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Run Capacitor

What is a run capacitor?

Much like a battery, a capacitor is an electrical device designed to store an electric charge. However, they are different in the way they store energy. While a capacitor cannot store as much energy as a battery, it is capable of charging and releasing energy significantly faster. This characteristic is decidedly useful in the context of HVAC systems, as a capacitor can provide the energy needed to activate a cooling or heating cycle, and ensure that the system is administered with sufficient power for continuous optimal operation.

There are two different types of capacitors: start capacitors and run capacitors. Although the concept behind both devices is similar in the way they charge energy, they serve different purposes, and cannot be interchanged. A run capacitor is designed to continuously regulate the current or phase shift to the windings of a motor or engine with the aim of optimizing its energetic efficiency, and overall performance. On the other hand, start capacitors are not designed for continuous duty, but rather to provide a jolt of built-up energy as required by the system. In the context of HVAC systems, run capacitors play an important role as they enable the storage of energy for later use. They can therefore ensure that a cooling or heating system is always supplied with the adequate amount of power for continuous optimal operation.

How does a run capacitor work?

The purpose of a run capacitor is to accumulate an energetic charge from its source and store it, and release it whenever it is required by the circuit. Run capacitors create a charge, or current to voltage lag, in the detached start windings of a motor or engine. In this way, run capacitors can ensure that a system is continuously provided with the sufficient power to operate optimally. Therefore, they are constantly energized while the system is running. Since they are designed for continuous duty, one of the main differences with start capacitors is that run capacitors have significantly lower failure rates than their counterparts, and a much lower capacitance value.

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How many types of run capacitors exist?

It is important to be aware of the different types of run capacitors, as according to the specifications of a HVAC, a specific run capacitor will be needed. Generally, run capacitors are rated by their voltage classification and microfarad (uF) rating. There are usually three main voltage classifications for run capacitors, of 250, 370 and 440 V. The range for microfarad rating can vary from 1.5 to 100 µ.

How to test a run capacitor?

Testing a run capacitor is of vital importance to make sure that a HVAC is operating efficiently and without energy losses. In addition, the state of a run capacitor can have, to a large extent, a significant impact on the life expectancy of the system motor. For instance, a run capacitor in poor condition can cause the running amperage to be higher than it should be, therefore heating up the motor more than necessary and with inefficient energy outflow. The way to test if a run capacitor is operating properly is by means of a tool called a multimeter, multitester or VOM. It is a useful tool as multiple types of measurements can be made with the same device: current, voltage and resistance.

All you have to do is attach the multimeter cables to the capacitor terminals and make sure the MFD rating displayed on the multimeter is equal to the capacitance value indicated by the manufacturer. If the MFD rating displayed is lower, it means that the capacitor is failing and needs maintenance or replacement.