Voltage abnormalities can affect facility electrical systems, and understanding them is important to reducing the cost of downtime. The many types of voltage deviations can affect many types of facilities, and each has a different cause and solution.

High Voltage

Motors perform best at their rated voltage. While motors can tolerate some voltage above their design values, high voltage can cause high current as it attempts to magnetize the steel beyond the saturation point. This is especially true in small motors. Single-phase motors tend to be more sensitive to overvoltage than three-phase motors. Premium efficiency (Super-E) motors are less sensitive to overvoltage than standard efficiency motors.

A 440-volt (V) motor with a voltage tolerance band of 10% can typically tolerate normal line voltage variations from 396 to 484 V, which does not mean that the motor can be operated on any constant voltage in this range. Current above the nameplate rating overheats the motor and shortens its life.

Variations in line voltage affect the light output of incandescent lamps. Premature incandescent lamp failure is often due to higher than normal line voltage, but could also be caused by vibration. In cases where there is high voltage, the use of 130-V rated lamps is common practice.

Low Voltage

On lightly loaded motors with easy-to-start loads, low voltage has little effect. Low voltage can reduce the starting ability of motors with heavy loads and shorten their life due to overheating. Motors that drive fixed loads draw a fixed amount of power from the line. When operated at a voltage below their nameplate rating, these draw more current to compensate for the low voltage to provide the same amount of power. Under heavy mechanical loads, the current will increase even more. When the motor current exceeds the nameplate rating, heat will build up in the motor that if sustained can damage the motor insulation.

Low voltage reduces the power needed by incandescent lamps and their light output. Voltage variations also affect fluorescent and high-intensity discharge (HID) luminaires, such as high-pressure sodium (HPS), metal halide, and mercury-vapor systems by affecting how their ballasts regulate the light output in response to line variations.

Usually sensitive electronic loads use a power supply that converts ac voltage is to pulsed dc using a bridge rectifier. The pulsing dc is stored in a filter capacitor that supplies constant dc for the rest of the power supply, including voltage regulators.

When the dc supplied by the filter capacitor falls below a critical level, the regulators can’t deliver the design voltage and the system fails. Since the filter capacitor stores energy, there is an inherent ability to ride through short sags, but deep sags that last long enough will drop the system dc voltage below the critical level.

Voltage Sags

Voltage sags (or dips) are brief voltage reductions—usually lasting anywhere from a cycle to a second—that are caused by sudden increases in current when internal loads, such as large motors and electric heaters are turned on, or circuit faults that may be caused by loose or defective wiring. Do not confuse sags with voltage drop. Voltage drop is a static value that results from line resistance. Voltage sags are dynamic values that can sometimes be caused externally on the utility electric power system by events such as faults on distant circuits or voltage regulator failures.

Sags do not usually interrupt the operation of fluorescent lighting systems, motors, or electric heating equipment. HID lamps can extinguish from a sag of about one cycle and then need to cool down before they can re-strike. Some of the new electronic HID ballasts have sufficient internal energy storage to ride through brief sags in the supply voltage.

Eliminating voltage sags requires an understanding of the source of the sag, because the wrong solution can make the problem worse. For example, installing a ferroresonant transformer as a voltage regulator or an uninterruptible power supply (UPS) upstream from a motor load that is causing the sags will not fix it; it will only make it worse.

Electrical equipment can also fail because of protection relay trips. Voltage sags on three-phase systems often affect one or two phases more than on the remaining phases. An unbalance relay that shuts down the system when the voltage exceeds a preset threshold is a good solution, as three-phase motors and transformers can be damaged by a continuous voltage unbalance.

A voltage sag that causes a 25-50% unbalance of only a second or two will not overheat motors or transformers. Unbalance relays with short delays can cause unnecessary shutdowns when there are minor sags.

Failures can also be caused by voltage monitor relays. These circuit monitors protect against insufficient voltage. When the equipment voltage requirements are not matched with the voltage monitor relays, circuit monitors can shut off power to equipment, although the equipment may be able to operate without a problem.

There are several ways to increase voltage sag immunity. These fixes can be found at: http://powerstandards.com/tutorials/immunity.htm

Voltage Adjustments

Swells are brief voltage increases over the same time period as sags that can be caused by abrupt load reductions on circuits with no voltage regulation or by loose neutral connections. They are usually not as important as voltage sags.

The electrical equipment in a facility will operate better if the voltage is optimized to a value that is close to the equipment ratings. In older facilities where there is a mix of both old 220/440-V and new 230/460-V motors, a compromise value close to 225 or 450-V can usually be obtained by changing the taps on power transformers.

Locating Problems

Many facilities have permanently installed power monitoring/metering equipment that comes with power quality measurement capability. These meters can be used to record and track voltage abnormalities, help locate the problems, and fix them. When thresholds are not properly set, there will be too much data, and not all the recorded events will be important enough to require attention, only those that can cause problems.

Voltage abnormalities on electrical distribution systems can cause equipment problems. Finding and reducing or eliminating them will allow equipment to operate more efficiently for a longer period of time and will reduce downtime.



Reprints of this article are available by contacting Jill DeVries at devriesj@bnpmedia.com or at 248- 244-1726.

Test Your Knowledge

1. Voltage sags are caused by
   1. abrupt increases in current
   2. abrupt decreases in current
   3. neither a nor b


2. Sags
   1. seldom cause equipment problems
   2. can cause equipment problems
   3. are not usually caused by internal loads


3. Low line voltage
   1. decreases motor current and there is no problem
   2. increases motor current that can cause motor overheating
   3. tends to saturate the motor magnetics


4. Voltage sags are more important than swells.
   • true
   • false
5. High line voltage can
   1. shorten motor life
   2. shorten incandescent lamp life
   3. both a and b

Answers

1. Voltage sags are caused by
   a. abrupt increases in current


2. Sags
   b. can cause equipment problems


3. Low line voltage
   b. increases motor current that can cause motor overheating


4. Voltage sags are more important than swells.
   true
5. High line voltage can
   c. both a and b