[EVDL List]

EVDL Administrator wrote
>
> ...They're not the same thing. 1.4 times higher is equivalent to 2.4
> times as
> high as.

I'm not following your parsing here.... Peak current is 1.414 times higher
than RMS. RMS is 0.707 times peak. Not sure what grammar rule gets you to
2.4x.

(NB - the peak and RMS refer to the low frequency sinusoidal current through
the AC motor, not the high frequency PWM'ed voltage that results in said low
frequency current.)


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This is an argument for keeping the tranny. With the torque multiplication of the gears, you need about 1/2 the current to pull onto a trailer (compared to a single gear system), meaning 1/4 the heating. The motor will also be spinning about twice as fast once moving, spreading the load better amongst the commutator bars.
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[quote]Morgan LaMoore wrote:
>> With an AC drive, the peak current through a phase (transistor and
>> winding) is 1.4 times higher than the RMS current. ...
>>
>> The same thing can happen with an AC induction motor if it is moving
>> backwards at exactly the slip speed; in that case, one phase sees DC
>> and could see a DC current of 1.4 times the RMS design value.

EVDL Administrator wrote:
> Wait, I'm confused here. Is it 1.4 times HIGHER, or 1.4 times AS HIGH AS?
>
> They're not the same thing. 1.4 times higher is equivalent to 2.4 times =
as
> high as.

No; "1.4 times higher" and "1.4 times as high" mean the same thing -- =

multiplying something by 1.4. If the motor current was 100 amps RMS, the =

peak is 141 amps.

Morgan's explanation was excellent. I would only add that at stall the =

winding *sits* at a DC current that is 1.4 times the RMS AC current that =

would have flowed if the motor were turning to produce that same torque. =

But in this DC case, one inverter transistor (out of the six) is =

carrying *all* that current at a 100% duty cycle. The transistors would =

normally be on only half the time (50% duty cycle) when producing AC, so =

this is double its normal peak, i.e. 2 x 1.4 =3D 2.8 times the normal AC =

RMS current.

-- =

A designer knows he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint Exup=E9ry
--
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[quote]On 25 Apr 2012 at 11:35, Jeffrey Jenkins wrote:

> I'm not following your parsing here.... Peak current is 1.414 times higher
> than RMS. RMS is 0.707 times peak. Not sure what grammar rule gets you to
> 2.4x.

Yes, that's the rule I was thinking of. I just looked it up.

I know this seems like I'm being pedantic here, and I guess in a way I am,
but this really can be confusing. There's a big semantic difference between
"higher than" and "as high as." Peak current is 1.414 times >AS HIGH AS<
RMS. Saying peak current is 1.414 times higher than RMS is >not< the same
thing.

If you said that peak current is 1.414 times higher than RMS, it would mean
that peak current is the original RMS value PLUS 1.414 times the original
RMS value - that is, 2.414 times as much.

The problem is that a lot of people say "higher than" when they really mean
"as high as." But not everyone does. Because of this, "higher than" will
always be ambiguous.

I think that when we are comparing things, we should just stop saying "more
than" or "higher than" and always use "as much as" to avoid confusion.

But I'm probably tilting at windmills. :-\

David Roden
EVDL Administrator
http://www.evdl.org/


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EVDL Administrator wrote
>
> If you said that peak current is 1.414 times higher than RMS, it would
> mean
> that peak current is the original RMS value PLUS 1.414 times the original
> RMS value - that is, 2.414 times as much.
>
> The problem is that a lot of people say "higher than" when they really
> mean
> "as high as." But not everyone does. Because of this, "higher than" will
> always be ambiguous.

I get the distinction you are trying to make, but I think you are
misapplying it. Your interpretation would be correct if the phrasing was,
"peak current is 41.4% higher than RMS." When the word, "times," is
included, however, the meaning of the phrase changes from addition to
multiplication. E.g. - "peak current is 1.414 times higher than RMS."


EVDL Administrator wrote
> But I'm probably tilting at windmills. :-\

Aye, Don Quixote - put down your sword... ;-)



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Lee Hart wrote
>
> Morgan's explanation was excellent. I would only add that at stall the
> winding *sits* at a DC current that is 1.4 times the RMS AC current that
> would have flowed if the motor were turning to produce that same torque.
> But in this DC case, one inverter transistor (out of the six) is
> carrying *all* that current at a 100% duty cycle. The transistors would
> normally be on only half the time (50% duty cycle) when producing AC, so
> this is double its normal peak, i.e. 2 x 1.4 = 2.8 times the normal AC
> RMS current.
>

I think both you and Morgan are making some assumptions here that may or not
apply.

If the inverter employs FOC then the stator phases will still be rotating
electrically at the maximum slip frequency when the rotor is stalled
otherwise the rotor won't be exerting any torque.

A minor point of correction is that all of the switches in an inverter are
pulse-width modulated, even if tasked with delivering an effective DC
current to just one phase, so they are never 100% on. In some ways this is
worse, though, as it means there are switching losses on top of the
conduction losses.


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[quote]On 25 Apr 2012 at 17:43, Lee Hart wrote:

> "1.4 times higher" and "1.4 times as high" mean the same thing --
> multiplying something by 1.4.

That's not what my technical writing prof drilled into my head 38 years ago!

In truth, I have to admit that style manuals differ on this point. Of
course I prefer to cite the ones that agree with me and my prof. ;-)

Here's The Economist's :

"Times: Take care. Three times more than x means four times as much as x."

http://www.economist.com/styleguide/t

By and large, language experts agree that while "times more" >may< mean the
same thing as "times as much," it's ambiguous. Some readers will interpret
"1.4 times more" as (x + 1.4x) while others will interpret it as simply
(1.4x).

This is the position that the New York Times's style manual takes - that
"precise readers" may interpret "more than" to mean (base + n * base) while
other readers will read it as (n * base). Thus Times writers should stick
to using "as much as," "as fast as," and so on since the meaning is clearer.

We're discussing technical matters - math and science - on the EVDL. In
that context, I would argue that we should junk anything that creates
ambiguity.

Therefore I ask that we please try to use "as much as" and avoid "more
than."

And then you, and I, and complete newbies will all know that

peak = 1.414 * RMS

Thank you.

David Roden
EVDL Administrator
http://www.evdl.org/


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[quote]Christopher Zach wrote:
Lee Hart wrote:
>> For an induction motor, worst case is with the rotor rotating
>> *backwards* as the slip speed when the motor is commanded to produce
>> forward torque. This situation requires DC on a phase winding.

> Sure, however if you have a vector motor you can program it to limit the
> motor current and keep the stator from melting/phase wire from overloading.

It has nothing to do with whether it's a vector drive or not. It's a
matter of whether the control program (or hardware) takes it into
account, and limits transistor current accordingly.

Hmm... it just occurred to me that some designs use current transformers
for sending phase currents. These won't work for the special case of DC
on a winding!

> I'll have to try this, roll backward while applying throttle. What
> should I be looking for?

If the drive works, it will generate torque in the correct direction
regardless of which way the motor is turning (moving slowly ahead,
stalled, or moving slowly backward while applying forward throttle). And
of course, it doesn't burn something out trying to do it!

>> That's true, though the duty cycle will be essentially 100% for one
>> transistor, and 1% for its twin.
>
> Ok, and if the IGBTs are rated at 300a continuous and max current is
> capped at 200a, what could happen?

A good motor designer will know all this, and take it into account.

A beginner might miss it, and think that since the motor is always
turning, the phases are always changing, and the duty cycle on any
individual transistor won't exceed 50%.

--
*BE* the change that you wish to see in the world.
-- Mahatma Gandhi
--
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Lee Hart wrote
>
> For a PM motor, worst case is at stall, which requires DC current in a
> phase winding.
>
> For an induction motor, worst case is with the rotor rotating
> *backwards* as the slip speed when the motor is commanded to produce
> forward torque. This situation requires DC on a phase winding.

Gotcha. It was ambiguous before (a genuine ambiguity, and not a manufactured
one... ahem) whether this was a PMSM or ACIM and whether the motor was
stalled and the inverter was still trying to exert force, or the motor was
rotating slightly backwards while the inverter was "stalled" trying to hold
the rotor still.


Lee Hart wrote
>
>> A minor point of correction is that all of the switches in an inverter
>> are
>> pulse-width modulated, even if tasked with delivering an effective DC
>> current to just one phase, so they are never 100% on. In some ways this
>> is
>> worse, though, as it means there are switching losses on top of the
>> conduction losses.
>
> That's true, though the duty cycle will be essentially 100% for one
> transistor, and 1% for its twin.

Are you referring to "six-step" PWM? I ask because the modulation strategy
in the inverter dictates the maximum output voltage that can be delivered to
any one phase, even if that phase is supposed to be supplied with a DC
current. Six-step PWM can go all the way to 100% of the DC bus voltage, but
every other modulation strategy requires turning the switches off for some
percentage of time (10% to 30% being typical, depending on the strategy).





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[quote] Lee Hart <xxx@xxx.xxx> wrote:
>> A minor point of correction is that all of the switches in an inverter are
>> pulse-width modulated, even if tasked with delivering an effective DC
>> current to just one phase, so they are never 100% on. In some ways this is
>> worse, though, as it means there are switching losses on top of the
>> conduction losses.
>
> That's true, though the duty cycle will be essentially 100% for one
> transistor, and 1% for its twin.

That enormously depends on the method of pulse width modulation.

A classic trapezoidal PWM scheme will have a duty cycle near 100% like
you describe.

In a simple sinusoidal implementation, 50% duty cycle will likely be
used as an artificial neutral point. With a low output voltage, all
duty cycles are near 50%; duty cycles only approach 0% or 100% when
outputting near maximum AC voltage, which would only happen at high
frequency.

Some space vector modulation routines also keep the artificial neutral
near 50%, so they also only approach 100% duty cycle at high
frequency.

A more clever space vector modulation scheme may always keep one of
the six transistors at 100% duty cycle to eliminate switching losses
in that phase; in that case, they would have to consider the effect of
stall on the transistor current ratings more. Hopefully if they are
optimizing their algorithm to reduce switching losses, they will also
think about these types of problems.

-Morgan LaMoore

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