[Fwd: Fw: [Boatanchors] rotor starting capacitor]

[Arthur Shulman]

Arthur Shulman wrote:

> Thank you for the explanation, Brian. Your argument makes sense, but leads to
> another question.
> To wit - what distinguishes an AC rated capacitor from the back-to-back
> solutions proposed? In other words, what different manufacturing or assembly
> process is involved?
> If we can acceptably 'fake' an AC capacitor by using a diode bridge, that's
> convenient, but surely manufacturers do not use internal diode bridges in their
> motor-start capacitors.
> Arthur Shulman
>
> Brian Clarke wrote:
>
> > Hi Peter,
> >
> > A dc electrolytic capacitor has one of its [usually rolled] plates coated in
> > aluminium oxide to form the insulator and dielectric. This oxide coating is
> > formed
> > by an electrolytic process - hence the name; similar kind of thing to
> > plating metals,
> > in a way. Hence, it is polarised - ie, it works when the voltage is applied
> > in one
> > favoured direction. The oxide layer is very thin - the actual thickness
> > determines to
> > a large extent the voltage rating of the capacitor. When a reverse voltage
> > is applied
> > to the capacitor, the electrolytic process reverses, the oxide layer gets
> > eaten away
> > and eventually the plates meet => short circuit => the 'attendant
> > consequences'.
> >
> > If you think that using two polarised capacitors back-to-back = depolarised,
> > I do
> > believe you are mistaken. You are using two polarised capacitors. If you
> > depolarise
> > them, they will eventually short.
> >
> > Let's try a little science. Measure the capacitance and Equivalent Series
> > Resistance [ESR] of each capacitor at installation and then every, say, 25
> > hours of
> > operation. I suspect the reason you and many others claim to have had such a
> > trouble-free run is that the actual time during which the capacitors are
> > reverse
> > polarised is very small, and so, the loss of oxide coating is fairly small -
> > so far. And
> > none of you has measured the capacitance or ESR over time to detect when
> > danger
> > is approaching.
> >
> > Let's really put our money where our mouths are. Get a dc electrolytic
> > capacitor
> > and connect it to a low impedance ac source whose voltage is near the
> > operating
> > voltage of the capacitor. Tell me how long it takes for the 'attendant
> > consequences'.
> > I'ld suggest a large Pyrex dish be inverted over the experiment to contain
> > the
> > 'attendant consequences'. I've done it in teaching electrotechnology
> > students - time
> > to failure? - about a couple of seconds! You can put a current limiting
> > resistor in
> > series if you want to reduce the clean-up time.
> >
> > OK, let's look at this 'use the same voltage rating' hypothesis. What
> > happens when
> > you put two capacitors in series? If they have the same capacitance, they
> > share the
> > applied voltage equally. If they are of different capacitance, the smaller
> > capacitor
> > gets the larger share of the voltage. So, it's the ACTUAL [read 'measured'
> > rather
> > than what's printed on the outside] capacitance that matters, rather than
> > the voltage
> > rating. Of course, it would be unwise to over-stress a capacitor - you'll
> > get
> > punch-through of the insulator/dielectric, local carbonisation, then gradual
> > spreading and failure.
> >
> > One respondent to this group suggested running the capacitors at way below
> > their rated voltage.This may slow down the measurable loss of oxide. But how
> > many of you have measured the capacitance at different operating voltages?
> > Electrolytic capacitors achieve their specified [+100%, -50%] capacitance at
> > near their rated operating voltage. So, if you use a pair of capacitors
> > whose
> > characteristics you have not measured, at well below their rated voltage, in
> > a
> > motor start circuit, what happens to the phase of the current in the motor
> > windings? The motor has to work much harder to achieve starting torque,
> > and you may well burn out the running winding. Is this a wonderful idea??
> >
> > Next time you inspect an ac motor start capacitor, have a close look at the
> > tolerance - you'll find it is much tighter than on the dc electrolytics we
> > amateurs are used to consuming. Why? So that the motor doesn't get its
> > starting current exceeding the windings' current carrying capability.
> >
> > Now, go ahead and connect your dc electrolytic capacitors in series reverse
> > polarity - and the very best of luck to you. Most of the time, the laws of
> > physics are not based on luck. Albert Einstein was well aware of this, when
> > he said 'God does not play tricks.'
> >
> > Another respondent to this group suggested using a pair of steering diodes
> > around your pair of reversed dc capacitors. This is close to a good
> > solution.
> > Philips, in their book on diodes, published about 30 years ago, suggested
> > putting a dc electrolytic capacitor inside a diode bridge. This way the
> > capacitor
> > is always polarised correctly, you only need one capacitor, and you can
> > probably afford 4 diodes. My only caveat would be to measure the
> > capacitance to be sure it's within cooee of the motor's requirement before
> > re-installing the whole lot at the top of your tower. Measure the
> > capacitance
> > OUTSIDE the diode bridge, ie, the way the motor and the ac mains will see
> > it.
> >
> > 73 de Brian, VK2GCE.
> >
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