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Magic Number 2652 Explained

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

Air Conditioner Can’t Keep Up

Currently, Air Conditioning Technicians are receiving many complaints about air conditioning systems that can’t seem to keep up with the heat. Typically, technicians check refrigerant pressures, superheat, subcooling, temperature splits, and system airflow. But these don’t always identify the problem. What happens when the system is performing as it should but the customer is still unhappy with the results? One way to explain what is happening is to use the analogy of a boat with a bilge pump. Bilge is the nasty water that collects in the lower reaches of boats. Bilge pumps are designed to pump that water out. However, if the water coming into the boat exceeds the capacity of the pump to remove it, the boat will sink. There are two solutions: either get a bigger pump or fix the leaks. The house is like a leaky boat and the air conditioning system is the bilge pump. When we focus all our attention on the air conditioning unit, we are locked into the “get a bigger pump solution” while ignoring the more obvious solution: fix the leaks. If you had a boat with a big hole, would your first thought be “I need a bigger pump”? I would want to fix the leak.

Water leaks in boats are pretty easy to spot, but air leaks in ducts and houses are a bit harder to locate. That is why they are often ignored until the ship is sinking. Most of the time a properly functioning air conditioning system can overcome the added heat load from leaks in the house, leaks in the ductwork, and poorly applied insulation. However, when temperatures rise to the outdoor design point or higher, the system gets swamped with the extra heat load.

So how do we find the leaks? A blower door can be used to determine the amount of house leakage, a duct blaster can be used to identify the amount of duct leakage, and an infra-red camera can identify areas that are improperly insulated. Ideally, you would check these things during and immediately after construction so heat leaks can be addressed. However, many homes have never been checked. It makes sense to locate and correct heat leaks in the homes of customers whose systems are operating correctly but not maintaining temperature.  Doesn’t this cost the customer money? Well, yes it does initially. However, it saves money continuously thereafter. You should of course make sure the air conditioning unit is functioning correctly, but don’t ignore the effect of the leaks.

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