September 2013 Electrical Apparatus

September 2013 Electrical Apparatus

This is a summary of the Electrical Apparatus September 2013 featured technical article,  by Richard L. Nailen, P.E.  

One of the earliest and simplest electrical devices, the capacitor is nevertheless subject to misuse and abuse in many ways that can result in failure to perform as expected or in premature failure. Power capacitors—so-called because of their use in power distribution networks rather than in electronic apparatus—serve several purposes. Best known as a means of improving inductive circuit power factor, they also act to minimize voltage drop during large motor starting, and as transient voltage surge protection at rotating machine terminals.

Long service requires operation of capacitors within temperature limits. Mounting them out of the way near a ceiling, without ventilation, or outdoors in hot climates, can be troublesome. To clear internal faults, capacitors should be protected by current-limiting fuses, monitored by blown fuse indicators or periodic tests. Fuse ratings must be coordinated with the capacitor tank rupture characteristics.

Several maintenance checks are recommended at six-month to one-year intervals. Infrared thermography scans of energized units are important to verify terminal connection integrity. To prevent pressure generated by an internal fault from rupturing the case, capacitors often include an internal switch to open the circuit when pressure rises. Hence, fuse integrity alone does not assure that the capacitor remains connected. Current measurements are needed, particularly to reveal any unbalance between phases in a polyphase bank.

For off-line measurements, all capacitor terminals should be short-circuited together and solidly grounded. Standards require that capacitors be equipped with internal discharge resistors capable of dissipating the stored energy soon after de-energization. The maximum allowable residual voltage is 50 in the U.S., up to 75 elsewhere. However, that alone does not guarantee safe handling. Discharge resistors are physically small. Their lead wires can become disconnected. Some authorities recommend periodic measurement of discharge resistance. The ohmic value chosen by the manufacturer will vary with connection of a three-phase unit as well as the voltage and kvar rating.

Insulation resistance can then be measured between terminals and the enclosing case, checking the values against manufacturer limits. Capacitance itself may be measured using appropriate metering.

Simple as they are in theory, capacitors do require attention that is all too often lacking in industrial installations.

So while the Motor Primer offers many useful insights, readers will need to look elsewhere for information about a number of subjects, including vertical high-thrust motors, belt drives, inverter usage, hazardous area requirements, starting methods, ventilation requirements, and enclosure variations.

To order a back issue with the full article, “Installation and maintenance determine capacitor longevity” call 312-321-9440 or visit our online webstore.

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