On Monday, August 29, 2016 at 5:29:05 PM UTC-4, Diesel wrote:
trader_4
Sun, 28
Aug 2016 13:16:53 GMT in alt.home.repair, wrote:
Agree with your other points, but not this one. When you put a
cap in a DC circuit on the power input to an amplifier, it does
act as a reservoir. It's fully charged during light or normal
loads, when the amp has a sudden increase in power needed, that
cap can supply some of it and being close to the destination, it
can do so without the impedance effects of many feet of wire.
It may not have been the best example. was just the easiest to
explain...
However, I don't think a hard start kit works that way. If it
did, it could only be effective for 1/120 of a second, because
that's all the initial charge that's there prior to starting will
last.
The hard start cap is only good for momentary contact. It's to be
disconnected as soon as the motor has achieved 'run' rpms. Otherwise,
you'd toast the cap.
Yes, I know that. The point is the start cap is not holding a charge
to send to the motor. As I showed you, all the cap would have in
the way of charge is whatever it accumulates in under 1/120 of a
second and that's insignificant.
IE; it sends a surge of current to the start
windings to get the motor up and going,
There is no charge to make a surge. It works by creating a phase
shift, which is necessary to get the motor rotating. Where is this
alleged surge coming from? The start cap has no power to it prior
to the motor attempting to start, so it has no charge store.
It can't get but a tiny, insignificant charge from the AC, because
in under 1/120 of a second, the AC voltage reverseds and what's
been charged in one direction is now being discharged in the other.
And whatever is going on there, the motor has barely moved, if at
all at that point.
and within a few seconds, a
relay is supposed (in some) to kick it off the start cap/windings and
onto the run cap/run windings.
As I said though, hvac isn't my specialty and I fully admit I could
be wrong in how the cap is being used in this way.
You are.
http://www.capacitorformotor.com/start_capacitor.html
This website seems to agree with me though...
What is a Start Capacitor?
Designed for momentary use, the start capacitor is what lets a motor
start up instantly instead of taking a long time to come up to speed
before it can be used. It stays energized long enough to rapidly
bring the motor to 3/4 of its full speed and is then taken out of the
circuit. It will briefly increase motor starting torque and allow a
motor to be cycled on and off rapidly, but is not meant to be used
for more than a few seconds.
What exactly does it do?
Single phase motors will commonly have both a start and run
capacitor. The difference between the two is that the start capacitor
has a much higher ability to store charge, also known as its
capacitance rating, for its size than that of the run capacitor.
Essentially, the start cap gives the motor a bolt of energy in order
to get it up and running, while the run capacitor keeps the motor
going after the start capacitor shuts down.
Sometimes even folks that should know better get it wrong. They're a
supplier, but they have it wrong. There is no "surge", it's a
phase shift. They only understand the static concept of a cap,
ie how it stores charge. They assume it's releasing that energy
somehow to make the motor go. They don't understand that a capacitor
in an AC circuit causes the current to lead the voltage by 90 degrees.
It's that effect that is used to create a second magnetic field that
is phase shifted in the motor.
The above is also consistent with the fact that many motors have
a run capacitor. If caps work in motors by doing a "surge",
then what is one doing in some motors all the time? The answer
is those motors need that phase shift to run.