|Item Title:||HVAC AC/Furnace Blower Fan Motor run/start Capacitor, 5 uFd(mFd), Oval; 370V, 50/60Hzl, $20 value|
|S&H:|| $3.95, Continental USA, S&H varies to other areas and countries.
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CAPACITOR SIZING DILEMMAS
The simplest way to explain the mechanics of a capacitor would be
to compare it to a battery. Both store and release electricity.
Capacitors are charged with electricity, then releases its stored
energy at a rate of sixty times per second in a 60 cycle alternating
current system. The sizing is critical to motor efficiency just as
sizing of batteries is critical to a radio. A radio that requires a 9V
battery will not work with a 1.5V size battery. Thus, as the battery
becomes weaker the radio will not play properly. A motor that requires
a 7.5 mfd capacitor will not work with a 4.0 mfd capacitor. Much the
same way, a motor will not run properly with a weak capacitor. This is
not to imply bigger is better, because a capacitor that is too large
can cause energy consumption to rise. In both instances, be it too
large or too small, the life of the motor will be shortened due to
overheated motor windings. Motor manufacturers spend many hours testing
motor and capacitor combinations to arrive at the most efficient
combination. There is a maximum of +10% tolerance in microfarad rating
on replacement start capacitors, but exact run capacitors must be
replaced. Voltage rating must always be the same or greater than
original capacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application.
Capacitors contain two metal plates insulated from each other
(See Figure 1). When opened, the inside looks like two sheets of foil
with wax paper in between them and rolled tight, similar to a roll of
paper towel. Years ago the oil filled type used PCB's as a coolant.
Today most capacitors are the dry type.
Two basic types are used in electric motor:
1) Run capacitors are rated in a range of 3-70 microfarad
(mfd). Run capacitors are also rated by voltage classification. The
voltage classifications are 370V and 440V(other voltages rare but do exist). Capacitors with ratings
above 70 microfarad (mfd) are starting capacitors. Run capacitors are
designed for continuous duty, and are energized the entire time the
motor is running. Single phase electric motors need a capacitor to
energize a second phase winding. This is why sizing is so critical. If
the wrong run capacitor is installed, the motor will not have an even
magnetic field. This will cause the rotor to hesitate at those spots
that are uneven. This hesitation will cause the motor to become noisy,
increase energy consumption, cause performance to drop, and cause the
motor to overheat.
Dual Capacitors are 2 running capacitors made into one package with 2 different capacitances. They are designed this way to make wiring a little easier, the bigger capacitance is for the compressor motor while the smaller one for the fan motor.
2) Starting capacitors are housed in a black plastic
case and have a mfd range as opposed to a specific mfd rating on run
capacitors. Start capacitors (ratings of 70 microfared or higher) have
three voltage classifications: 125V, 250V, and 330V. Examples would be
a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V start
capacitor. Start capacitors increase motor starting torque and allow a
motor to be cycled on and off rapidly. Start capacitors are designed
for momentary use. Start capacitors stay energized long enough to
rapidly bring the motor to 3/4 of full speed and are then taken out of
Potential relays are also as important. Potential relays are
used to electronically connect and disconnect to starting capacitors
from the motor circuit (See Figure 2). Each relay has a specific
voltage rating to place the start capacitor in series with the start
winding and a specific voltage to take it out of the circuit. Each
rating is based on the electromagnetic field generated by the rotation
of the motor. The motor manufacturer studies the effect of placing in
and taking out the capacitor to increase starting torque with as little
winding flex as possible. Potential relays have four ratings; (1)
continuous coil voltage, (2) minimum pick-up voltage, (3) maximum
pick-up voltage, and (4) drop out voltage. A potential relay is
difficult to check and should always be replaced when a start capacitor
is replaced. The exact size designed for that particular motor must be
reinstalled. The potential relay must also be replaced if contacts are
found to be open.
For Informational purpose only. Consult with your equipment manufacturer if you are not sure in changing a capacitor or any other part.