----- Original Message -----
From: "Laurent Ferracci" <l.ferracci@comeca-group.com>
To: <towertalk@contesting.com>
Sent: Thursday, December 02, 2004 8:43 AM
Subject: [TowerTalk] Problem with KENPRO KR600X
First, let me introduce myself, as i'm a new Tower-talker. I'm Laurent -
F1JKJ, located in south of France. Sorry for limited english. French
"no-code" hams have been authorized on HF bands a few monthes ago, and
as i bought a new house in september, i now have a (little) garden for
an antenna. I've got a 12meters (~36feet) tower and a KLM KT34 in the
garden, and i'm planning to get it erected in a few weeks.
About the rotator: It's a Kenpro 5600 i used a few years ago on
satellites. In fact, it's a combination of KR600 (azimuth) and KR500
(evelation). I'll use the KR600 alone.
As it didn't turn, i've disassembled it to see what happened. I have two
problems with it: With the motor cap, and with the motor itself.
Motor cap: The cap is located in the rotor itself. I've read recent
messages about running/starting motor cap, but i still don't get it:
What's the use of that cap ? On the schematics i found, the cap is
labelled "100µF, 63V". OK for the values, but i'm in trouble with the
type of capacitor. I guess it's not polarized, right ? But if i'm
looking for another 100µF 63V not polarized cap, it's ridiculously
small compared to the one that i want to replace !
>>> As you've noticed, there's more to a capacitor than just capacitance and
voltage rating. There's also a current rating, which is determined by two
aspects. The first is the peak current handling ability. Some capacitors
have very fine wire or thin plated connections internally, with very low
thermal mass, so a high current spike will kill them. The other, and more
relevant for motor capacitors, is a power dissipation limit. All capacitors
(especially big ones) have some loss, either from leakage through the
electrolyte, dielectric losses, or resistive losses in the plates/internal
connections. You'll see terms like "loss tangent" or "ESR"(= Equivalent
Series Resistance) in this connection. When you pass significant current
through the capacitor some power is dissipated, heating the capacitor up.
And, just like a resistor, a physically large capacitor heats up slower than
a small one (for the same dissipated power).
For motor duty, there's start capacitors, intended for short duty cycle (<<
1%, typically), where the current is only flowing through the capacitor for
a few seconds at a time; and run capacitors, 100% duty cycle, where the
current is flowing all the time. In the start cap case, you can tolerate a
fairly lossy (read: inexpensive and high capacitance/volume ratio)
capacitor, because the cap won't get too hot, even though there's a fair
amount of peak power dissipated. The start cap never reaches thermal
equilibrium or steady state. In the run cap case, though, the capacitor has
to be designed so that that the power being dissipated internally can be
transferred to somewhere else (i.e. the air, the motor case, etc.) without
cooking the inside of the cap in a steady state sort of way. They do this
by a)reducing the power dissipation (using a lower loss dielectric system:
oil&paper or polypropylene instead of electrolytic) and b)making the
capacitor bigger (for a given dissipation) (increasing the surface area for
heat transfer, increasing the mass, etc.).
A reversible split phase AC motor has two windings, roughly 90 degrees out
of phase. By feeding one winding with current that is 90 degrees lagging or
leading the other, a rotating magnetic field is created, making the motor
turn. The direction of the field rotation determines the direction of the
motor turning. (Three phase motors do exactly the same thing.. Both two and
three phase motors require 3 wires, and in a three phase system, all the
wires carry the same current, so there's less IR loss than in a two phase
system, where one wire carries 1.4 times the current. Either way, the
"phase sequence" in the windings determines the rotation direction.
So, since most of us don't have 2 or 3 phase power available, they use a
series capacitor to create the phase shifted current for the second phase.
The value of the capacitor is chosen so that current through the winding
with the capacitor in series is 90 degrees out of phase with the
"non-capacitor" winding. Since the circuit is a fairly complex LRC (with
interaction between the two windings and the motor's rotor as well), the
precise capacitor value varies with the load and speed of the motor.
Fortunately, most motor applications where they are using this scheme aren't
super critical, and if the phase is a little bit wrong, it's no big deal. In
the commercial world, you hook up a watt meter to the motor on a dynamometer
(or with the actual load), and try different capacitors with a substitution
box until you get the torque/speed/efficiency characteristics you want. For
a reasonable volume application (>several hundred units), you can actually
have the motors wound to have the right balance of winding inductance, rotor
characterstics, etc.
If you're always turning one direction, and you want a smooth running quiet
(low-vibration) motor (compared to other single phase motors), you use a
special variant called a PSC motor, where the phase shifted winding is
smaller than the primary winding, carrying less current, requiring a smaller
capacitor, etc.
OK.. so in the classic split phase application (reversible pump or fan),
half the motor current is going through the capacitor, ALL the time. So, it
would require a "RUN" capacitor.
In the ham rotator case, though, you've got a low duty cycle application.
Run for a few seconds, then stop for many minutes. Here, you can use a
"start" capacitor. However, get too busy on the rotator switch, or have the
motor torque requirement increase a whole lot (frozen bearing, wind loads,
etc.), so the current is increased (or it turns slowly, so you run longer),
and the "low duty cycle" assumption isn't met any more, and you cook the
capacitor. I could conceive that cold weather hits you with a double
whammy: the lubricant is stiffer and/or there's ice; and the capacitor is
very cold, which might affect its internal structure or loss properties. Low
temperature also decreases the resistance of the motor windings, so the
current limiting from that source is reduced.
To return to Laurent's question... The original capacitor was chosen for a
particular duty cycle and (probably) low cost. Say you need 100uF at 60V.
You could buy a standard motor cap for a few dollars in quantities of
thousands. You could also buy a low loss non-polarized tantalum capacitor
with the same ratings for a whole lot more money. It would be smaller,
etc., and isn't what the mfr (who is very price sensitive) would choose, but
it would work. The key is looking at the current ratings and thermal
properties. Also, the Ta cap might not have the temperature range spec that
the original polypropylene/foil/oil capacitor does.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|