Re: Power Supply Rectification and Smoothing

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

On Wed, 17 Oct 2007 07:56:41 GMT, Ross Herbert
<rherber1@xxxxxxxxxxxxxx> wrote:

What you don't seem to understand is that the amplitude of a mains
spike will be reduced by the turns ratio of the transformer at the
secondary. In your case about 6.5 times reduction.

But let's see, its energy content will remain about the same, and
according to most experts in this area that I've read, electrolytic
cap life is reduced by spikes, especially when the rated voltage is
so close to the actual working voltage. (50V caps for a 36V supply)
Now, I don't know how much effect mains spikes will have on the
lifespan of caps in my application, but I have had variable advice,
and am merely trying to clarify this.

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.

There are several things you have to consider though. The first is the
ability of your transformer to supply a relatively constant secondary
voltage at the nominal current requirement without excessive core
temperature rise together with a relatively small output voltage rise
when lightly loaded. Since you are winding the secondary to produce a
nominal 36Vdc at 18A, and if the transformer has poor load regulation,
it might produce around 45Vrms when lightly loaded. These are tests
you should run to determine the load regulation capability of your
transformer before you start selecting capacitors.

At 45Vrms this means the filtered voltage across the electrolytic caps
will be around 63Vdc when lightly loaded, and as such, you would then
require 100Vdc rated electro caps. A well designed transformer will
have good load regulation capability and a quite small voltage
variation between lightly loaded and fully loaded conditions and you
could then use 50Vdc rated caps. 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.

While it is true that electrolytic caps may present significanly
higher impedance to sharp rising transient spikes than a lead acid
battery, the fact that you have quite a few electro's in parallel will
reduce the capacitor impedance to a very low value. If you are
concerned about it then connect a couple of polypropylene caps (AC
motor start is fine) in parallel with the electro's to present a lower
impedance path for the spikes.



Next thing, the impedance of your capacitor smoothed DC supply will
be
so low that any transient spike induced in the secondary will not
appear across the output capacitor since it looks like a short
circuit
to the transient.

Of course, but adding spike energy to the cap - my concern at this
point.

The impedance of the secondary winding will be far
greater than that of the capacitor filter itself and thus the
transient voltage spike will be dissipated in the winding via
leakage

capacitance etc.

That's interesting, thanks. So a lot of the spike energy is
dissipated in heating up the secondary winding of the transformer
(not that there is likely to be that much energy there) and of
course, my secondary windings are way more robust than delicate
electrolytic caps as I'm using 41 0.32 30A rated flex.

Spikes are generally of such small time duration and the thermal
inertia of the transformer is so large, that the increase in
temperature due to this factor will be almost nil.

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. It is not a good
material for winding a secondary since adjacent turns are separated by
twice the jacket thickness and magnetic efficiency will consequently
be lower than it should be. 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.


My main problem of ignorance (I've plenty of these :) now is how
smooth a current these motor speed controllers demand. I really don't
want to suck it and see. I once knew of an EE who has built many, and
modified the Chinese and other controllers. He's not on the group he
used to be but I think I have his website somewhere. I'll try to find
him and ask. He's usually terribly helpful.

The main purpose of the DC supply is to provide the required field
current and I doubt that it needs to be perfectly ripple free. Once
the motor is spinning the inertia of the armature will overcome any
minor variations due to DC ripple effect. Without knowing the method
of speed control used in your application I can't be specific.
However, if using PWM or other chopper type speed control then I doubt
that the smoothness of the DC supply will be critical at all.


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. Cheers, jack
.

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