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'[EE] Switching unmatched Voltage Current Combinati'
2011\02\11@071411 by Tobias Gogolin

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I'm thinking about proposing a new way to look at EV Battery banks (as I
have messaged about previously here). And my thought is to not insist on a
prefect unfailing Voltage/Current matched battery bank and motor
combination, but rather to completely avoid parallel cells in favor of a
higher voltage (lower current) serial string, in which any discharged cell
is bridged out by the means of FET or Diode. The bank should have a parallel
capacitor so that in occasions when there is less continuous current
required even a cell that cant participate in the current most of the time
can invest its energy into charge of this capacitor bank. The question I
have is, what kind of switching power supply architecture would be
recommendable to match lets say a 50V motor (BLDC or Asynchronous) which
would be fed by a 300V Bank that cant really supply the max. power current
of the motor?
How reasonable is it to hope that such an approach could actually be
beneficial and eliminate key problems of battery systems (matching,
balancing, cell failures).

Thanks guys for any enlightenment on the subject

2011\02\11@080952 by Isaac Marino Bavaresco

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Em 11/2/2011 10:14, Tobias Gogolin escreveu:
> The question I have is, what kind of switching power supply architecture would be recommendable to match lets say a 50V motor (BLDC or Asynchronous) which would be fed by a 300V Bank that cant really supply the max. power current of the motor?

A buck step-down converter?

What is the power of this motor? Keep in mind that a 20kW motor at 50V
will need 400A.


> How reasonable is it to hope that such an approach could actually be
> beneficial and eliminate key problems of battery systems (matching,
> balancing, cell failures).

I would use a higher voltage motor to keep the current at lower levels
and thus the losses low. Remember that the step-down converter also
introduces some losses and such losses are proportional to the current.


Best regards,

Isaac

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2011\02\11@085608 by alan.b.pearce

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> > The question I have is, what kind of switching power supply architecture
> > would be recommendable to match lets say a 50V motor (BLDC or Asynchronous)
> > which would be fed by a 300V Bank that cant really supply the max. Power
> current of the motor?
>
> A buck step-down converter?

I am guessing you are thinking 'electric vehicle' here ...

My reaction would be a multiphase buck switcher. These seem to be preferred for high power applications.
If starting from 300V then anything designed to work from 240V mains would be a starting point, as that is around 300V peak, so once rectified a typical SMPS will operate from around 300V DC.
-- Scanned by iCritical.

2011\02\11@091522 by Olin Lathrop

face picon face
Isaac Marino Bavaresco wrote:
>> The question I have is, what kind of switching power supply
>> architecture would be recommendable to match lets say a 50V motor
>> (BLDC or Asynchronous) which would be fed by a 300V Bank that cant
>> really supply the max. power current of the motor?
>
> A buck step-down converter?

You shouldn't need that.  A motor can be its own filter.  You can chop the
motor from substantially higher voltage than what it is intended to run at,
just make really really sure the duty cycle is low enough so that the
average is right.  Put another way, you need a buck converter, but the coils
in the motor can be the inductor and the final load at the same time.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\11@095630 by Dave Tweed

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flavicon
face
Olin Lathrop wrote:
> Isaac Marino Bavaresco wrote:
> >> The question I have is, what kind of switching power supply
> >> architecture would be recommendable to match lets say a 50V motor
> >> (BLDC or Asynchronous) which would be fed by a 300V Bank that cant
> >> really supply the max. power current of the motor?
> >
> > A buck step-down converter?
>
> You shouldn't need that.  A motor can be its own filter.  You can chop the
> motor from substantially higher voltage than what it is intended to run at,
> just make really really sure the duty cycle is low enough so that the
> average is right.  Put another way, you need a buck converter, but the coils
> in the motor can be the inductor and the final load at the same time.

But the premise was that the battery couldn't supply the peak current directly,
so you'll need a VERY hefty capacitor in parallel with it (or better yet, an
L-C low-pass filter between the battery and the switch) in order to handle the
ripple current.

-- Dave Twee

2011\02\11@102646 by N. T.

picon face
Olin Lathrop wrote:
> Isaac Marino Bavaresco wrote:
>>> The question I have is, what kind of switching power supply
>>> architecture would be recommendable to match lets say a 50V motor
>>> (BLDC or Asynchronous) which would be fed by a 300V Bank that cant
>>> really supply the max. power current of the motor?
>>
>> A buck step-down converter?
>
> You shouldn't need that.  A motor can be its own filter.  You can chop the
> motor from substantially higher voltage than what it is intended to run at,
> just make really really sure the duty cycle is low enough so that the
> average is right.  Put another way, you need a buck converter, but the coils
> in the motor can be the inductor and the final load at the same time.

I am not sure what duty cycle are you talking about. The motor coils
operate in continuous current mode. To do so you need to switch a coil
from 0 to 300V to let it "suck" extra current from ground. The specs
may feel bad about high frequency 300V signal on 50V DC motor coils
(isolation).

2011\02\11@103601 by Olin Lathrop

face picon face
Dave Tweed wrote:
> But the premise was that the battery couldn't supply the peak current
> directly, so you'll need a VERY hefty capacitor in parallel with it
> (or better yet, an L-C low-pass filter between the battery and the
> switch) in order to handle the ripple current.

Yes, but that depends on whether he meant instantaneous peak, like a few
microseconds, or peak as in max current the motor could draw under load.

If the battery and possibly cap in parallel with it can't supply the
instantaneous peak currents required for each switching pulse, you're
screwed either way.  Whether you drive the motor directly or put a
intermediate buck converter in there, the pulses will look about the same to
the battery.  A good portion of the current out the buck converter or thru
the motor windings will come from ground via the catch diode.  The average
current from the battery will therefore be lower, but the peak output can't
exceed the peak input current per pulse.

Either way, we are making the assumption that the battery/cap combination
can produce the peak output current for a few microseconds at a time during
the on phase of the buck pulse.  The average current over a whole buck pulse
will be significantly less, which is all the battery will see over any
period of a few 10s of microseconds or more.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\11@104855 by Olin Lathrop

face picon face
N. T. wrote:
>> You shouldn't need that. A motor can be its own filter. You can chop
>> the motor from substantially higher voltage than what it is intended
>> to run at, just make really really sure the duty cycle is low enough
>> so that the average is right. Put another way, you need a buck
>> converter, but the coils in the motor can be the inductor and the
>> final load at the same time.
>
> I am not sure what duty cycle are you talking about. The motor coils
> operate in continuous current mode.

Sortof.  The motor's ability to see current changes goes down as the
frequency goes up.  At a sufficiently high frequency, only the average
current matters from the motor's point of view.  The inductance of the motor
coils will smooth out the current to a good extent, and the mechanical
inertia can't respond to torque changes in a meaningful way usual over a few
100 Hz.  The biggest problem is usually the force on individual wires of the
motor coils.  These can vibrate a lot faster than the motor can move, which
can stress them and cause annoying audible whine.  With a switching
frequency of a few 10s of KHz, these effects are usually reduced to the
point of irrelevance.

> To do so you need to switch a coil
> from 0 to 300V to let it "suck" extra current from ground.

Yes, that is the essence of a buck converter.  But note that this coil can
just as well be a motor winding than a deliberate external inductor in a
separate buck converter.  The former will be more efficient.

> The specs
> may feel bad about high frequency 300V signal on 50V DC motor coils
> (isolation).

This may be true.  It depends on the motor, of course.  Most motors can be
chopped at substantially higher than their normal operating voltages, but of
course you have to check.  In this case we're asking for 6x insulation
capability over normal operation.  Probably that's OK, but again, you have
to check the specs for your specific motors.  I never meant that you should
just assume your motor can handle this in any particular case.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2011\02\14@065238 by Tobias Gogolin

picon face
>You shouldn't need that.  A motor can be its own filter.  You can chop the
>motor from substantially higher voltage than what it is intended to run at,
>just make really really sure the duty cycle is low enough so that the
>average is right.  Put another way, you need a buck converter, but the
coils
>in the motor can be the inductor and the final load at the same time.

That's what I was hoping to hear!
The idea is to be able to upgrade a battery bank simply by hooking up the
new batteries in series. As I mentioned before it's for an open source
E-bike design inspired by something in between Blade 04 and Erockit:
http://www.youtube.com/watch?v=YPHThtr5GME
http://www.youtube.com/watch?v=tzsHFka92X8

<http://www.youtube.com/watch?v=tzsHFka92X8>Thanks for your answers
everyone! Have to admit I should not be the one designing this part, as this
is far beyond my expertise...
This is more like a technology question, the effective wattage a cell array
can provide is an obvious imitation, and I have no illusions that there wont
be losses, or that the batteries be cheap! But the issue at hand is to
imagine a system that has flexibility and high reliability, and
can start with a minimal battery and can be scaled to be an exciting tool!

I have another interesting related engineering challenge: Braking Energy
recovery: to stop an electric vehicle the initial voltage form the motor as
generator is always the biggest when the speed is highest, but as one is
getting closer to a stop, whatever is being used to receive the energy is
fullest, and the voltage is the smallest - any suggestions what to do with
that last bit of energy? Mechanical brakes I suppose ?


On Fri, Feb 11, 2011 at 6:15 AM, Olin Lathrop <spam_OUTolin_piclistTakeThisOuTspamembedinc.com>wrote:

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