5 May
2021
5 May
'21
8:07 a.m.
I have found the Motor Generator thread to be fascinating and
enlightening. But it has made many a reference to the 400 Hz or other
frequency much higher than mains line frequency. Despite the comments
about the frequency, I'm still confused as to why the higher than mains
frequency was used.
Were the higher frequencies used because it directly effected the amount
of time / duration in (fractions of) seconds between peaks of rectified
(but not yet smoothed) power?
I ask because it seems to me like the percentage of time / duty cycle of
raw rectified but not yet smoothed) power would be the same at any and
all frequencies. Is this assumption / understanding correct or
completely off the mark?
A few different people made references to the amount of capacitance
needed at 400 Hz et al. vs 50/60 Hz mains frequency. Someone even spoke
about high power DC being produced by polyphase converters and the
possibility to tweak tweak winding voltages in order to possibly do away
with the need for capacitors.
Am I starting to understand the motivation behind the 400 Hz or is there
something else behind it? Is this really playing to the (dis)charge
time of capacitors in between peaks of rectified (but not smoothed) sources?
Aside: I started a new thread for this very specific minutia to not
mire the other Motor Generator thread down.
Thank you for all the comments and those who respond to help me learn
something new today. :-)
--
Grant. . . .
unix || die
5 May
5 May
8:24 a.m.
On May 5, 2021, at 11:07 AM, Grant Taylor via cctalk
<cctalk at classiccmp.org> wrote:
I have found the Motor Generator thread to be fascinating and enlightening. But it has
made many a reference to the 400 Hz or other frequency much higher than mains line
frequency. Despite the comments about the frequency, I'm still confused as to why the
higher than mains frequency was used.
Were the higher frequencies used because it directly effected the amount of time /
duration in (fractions of) seconds between peaks of rectified (but not yet smoothed)
power?
I ask because it seems to me like the percentage of time / duty cycle of raw rectified
but not yet smoothed) power would be the same at any and all frequencies. Is this
assumption / understanding correct or completely off the mark?
A few different people made references to the amount of capacitance needed at 400 Hz et
al. vs 50/60 Hz mains frequency. Someone even spoke about high power DC being produced by
polyphase converters and the possibility to tweak tweak winding voltages in order to
possibly do away with the need for capacitors.
There are a couple of considerations: transformers, filter capacitors, and ripple
amplitude.
Ripple amplitude is affected by the number of phases and by whether you use half wave or
(the normal) full wave rectifiers. For example, a full wave single phase rectifier
produces a abs(sin(2pi * f * t)) waveform. A multi-phase full wave rectifier produces the
max of these waveforms offset by the phase angles -- in other words, the
"valleys" are filled in by the "peaks" of some of the other phases.
The ripple filter then smooths that out into DC, or more precisely, makes the ripples a
lot smaller. What exactly those waveforms then look like depend on the filter used.
So rectifying 3 phase power produces much smaller ripple than rectifying single phase.
For exotic applications where you can afford to deal with more than 3 phases you can make
the amplitude smaller still.
The above is independent of frequency.
Now for transformers. As the operating frequency increases, the amount of iron or other
core material needed goes down. So a 400 Hz transformer for a given amount of power can
be much smaller than a 60 Hz transformer for the same job. This is why modern power
supplies are "switching supplies": they convert the mains voltage into high
frequency power -- sometimes as high as a megahertz or so -- which allows the power
transformer to be tiny.
Finally, capacitors. The ripple attenuation of a filter depends on the impedance of the
filter elements. The most common is a capacitor filter, so the filter capacitors (their
AC impedance) is in parallel with the load impedance. The ripple attentuation is
basically the ratio of filter capacitor impedance to load impedance. Capacitor impedance
is inversely proportional to frequency, so the use of a higher frequency allows the use of
smaller capacitors. This too is used in modern switching regulators, where the capacitors
are often just a few microfarads and are generally ceramic, not electrolytic.
In all this there are limits on how far you can usefully go. Motor generators above a few
hundred hertz are hard to build, though there exist alternators that produce
multiple-kilohertz output (look up Alexanderson machines). Switching regulators can to
into the MHz range, but if you go too high your benefits stop because the transformers and
capacitors are no longer close enough to "ideal components". For example, the
impedance of a capacitor stops dropping at some frequency that depends on its design, this
is the ESR (effective series resistance) of the part. Near or above that frequency is is
no longer a good filter capacitor, or for that matter a good capacitor for many other
purposes.
paul
8:25 a.m.
On 05/05/2021 16:07, Grant Taylor via cctalk wrote:
Were the higher frequencies used because it directly
effected the amount
of time / duration in (fractions of) seconds between peaks of rectified
(but not yet smoothed) power?
Haven't read the rest of the thread and so at the risk of being
profoundly wrong... Benefit of 400Hz mains is that transformers can be
much smaller. Think of switching power supplies that rectify to DC and
then switch up into kHz, which are then able to use far smaller
transformer cores than an old linear PSU. At least this is a key
motivation with 115V/400Hz 3-phase aviation power AFAIK.
By coincidence we've just built a big 28VDC power supply, so that we can
run a vintage 400Hz aircraft rotary inverter, which will then be used to
power up old mil surplus kit that wants this. A classic adventure in yak
shaving. Anyway, here's the 28VDC bit.
https://www.rs-online.com/designspark/constructing-a-high-current-28v-dc-po…
Andrew
8:37 a.m.
On May 5, 2021, at 11:25 AM, Andrew Back via cctalk
<cctalk at classiccmp.org> wrote:
On 05/05/2021 16:07, Grant Taylor via cctalk wrote:
Incidentally, a way to get three phase power at a frequency of your choice is to use a
"variable frequency drive". That's basically a high power solid state
inverter intended to drive three-phase motors with a chosen frequency resulting in the RPM
of your choice. Depending on the model, those can go up to 120 Hz or so, or all the way
to somewhere around 400 Hz. I have a very cheap one at home that runs on single phase 220
volt power, producing up to 3 kW or so at anywhere up to 120 Hz. (Made by a company named
Teco, amusingly.) Most of these and especially the larger ones want three phase mains
input, though I'm told that even for those you can typically just connect them to
single phase power (between two of the three inputs, leaving the third unconnected) at
reduced power ratings. These devices are surprisingly cheap, in particular they tend to
be cheaper than "rotary phase converters" which is how machine shops
traditionally produce three phase power when their mains is just single phase.
paul
Were the higher frequencies used because it
directly effected the amount
of time / duration in (fractions of) seconds between peaks of rectified
(but not yet smoothed) power?
Haven't read the rest of the thread and so at the risk of being
profoundly wrong... Benefit of 400Hz mains is that transformers can be
much smaller. Think of switching power supplies that rectify to DC and
then switch up into kHz, which are then able to use far smaller
transformer cores than an old linear PSU. At least this is a key
motivation with 115V/400Hz 3-phase aviation power AFAIK.
By coincidence we've just built a big 28VDC power supply, so that we can
run a vintage 400Hz aircraft rotary inverter, which will then be used to
power up old mil surplus kit that wants this. 
8:50 a.m.
On 5/5/21 8:37 AM, Paul Koning via cctalk wrote:
Incidentally, a way to get three phase power at a
frequency of your choice is to use a "variable frequency drive". That's
basically a high power solid state inverter intended to drive three-phase motors with a
chosen frequency resulting in the RPM of your choice. Depending on the model, those can
go up to 120 Hz or so, or all the way to somewhere around 400 Hz. I have a very cheap one
at home that runs on single phase 220 volt power, producing up to 3 kW or so at anywhere
up to 120 Hz. (Made by a company named Teco, amusingly.) Most of these and especially
the larger ones want three phase mains input, though I'm told that even for those you
can typically just connect them to single phase power (between two of the three inputs,
leaving the third unconnected) at reduced power ratings. These devices are surprisingly
cheap, in particular they tend to be cheaper than "rotary phase converters"
which is how machine shops traditionally produce three phase power when their mains is
just single phase.
--
--Chuck
Sent from my digital computer
8:53 a.m.
On 5/5/21 8:37 AM, Paul Koning via cctalk wrote:
Incidentally, a way to get three phase power at a
frequency of your
choice is to use a "variable frequency drive". That's basically a
high power solid state inverter intended to drive three-phase motors
with a chosen frequency resulting in the RPM of your choice.
Depending on the model, those can go up to 120 Hz or so, or all the
way to somewhere around 400 Hz. I have a very cheap one at home that
runs on single phase 220 volt power, producing up to 3 kW or so at
anywhere up to 120 Hz. (Made by a company named Teco, amusingly.)
Most of these and especially the larger ones want three phase mains
input, though I'm told that even for those you can typically just
connect them to single phase power (between two of the three inputs,
leaving the third unconnected) at reduced power ratings. These
devices are surprisingly cheap, in particular they tend to be cheaper
than "rotary phase converters" which is how machine shops
traditionally produce three phase power when their mains is just
single phase.
Please forgive the null message--too fast with the mouse.
A simple single-to-three phase converter used by many home shops is
simply a 3-phase motor fed single-phase power to one pair and a large
capacitor connected between the remaining lead and the "hot" side of the
power line. Such a motor is an "idler", run no-load--basically a
rotary transformer. Of course, you don't get a precise 120 degree
phase relationship but it's adequate for powering a lathe or mill.
--Chuck
9:22 a.m.
On 05/05/2021 10:37 AM, Paul Koning via cctalk wrote:
Incidentally, a way to get three phase power at a
frequency of your choice is to use a "variable frequency
drive". That's basically a high power solid state inverter
intended to drive three-phase motors with a chosen
frequency resulting in the RPM of your choice. Depending
on the model, those can go up to 120 Hz or so, or all the
way to somewhere around 400 Hz. I have a very cheap one at
home that runs on single phase 220 volt power, producing
up to 3 kW or so at anywhere up to 120 Hz. (Made by a
company named Teco, amusingly.) Most of these and
especially the larger ones want three phase mains input,
though I'm told that even for those you can typically just
connect them to single phase power (between two of the
three inputs, leaving the third unconnected) at reduced
power ratings. These devices are surprisingly cheap, in
particular they tend to be cheaper than "rotary phase
converters" which is how machine shops traditionally
produce three phase power when their mains is just single
phase. paul
Note that VFDs are designed to run motors exclusively. They
approximate a sine wave with pulse width modulated 400 V
pulses. DON'T EVER try to run electronic gear with a VFD, at
least without a massive smoothing filter to convert to a
true sine wave. With a motor, the winding inductance solves
the problem, and the coils see nice sine-wave currents.
Jon
9:39 a.m.
On May 5, 2021, at 12:22 PM, Jon Elson <elson at
pico-systems.com> wrote:
On 05/05/2021 10:37 AM, Paul Koning via cctalk wrote:
I've heard that. But why? It's not like the electronics we're talking about
actually runs on AC. Instead, it goes right into a transformer (an inductive load not
much different from a motor) and after that into a ripple filter. That filter IS the
"massive smoothing filter" you're talking about.
It's certainly worth a test especially before connecting something rare and ancient,
but I see no theoretical reason for it to cause trouble.
The voltage issue is a different one. I've never seen a VFD that offered anything
other than frequency change -- indeed, it produces the same RMS output voltage as what you
feed it, and it isn't insulated. Again, for the application we're talking about
that is likely to be fine. For example, the CDC 6600 wants 208 volt three-phase (in the
US model) both at 60 Hz for the motors and at 400 Hz for the main power.
Chuck mentioned an "idler motor", that's what is sold commercially as a
"rotary converter". Those are nice and can be had for quite high power, but
often a VFD does the job more cheaply (and with the added benefit of speed control) for
much less. I think my Teco converter cost about $100, way less than a commercial rotary
converter rated at 3 hp as that one was, and for a lot less effort than home-building a
rotary converter would be.
paul
Incidentally, a way to get three phase power at a
frequency of your choice is to use a "variable frequency drive". That's
basically a high power solid state inverter intended to drive three-phase motors with a
chosen frequency resulting in the RPM of your choice. Depending on the model, those can go
up to 120 Hz or so, or all the way to somewhere around 400 Hz. I have a very cheap one at
home that runs on single phase 220 volt power, producing up to 3 kW or so at anywhere up
to 120 Hz. (Made by a company named Teco, amusingly.) Most of these and especially the
larger ones want three phase mains input, though I'm told that even for those you can
typically just connect them to single phase power (between two of the three inputs,
leaving the third unconnected) at reduced power ratings. These devices are surprisingly
cheap, in particular they tend to be cheaper than "rotary phase converters"
which is how machine shops traditionally produce three phase power when their mains is
just single phase. paul
Note that VFDs are designed to run motors exclusively.
They approximate a sine wave with pulse width modulated 400 V pulses. DON'T EVER try
to run electronic gear with a VFD, at least without a massive smoothing filter to convert
to a true sine wave. With a motor, the winding inductance solves the problem, and the
coils see nice sine-wave currents.
Jon 
11:39 a.m.
k
> Note that VFDs are designed to run motors
exclusively. They
> approximate a sine wave with pulse width modulated 400 V pulses. DON'T
> EVER try to run electronic gear with a VFD, at least without a massive
Yes, that is what I suggested; these filters are called "Sinus Filters"
in the field of driving even motors over longer cables. One does not want
to transmit the 10kHz PWM over longer distrances.
I've heard that. But why? It's not like the
electronics we're talking about actually runs on AC.
(a) Input filters often contain caps...
https://uk.tdk-lambda.com/content/faq/130796117766108286_FilterFAQ_how%20Im…
With 50Hz or 60Hz they create a little current which does not hurt.
Applying a square wave of 15kHz has harmonics up to 100kHz and more.
That means, that the current in these is more than 100 times bigger - BAM!
Instead, it goes right into a transformer (an
inductive load not much
different from a motor)
(b) Yes, a transformer usually has an optimum frequency range. Above
it does not deliver the energy to the output. Why? (1) Because of stray
inductivity. That is what helps you in filtering ther higer frequencies.
But there is also (2) capacity between windings shorting the higher
frequencies -> Heat and stress for the insulation. And finally (3) each
time you are changing the magnetic field in the core, it causes loss
and heat. The more often one tries to do that, the more loss ->
Additional heat.
Summary: Depends on the transformer what happens. May be OK, may
fail after some time duer to heat or the insulation may break
down.
and after that into a ripple filter. That
filter IS the "massive smoothing filter" you're talking about.
(c) After the rectifier, you have got the caps. They are no
problem, but the rectifiers (e.g. 1N4007 or similar) are optimited
for lower frequencies. They to not like higher frequencies: Esp.
seitching them off causes some current in the wrong direction
eventually heating the diode. May be a problem, but have not
had trouble with this case yet...
The voltage issue is a different one. I've never
seen a VFD that
offered anything other than frequency change -- indeed, it produces the
same RMS output voltage as what you feed it, and it isn't insulated.
Look to the link in my other email. The device there is around
USD100 and you can select frequency (10..400Hz) and Uout (10%...100%)
and also go ramps etc.
Again good luck ;-)
Erik.
''~``
( o o )
+------------------------.oooO--(_)--Oooo.--------------------------+
| Dr. Erik Baigar Inertial Navigation & |
| erik at baigar.de .oooO Vintage Computer |
| www.baigar.de ( ) Oooo. Hobbyist |
+---------------------------\ (----( )----------------------------+
\_) ) /
(_/
12:46 p.m.
Dr. Erik Baigar via cctalk wrote:
Additionally, output devices with really fast switching times can
trigger reflections back from the load, and these can potentially expose
these devices to very high reflected voltages, which will trigger their
breakdown.? I've seen it happen in oil field equipment, with IGBTs.
Carlos.
I've heard
that.? But why?? It's not like the electronics we're
talking about actually runs on AC.
(a) Input filters often contain caps...
https://uk.tdk-lambda.com/content/faq/130796117766108286_FilterFAQ_how%20Im…
With 50Hz or 60Hz they create a little current which does not hurt.
Applying a square wave of 15kHz has harmonics up to 100kHz and more.
That means, that the current in these is more than 100 times bigger -
BAM!
6 May
6 May
12:59 a.m.
Applying a
square wave of 15kHz has harmonics up to 100kHz and more.
That means, that the current in these is more than 100 times bigger - BAM!
Additionally, output devices with really fast switching times can trigger
reflections back from the load, and these can potentially expose these
devices to very high reflected voltages, which will trigger their breakdown.? 
7 May
7 May
2:59 p.m.
I will add that aircraft are one of the main users of 400 Hz. This is because weight is
always an critical design consideration. So with smaller transformers, smaller capacitors,
etc, you can save a LOT of weight on electronic devices in an aircraft.
73 Eugene W2HX
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Andrew Back via cctalk
Sent: Wednesday, May 5, 2021 11:26 AM
To: cctalk at classiccmp.org
Subject: Re: 400 Hz
On 05/05/2021 16:07, Grant Taylor via cctalk wrote:
Were the higher frequencies used because it directly
effected the
amount of time / duration in (fractions of) seconds between peaks of
rectified (but not yet smoothed) power?
Haven't read the rest of the thread and so at the risk of being profoundly wrong...
Benefit of 400Hz mains is that transformers can be much smaller. Think of switching power
supplies that rectify to DC and then switch up into kHz, which are then able to use far
smaller transformer cores than an old linear PSU. At least this is a key motivation with
115V/400Hz 3-phase aviation power AFAIK.
By coincidence we've just built a big 28VDC power supply, so that we can run a vintage
400Hz aircraft rotary inverter, which will then be used to power up old mil surplus kit
that wants this. A classic adventure in yak shaving. Anyway, here's the 28VDC bit.
https://www.rs-online.com/designspark/constructing-a-high-current-28v-dc-po…
Andrew
4:35 p.m.
On 05/07/2021 04:59 PM, W2HX via cctalk wrote:
I will add that aircraft are one of the main users of
400 Hz. This is because weight is always an critical design consideration. So with smaller
transformers, smaller capacitors, etc, you can save a LOT of weight on electronic devices
in an aircraft.
73 Eugene W2HX
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Andrew Back via
cctalk
Sent: Wednesday, May 5, 2021 11:26 AM
To: cctalk at classiccmp.org
Subject: Re: 400 Hz
On 05/05/2021 16:07, Grant Taylor via cctalk wrote:
Interestingly, a lot of other military gear also uses 400 Hz
power for the same reasons.
I tore down a Nike radar van a long time ago, everything ran
off 400 Hz.
Jon
Were the higher frequencies used because it
directly effected the
amount of time / duration in (fractions of) seconds between peaks of
rectified (but not yet smoothed) power?
Haven't read the rest of the thread
and so at the risk of being profoundly wrong... Benefit of 400Hz mains is that
transformers can be much smaller. Think of switching power supplies that rectify to DC and
then switch up into kHz, which are then able to use far smaller transformer cores than an
old linear PSU. At least this is a key motivation with 115V/400Hz 3-phase aviation power
AFAIK.
By coincidence we've just built a big 28VDC power supply, so that we can run a
vintage 400Hz aircraft rotary inverter, which will then be used to power up old mil
surplus kit that wants this. A classic adventure in yak shaving. Anyway, here's the
28VDC bit.
https://www.rs-online.com/designspark/constructing-a-high-current-28v-dc-po…
Andrew
4:55 p.m.
Hmmm. If they ever use single phase, instead of 3 phase, . . .
A4 (A above middle C) is nominally 440Hz, but not everybody agrees.
NY Phil uses 442, Boston uses 441, and many argue for 432.
But, 400 would be A-flat. With a constant A flat droning in the
background, . . .
6:56 p.m.
On 5/7/21 4:55 PM, Fred Cisin via cctalk wrote:
Hmmm.? If they ever use single phase, instead of 3
phase, . . .
A4 (A above middle C) is nominally 440Hz, but not everybody agrees.
NY Phil uses 442, Boston uses 441, and many argue for 432.
But, 400 would be A-flat.? With a constant A flat droning in the
background, . . .
I used to play (tuba) just below a fluorescent fixture that buzzed at
120 Hz. Very annoying--a flat B-natural or a sharp B-flat. Messed with
my head.
--Chuck
8 May
8 May
10 a.m.
On 05/07/2021 06:55 PM, Fred Cisin via cctalk wrote:
Hmmm. If they ever use single phase, instead of 3
phase,
. . .
A4 (A above middle C) is nominally 440Hz, but not
everybody agrees.
NY Phil uses 442, Boston uses 441, and many argue for 432.
But, 400 would be A-flat. With a constant A flat droning
in the background, . . .
Yes, large 400 Hz power systems can be QUITE irritating,
especially with harmonics added.
The whine from a huge 415 Hz UPS running a large computer
installation is totally deafening.
Jon
5 May
5 May
8:29 a.m.
On 5/5/21 8:07 AM, Grant Taylor via cctalk wrote:
I have found the Motor Generator thread to be
fascinating and
enlightening.? But it has made many a reference to the 400 Hz or other
frequency much higher than mains line frequency.? Despite the comments
about the frequency, I'm still confused as to why the higher than mains
frequency was used.
Iron and filtering. 400Hz means far less iron needed in transformers
and variacs (CDC used a three-phase variac at the CPU cabinet for
adjustment of voltage). Compare the sizes and it's obvious.
Filtering the ripple out of a 2400 Hz 92% DC rectified supply is
comparatively simple. A 360Hz supply, less so.
In my salad days, I worked at a steel mill where a fair amount of
equipment ran from 25 Hz voltage. The transformers were huge in
comparison to their 60 Hz cousins.
--Chuck
9:14 a.m.
On 05/05/2021 10:07 AM, Grant Taylor via cctalk wrote:
I have found the Motor Generator thread to be
fascinating
and enlightening. But it has made many a reference to the
400 Hz or other frequency much higher than mains line
frequency. Despite the comments about the frequency, I'm
still confused as to why the higher than mains frequency
was used.
Were the higher frequencies used because it directly
effected the amount of time / duration in (fractions of)
seconds between peaks of rectified (but not yet smoothed)
power?
It made it possible for transformers, inductors and
capacitors to be much smaller. The 2500 Hz power supplies
in the 360's delivered 40 A at 6 V, and used inductor-input
filters and a final series-pass germanium transistor for
regulation. They were about the same size as a 6V 2 A
regulated power supply running on 60 Hz.
So, to an extent, the answer to your question is yes. But,
not exactly just that, because the size of the transformer
scaled inversely with frequency.
Jon
1329
days inactive
1332
days old
17 comments
9 participants
participants (9)
-
andrew＠carrierdetect.com -
cclist＠sydex.com -
cctalk＠gtaylor.tnetconsulting.net -
ce.murillosanchez＠gmail.com -
cisin＠xenosoft.com -
elson＠pico-systems.com -
erik＠baigar.de -
paulkoning＠comcast.net -
w2hx＠w2hx.com