One of the most common faulty components on single-phase HVAC systems is operating capacitors, so much so that we sometimes refer to junior technicians as “capacitor changers.” Although capacitors may be easy to diagnose and replace, there are many things that technicians may not know.
A capacitor is a device that stores differential charges on opposing metal plates. Although capacitors can be used in circuits that boost voltage, they do not actually increase the voltage by themselves. We often see that the voltage across the capacitor is higher than the line voltage, but this is due to the back electromotive force (back electromotive force) generated by the motor, not the capacitor.
The technician noticed that the side of the power supply is connected to the C terminal or the side opposite to the running winding. Many technicians imagine that this energy “feeds” into the terminal, gets boosted or transferred, and then enters the compressor or motor through the other side. Although this may make sense, it is not actually how capacitors work.
A typical HVAC operating capacitor is just two long thin metal sheets, insulated with a very thin plastic insulation barrier, and immersed in oil to help dissipate heat. Just like the primary and secondary of a transformer, these two pieces of metal have never actually been in contact, but electrons do accumulate and discharge with each cycle of alternating current. For example, electrons gathered on the “C” side of the capacitor will never “pass” the plastic insulating barrier to the “Herm” or “Fan” side. These two forces simply attract and release the capacitor on the same side where they enter.
On a properly wired PSC (Permanent Separate Capacitor) motor, the only way the start winding can pass any current is to store and discharge the capacitor. The higher the MFD of the capacitor, the greater the stored energy and the greater the amperage of the starting winding. If the capacitor fails completely under zero capacitance, it is the same as the start winding open circuit. Next time you find that the running capacitor is malfunctioning (there is no starting capacitor), use pliers to read the amperage on the starting winding and see what I mean.
This is why an oversized capacitor can quickly damage the compressor. By increasing the current on the start winding, the compressor start winding will be more prone to early failure.
Many technicians think they must replace 370v capacitors with 370v capacitors. The rated voltage shows “must not exceed” the rated value, which means you can replace 370v with 440v, but you cannot replace 440v with 370v. This misunderstanding is so common that many capacitor manufacturers started stamping 440v capacitors with 370/440v just to eliminate confusion.
You just need to measure the current (amperes) of the motor’s start winding flowing from the capacitor and multiply it by 2652 (3183 at 60hz power, and at 50hz power), then divide that number by the voltage you measured across the capacitor .
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Bryan Orr is an HVAC and electrical contractor in Orlando, Florida. He is the founder of HVACRSchool.com and HVAC School Podcast. He has been involved in technician training for 15 years.
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Post time: Nov-25-2021