Testing a capacitor under load means testing it while it is in an operating circuit. To do this you measure the operating capacitor amp draw and voltage and then apply them to the formula
Why does this work and where does the 2652 come from? To answer these questions, we need to understand what a capacitor does in an AC motor circuit.
What Does a Capacitor Do?
A run capacitor’s job is to add enough capacitive reactance to offset the inductive reactance of the winding it is in series with. Current in an inductive (magnetic) load lags the voltage. This means that the current peaks AFTER the voltage. Since the current and voltage are out of phase with each other, they don’t work together, causing inefficiency. Adding a capacitor in series with an inductive (magnetic) load helps correct this because capacitors cause the current to peak BEFORE the voltage. The amount of capacitive reactance needed depends upon the inductive reactance of the motor.
Like resistance, capacitive reactance is measured in ohms. The capacitive reactance produced by a particular capacitor varies with both the frequency of the AC current and the microfarad capacity of the capacitor. Higher frequencies and higher microfarad capacity both decrease capacitive reactance. The formula is
This means that capacitive reactance is equal to the inverse of the product of 2 x pi x frequency x Farad rating. Through the magic of algebra we know that we can swap the XC (capacitive reactance) and C (capacitance) terms to get our formula for capacitance. That gives us the formula
This can be rewritten as
In this formula, 2π is a mathematical expression for a cycle. Recalling that the circumference of a circle is twice the radius times π, the expression 2π represents a complete turn of a circle if we are not concerned with the circle’s radius. Frequency is represented by f, which is always 60 cycles in North America. Together, 1/2πf calculates the effect of frequency on capacitive reactance.
works out to 0.00265258 for 60 cycle power. This would produce an answer in Farads, but we normally work with microfarads. Multiplying this by 1000 to get 2652.58 produces an answer in microfarads. This is usually rounded to 2652.
What about the capacitive reactance, XC? Remember that capacitive reactance is measured in ohms and that ohms can be found by dividing volts by amps. So we can substitute the capacitor voltage divided by the capacitor amps for the capacitive reactance. However, since 1/XC is the inverse of capacitive reactance, the fraction is flipped to perform the multiplication, placing amps on top. Together the two terms become
