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HVAC School - For Techs, By Techs

Nov 28, 2023

In this short episode, Bryan explains the fundamentals of capacitance, focusing on the unit of measure: farads, including micro and pico.

Farads are named after scientist Michael Faraday and measure capacitance; one farad represents the capacitance of a capacitor in which one coulomb of charge causes a potential difference of one volt across the plates. Farads measure the storage of electrical energy and indicate the capacitor's ability to create a phase shift.

Since farads are large units, our capacitors are rated in microfarads (1/1,000,000 farads). Bigger capacitors have higher microfarad ratings and store more charge.

Capacitors create a phase shift and limit current on the start or auxiliary winding. (You'll read less current across the start winding than the run winding or common when a run capacitor is in the circuit.) The start winding helps get a single-phase motor up and running (but it isn't present on all motors). Three-phase power has three windings, and it has three sine waves 120 degrees out of phase with each other, all of which can apply directional force. A single-phase motor has two windings and only one sine wave, so it doesn't have that phase difference, making it difficult to start a motor. Capacitors charge and discharge at a different point of the sine wave, causing a phase shift.

A picofarad is 1/1,000,000,000,000 farad, which is smaller than the microfarads we use. However, our meters can auto-range into the picofarad scale if they read a very weak capacitor. You'll have to make sure your meter is reading in the microfarad scale, not the picofarad scale.


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