the motors for
> > voltages of 144 or less. Big motors (11" or
> bigger)
> > crap out below 3000
> > RPM, small ones then out pull them to 5500. The
> sum
> > of the two might be
> > interesting. I need to draw some composite curves
> > before I rule the idea
> > out. The combination wouldn't produce more peak
> > power at the point the
> > controller is just coming out of current limiting,
> > because that's determined
> > mostly by the battery and controller capabilities
> > anyway, not the motor, but
> > I really think it might provide decent power over
> a
> > wider RPM range,
> > counteracting a big motor's tendency to go flat as
> > RPM rises because of back
> > EMF limiting current, which might benefit
> > eliminating the trans. Our
> > problem with running without a trans is that the
> > controller current limit
> > limits power at low RPM when current is high, and
> > the pack voltage limits
> > power at higher RPM. I think that with two
> > different size motors in
> > parallel, the big one would automatically dominate
> > at low RPM, and as RPM
> > rises, the smaller one would take over. I suspect
> > it would act a little
> > like a CVT in terms of matching battery capability
> > to road speed. The
> > motors would have to stay electrically in
> parallel,
> > or the power would
> > always go to the less capable motor instead.
> >
> > Jim is absolutely correct on one point, that the
> > smaller motor could easily
> > be overworked. As RPM rises, the current, and
> heat,
> > migrates to the small
> > motor because of lower back EMF, so the small
> motor
> > would have to have
> > enough capacity to do most of the work at higher
> > RPM. This wouldn't be a
> > setup for Bonneville speed record attempts or
> > highway speed mountain driving
> > without a trans. Here in Illinois? Maybe.
> >
> > ----- Original Message -----
> > From: "Jim Husted" <[email protected]>
> > To: "Electric Vehicle Discussion List"
> > <[email protected]>
> > Sent: Thursday, August 16, 2007 10:13 AM
> > Subject: Re: [EVDL] twin, different motors?
> >
> >
> > > Hey Jeff
> > >
> > > I couldn't have said it better 8^) BTW I'm not
> > > scrawny I consider it more like being bulk
> > challenged
> > > 8^o
> > >
> > > I will say some of those here are not overly
> > concerned
> > > about having to warranty a motor 8^P
> > >
> > > Anyway I feel the wheelbarrow stories help
> people
> > to
> > > see how things work duty cycle wise. Maybe we
> > should
> > > write a wheelbarrows guide to motor usage, LMAO.
> > >
> > > Had fun
> > > Cya
> > > Jim Husted
> > > Hi-Torque Electric
> > >
--- Jeff Major <[email protected]> wrote:
> > >
> > >> Hey Marty,
> > >>
> > >> Instead of talking volts and amps, I'll try it
> > like
> > >> some other guys explain stuff.
> > >>
> > >> Say Dad has a big pile of dirt in the yard he
> > needs
> > >> moved. He gets his twin sons, Bill and Ted,
> out
> > >> there
> > >> to move half of it. But he has just one
>
=== message truncated ===




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Discussion Starter #11
Now this just caused a thought to pop into my twisted little head
(never a safe thing :) I was also looking at the White Zombie
reverse circuit on my computer screen and the 2 kinda merged in my mind.

How about field weakening for high end gains, with an inductor in the
circuit? What popped into my head is along the same lines as the WZ
reverse. Take one motor's armature out of the circuit but leave field
windings in it. Put the field weakening resistor across the others
motors field. Now the controller should be able to control a motor
with a field weakening resistor. It would force the car to use a
single motor at the far end of the track but should allow that motor
take plenty of amps at high rpms.

Now back to your regularly scheduled (sane) posters!

Paul "neon" Gooch


Jim Husted wrote:

--- Jeff Major <[email protected]> wrote:

[snip]

> You mention "having a good torque over wider RPM
> range, like an AC motor." I have done both, DC and
> AC, motors. When you push an AC motor to its
> limits,
> the resultant speed/torque map shape closely
> resembles
> that of a series wound DC motor. At the higher
> frequencies (RPM) with the AC motor, you end up
> field
> weakening (or reducing the volts/hertz), so you lose
> torque.

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Discussion Starter #12
always pick the first half cause then you can tease the girls?

Rush
Tucson AZ
www.ironandwood.org
www.TEVA2.com
www.Airphibian.com


----- Original Message -----
From: "Jeff Major" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Thursday, August 16, 2007 7:26 AM
Subject: [EVDL] twin, different motors?


> Hey Marty,
>
> Instead of talking volts and amps, I'll try it like
> some other guys explain stuff.
>
> Say Dad has a big pile of dirt in the yard he needs
> moved. He gets his twin sons, Bill and Ted, out there
> to move half of it. But he has just one wheelbarrow.
> He puts Ted on one handle and Bill on the other. Dad
> fills the wheelbarrow with dirt, he has a skidloader,
> he's not stupid. Then Bill and Ted push the loaded
> wheelbarrow over to the other side of the yard. They
> both go at the same speed, obviously. Bill and Ted
> are the same size boys and do an equal amount of work
> and get equally exhausted.
>
> Now, half the pile is moved and Bill and Ted go off
> down the street to tease the girls. Dad then calls
> out his other two sons, Jim and Marty, who are not
> twins. Jim is a little scrawny boy and Marty is a
> husky dude. Just like with the twins, Jim takes one
> handle and Marty the other. Now, when they start
> pushing, Marty really leans into it and Jim does all
> he can. Still going at equal speeds, they get it up
> to pace and continue across the yard. But Jim, the
> scrawny kid, is really working hard now, while Marty
> is just cruising. They get the job done. Marty goes
> off to the gym for his workout. But poor Jim is lying
> on the ground moaning. Dad takes Jim to the hospital
> and checks him in for a rewind.
>
> Get my meaning?
>
> Jeff M
>
>
>
>
>
>
>
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