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5 Capacitor Facts You Should Know | 2019-06-10 | ACHR News

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Running phase capacitors are one of the most common failures in single-phase HVAC systems, so much so that we sometimes refer to junior technicians as "capacitor changers." Although capacitors are easy to diagnose and replace, many technicians may not know this.

A capacitor is a device that stores differential charges on opposite metal plates. Although capacitors can be used in circuits that increase 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 rather than the capacitor.

The technician noticed that one side of the power supply is connected to the C terminal or the side opposite to the running winding. Many technicians believe that this power will "feed" into the terminal, be augmented or transferred, and then enter the compressor or motor from the other side. Although this may make sense, it is not actually how the capacitor works.

A typical HVAC operating capacitor has only two long thin metal sheets, insulated with an insulating layer made of very thin plastic, and immersed in oil to help dissipate heat. Just like the primary and secondary of a transformer, the two pieces of metal have never actually touched, but electrons do accumulate and discharge in 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 only attract and release the capacitor on the same side where they enter.

On a properly wired PSC (Permanent Shunt Capacitor) motor, the only way any current can flow through the start winding is to store and discharge the capacitor. The higher the MFD of the capacitor, the greater the energy stored, and the greater the amperage of the starting winding. If the capacitor fails completely at zero capacitance, it is the same as an open winding. Next time you find that the running capacitor (without starting capacitor) is malfunctioning, please use the clamp to read the amperage on the starting winding to understand what I mean.

This is why too much capacitor will 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 believe that 370v capacitors must be replaced with 370v capacitors. The rated voltage shows "not exceeding" the rated value, which means you can replace 440v with 370v, but not 370v with 440v. This misunderstanding is very common, and many capacitor manufacturers have begun to imprint 440v capacitors with 370/440v, just to eliminate confusion.

You only need to measure the current (amperes) of the motor starting winding flowing from the capacitor, then multiply it by 2652 (3183 on a 60hz power supply and 3183 on a 50hz power supply), and then divide that number by the measurement on the capacitor The voltage can be. 

Release Date: 6/10/2019

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Bryan Orr is an HVAC and electrical contractor in Orlando, Florida. He is the founder of

And the HVAC School podcast. He has been training technicians for 15 years. 

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