- Greetings -
The exchanges regarding starting caps vs run caps have been interesting.
Let's make certain that there are no errors that can lead one down a
faulty road.
Several on this list have stated most correctly that the motor operating
on a single phase supply needs a second phase to know which rotation
to run. My 1/3 hp lathe motor has a main winding for all running purposes
and a starting winding which is in series with the starting capacitor
and the starting switch. When the motor is turned on, the starting switch
is closed. Current flows thru the main winding and lags the voltage
waveform by some 10's of degrees. Current also flows thru the starting
capacitor and starting winding and leads the voltage waveform by some
10's of degrees. The starting winding characteristics along with the
value of capacitance cause the current to lead that in the main winding
by close to 90 degrees. Somewhere around 75% full load speed the switch
opens due to a centrifugal mechanism. This motor has a lot of starting
torque due to the favorable phase shift.
The starting capacitor electrodes have very thin layers of oxide or
metallic salt serving as insulators between the electrodes. These layers
are formed directly on the electrodes by electrolytic chemistry. They
are lossy and will explode if overheated by alternating current or
overvoltage. The advantage is that lots of capacitance can be packed
into a small package due to the thinness of the layers. In starting
service they are in use for no more than a few seconds, maybe tens of
seconds.
The condenser fan motor in the air conditioner is a capacitor run motor.
It does not need a lot of torque and can make do with an oil-filled
paper capacitor. The main winding once again has current which lags
the voltage waveform. The "starting" winding in series with the capaci-
tor has current which may be almost in phase with the voltage waveform.
Because the capacitance is small, say 6 uF, the current in this circuit
is also small and the starting torque is small, but more than enough
for a fan. Advantage is that no starting switch is needed.
The oil-filled paper capacitor is named because the insulator between
the aluminum electrodes is paper, soaked with oil to increase the dielec-
tric constant and the capacitance per unit volume. It is less lossy and
should operate cooler than the electrolytic cap in the same application.
The Elec-Trak battery charger provides a near-constant charging voltage
despite power line voltage variations by the use of a ferro-resonant trans-
former which has a capacitive load on one winding. This is a continuous
duty operation which requires a non-electrolytic capacitor. The cap is
commonly of oil-filled paper design.
The radial arm saw has two caps; an electroytic to get higher starting
torque and a "run" capacitor to make use of the second motor winding for
more horsepower. At about 75% full speed the starting cap is switched
out of the circuit, the run cap remains in series with the winding so
that the motor continues in some manner as a two-phase machine.
More than you ever wanted to know. Hope this helps. Keep on 'Traking.
Rhett George
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