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8 Fluorescent Lamp Ballasts
A ballast is a device used with a gas discharge lamp to provide the necessary starting and operating electrical conditions.
- The ballast supplies the right voltage to start and operate the lamp.
- The ballast limits current to a gas discharge lamp during operation – the resistance of a gas discharge lamp becomes negligible once the arc has been struck.
- The ballast prevents any voltage or current fluctuations caused by the arc discharge from reflecting into the line circuit.
- The ballast compensates for the low power factor characteristic of the arc discharge.
- A simple standard ballast is a core and coil assembly.
- The core is made of laminated transformer steel.
- The coil consists of copper or aluminum wire which is wound around the core.
- The core–coil assembly is impregnated with a nonconductor to provide electrical insulation and aid in heat dissipation.
- Capacitors may be included in the ballast circuit to assist in providing sufficient voltage, start the lamp, and/or correct power factor.
- Some ballasts are housed inside the lighting fixture.
Simple Ballast Illustrations
Typical Wiring Diagrams
- A ballast, as an electric circuit, has electric energy losses.
- Ballast losses are obtained from catalogues of ballast manufacturers.
- Energy efficient ballasts have lower losses.
- Basic types of ballasts based on ballast construction and efficiency are:
- energy efficient ballasts (core–coil magnetic);
- electronic ballasts (solid–state);
- standard magnetic ballast (core–coil design).
- Ballasts are also classified by the type and function of their electric circuit.
- Note that electro–magnetic fluorescent ballasts are gradually being removed from the market place by energy regulations.
- Each ballast is designed to be used with a specific type and size (wattage) of lamp.
- The lamp type and size compatible with the ballast are listed on the ballast label.
- Ballasts should meet ANSI (American National Standards Institute) specifications for proper lamp performance. The Canadian standard for ballast efficiency is CAN/CSA–C654–M91 Fluorescent Lamp Ballast Efficacy Measurements.
- The CBMA (Certified Ballast Manufacturers Association) label indicates that the ballast has been tested and meets ANSI specifications.
- The UL (Underwriters Laboratories ) label indicates that the ballast has been tested and meets UL safety criteria (US standard) as well as the Canadian CAN/CSA–C654–M91 criteria.
- The CSA (Canadian Standards Association) label indicates that the ballast has been tested and meets CSA safety criteria.
- Under the North American Free Trade Agreement, both UL CSA can certify electrical products for sale in both countries.
- The NEC (US National Electrical Code) and the Canadian Electrical Code require that all indoor ballasts must be thermally protected.
- This is accomplished by a thermal switch in the ballast which turns power off above a maximum temperature (1050°C approximately).
- Ballasts meeting this standard for protection are designated Class P.
- A cycling ballast, which turns power off and on, indicates an overheating problem.
- All core–coil ballasts produce a sound commonly described as a “hum”.
- Manufacturers give the ballasts a sound rating from A to F.
- An A ballast produces the least hum, and should be used in quiet areas (offices, homes).
- An F ballast produces the most audible hum, and may be used in places where noise is acceptable (factories, outdoors).
- Most ballasts are designed for about 50,000 hours under standard conditions.
- If ballast and lamp heat is not dissipated properly ballast life is reduced.
- An 8–10°Ca increase over rated temperature on the case will cut ballast life in half.
- Ballasts are rated typically for 75°C. 90°C ballasts are a special design called “Extreme Temp”. Some manufacturers list 8°C instead of 10°C.
- Similarly, a 100°C decrease will approximately double ballast life.
b. Electronic Ballasts for Gas Discharge Lamps
Typical Circuit Component Diagram
Functional Block Diagram
- Some ballasts have fewer components.
- Some ballasts have components to reduce total harmonic distortion, improve power factor and provide thermal protection.
- A rapid start ballast starts one or more gas discharge lamps by first heating the electrodes of the lamps to the proper electron emission temperature before initiating the arc.
- An instant start ballast does not preheat the electrodes but initiates the arc by a higher starting voltage.
- A modified start ballast starts the lamp in the same way as the rapid start ballast. It then reduces or cuts off the electrode heating voltage after the lamp arc has stabilized.
- Both types of ballast stabilize the arc by limiting the current to proper levels.
- Older technology (i.e., electromagnetic) ballasts are made of laminated cores wound with copper or aluminum wires; some have capacitors to control voltage and/or to correct power factor.
- Electromagnetic ballasts operate the lamps at line frequency, 60 Hz.
- Electronic ballasts for fluorescent lamps have electronic or solid–state components.
- Electronic ballasts operate the lamps at a high current frequency, typically from 25–50 kHz.
- Electronic ballasts in both the rapid start, instant start and ‘program start’ modes are available.
- Operation of rapid start lamps by instant start or modified start ballasts can potentially shorten lamp life if combined with other control technologies such as occupancy sensors. Refer to the ballast and lamp manufacturers’ data.
- In comparison with the electromagnetic ballast, the electronic ballast weighs less, operates at lower temperatures and at a lower noise level, and is more energy efficient, but costs more.
- It is essential to match the electrical characteristics of both lamps and ballasts.
- Models are available for one–lamp, two–lamp, three–lamp or four–lamp fixtures.
- Available in 120 volts, 277 volts and 347 volts. Some ballasts are now available for universal voltage, i.e., 120 V to 277 V, and less common voltages such as 240 V.
- Ballast specification is based on: number of lamps, lamp type (F32T8/841 or other) and line voltage.
- Example: two–lamp F32T8/841 120V electronic ballast
- Some electronic ballasts are dimmable.
- The efficacy of electronic ballasts is 21% to 43% better than electromagnetic ballasts.
- Total harmonic distortion (THD) indicates the strength of electromagnetic noise generated.
- Lower ballast temperature means lower electrical losses and a smaller cooling load.
Total Harmonic Distortion
- Harmonics are frequencies that are integral multiples of the fundamental frequency.
- For a 60 Hz fundamental frequency, the second harmonic is 120 Hz, and the third is 180 Hz.
- Harmonics can be present in voltage and/or current.
- Harmonics occur whenever the wave shape is distorted from a pure sine wave.
- Electric utilities supply voltage and current very close to the sinusoidal wave form.
- If the user’s load is nonlinear, drawing short pulses of current within each sine wave cycle, the sinusoidal current wave shape will be distorted and a harmonic current will be present.
- The characteristics of the nonlinear load determine the form of the distortion, the magnitude of each harmonic and the corresponding harmonic current.
- Total current is a combination of the fundamental frequency and a contribution from each of the harmonics.
- THD in the current is the root mean square (rms) of all the harmonic currents as a percentage of the fundamental current, and is defined as follows:
- IEEE Standard 519–1981 refers to the Distortion Factor (DF) which equals the THD. However, THD is the preferred term in this guide as it is more descriptive.
- Most electromagnetic ballasts have THD between 18% and 35%.
- Electronic ballasts generate less than 32% THD. Most of them are below 20%. Some are below 10%.
- Due to higher efficiency, the T8 electronic ballast system typically draws 30% less current than the conventional electromagnetic ballast system.
Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI
- EMI/RFI may cause interference with communication equipment, such as radio, TV, computer.
- Fluorescent lamps energized by electromagnetic or electronic ballasts radiate EMI directly into the air.
- EMI from the lamps may feed back to the line conductors via the ballasts.
- EMI at the electronic ballast fundamental frequency and its harmonics propagate from the ballast’s electronic circuits to the line conductors. This EMI may interfere with other electrical equipment on the same distribution network.
- EMI may radiate from the line conductor into the air.
- EMI may be radiated from the high frequency electronic components of the electronic ballast.
- In the US, electronic ballasts must comply with Federal Communications Commission Part 18, Subpart C, Class A for industrial and commercial applications, or Class B for residential applications. As yet no Canadian standard has been set.
- Power factor can be calculated by two methods:
- wattage (W), voltage (V) and current (I).
- wattage (W) and reactive power (VAR).
- If calculated correctly – the results should be the same using both methods.
- A low power factor will increase the demand component of your electricity bill for a given lighting load.
Rated Average Life
- Ballasts are designed to operate for about 50,000 hours.
- Lower ballast operating temperature reduces air–conditioning load.
- The early models had lower reliability than the present ballasts.
- When used with light sensors, dimmable electronic ballasts can reduce the lighting load by providing just the required light level, if other light sources exist.
- Similarly, an energy management and control system uses dimmable ballasts to partially shed the lighting load.