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Understanding Electrolytic Capacitors in UPS Systems

Electrolytic capacitors are a vital component in virtually all electronics. The most common application is filtering rectified AC input voltage for power supplies. Consequently, Uninterruptible Power Supply (UPS) systems use many large electrolytic capacitors. Our engineers receive a lot of questions about electrolytic capacitors and it is always a hot topic during our training courses. Here are some of the most commonly asked questions we receive:   

What purpose does electrolytic capacitors serve in the UPS system? All AMETEK Solidstate Controls’ UPS systems rectify the AC input voltage (usually 480 volts 3 phase) to DC. The rectified DC maintains the charge on the batteries and feeds DC to the inverter. The inverter converts the DC back to AC to feed the load. The electrolytic capacitors in a UPS smooth the rectified DC voltage. Since batteries have similar properties to capacitors, a battery will also filter the rectified voltage if there are inadequate or failing capacitors. This results in premature battery failure.

What makes an electrolytic capacitor? Electrolytic capacitors consist of an anode (positive) foil separated with a thin paper, that is saturated with an electrolyte, and a cathode (negative) foil. The capacitance is between the anode and the electrolyte. The cathode connects to the electrolyte. Capacitance is determined by the surface area of the foil. The foil is etched to increase the surface area. The voltage rating is determined by the thickness of the paper, meaning a higher voltage equals thicker paper which reduces capacitance.

Internal Construction of an Electrolytic Capacitor

What is the purpose of an electrolytic capacitor? Although filtering of the rectified voltage is the primary purpose of electrolytic capacitors, they also absorb voltage spikes inherent from the inverter which switches the DC off and on at high frequencies. This is critical to long term inverter reliability.

How does an electrolytic capacitor age? The natural aging process of all electrolytic capacitors, large and small, is that the electrolyte dries out. The rate of which this occurs is determined by the operating temperature and the ripple current. Ripple current is the constant charge and discharge of the capacitor each 60 Hz cycle.

What happens when an electrolytic capacitor fails? Internal failure results in pressure inside the can. Large electrolytic capacitors have a pressure relief valve in the top of the can to release the pressure to avoid explosion. Once the valve opens it does not re-close and the capacitor quickly dries out and becomes an open circuit.

When should electrolytic capacitors be replaced? Whether the capacitor is the large can type found in a UPS or a small capacitor on a circuit board, life expectancy is still determined by ambient temperature and ripple current.Most manufacturers have a preventive maintenance schedule to replace capacitors before they cause failure. Some manufacturers have a schedule in the 6 – 8 year range.AMETEK Solidstate Controls’ systems are conservatively rated for a 40°C (105°F) environment, with a 10-year replacement schedule. Adequate numbers of capacitors are required to minimize the ripple current and meet the 10-year design life. This increases the cost but supports long term reliability, which is one of our primary goals when designing our UPS systems.

Electrolytic Capacitor Failure

If you would like to discuss electrolytic capacitors more in-depth, join us at one of our upcoming Uninterruptible Power Supply System training classes. If you have an immediate need with your capacitors, contact our service department for assistance.

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