How about a CVT and constant speed motor?

[footer746]

Theoretical question:
 
What if you put a constant speed motor mated to a constant velocity transmission, not for the typical gear reduction of an ICE, but to change (simplify?) controller requirements?  The go pedal would drive the electronics in the CVT, rather than high current PWM circuitry?  Not sure if you gain anything - just a thought.
 
 
Matt Albertson  
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[fsabolich]

This would only simplify the control logic, the costly stuff - high power transistors, DC link capacitors, heat sink, etc. - would remain the same.  You would shave a few bucks off the controller at most.

Fran

footer746 wrote:

Theoretical question:
 
What if you put a constant speed motor mated to a constant velocity transmission, not for the typical gear reduction of an ICE, but to change (simplify?) controller requirements?  The go pedal would drive the electronics in the CVT, rather than high current PWM circuitry?  Not sure if you gain anything - just a thought.
 
 
Matt Albertson  

[Lee Hart]

Matt Albertson wrote:
> What if you put a constant speed motor mated to a constant velocity
> transmission, not for the typical gear reduction of an ICE, but to
> change (simplify?) controller requirements?  The go pedal would drive
> the electronics in the CVT, rather than high current PWM circuitry?
> Not sure if you gain anything - just a thought.

I think there is hope for that approach. An ICE normally runs in a
fairly narrow speed range, like 1000-3000 RPM. The transmission converts
this so you can drive at any speed you like.

EVs can obviously be built without a transmission. Both a series DC
motor and an AC induction motor can cover the whole range from 1-100
mph. But, going transmissionless does require a bigger motor and
controller; this adds cost, size, and weight.

If you had an "ideal" transmission that efficiently covered a very wide
range of speeds with a narrow motor RPM range, you could indeed use
smaller electric motors and controllers. For example, PM DC motors have
very high efficiencies, but don't work well over a wide speed range.
Likewise, the very simple AC inverters we have been discussing work best
over a limited speed range.

The big unknown is whether you gain enough in the motor/controller to
pay for the losses in the transmission.
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[Lee Hart]

footer746 wrote:
>> What if you put a constant speed motor mated to a constant velocity
>> transmission...

fsabolich wrote:
> This would only simplify the control logic, the costly stuff - high
> power transistors, DC link capacitors, heat sink, etc. - would remain
> the same. You would shave a few bucks off the controller at most.

Actually, it simplifies a lot of things for the motor and controller.

If you have an ideal transmission, the motor only has to run over a
narrow range of speeds. For a DC motor, your "controller" could be as
simple as a big contactor to switch it straight to the battery (with
perhaps a second contactor and starting resistor to get it going without
such a lurch). A PM or shunt motor would work fine.

For an AC system, this ideal transmission lets you use a plain old
off-the-shelf AC motor. It doesn't need to run at 0-13,000 RPM like the
GM EV1 and other AC EVs; 1200-3600 RPM is fine, which can be achieved
with 40-120 Hz to a 60 Hz motor.

In motors, voltage is proportional to speed, and current is proportional
to torque. Without a transmission, your controller needs to handle high
voltage to reach high speeds, and high currents to provide high torque.
This is what makes it so expensive. With a transmission to narrow the
speed range, the transistors and diodes can be much smaller and cheaper.

With an induction motor, voltage control is not strictly necessary.
Efficiency is improved if you can pick the voltage to suit the load and
speed; but if you get it wrong, the efficiency drop is not large if you
keep the range small. For example, suppose you have an 85% efficient 60
Hz 120 VAC motor; if you apply a fixed 120 VAC but vary the frequency
over a 40-120 Hz range, its efficiency will be still be over 80% across
this range. A highly optimized inverter gives you the full 85% over this
range; only 5% better.

So, an ideal transmission can allow a cheaper motor and a much simpler
cheaper controller for the same horsepower.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
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[fsabolich]

Well, I suppose he was talking about actual/practical CVT.

If you simply have a contactor, the CVT would have to have to have almost an infinite "gear ratio" range.  Imagine driving at a snail's pace in a traffic jam.  Your CVT could not match motor speed to wheel speed.  So all of a sudden you need a clutch.  In this situation an electric motor should excel, but in this case you end up heating the clutch (just like ICE vehicles).

As for high speed, you don't need high voltage.  You can simply weaken the field.  In a series field motor this happens automatically as the current drops (it kind of has a built in transmission).  In a shunt field motor you weaken the field directly.  In a AC induction motor with field oriented control you simply weaken the D component which magnetizes the squirrel cage.

I doubt you can get away with a contactor and no clutch.  Once you have power transistors the processing power to control them intelligently is cheap.

Fran

Lee Hart wrote:

footer746 wrote:
>> What if you put a constant speed motor mated to a constant velocity
>> transmission...

fsabolich wrote:
> This would only simplify the control logic, the costly stuff - high
> power transistors, DC link capacitors, heat sink, etc. - would remain
> the same. You would shave a few bucks off the controller at most.

Actually, it simplifies a lot of things for the motor and controller.

If you have an ideal transmission, the motor only has to run over a
narrow range of speeds. For a DC motor, your "controller" could be as
simple as a big contactor to switch it straight to the battery (with
perhaps a second contactor and starting resistor to get it going without
such a lurch). A PM or shunt motor would work fine.

For an AC system, this ideal transmission lets you use a plain old
off-the-shelf AC motor. It doesn't need to run at 0-13,000 RPM like the
GM EV1 and other AC EVs; 1200-3600 RPM is fine, which can be achieved
with 40-120 Hz to a 60 Hz motor.

In motors, voltage is proportional to speed, and current is proportional
to torque. Without a transmission, your controller needs to handle high
voltage to reach high speeds, and high currents to provide high torque.
This is what makes it so expensive. With a transmission to narrow the
speed range, the transistors and diodes can be much smaller and cheaper.

With an induction motor, voltage control is not strictly necessary.
Efficiency is improved if you can pick the voltage to suit the load and
speed; but if you get it wrong, the efficiency drop is not large if you
keep the range small. For example, suppose you have an 85% efficient 60
Hz 120 VAC motor; if you apply a fixed 120 VAC but vary the frequency
over a 40-120 Hz range, its efficiency will be still be over 80% across
this range. A highly optimized inverter gives you the full 85% over this
range; only 5% better.

So, an ideal transmission can allow a cheaper motor and a much simpler
cheaper controller for the same horsepower.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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[Morgan LaMoore]

On Thu, Jan 31, 2008 at 11:48 AM, fsabolich <[hidden email]> wrote:
>  As for high speed, you don't need high voltage.  You can simply weaken the
>  field.

But high speed does require a motor that's designed to handle high
frequencies. Normal industrial AC motors will have excessively high
core losses at higher frequencies (leading to excessive heating and
reduced safe power levels); you probably can't take them over 120Hz.

-Morgan LaMoore

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[Lee Hart]

fsabolich wrote:
> Well, I suppose he was talking about actual/practical CVT.

I think he said "theoretically"; but I agree that it is more useful to
discuss it in terms of CVTs you can actually get.

> If you simply have a contactor, the CVT would have to have to have almost an
> infinite "gear ratio" range.  Imagine driving at a snail's pace in a traffic
> jam.  Your CVT could not match motor speed to wheel speed.  So all of a
> sudden you need a clutch.  In this situation an electric motor should excel,
> but in this case you end up heating the clutch (just like ICE vehicles).

My ElecTrak (electric garden tractor) has a contactor controller (just
off/on), a PM motor, and a 3-speed transmission with a slip-the-b-belt
clutch. Crude; but it works.

I've driven a "Freeway" EV, which has a fixed-speed PM motor, contactor
controller, and snowmobile-type CVT. It is also crude; but effective.

Vehicles with hydrostatic CVTs often run the motor at constant speed,
and do all speed control (and forward/reverse) just with the CVT. They
aren't particularly efficient, but work very well.

> As for high speed, you don't need high voltage. You can simply weaken
> the field...

Sometimes you can. But most of the time, trying to weaken the field to
get higher RPM also reduces torque, and thus horsepower. The motor can
"wind up" to high RPM downhill, but can't produce enough torque to
maintain that speed on the level.

Once you've reached "top end", the only way to go faster is to raise
pack voltage, or shift the transmission.

> I doubt you can get away with a contactor and no clutch.  Once you have
> power transistors the processing power to control them intelligently is
> cheap.

Well, literally millions of EVs have been built with contactor
controllers and no clutch. They aren't sophisticated or high tech; but
they're cheap and they work.

The usual problem with CVTs, and with contactor controllers is that
someone did it to be cheap -- not good. But if you get the details
right, I think combining a contactor controller with a good CVT could
make quite a nice, drivable vehicle. Bob McKee's Sundancer EV comes to mind.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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[fsabolich]

If you look at an ideal motor, reducing field strength won't reduce horsepower, it'll simply keep it constant.  So when you look at the power curve you have constant torque from 0 rpm to the rpm where counter EMF equals pack voltage.  To go faster you must weaken the field; this puts you in constant horsepower region where RPM * torque is constant.

My experience is mostly in industry with large DC shunt wound motors, and they pretty much behave like this.

Of course if you have no transmission, you have to have more power to compensate.  Removing the transmission saves space and weight and allows for a bigger motor.

Currently I'm working with PM synchronous motors.  Compared to induction motors they are significantly smaller and lighter.  So with them you can simply have lots of power without having big and heavy motors - so it is possible to work without a transmission.  But anyways, I'm drifting off topic.

Fran

[Josh and Jenifer]

What about using a large centrifical clutch behind the motor to a direct
drive to a low geared rear end?  Could you use a smaller say 9" motor?  I'm
thinking the motor could turn 300 to 400 rpm before the clutch would engage,
then allow the vehicle to start to move with out puting a stall load on the
motor.  but when running down the road at speed, the clutch would be fully
engaged.  Any thoughts?

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[gottdi]

Actually thought of that for an electric go cart. I have one  
(centrifical clutch) and a motor that would work for a go cart. Seems  
like it would work. Just change the spring tension.

:  )



On Jan 31, 2008, at 7:59 PM, Josh Creel wrote:

>
> What about using a large centrifical clutch behind the motor to a  
> direct
> drive to a low geared rear end?  Could you use a smaller say 9"  
> motor?  I'm
> thinking the motor could turn 300 to 400 rpm before the clutch  
> would engage,
> then allow the vehicle to start to move with out puting a stall  
> load on the
> motor.  but when running down the road at speed, the clutch would  
> be fully
> engaged.  Any thoughts?
>
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[Josh and Jenifer]

> Actually thought of that for an electric go cart. I have one
> (centrifugal clutch) and a motor that would work for a go cart. Seems
> like it would work. Just change the spring tension.

It would probably take some special machine work to build one for a vehicle.
Has anyone heard of a large one?

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[Jeff Major]

--- Morgan LaMoore <[hidden email]> wrote:
.

> But high speed does require a motor that's designed
> to handle high
> frequencies. Normal industrial AC motors will have
> excessively high
> core losses at higher frequencies (leading to
> excessive heating and
> reduced safe power levels); you probably can't take
> them over 120Hz.
>
> -Morgan LaMoore
>

Hi Morgan,

The DC motors I see most widely used on EVs are of
"normal industrial" quality.  What is the difference
of running a 4 pole DC motor 3600 RPM and higher
compared to an AC motor WRT core loss?

Regards,

Jeff M


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[Lee Hart]

Josh Creel wrote:
> What about using a large centrifical clutch behind the motor to a direct
> drive to a low geared rear end?  Could you use a smaller say 9" motor?  I'm
> thinking the motor could turn 300 to 400 rpm before the clutch would engage,
> then allow the vehicle to start to move with out puting a stall load on the
> motor.  but when running down the road at speed, the clutch would be fully
> engaged.  Any thoughts?

You could do this. It works fine on all sorts of small vehicles
(minibikes, go-karts, etc.).

But it is less useful for EVs. Electric motors can produce torque at 0
speed, so they don't have the problem starting out from a dead stop that
ICEs do (though you might with a single-phase AC motor). A centrifugal
clutch does not multiply torque, so you don't get any transmission-like
benefits.

With an electric motor, I think it makes more sense to consider a torque
converter. On a small EV, this could be a variable-speed belt like the
Comet CVTs. On a larger EV, it could be a locking torque converter from
an automatic transmission. These can multiply the torque as well as
reduce the speed, giving you the benefits of a transmission without the
weight and shifting.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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[Lee Hart]

Morgan LaMoore wrote:
> But high speed does require a motor that's designed to handle high
> frequencies. Normal industrial AC motors will have excessively high
> core losses at higher frequencies (leading to excessive heating
> and reduced safe power levels); you probably can't take them over
> 120Hz.

That's exactly what they do with industrial inverters; they run 60 Hz
motors at 120 Hz maximum, because that's about as high as you can go
with normal laminations.

Indeed, I find that a normal 60 Hz motor runs at almost exactly the same
temperature at 120 Hz. Its core losses double, but because it's running
at double the RPM, it also gets twice the cooling from its internal fan.

If you start with a 4-pole motor (1750 RPM at 60 Hz), then 120 Hz is
about 3500 RPM. This is no strain on the bearings or rotor strength, either.

A special purpose AC EV motor is designed to run at 4 times that speed
(13,000 RPM for the GM EV1, for example). This requires much higher
frequencies, and so much better core materials are needed. Virtually
nothing in these motors is the same as a garden variety 60 Hz motor.

--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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