Fw: [Boatanchors] rotor starting capacitor

[Dave Brown]

An option no one has mentioned yet is the use of the readily available
'non-polarised' caps used in speaker crossover networks. These are typically
100 volt 85 degreeC rated caps in a range of values from 1uF to 400 uF.  Not
sure of their current ratings, but they are used in high power audio
applications where the currents will be significant-especially at the woofer
and sub-woofer end of the spectrum. Prices here vary from a dollar up to $16
or so for the high cap values.
73
 Dave, ZL3FJ

----- Original Message ----- 
From: "Peter Markavage" <manualman at juno.com>
To: <boatanchors at mailman.qth.net>
Sent: Saturday, July 10, 2004 3:11 AM
Subject: Re: Fw: [Boatanchors] rotor starting capacitor


> Brian:
> As with all Cornell Dubilier (now made by HY-Gain/MFJ) type rotors, the
> motor start capacitor is located in the control box at the operating
> position. As you indicate, the low voltage capacitors are probably more
> prone to failure especially in even these types of consumer applications.
> I would suspect if manufacturers used much higher rated AC capacitors in
> these rotor applications, they would not be such a common failure. The
> back to back capacitors that I use are rated for twice the capacitance of
> the original and 350 V DC. Requires a little more room than the original
> but generally there is adequate room in the control box.
>
> Pete, wa2cwa
>
>
> On Fri, 9 Jul 2004 17:19:02 +1000 "Brian Clarke"
> <brianclarke01 at optusnet.com.au> writes:
> Hi Peter,
>
> Did you know that it is mostly the low voltage capacitors that cause
> problems in
> domestic appliances, such as TV sets. The higher voltage caps have much
> thicker
> oxide coatings and seem not to be so prone to early failure. Ask any TV
> techo -
> he'll tell you he replaces all the electrolytics BEFORE doing any
> diagnosis.
>
> I suspect it is the intermittent usage that contributes to the long life
> - however, if
> the motor start cap is up at the top of your tower in the sun and snow,
> it is being
> stressed more than any device inside the home.
>
> It's not the age - it's the time under stress that counts.
>
> 73 de Brian, VK2GCE.
> ----- Original Message ----- 
> From: Peter Markavage
> To: boatanchors at mailman.qth.net
> Sent: Friday, July 09, 2004 4:30 PM
> Subject: Re: Fw: [Boatanchors] rotor starting capacitor
>
>
> Brian:
> Thanks for the dissertation on the care and feeding of polarized
> capacitors. However, in the initial discussion we were targeting motor
> start capacitors in Cornell Dubilier Ham/CB/TV (consumer grade, low
> voltage approx. 24 V AC to the motor winding) type rotors which generally
> have intermittent type operation. The VAC on the capacitor generally was
> around 50VAC. Tight component tolerances were not high on their design
> criteria. In industrial or consumer type applications, of constant or
> high motor start activity, the proper AC motor start capacitor has to be
> considered.
>
> I'll let you know if and when I blow out any of my "back to back" rotor
> motor start capacitor replacements. One is already 30 years old so I
> should get another set ready to put in.
>
> Pete, wa2cwa
> Wait, I just heard a pop! Sorry, it was the dog barking.
>
> On Thu, 8 Jul 2004 10:35:28 +1000 "Brian Clarke"
> <brianclarke01 at optusnet.com.au> writes:
> > 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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