Pendulum capacitor failing...

[dersh.z]

Hi Everyone, first time post... Mr. Goldwasser suggested I run this
question by this group...

I work at a small college with a bottom electromagnet driven pendulum.

The electromagnet consists of a large coil (.75H) with an iron core
and a 10uf capacitor in series with the hot leg (110vac). The pendulum
is an 8lb. iron shotput. Works great 24/7 except...

The caps, I've been using (GE 10uf @ 370VAC motor cap), last about 4
months before the circuit becomes "un-tuned" (measured capacitance
decreases - pendulum swing gradually increases then will eventually
stop and get stuck to the magnet).

I get the best swing using these params... l=.75H, C= 9.7uf, f= 59Hz.
(most of the 10uf caps have measured 9.6 - 9.8uf)

My question... can you recommend a higher reliability capacitor, one
that might last a few years instead of a few months?

Thank you for your time.

 

[Jim Adney]

On Tue, 26 Aug 2008 21:02:30 -0700 isw <isw@witzend.com> wrote:

In article
bb99c2cf-8566-4e58-ad54-aaf1bbf6dc0a@q5g2000prf.googlegroups.com>,
"dersh.z" <dersh.z@gmail.com> wrote:

Hi Everyone, first time post... Mr. Goldwasser suggested I run this
question by this group...

I work at a small college with a bottom electromagnet driven pendulum.

The electromagnet consists of a large coil (.75H) with an iron core
and a 10uf capacitor in series with the hot leg (110vac). The pendulum
is an 8lb. iron shotput. Works great 24/7 except...

The caps, I've been using (GE 10uf @ 370VAC motor cap), last about 4
months before the circuit becomes "un-tuned" (measured capacitance
decreases - pendulum swing gradually increases then will eventually
stop and get stuck to the magnet).

I get the best swing using these params... l=.75H, C= 9.7uf, f= 59Hz.
(most of the 10uf caps have measured 9.6 - 9.8uf)

My question... can you recommend a higher reliability capacitor, one
that might last a few years instead of a few months?

Thank you for your time.

Something like 600V film caps should last nearly forever. You'll
probably have to parallel a few to get the capacitance you want, though.

Putting several in parallel would also give him the advantage of being
able to fine tune the total capacitance.

-
-----------------------------------------------
Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Ross Herbert]

On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:

:Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:question by this group...
:
:I work at a small college with a bottom electromagnet driven pendulum.
:
:The electromagnet consists of a large coil (.75H) with an iron core
:and a 10uf capacitor in series with the hot leg (110vac). The pendulum
:is an 8lb. iron shotput. Works great 24/7 except...
:
:The caps, I've been using (GE 10uf @ 370VAC motor cap), last about 4
:months before the circuit becomes "un-tuned" (measured capacitance
:decreases - pendulum swing gradually increases then will eventually
:stop and get stuck to the magnet).
:
:I get the best swing using these params... l=.75H, C= 9.7uf, f= 59Hz.
most of the 10uf caps have measured 9.6 - 9.8uf)
:
:My question... can you recommend a higher reliability capacitor, one
:that might last a few years instead of a few months?
:
:Thank you for your time.

Data sheet for GE (Regal Beloit) motor run capacitors in the GEM series (which
includes the 97F9002)
http://www.alliedelec.com/Images/Products/Datasheets/BM/GENERAL_ELECTRIC_CAPACITOR/GE_CAP_5910097.PDF

Assuming there is very little series resistance (other than the dc resistance of
the inductor) you are left with a series resonant circuit which is resonant at
approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
current through the circuit at more than 6A so it might be dissipating a fair
amount of heat depending upon duty cycle. The pulsed nature of the drive voltage
might also contribute to premature failure.

Normally, a motor run capacitor is connected across both legs of the ac supply
in series with an auxiliary winding in an electric motor in a similar fashion to
your circuit but I would hazard a guess that the current through the motor
winding would be much less than in your arrangement with 0.75H.

Does the capacitor get hot?

 

[Michael A. Terrell]

"dersh.z" wrote:

Thanks Ross, et.al,

The capacitor does get warm but not hot, the coil does get quite hot
tho without a cooling fan.

Your calculations are what I get as well, I found this thing swings
quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
but, I' m having the cap failures at a much higher rate than the old
setup. I might have changed the original caps every 10years or so.

Both setups had the cap in series with the coil, the only major change
in my setup is the coil - the old coil lost it's smoke, couldn't get
it back in... so I had a new coil made. The old coil was hand wound
with a solid steel core, this new coil was machine wound (epoxied,
etc...) and I used 1/16" (no flux) welding rods tightly bundled into
an aprox. 1.25" core diameter (I also have a few brass tubes creating
air gaps in the core for forced air cooling from below - will cook
without the forced air cooling tubes).

I have been thinking Ceramic but am unsure as to which I should use in
this sort of an application. I generally don't do the "industrial"
stuff, I'm more of a component level tech - these motor caps are not
something I normally deal with.

Thanks to all for the repies, very much appreciated.

Are you useing motor start or motor run capacitors?


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.

 

[dersh.z]

On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com> wrote:

:Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:question by this group...
:
On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:

Assuming there is very little series resistance (other than the dc
resistance of

the inductor) you are left with a series resonant circuit which is resonant at
approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
current through the circuit at more than 6A so it might be dissipating a fair
amount of heat depending upon duty cycle. The pulsed nature of the drive voltage
might also contribute to premature failure.

Normally, a motor run capacitor is connected across both legs of the ac supply
in series with an auxiliary winding in an electric motor in a similar fashion to
your circuit but I would hazard a guess that the current through the motor
winding would be much less than in your arrangement with 0.75H.

Does the capacitor get hot?

Thanks Ross, et.al,

The capacitor does get warm but not hot, the coil does get quite hot
tho without a cooling fan.

Your calculations are what I get as well, I found this thing swings
quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
but, I' m having the cap failures at a much higher rate than the old
setup. I might have changed the original caps every 10years or so.

Both setups had the cap in series with the coil, the only major change
in my setup is the coil - the old coil lost it's smoke, couldn't get
it back in... so I had a new coil made. The old coil was hand wound
with a solid steel core, this new coil was machine wound (epoxied,
etc...) and I used 1/16" (no flux) welding rods tightly bundled into
an aprox. 1.25" core diameter (I also have a few brass tubes creating
air gaps in the core for forced air cooling from below - will cook
without the forced air cooling tubes).

I have been thinking Ceramic but am unsure as to which I should use in
this sort of an application. I generally don't do the "industrial"
stuff, I'm more of a component level tech - these motor caps are not
something I normally deal with.

Thanks to all for the repies, very much appreciated.

 

[default]

On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z"
<dersh.z@gmail.com> wrote:

On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com> wrote:

:Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:question by this group...
:
On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
Assuming there is very little series resistance (other than the dc
resistance of
the inductor) you are left with a series resonant circuit which is resonant at
approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
current through the circuit at more than 6A so it might be dissipating a fair
amount of heat depending upon duty cycle. The pulsed nature of the drive voltage
might also contribute to premature failure.

Normally, a motor run capacitor is connected across both legs of the ac supply
in series with an auxiliary winding in an electric motor in a similar fashion to
your circuit but I would hazard a guess that the current through the motor
winding would be much less than in your arrangement with 0.75H.

Does the capacitor get hot?

Thanks Ross, et.al,

The capacitor does get warm but not hot, the coil does get quite hot
tho without a cooling fan.

Your calculations are what I get as well, I found this thing swings
quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
but, I' m having the cap failures at a much higher rate than the old
setup. I might have changed the original caps every 10years or so.

Both setups had the cap in series with the coil, the only major change
in my setup is the coil - the old coil lost it's smoke, couldn't get
it back in... so I had a new coil made. The old coil was hand wound
with a solid steel core, this new coil was machine wound (epoxied,
etc...) and I used 1/16" (no flux) welding rods tightly bundled into
an aprox. 1.25" core diameter (I also have a few brass tubes creating
air gaps in the core for forced air cooling from below - will cook
without the forced air cooling tubes).

I have been thinking Ceramic but am unsure as to which I should use in
this sort of an application. I generally don't do the "industrial"
stuff, I'm more of a component level tech - these motor caps are not
something I normally deal with.

Thanks to all for the repies, very much appreciated.

Series resonant circuits can generate some pretty high voltages across
the coil and cap. Is it within specification for the cap?

The brass tubes are shorted turns and are eating some of your power,
can you sub some non-conductive tubes? Or how about leaving a space
between the core and bobbin and force the air in coaxially? That
would take some of the load off the cap and improve the Q of the
circuit.

The cap you refer to seems like just what I would use in that
application - polypropylene film has a very low dielectric loss to
cause heating, yet you say its getting warm? And you notice the graph
showing life versus temperature? (they don't exactly say how it
fails, so loss of capacity might be the failure mode)

The circulating current is too high for the cap. Remember they say
the conductive film is only a few microns thick - depending on the
construction, 6 amps might just be vaporizing parts of the plates.

Like someone else suggested you are probably better off with several
smaller caps than one large one. If it still drops in capacity you
then have the option of adding a cap every few months - but with more
leads and more caps the current through any one cap may be within its
current carrying capacity.

Transmitter tank caps are designed for heavy current - but probably
don't come in that size. Another cap to look for is called a
"commutating" cap (GE does make those too). They are beefed up to
present very low resistance and able to withstand high peak currents.

I'd go for multiple smaller (20-50)caps also with polypropylene
dielectric material.
--


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[Franc Zabkar]

On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z"
<dersh.z@gmail.com> put finger to keyboard and composed:

Hi Everyone, first time post... Mr. Goldwasser suggested I run this
question by this group...

I work at a small college with a bottom electromagnet driven pendulum.

The electromagnet consists of a large coil (.75H) with an iron core
and a 10uf capacitor in series with the hot leg (110vac). The pendulum
is an 8lb. iron shotput. Works great 24/7 except...

The caps, I've been using (GE 10uf @ 370VAC motor cap), last about 4
months before the circuit becomes "un-tuned" (measured capacitance
decreases - pendulum swing gradually increases then will eventually
stop and get stuck to the magnet).

I get the best swing using these params... l=.75H, C= 9.7uf, f= 59Hz.
(most of the 10uf caps have measured 9.6 - 9.8uf)

My question... can you recommend a higher reliability capacitor, one
that might last a few years instead of a few months?

Thank you for your time.

It seems to me that a perfectly tuned LC circuit would require a
capacitance of 9.38uF at 60Hz.

2.pi.f = 1/sqrt(LC)

So as the capacitance falls from your initial value of 9.7uF, the
circuit becomes better tuned and the current increases. AISI, this
increase in current would accelerate the capacitor's failure. Would it
not be possible to detune the circuit to an initial frequency of 61Hz
and extend the life of the pendulum that way? Would the performance at
61Hz be as good as at 59Hz?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Wed, 27 Aug 2008 13:45:22 GMT, Ross Herbert
<rherber1@bigpond.net.au> put finger to keyboard and composed:

On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:

:Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:question by this group...
:
:I work at a small college with a bottom electromagnet driven pendulum.
:
:The electromagnet consists of a large coil (.75H) with an iron core
:and a 10uf capacitor in series with the hot leg (110vac). The pendulum
:is an 8lb. iron shotput. Works great 24/7 except...
:
:The caps, I've been using (GE 10uf @ 370VAC motor cap), last about 4
:months before the circuit becomes "un-tuned" (measured capacitance
:decreases - pendulum swing gradually increases then will eventually
:stop and get stuck to the magnet).
:
:I get the best swing using these params... l=.75H, C= 9.7uf, f= 59Hz.
most of the 10uf caps have measured 9.6 - 9.8uf)
:
:My question... can you recommend a higher reliability capacitor, one
:that might last a few years instead of a few months?
:
:Thank you for your time.

Data sheet for GE (Regal Beloit) motor run capacitors in the GEM series (which
includes the 97F9002)
http://www.alliedelec.com/Images/Products/Datasheets/BM/GENERAL_ELECTRIC_CAPACITOR/GE_CAP_5910097.PDF

Assuming there is very little series resistance (other than the dc resistance of
the inductor) you are left with a series resonant circuit which is resonant at
approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
current through the circuit at more than 6A so it might be dissipating a fair
amount of heat depending upon duty cycle. The pulsed nature of the drive voltage
might also contribute to premature failure.

If the circuit current is 6A, then the voltage across the capacitor
(and inductor) must be ...

Vc = I x Xc = I x 1/(2pi.f.C) = 1600V

Are you sure your calculation is correct, in which case the capacitor
is doomed, or am I visualising the circuit incorrectly?

Normally, a motor run capacitor is connected across both legs of the ac supply
in series with an auxiliary winding in an electric motor in a similar fashion to
your circuit but I would hazard a guess that the current through the motor
winding would be much less than in your arrangement with 0.75H.

Does the capacitor get hot?

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[default]

On Wed, 27 Aug 2008 13:00:54 -0700 (PDT), "dersh.z"
<dersh.z@gmail.com> wrote:

The brass tubes are shorted turns and are eating some of your power, can you sub some non-conductive tubes?


The brass tubes are in the core, not the coil. Without them, the core
gets very hot.

"Without them the core gets very hot" and the coil is cool? If the
core is hotter than the coil, it would suggest high hysteresis loss or
the diameter of the core wires is too large. I'd use wire diameter of
~ 1-2 mm to limit core loss. Welding rod?

The core wires should also be insulated from one another. If the wire
is galvanized - an acid treatment or just leaving them out in the
weather for a few months (like electric fence wire - cheap small
diameter and galvanized). Throwing iron wire in a BBQ will help form
an oxide coating, and will anneal it -soft wire has lower losses than
hard drawn wire.

Hard stiff wire has higher hysteresis loss. The wire doesn't switch
poles easily so it acts like it isn't there until the magnetic force
is high enough to turn it over. A test is to see how much magnetic
remanence it has: Stroke a piece of wire with a magnet and see if it
becomes a magnetized. If it does, the hysteresis loss will be higher
- annealing the wire helps there.

The iron has higher permeability than air so most of the flux should
go though the iron but some will still be dissipated as heat in the
brass tubes - larger diameter tubes, higher loss.

Last but not least, the core should protrude from the ends of the coil
- the magnetic lines of force expand as they get to the center of the
coil (on the outside of the coil). Stretching the length of the core
raises inductance for the same number of turns. I like at least one
diameter of core to protrude from each end of the coil and 2D is
better - for induction coils.


--


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[default]

On Wed, 27 Aug 2008 13:00:54 -0700 (PDT), "dersh.z"
<dersh.z@gmail.com> wrote:

The brass tubes are shorted turns and are eating some of your power, can you sub some non-conductive tubes?


The brass tubes are in the core, not the coil. Without them, the core
gets very hot.

"Without them the core gets very hot" and the coil is cool? If the
core is hotter than the coil, it would suggest high hysteresis loss or
the diameter of the core wires is too large. I'd use wire diameter of
~ 1-2 mm to limit core loss. Welding rod?

The core wires should also be insulated from one another. If the wire
is galvanized - an acid treatment or just leaving them out in the
weather for a few months (like electric fence wire - cheap small
diameter and galvanized). Throwing iron wire in a BBQ will help form
an oxide coating, and will anneal it -soft wire has lower losses than
hard drawn wire.

Hard stiff wire has higher hysteresis loss. The wire doesn't switch
poles easily so it acts like it isn't there until the magnetic force
is high enough to turn it over. A test is to see how much magnetic
remanence it has: Stroke a piece of wire with a magnet and see if it
becomes a magnetized. If it does, the hysteresis loss will be higher
- annealing the wire helps there.

The iron has higher permeability than air so most of the flux should
go though the iron but some will still be dissipated as heat in the
brass tubes - larger diameter tubes, higher loss.

Last but not least, the core should protrude from the ends of the coil
- the magnetic lines of force expand as they get to the center of the
coil (on the outside of the coil). Stretching the length of the core
raises inductance for the same number of turns. I like at least one
diameter of core to protrude from each end of the coil and 2D is
better - for induction coils.
--


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[dersh.z]

On Aug 27, 10:21 am, "Michael A. Terrell" <mike.terr...@earthlink.net>
wrote:

"dersh.z" wrote:

Thanks Ross, et.al,

The capacitor does get warm but not hot, the coil does get quite hot
tho without a cooling fan.

Your calculations are what I get as well, I found this thing swings
quite nicely when tuned to aprox. 58-59Hz.  The old setup (1960's
design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
but, I' m having the cap failures at a much higher rate than the old
setup.  I might have changed the original caps every 10years or so.

Both setups had the cap in series with the coil, the only major change
in my setup is the coil - the old coil lost it's smoke, couldn't get
it back in... so I had a new coil made.  The old coil was hand wound
with a solid steel core, this new coil was machine wound (epoxied,
etc...) and I used 1/16" (no flux) welding rods tightly bundled into
an aprox. 1.25" core diameter (I also have a few brass tubes creating
air gaps in the core for forced air cooling from below - will cook
without the forced air cooling tubes).

I have been thinking Ceramic but am unsure as to which I should use in
this sort of an application.  I generally don't do the "industrial"
stuff, I'm more of a component level tech - these motor caps are not
something I normally deal with.

Thanks to all for the repies, very much appreciated.

   Are you useing motor start or motor run capacitors?

They are Motor Run caps.

thanks

 

[dersh.z]

On Aug 27, 2:43 pm, default <defa...@defaulter.net> wrote:

The brass tubes are shorted turns and are eating some of your power, can you sub some non-conductive tubes?

The brass tubes are in the core, not the coil. Without them, the core
gets very hot.

Like someone else suggested you are probably better off with several smaller caps than one large one.

Seems easy enough, I will try using several caps instead of just the
one.

Transmitter tank caps are designed for heavy current - but probably don't come in that size. Another cap to look for is called a
"commutating" cap (GE does make those too).

Thanks, I'll look in to them.

 

[Ross Herbert]

On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:

:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com>
wrote:
:>
:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:> :question by this group...
:> :
:On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
: Assuming there is very little series resistance (other than the dc
:resistance of
:> the inductor) you are left with a series resonant circuit which is resonant
at
:> approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
:> current through the circuit at more than 6A so it might be dissipating a fair
:> amount of heat depending upon duty cycle. The pulsed nature of the drive
voltage
:> might also contribute to premature failure.
:>
:> Normally, a motor run capacitor is connected across both legs of the ac
supply
:> in series with an auxiliary winding in an electric motor in a similar fashion
to
:> your circuit but I would hazard a guess that the current through the motor
:> winding would be much less than in your arrangement with 0.75H.
:>
:> Does the capacitor get hot?
:
:Thanks Ross, et.al,
:
:The capacitor does get warm but not hot, the coil does get quite hot
:tho without a cooling fan.
:
:Your calculations are what I get as well, I found this thing swings
:quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
:design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
:but, I' m having the cap failures at a much higher rate than the old
:setup. I might have changed the original caps every 10years or so.
:
:Both setups had the cap in series with the coil, the only major change
:in my setup is the coil - the old coil lost it's smoke, couldn't get
:it back in... so I had a new coil made. The old coil was hand wound
:with a solid steel core, this new coil was machine wound (epoxied,
:etc...) and I used 1/16" (no flux) welding rods tightly bundled into
:an aprox. 1.25" core diameter (I also have a few brass tubes creating
:air gaps in the core for forced air cooling from below - will cook
:without the forced air cooling tubes).
:
:I have been thinking Ceramic but am unsure as to which I should use in
:this sort of an application. I generally don't do the "industrial"
:stuff, I'm more of a component level tech - these motor caps are not
:something I normally deal with.
:
:Thanks to all for the repies, very much appreciated.


Stick to polypropylene filmcaps instead of ceramic. I would suggest a better
capacitor to use is one made by American Capacitor. The VW2M106K (1000V) which
has rated RMS current of 19A and a peak of 2910A with high dV/dT.
http://www.americancapacitor.com/PDF/VData.pdf

You will have to contact them to get details of purchasing.
http://www.americancapacitor.com/index.htm

 

[Franc Zabkar]

On Thu, 28 Aug 2008 02:25:13 GMT, Ross Herbert
<rherber1@bigpond.net.au> put finger to keyboard and composed:

On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:

:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com
wrote:
:
:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:> :question by this group...
:> :
:On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
: Assuming there is very little series resistance (other than the dc
:resistance of
:> the inductor) you are left with a series resonant circuit which is resonant
at
:> approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
:> current through the circuit at more than 6A so it might be dissipating a fair
:> amount of heat depending upon duty cycle. The pulsed nature of the drive
voltage
:> might also contribute to premature failure.
:
:> Normally, a motor run capacitor is connected across both legs of the ac
supply
:> in series with an auxiliary winding in an electric motor in a similar fashion
to
:> your circuit but I would hazard a guess that the current through the motor
:> winding would be much less than in your arrangement with 0.75H.
:
:> Does the capacitor get hot?
:
:Thanks Ross, et.al,
:
:The capacitor does get warm but not hot, the coil does get quite hot
:tho without a cooling fan.
:
:Your calculations are what I get as well, I found this thing swings
:quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
:design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
:but, I' m having the cap failures at a much higher rate than the old
:setup. I might have changed the original caps every 10years or so.
:
:Both setups had the cap in series with the coil, the only major change
:in my setup is the coil - the old coil lost it's smoke, couldn't get
:it back in... so I had a new coil made. The old coil was hand wound
:with a solid steel core, this new coil was machine wound (epoxied,
:etc...) and I used 1/16" (no flux) welding rods tightly bundled into
:an aprox. 1.25" core diameter (I also have a few brass tubes creating
:air gaps in the core for forced air cooling from below - will cook
:without the forced air cooling tubes).
:
:I have been thinking Ceramic but am unsure as to which I should use in
:this sort of an application. I generally don't do the "industrial"
:stuff, I'm more of a component level tech - these motor caps are not
:something I normally deal with.
:
:Thanks to all for the repies, very much appreciated.


Stick to polypropylene filmcaps instead of ceramic. I would suggest a better
capacitor to use is one made by American Capacitor. The VW2M106K (1000V) which
has rated RMS current of 19A and a peak of 2910A with high dV/dT.
http://www.americancapacitor.com/PDF/VData.pdf

You will have to contact them to get details of purchasing.
http://www.americancapacitor.com/index.htm

It seems to me that choosing a better spec cap is still only delaying
the inevitable. IMHO a better approach would be to choose a 9.1uF cap,
polypropylene film or otherwise. This assumes that the capacitance
required for resonance at 60Hz is 9.4uF. As it is now, the circuit is
slightly detuned above its resonant frequency. AFAICS this means that
when the 9.7uF cap inevitably degrades, the circuit drifts *toward*
resonance, in which case the capacitor's current and voltage both
*increase*, resulting in further degradation and even more drift
toward resonance. If, however, the circuit were to be detuned on the
other side of resonance, then any degradation would result in a drift
*away* from resonance, with a *reduction* in current and voltage, and
this would in turn would slow the rate of degradation of the cap.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Franc Zabkar]

On Thu, 28 Aug 2008 06:01:53 +1000, Franc Zabkar
<fzabkar@iinternode.on.net> put finger to keyboard and composed:

Assuming there is very little series resistance (other than the dc resistance of
the inductor) you are left with a series resonant circuit which is resonant at
approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
current through the circuit at more than 6A so it might be dissipating a fair
amount of heat depending upon duty cycle. The pulsed nature of the drive voltage
might also contribute to premature failure.

If the circuit current is 6A, then the voltage across the capacitor
(and inductor) must be ...

Vc = I x Xc = I x 1/(2pi.f.C) = 1600V

When I input the OP's numbers into the following QBASIC formulae, I
get only 21.3mA for the current. I'm wondering whether I've
misunderstood the OP's circuit.

PI = 3.14159265#
C = 9.7 * .000001
L = .75
F = 60
W = 2 * PI * F
XL = W * L
XC = 1 / W / C
Z = XL * XL - XC * XC
I = 110 / Z
VC = I * XC
VL = I * XL
E = .5 * L * I * I
PRINT C, L, XL, XC, Z, I, VC, VL, E

It seems to me that the OP should at least take some measurements to
determine what currents and voltages are present in his circuit,
especially at or near the time of failure.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

 

[Ross Herbert]

On Thu, 28 Aug 2008 16:12:32 +1000, Franc Zabkar <fzabkar@iinternode.on.net>
wrote:

:On Thu, 28 Aug 2008 02:25:13 GMT, Ross Herbert
:<rherber1@bigpond.net.au> put finger to keyboard and composed:
:
:>On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:
:>
:>:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com>
:>wrote:
:>:>
:>:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:>:> :question by this group...
:>:> :
:>:On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
:>: Assuming there is very little series resistance (other than the dc
:>:resistance of
:>:> the inductor) you are left with a series resonant circuit which is resonant
:>at
:>:> approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
:>:> current through the circuit at more than 6A so it might be dissipating a
fair
:>:> amount of heat depending upon duty cycle. The pulsed nature of the drive
:>voltage
:>:> might also contribute to premature failure.
:>:>
:>:> Normally, a motor run capacitor is connected across both legs of the ac
:>supply
:>:> in series with an auxiliary winding in an electric motor in a similar
fashion
:>to
:>:> your circuit but I would hazard a guess that the current through the motor
:>:> winding would be much less than in your arrangement with 0.75H.
:>:>
:>:> Does the capacitor get hot?
:>:
:>:Thanks Ross, et.al,
:>:
:>:The capacitor does get warm but not hot, the coil does get quite hot
:>:tho without a cooling fan.
:>:
:>:Your calculations are what I get as well, I found this thing swings
:>:quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
:>:design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
:>:but, I' m having the cap failures at a much higher rate than the old
:>:setup. I might have changed the original caps every 10years or so.
:>:
:>:Both setups had the cap in series with the coil, the only major change
:>:in my setup is the coil - the old coil lost it's smoke, couldn't get
:>:it back in... so I had a new coil made. The old coil was hand wound
:>:with a solid steel core, this new coil was machine wound (epoxied,
:>:etc...) and I used 1/16" (no flux) welding rods tightly bundled into
:>:an aprox. 1.25" core diameter (I also have a few brass tubes creating
:>:air gaps in the core for forced air cooling from below - will cook
:>:without the forced air cooling tubes).
:>:
:>:I have been thinking Ceramic but am unsure as to which I should use in
:>:this sort of an application. I generally don't do the "industrial"
:>:stuff, I'm more of a component level tech - these motor caps are not
:>:something I normally deal with.
:>:
:>:Thanks to all for the repies, very much appreciated.
:>
:>
:>Stick to polypropylene filmcaps instead of ceramic. I would suggest a better
:>capacitor to use is one made by American Capacitor. The VW2M106K (1000V) which
:>has rated RMS current of 19A and a peak of 2910A with high dV/dT.
:>http://www.americancapacitor.com/PDF/VData.pdf
:>
:>You will have to contact them to get details of purchasing.
:>http://www.americancapacitor.com/index.htm
:
:It seems to me that choosing a better spec cap is still only delaying
:the inevitable. IMHO a better approach would be to choose a 9.1uF cap,
olypropylene film or otherwise. This assumes that the capacitance
:required for resonance at 60Hz is 9.4uF. As it is now, the circuit is
:slightly detuned above its resonant frequency. AFAICS this means that
:when the 9.7uF cap inevitably degrades, the circuit drifts *toward*
:resonance, in which case the capacitor's current and voltage both
:*increase*, resulting in further degradation and even more drift
:toward resonance. If, however, the circuit were to be detuned on the
ther side of resonance, then any degradation would result in a drift
:*away* from resonance, with a *reduction* in current and voltage, and
:this would in turn would slow the rate of degradation of the cap.
:
:- Franc Zabkar


I doubt that "losing capacitance" is the only symptom of the capacitor failing
in this circuit - it's just that we don't know what else is happening to the
capacitor in the application. The only data available for the GE97F9002 cap
doesn't give details of rated or peak current or dV/dT rating. All we know is
that it is specced as a motor run capacitor. The normal use for this capacitor
type is in series connection with a motor winding having considerable inductive
reactance across an ac supply where the peak inrush current period is limited
and infrequent in nature, and normal operating current is not likely to be
anywhere near 6A. The OP is endeavouring to use it in a circuit where it
receives frequent high peak inrush current and a large inductive kick when the
circuit is broken, at each swing of the pendulum. I doubt that these conditions
are ideal for maximising the life of this type of capacitor. While the original
GE capacitors might have done the job for 10 years in the OP's first coil
design, these later incarnations made by a different process by Regal Benoit may
not be as good as the originals - ie. they are built down to a price.

While using a better capacitor might not be the most elegant solution in
engineering terms, if it works without having to change the current coil design
then it is still an effective solution. My guess is that the GE97F9002 cap
doesn't like the constant hammering of pulsed ac operation where the inrush
current is surely quite large and the current at resonance is still rather high
for a capacitor which is meant to be permanently connected across an ac supply
in series with a higher impedance winding than the 0.75H coil, and having a
relatively small current swing. The alternative cap I suggested does come with
the necessary data and is designed to handle high currents and with a large
dV/dT rating. Surely this is a good starting point.

Even if we were to find exactly what the failure mechanism for the GE cap is, it
would most likely turn out that a better specced capacitor is the only solution
anyway, so why not anticipate this necessity. It might give the electronics
perfectionist a warm glow to know exactly why a component fails but in practical
terms the end user only cares whether the item works and that it keeps on
working.

If the circuit performance doesn't deteriorate over time with the use of a
better specced cap it will have proved by empirical method to be an effective
solution. If the OP later decides to design a better coil then this cap will be
even better suited since it will possibly have to endure even lower stress.

 

[PeterD]

On Thu, 28 Aug 2008 16:12:32 +1000, Franc Zabkar
<fzabkar@iinternode.on.net> wrote:

It seems to me that choosing a better spec cap is still only delaying
the inevitable. IMHO a better approach would be to choose a 9.1uF cap,
polypropylene film or otherwise. This assumes that the capacitance
required for resonance at 60Hz is 9.4uF. As it is now, the circuit is
slightly detuned above its resonant frequency. AFAICS this means that
when the 9.7uF cap inevitably degrades, the circuit drifts *toward*
resonance, in which case the capacitor's current and voltage both
*increase*, resulting in further degradation and even more drift
toward resonance. If, however, the circuit were to be detuned on the
other side of resonance, then any degradation would result in a drift
*away* from resonance, with a *reduction* in current and voltage, and
this would in turn would slow the rate of degradation of the cap.

Using that logic, then while the circuit drifts towards resonance, it
tends to degrade faster towards resonance, until it crosses over the
resonance point, then degradation slows. (or it fails at resonance!)

- Franc Zabkar

 

[David]

When I input the OP's numbers into the following QBASIC
formulae, I
get only 21.3mA for the current. I'm wondering whether
I've
misunderstood the OP's circuit.

PI = 3.14159265#
C = 9.7 * .000001
L = .75
F = 60
W = 2 * PI * F
XL = W * L
XC = 1 / W / C
Z = XL * XL - XC * XC
I = 110 / Z
VC = I * XC
VL = I * XL
E = .5 * L * I * I
PRINT C, L, XL, XC, Z, I, VC, VL, E

It seems to me that the OP should at least take some
measurements to
determine what currents and voltages are present in his
circuit,
especially at or near the time of failure.

- Franc Zabkar

Frank,
You made a major mistake calculating things with your
program. XL is i*W*L (or j*W*L if you are an EE) which is an
imaginary number where i or j = sqrt(-1). Also, XC is
1/(i*W*C) that is again an imaginary number. You need to do
the math using complex arithmetic. If the circuit is in
exact resonance the current will approach infinity and the
voltages across each element will as well. There is
obviously series and effective shunt resistance in the
circuit elements to keep things within some reasonable
bounds at resonance.

David

 

[greenpjs]

On Thu, 28 Aug 2008 02:25:13 GMT, Ross Herbert
<rherber1@bigpond.net.au> wrote:

On Wed, 27 Aug 2008 08:17:39 -0700 (PDT), "dersh.z" <dersh.z@gmail.com> wrote:

:> On Tue, 26 Aug 2008 08:54:50 -0700 (PDT), "dersh.z" <ders...@gmail.com
wrote:
:
:> :Hi Everyone, first time post... Mr. Goldwasser suggested I run this
:> :question by this group...
:> :
:On Aug 27, 8:45 am, Ross Herbert <rherb...@bigpond.net.au> wrote:
: Assuming there is very little series resistance (other than the dc
:resistance of
:> the inductor) you are left with a series resonant circuit which is resonant
at
:> approx 58Hz for the values given (L = 0.75H and C = 10uF). I calculate the
:> current through the circuit at more than 6A so it might be dissipating a fair
:> amount of heat depending upon duty cycle. The pulsed nature of the drive
voltage
:> might also contribute to premature failure.
:
:> Normally, a motor run capacitor is connected across both legs of the ac
supply
:> in series with an auxiliary winding in an electric motor in a similar fashion
to
:> your circuit but I would hazard a guess that the current through the motor
:> winding would be much less than in your arrangement with 0.75H.
:
:> Does the capacitor get hot?
:
:Thanks Ross, et.al,
:
:The capacitor does get warm but not hot, the coil does get quite hot
:tho without a cooling fan.
:
:Your calculations are what I get as well, I found this thing swings
:quite nicely when tuned to aprox. 58-59Hz. The old setup (1960's
:design) swung about 3ft max @220VAC, I can now get 8ft+ @115VAC...
:but, I' m having the cap failures at a much higher rate than the old
:setup. I might have changed the original caps every 10years or so.
:
:Both setups had the cap in series with the coil, the only major change
:in my setup is the coil - the old coil lost it's smoke, couldn't get
:it back in... so I had a new coil made. The old coil was hand wound
:with a solid steel core, this new coil was machine wound (epoxied,
:etc...) and I used 1/16" (no flux) welding rods tightly bundled into
:an aprox. 1.25" core diameter (I also have a few brass tubes creating
:air gaps in the core for forced air cooling from below - will cook
:without the forced air cooling tubes).
:
:I have been thinking Ceramic but am unsure as to which I should use in
:this sort of an application. I generally don't do the "industrial"
:stuff, I'm more of a component level tech - these motor caps are not
:something I normally deal with.
:
:Thanks to all for the repies, very much appreciated.


Stick to polypropylene filmcaps instead of ceramic. I would suggest a better
capacitor to use is one made by American Capacitor. The VW2M106K (1000V) which
has rated RMS current of 19A and a peak of 2910A with high dV/dT.
http://www.americancapacitor.com/PDF/VData.pdf

You will have to contact them to get details of purchasing.
http://www.americancapacitor.com/index.htm

Hi all,
I am afraid I can't help the original poster, but I find this thread
very interesting. There is a pendulum at a local museum and I always
wondered how it worked. I assume that an electromagnet is used to
replace the small amount of energy lost during each swing of the
pendulum, but how is it actually connected and where is the
electromagnet? What does having an resonant circuit tuned to
slightly below the power line frequency have to do it? I guess I'm
asking for someone to explain the theory. I don't suppose there's a
"how stuff works" article on the subject, but I'll go check that out
now.

Thanks,
Pat

 

[Franc Zabkar]

On Thu, 28 Aug 2008 09:12:30 -0400, PeterD <peter2@hipson.net> put
finger to keyboard and composed:

On Thu, 28 Aug 2008 16:12:32 +1000, Franc Zabkar
fzabkar@iinternode.on.net> wrote:

It seems to me that choosing a better spec cap is still only delaying
the inevitable. IMHO a better approach would be to choose a 9.1uF cap,
polypropylene film or otherwise. This assumes that the capacitance
required for resonance at 60Hz is 9.4uF. As it is now, the circuit is
slightly detuned above its resonant frequency. AFAICS this means that
when the 9.7uF cap inevitably degrades, the circuit drifts *toward*
resonance, in which case the capacitor's current and voltage both
*increase*, resulting in further degradation and even more drift
toward resonance. If, however, the circuit were to be detuned on the
other side of resonance, then any degradation would result in a drift
*away* from resonance, with a *reduction* in current and voltage, and
this would in turn would slow the rate of degradation of the cap.

Using that logic, then while the circuit drifts towards resonance, it
tends to degrade faster towards resonance, until it crosses over the
resonance point, then degradation slows. (or it fails at resonance!)

The OP stated that the "pendulum swing gradually increases then will
eventually stop and get stuck to the magnet". I took this to mean that
the capacitor fails as the circuit approaches resonance. If we assume
a circuit resistance of 10 ohms, then, at resonance, this would result
in a current of 11A which would probably challenge the fuse. Moreover,
it would result in a capacitor voltage of around 3000V. I suspect that
the actual DC resistance of the coil is much less than 10 ohms.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.