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Single Phase Theory II

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Capacitors

Capacitors are devices that oppose a change of voltage.

Three factors that determine capacitance of a capacitor
1) this surface area of the plates( added area = added capacity)
2) the distance between the plates
3) the type of dielectric used( insulating material)

Current can only flow during the period of time that a capacitor is either charging or discharging.

A farad, F, has a measurement of 1 when a change of one Volt across its plates results in a movement of one coulomb. Microfarad

Polarized capacitors can be D C only
Non polarized capacitors can be a AC/DC

Capacitive reactance = XC
Reactive power = VARS
Apparent power = VA
True power = Watts or P

Click here to get the following table in PDF format





RLC Circuits

The voltage applied to all legs of an RLC circuit is equal. The same.

The current flow in the inductive leg will lag the voltage by 90 degrees.
The current flow in the resistive leg will be in phase with the voltage.
The current flow in the capacitive leg will be led by the voltage by 90 degrees.
Angle theta for the circuit is determined by the amounts of inductance and capacitance.




Power Factor

Motor power factor can be corrected by connecting capacitance in parallel with the motor. The same amount of capacitive VARS must be connected as inductive VARS. It is common practice to correct motor power factor to about 95%. To correct to unity, or 100 percent, is considered not practical. Please see three phase theory for a graphical demonstration of power factor.