Saturday, June 28, 2008

Air Conditioner Hard Start Capacitor

It's that time of the year, again. If you live in a warm area like I do, air conditioning is pretty much a necessity.
It's not so bad during the day, because there are any number of things you can do to beat the heat. The problem is sleeping; it's hard to get a good night's sleep when the house still holds residual heat from the long, hot day, and it's still 80 or 90 degrees outside.
Those who live in a house with central air conditioning don't have to worry. But for those of us who must rely on a small window unit or RV air conditioner, this time of the year is when we face the question of whether it is even gonna start and cool.

Those who live offgrid especially, whether fulltiming in an RV or homesteading in a fixed location, are familiar with the question of whether their generator or inverter has enough oomph to start the air conditioner. They get harder to start with age, and generators lose power with age. Adding to the difficulties, the power from small generators is hard on the start capacitor in air conditioners, because the voltage and frequency fluctuate every time the compressor cycles. What happens is the motor windings in the compressor initially draw heavy current, which causes the generator to slow down momentarily as it tries to pick up the increased demand. Then those same windings "kick back" with a voltage spike that can be much higher than the nominal supply voltage. This kick back is damaging to the capacitor (because it is too small to run on a marginal power supply), and also to other appliances on the circuit. Ever notice how the lights dim when you turn on the air conditioner? Well, the dimming is followed by a spike that you may or may not see, but every time it happens, it damages your light bulbs and, more importantly, your electronic appliances. This damage builds up over time, shortening the life of those appliances, and of your light bulbs.

There is a device called a hard start capacitor or start booster, that significantly relieves these problems. It is simply an additional, larger capacitor and a relay. The relay is necessary because the large capacitor cannot stay in the circuit all the time; it would be constantly "fighting" the inductance of the running compressor, causing both the capacitor and the windings to overheat and wasting energy. It only needs to be in the circuit when the windings kick back, so as to absorb that voltage spike and feed it back into the windings in the proper phase, when they can use it. The way it works is, most of the time the relay is off, keeping the hard start capacitor out of the circuit. When the compressor tries to start, the relay senses the voltage spike and turns itself on momentarily, just long enough to allow the start capacitor to absorb and then return the surge, giving the compressor the boost it needs to start turning. Then the relay turns back off and waits for the next time the compressor cycles.

If these things are so great, you may ask, why don't air conditioners come from the factory with them? Simple: they can get by without them, when the power is coming from a huge power grid with virtually unlimited current capability. Why would the factory spend the extra few dollars per unit, to help the minority of buyers who run their air conditioner on generator power? Thus, it is up to us to buy and install our own hard-start capacitors. Here's where to get them, in two different sizes:

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