Re: Power Supply Rectification and Smoothing

On Fri, 19 Oct 2007 05:38:46 +0900, spamfree@xxxxxxxxxxx wrote:

On Thu, 18 Oct 2007 08:20:43 GMT, Ross Herbert
<rherber1@xxxxxxxxxxxxxx> wrote:

On Wed, 17 Oct 2007 18:16:14 +0900, spamfree@xxxxxxxxxxx wrote:


Energy content of spikes on the secondary side is a non-issue.
Consider the filter caps as if they were a 36V lead acid battery.
This load would constitute the best output filter with the lowest
internal resistance possible. As far as spikes in the secondary are
concerned the battery is a dead short and the only place they can
then be dissipated is in the secondary winding and the transformer
leakage capacitance. Forget about spikes for your motor run
application.

Thanks Ross. Makes sense. Did you see my question I posed about
turning the mains off and then on quickly with the transformer core
still magnetised, and the phase angle at switchon such that the
transformer goes "boing"?

No, I didn't see that question. However, it is well known that rapid
on-off switching on the mains side of large transformer based power
supplies can result in unintended surges and overloads. Mostly, these
simply cause a circuit breaker to pop but there may be other
consequences depending on the psu design. It is generally accepted
that this practice is frowned upon and belongs in the realm of "worker
vandalism".


To avoid the need of caps, and keep the voltage stable, what about
another suggestion I've had to build "a simple series pass regulator
using a 36V zener like a 1N5936 and a 60A NPN transistor like a
MJ14002. it would be tons cheaper than the humongous capacitors you
would need. Since these need a couple of volts headroom, wind the
transformer to provide a slightly higher voltage say 32V or so. Also
the transistor must be put on a largish heatsink."

You could use this arrangement with a single large electrolytic across
the zener reference as a means of producing a "capacitor multiplying"
effect but you would still need a fairly hefty bulk storage capacitor
across the output to provide for instantaneous current demand due to
sudden load changes. You wouldn't save a great deal if any and in my
opinion it is not required for your application.

I doubt that a single series pass transistor would be suitable either.


Or even "to parallel the outputs of 5 or 6 LM338K 3 terminal
regulators. Since the max output is 32V you would have to float the
regulators above ground using a 3.9V zener like a 1n4730.
Again you would need a big heatsink."

Solid state linear regulators (well designed) for high power DC
supplies have been used successfully as far back as the 60's and are
still used today when the most rugged operational conditions are to be
endured. $ for $ they will withstand almost anything you can throw at
them and while heavy duty switch mode units are also very capable, the
inclusion of safeguards against sudden blow-ups makes them more costly
and complex to design.

A transformer having this capability
for your application will be quite large and very heavy. Since your
transformer is a rehash of a microwave oven unit it is not likely to
have good load regulation.

This one weighs 11 lb, and some others weigh up to 15. Have you a
ball park figure for a reasonably stable load regulation?

It depends to a large extent on how you are going to drive your motor.
If it is going to be running at near full load for several hours at a
time then an 11lb transformer is totally inadequate. For such a
requirement (36V @18A) I would specify a transformer of 1KVA to be on
the safe side. As an example, I have a 1KVA isolation transformer and
this weighs 16Kg (35lb), so your 11lb unit is a pip-squeak by
comparison.

I've just played with the first tranny I came across, but I have an
older much heavier microwave oven to junk. Is bigger the better a
reasonable maxim? The problem is, the price difference between an old
microwave and a large bought one is horrendous :)

I was only concerened that the margin of error for 50V caps was a bit
fine, especially when a poor tranny might raise that considerably
when lightly loaded. Unfortunately, only the 50V ones are offered at
this cheap price, and so running more in series pairs seemed to be a
compromise. With the balancing resistors, of course.

I mean, I can get 2 dozen of these for around $US18 . Cheapest
capacitance I can find. Even three 12V leadacids would cost much more
and last for much less a time.


It seems to me that you have yet to grasp that your project is not a
simple "use what you can find at the lowest cost" type of project.
What you are trying to do is build a fairly heavy duty transformer
based dc power supply and to do this you have to start out with an
appropriately rated transformer in the first instance. There is no
substitute for an adequate laminated core mass when good load
regulation is required. Grabbing an old microwave transformer and
slapping on a few turns of pvc flex as a secondary is totally
inadequate in my opinion. If you had used something like 10AWG Formvar
insulated magnet wire then you would achieve far greater magnetic
coupling efficiency and get a result closer to what you were looking
for.

You can't use a transformer with poor load regulation AND get by with
the lower voltage rated capacitors - it's that simple. You either use
a good transformer which produces little voltage sag between low load
and full load where lower voltage caps can be used or you cover the
available output voltage range using higher rated caps. It's one or
the other and you have a cost penalty either way.

What is the 30A flex jacket material made of and what is the
temperature rating of the material? 30A PVC insulated flex generally
has a 75C rating and the insulation is quite thick. Operating the
transformer at temperatures over 75C will soften the jacket and
possibly compromise insulation among other things. Ambient temp must
be considered here as well.

This stuff is automotive, I think. It has quite a thin PVC sheath.
Its overall diameter is a whisker over 3mm. Drawn to scale, I can fit
at least 54 turns in the transformers secondary window.
I intend to have a fan blowing continuously over the whole shebang,
caps, transformer, bridge rectifier and motor, and I will monitor the
temps of everything to begin with to make sure that nothing is
getting too hot.


It is a long time since I wound a secondary to get a desired voltage
but from memory I had to wind something between 3 and 5 turns per volt
(240Vac primary) to get what I wanted. Depending upon the number of
turns on your primary will determine your required turns per volt at
the secondary and you should experiment to determine this. 54T sounds
a bit on the low side for a 36V,18A load to me.

From my experience, depending upon the supplier, automotive shops
describe their flex with rather dubious current ratings. If the
overall diameter of your flex is 3mm the wire diameter will be about
half that at best and I would question its ability to conduct 30A for
any appreciable time without drastic losses and severe temperature
increase. See the chart here http://www.powerstream.com/Wire_Size.htm
to get an idea of wire gauge for a nominal current capacity. For
example, 30A power transmission conductor would be in the region of
3.7mm diameter without any insulation.

Thanks for that, Ross. The speed controller is described by my
friendly ee who has experience with them on his bike, as:
"they have big input caps on the supply voltage in the controller,
and the circuit after that can be thought of as a buck switching
supply. All the logic and control runs from a low voltage (14.4v in
Crystalyte) that is internally regulated, so as long as your ripple
does not drop below that or go above the voltage ratings of the fets
or caps, you should be fine."


Since it's a "chopper" type speed controller then a bit of ripple on
the dc output will not be a problem. Your main problem is your
transformer load regulation as I read it.

I've also got a couple of good quality brushless 24V 400W motors
with built in brushless controllers. He rebuilds these to reverse
the rotation. I will ask him about these too.

I've posed the question and await the reply.
These are beautiful machines - black, finned, machined aluminium with
just two heavy power leads emerging and a small control loom for a
pot or a Hall effect twist grip "throttle".
These are too nice for a lathe, best for my trike :)

I haven't any experience with this type of motor so I can't add
anything.
.

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