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Don't Let Surges and Sags Get You Down

Nov 1, 2000 12:00 PM, By Stan Turkel

Misdiagnosing routine power quality problems due to the effects of voltage surges or sags is easy to do, if you're unclear about the causes.

A large-scale insurance office has several computers malfunctioning. It seems they lock up in the middle of operations - most often when they start printing to the central laser printer. Maintenance checks out the problem by connecting a meter to the system and monitoring the situation for 24 hr. After concluding that surges on the system are the culprit, maintenance personnel orders surge suppressors to remedy the situation. Two weeks after the installation of the suppressors, the computers are still locking up. What else could be the problem?

Finally, someone decides to use a scopemeter to analyze the circuit, determining that the office copier is running off the same circuit and creating a voltage sag. The internal heater of the copier cycles on every minute for 1 sec to 2 sec, which draws about 25A of current. The laser engine is also kicking in on occasion, further reducing the system voltage. This compounds the problem. Unfortunately, all the suppressors in the world wouldn't have helped in this condition.

You can attribute many problems in plant operations to voltage sags or dips that are not destructive in nature - but are problematic. You need a clear understanding of surges and sags to isolate these types of problems. Let's take a closer look at voltage sags and surges and see how they affect other electrical equipment on your system.

A voltage sag (dip) is more than a 10% decrease below the normal rated line voltage - lasting anywhere from .5 cycles, up to 1 min in duration. If this condition lasts longer than a minute, it's an under voltage condition. We don't consider a complete loss (0V) to be a voltage sag. A voltage surge (swell) is where we exceed the normal voltage by 10% or greater, for a period of .5 cycles and less than 1 min. This condition typically occurs with the cyclic removal and addition of loads in our building. It's usually the source of many problems.

All equipment operates within a specific voltage range. Problems develop when you begin to leave the safe region of operation specified for that equipment. Industry standards suggest a deviation of 510% is acceptable for most equipment. However, you must examine this on a case-by-case basis, as installations and equipment are subject to individual circumstances.

It's easy to misdiagnose many routine problems in and around the plant, tracing the problems back to voltage sags - especially when this condition is cyclic in nature. Here are some common problems associated with voltage sags.

Lighting problems. Voltage sags below 80% of rated voltage can extinguish most HID lamps, requiring a restart that can take several minutes. Decreased voltage to fluorescent fixtures also has an adverse effect on the life and efficiency of the lamps. These problems begin with a voltage decrease as little as 7% to 8%, well within the acceptable 10% range.

Solenoid problems. It's common practice to replace solenoid valves for sticking or hanging up during operation. Unfortunately, the real problem with these units is probably due to an improper voltage source. A reduced voltage at the wrong time will prevent the coil from developing the needed magnetic force to operate the spool inside the valve, resulting in erratic operation. Many ice cube relays suffer premature failure under the same conditions, as they attempt to keep their contacts closed under load.

Motor problems. Motors can also suffer from voltage sags. The most common effect of voltage sags with motors is reduced torque capabilities and additional heating. A motor's current increases proportionately to the amount of reduced voltage. Thus, a 10% voltage drop will increase full load current by 10%. The same 10% voltage reduction can reduce motor torque by as much as 22%!

There's no simple solution to solving your power quality dilemmas. In some cases, common sense load distribution is all you need to solve your problem. Other times, voltage stabilizers or regulators may be the answer.

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© 2012 Penton Business Media, Inc.

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