Motor Start and Run Capacitors
Motor Start and Run Capacitors
Motor Start and Run Capacitors
Start Capacitors
Start capacitors are responsible for increasing the starting torque of a AC motor,
which in return cycles the AC motor on and off rapidly. Start capacitors stay in the
circuit long enough for the motor to reach a determined speed (typically 75% of full
power), and then it’s taken out of the circuit by a centrifugal switch. After the starting
process AC motors work more efficiently with run capacitors.
Start capacitors are electrochemical devices consisting of compactly wound
aluminum foil separated by layers of paper, which are impregnated with a
conducting electrolyte. Etching of the foil prior to formation and winding increases
both the effective foil surface area and the capacitance per unit volume of the
finished capacitor. The entire assembly is housed in a case of moisture and oil
resistant molded plastic. Start capacitors are rated for operation in ambient
temperatures from -40°C to +65°C and at a frequency of 50Hz to 60Hz
(applications at higher frequencies are not recommended).
Start Capacitors have a fixed capacitance and voltage. They typically have a
capacitance range rated above 70uF.
The most common voltages are:
125 VAC
165 VAC
250 VAC
330 VAC
Note: Any start capacitor that is rated above 20uF is a non polarized aluminum
electrolytic capacitor with a electrolyte that is not solid. Meaning that it's only
applicable for mommentary use.
Run Capacitors
In order to run many single phase AC motors need a rotating magnetic field. A run
capacitor is responsible for powering up the second phase winding (auxiliary coil)
in an AC motor, which in return creates a rotating magnetic field that keeps the
motor running.
Run capacitors are constructed to be used continuously while the AC motor is
running, unlike start capacitors which are only in the circuit for a short period of time
just to start the motor. This is why low loss polymer capacitors are used as run
capacitors because of a longer life time and lower loss of current, opposed to
electrolytic capacitors which are ideal for momentary use.
Run capacitors come in two different styles a wet style and a dry style. The wet style
run capacitor is filled with a liquid that prevents the capacitor from overheating. The
dry style comes with the same dielectric but it's not filled with a liquid which makes
this style weigh considerably less than the wet style. Currently most run capacitors
come with a film polypropylene or polyester dielectric.
Run capacitors have a fixed capacitance and voltage. The capacitance ranges
from 1.5uF to 100uF.
The most common voltages are:
330 VAC
370 VAC
440 VAC
660 VAC
Dual run capacitors are run capacitors that are able to power up two electric
motors instead of one. This capacitor basically saves you space when you utilize it
because it combines two capacitors in one case. Dual run capacitors typically have
at least three leads or terminals, which are labeled “C”, “FAN”, and “HERM”.
Common
FAN
HERMetically sealed compressor
They are rated with two capacitance values, allowing the capacitor to be used for
two different applications at the same time. An example would be 20uF + 5uF at
370VAC. Dual run capacitors are often found in air conditioners. They are used to
supply power to both the fan motor and the compressor motor.
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