Fw: [Boatanchors] rotor starting capacitor

[Peter Markavage]

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.