Voltage Variations And Arc Discharge Lamps

Mar 1, 1999 12:00 PM, By Joseph R. Knisley, Senior Editorial Consultant

How do line voltage excursions affect HID and fluorescent ballast regulation and lamp operation? Are the effects different for the two lamp/ballast systems?

Sure, manufacturers design their arc discharge lamps to perform best when operated on a line voltage within the range of ballast design (indicated on the ballast nameplate). But what about line voltage excursions? How do they affect HID and fluorescent lamps/ballast systems? Will the effects differ for each type of lamp and/or ballast? Can you troubleshoot problems based on behavioral symptoms? Read on and find out.

Fluorescent ballast behavior during voltage excursions. If your supply voltage is too low for a fluorescent lamp, it will have difficulty starting, especially if the humidity is high. This condition can cause the lamp to flash off and on without starting, which may slowly deteriorate the lamp's electrodes. Also, because of the reduced energy level in the mercury arc, you'll get less visible light.

What about too high a voltage? With a preheat or rapid start lamp, it will sometimes operate as an instant start lamp. As a result, the lamp's cathode coating will deteriorate because of the high-voltage pulses delivered to the cathode. Lumen maintenance will also suffer. At the same time, higher than normal lamp operating currents can cause premature lamp failure and overheating of ballasts.

A temporary voltage drop (sag) on the power system, even for a few cycles, will cause a fluorescent lamp to "drop out." If the voltage drops below 80% of the rated level of a rapid-start ballast (and most other ballasts), the arc stream within the lamp becomes unstable and extinguishes. Fortunately, the lamp will restrike almost immediately upon restoration of full voltage.

New versus old fluorescent ballast systems. Older electronic fluorescent ballasts can be more sensitive than their magnetic counterparts to a surge or a swell. These surges (and poor heat sinking of transistors) were main causes of electronic ballast failures in the early '80s. Designs now employ filters and voltage limiters at their input to protect internal transistors and rectifiers from electrical line surges.

Manufacturers design today's units with tight regulation, so there's virtually no change in light output over the entire 610% voltage input range to the ballast. This regulation assures the ballast maintains specified lighting levels.

HID ballast behavior during voltage excursions. With reactor ballasts, a +/- 5% change in line volts causes a variation of +/- 10 % change in lamps watts. You can use this type of ballast only on specified line voltages-usually 240V or 277V for 100W through 400W lamps; and 480V for 700W and 1000W lamps. Make sure you use this ballast only on circuits where you're certain to maintain +/- 5% of line volts. With regulator ballasts, +/- 13% change in line voltage results in 2% or 3% change in lamp watts. You can install it on any circuit without concern for voltage variations or sags. With auto-regulator ballasts, regulation is high: Line voltage changes of +/- 10% can result in changes in lamp watts of about 5%.

With lag reactor or high-reactance autotransformer ballasts, lamp wattage regulation is poor: A 5% change in line voltage can result in a 12% change in lamp wattage.

HID system options for voltage interruptions. Any interruption in the power supply, or even a serious voltage dip for a few cycles, causes any HID lamp to go out. It takes minutes before the lamp will restart, because the arc tube has to cool, and the internal vapor pressure has to decrease to the point when the arc can restrike. This restrike time, together with the warm-up period, means an 8 min to 15 min waiting period before full light returns.

To address this problem, the NEC requires some form of a backup lighting system immediately available in case you experience power interruptions with HID lighting systems.

Auxiliary tungsten halogen lamp option. You can order an industrial HID luminaire with an integral tungsten halogen lamp. Here, you provide a separate power source for the halogen lamp (which usually operates at 120VAC), such as an emergency power source. Whenever the light fixture ballast senses a loss of lamp current, the halogen source turns on. A time-delay following return of normal power maintains the tungsten lamp illumination until the HID lamp returns to full output.

Hot restrike option. A ballast component called a hot restrike device, which provides a higher-than-normal ignition pulse can be specified for some single- and double-ended MH lamps. The hot restrike feature delivers a high-voltage pulse to one of the electrodes, similar to the starting method for the HPS lamp. Thus, a hot restrike accessory can restart the MH lamp's arc almost immediately following a momentary voltage dip. You can find this feature on the high wattage (generally 1000W or 1500W) MH lamps at sports arenas and other high occupancy facilities.

Double arc-tube HPS lamp. To provide almost immediate light output following a voltage dip or momentary outage on a HPS lighting system, you could use a special lamp with a double arc-tube. When you energize this lamp, only one of its arc tubes ignites, and the parallel arc tube stays in a standby mode. When a loss of power of sufficient duration extinguishes the first arc tube, the second arc tube immediately ignites at partial output as soon as the power is reestablished. It will quickly come up to full brightness. With about a 50% cost premium over a standard HPS lamp, the double arc tube lamp has applications at prisons and related facilities.

Sidebar: HID Lamp and Ballast System Basics

There are three high intensity discharge (HID) lamps: mercury vapor (MV), metal halide (MH), and high-pressure sodium (HPS). Their starting method is completely different from the low intensity discharge (fluorescent) lamp.

Lamps. MV and MH lamps have a separate staring electrode, along with the two operating electrodes of the arc tube.

When you energize an HID ballast, it applies voltage between the starting electrode and the adjacent operating electrode, creating an emission of electrons that sets up a local glow. This causes the mercury within the lamp to slowly vaporize, allowing the arc to strike between the two operating electrodes.

HPS lamps use a small diameter arc tube that doesn't have room for a separate starting electrode. Instead, the ballast creates a high pulse of voltage every half-cycle with its integral special starting circuit. Peaking at 2500V and lasting one microsecond, these high-voltage pulses ionize the xenon gas, allowing the arc to strike between electrodes. Once it strikes, the starting pulses cease.

Even though an MH lamp is similar to an MV lamp, there are two differences: An MH lamp needs a higher ballast open-circuit voltage to start the arc as well as a higher re-ignition voltage. During warm-up, low conduction of the arc occurs. This means the re-ignition needed to establish/reestablish an arc after each half cycle is greater than what the voltage standard mercury ballast normally supplies.

Ballasts. The reactor ballast is the simplest magnetic ballast, consisting of a choke coil wound on an iron core (to limit the current).

A lag ballast combines a reactor and an autotransformer on a single winding. It's generally used on 120V circuits. The lag ballast doesn't have a capacitor added to correct the 50% power factor. However, it does have the same features as the reactor ballast. You can use it when the line voltage is lower or higher than the required starting voltage.

A regulator ballast (for both MV and MH lamps) consists of a capacitor in series with a lamp and either in series or parallel with two separate windings.

An auto-regulator, or constant wattage autotransformer ballast, combines a regulator circuit with an autotransformer. Part of the primary winding couples with the secondary winding, reducing ballast size. This is the most popular because it offers the best compromise between cost and performance.

The MH ballast has a peak-lead or lead peaked design. It's essentially the same as a MV auto-regulator type, the difference being the peak-lead ballast has one or more core slots on the secondary winding.

The HPS ballast has an auxiliary starting circuit to begin arc conduction. The circuit provides a high-voltage pulse between 2500V to 3000 V. HPS ballast types are: reactor, regulator, and auto-regulator.