Electrical characteristics of lamps
From Electrical Installation Guide
Incandescent lamps with direct power supply
Due to the very high temperature of the filament during operation (up to 2,500 °C), its resistance varies greatly depending on whether the lamp is on or off. As the cold resistance is low, a current peak occurs on ignition that can reach 10 to 15 times the nominal current for a few milliseconds or even several milliseconds.
This constraint affects both ordinary lamps and halogen lamps: it imposes a reduction in the maximum number of lamps that can be powered by devices such as remote-control switches, modular contactors and relays for busbar trunking.
Extra Low Voltage (ELV) halogen lamps
- Some low-power halogen lamps are supplied with ELV 12 or 24 V, via a transformer or an electronic converter. With a transformer, the magnetization phenomenon combines with the filament resistance variation phenomenon at switch-on. The inrush current can reach 50 to 75 times the nominal current for a few milliseconds. The use of dimmer switches placed upstream significantly reduces this constraint.
- Electronic converters, with the same power rating, are more expensive than solutions with a transformer. This commercial handicap is compensated by a greater ease of installation since their low heat dissipation means they can be fixed on a flammable support. Moreover, they usually have built-in thermal protection.
New ELV halogen lamps are now available with a transformer integrated in their base. They can be supplied directly from the LV line supply and can replace normal lamps without any special adaptation.
Dimming for incandescent lamps
This can be obtained by varying the voltage applied to the lamp
This voltage variation is usually performed by a device such as a Triac dimmer switch, by varying its firing angle in the line voltage period. The wave form of the voltage applied to the lamp is illustrated in Figure N38a. This technique known as “cut-on control” is suitable for supplying power to resistive or inductive circuits.
Another technique suitable for supplying power to capacitive circuits has been developed with MOS or IGBT electronic components. This techniques varies the voltage by blocking the current before the end of the half-period (see Fig. N38b) and is known as “cut-off control”.
|[a] cut-on control||[b] cut-off control|
Fig. N38: Shape of the voltage supplied by a light dimmer at 50% of maximum voltage with the following techniques:
Switching on the lamp gradually can also reduce, or even eliminate, the current peak on ignition.
As the lamp current is distorted by the electronic switching, harmonic currents are produced. The 3rd harmonic order is predominant, and the percentage of 3rd harmonic current related to the maximum fundamental current (at maximum power) is represented on Figure N39.
Note that in practice, the power applied to the lamp by a dimmer switch can only vary in the range between 15 and 85% of the maximum power of the lamp.
According to IEC standard 61000-3-2 setting harmonic emission limits for electric or electronic systems with current ≤ 16 A, the following arrangements apply:
- Independent dimmers for incandescent lamps with a rated power less than or equal to 1 kW have no limits applied
- Otherwise, or for incandescent lighting equipment with built-in dimmer or dimmer built in an enclosure, the maximum permissible 3rd harmonic current is equal to 2.30 A
Fluorescent and discharge lamps with magnetic ballast
Fluorescent tubes and discharge lamps require the intensity of the arc to be limited, and this function is fulfilled by a choke (or magnetic ballast) placed in series with the bulb itself (see Fig. N40).
This arrangement is most commonly used in domestic applications with a limited number of tubes. No particular constraint applies to the switches.
Dimmer switches are not compatible with magnetic ballasts: the cancellation of the voltage for a fraction of the period interrupts the discharge and totally extinguishes the lamp
The starter has a dual function: preheating the tube electrodes, and then generating an overvoltage to ignite the tube. This overvoltage is generated by the opening of a contact (controlled by a thermal switch) which interrupts the current circulating in the magnetic ballast.
During operation of the starter (approx. 1 s), the current drawn by the luminaire is approximately twice the nominal current.
Since the current drawn by the tube and ballast assembly is essentially inductive, the power factor is very low (on average between 0.4 and 0.5). In installations consisting of a large number of tubes, it is necessary to provide compensation to improve the power factor.
For large lighting installations, centralized compensation with capacitor banks is a possible solution, but more often this compensation is included at the level of each luminaire in a variety of different layouts (see Fig. N41).
Fig. N41: The various compensation layouts: a] parallel; b] series; c] dual series also called “duo” and their fields of application
The compensation capacitors are therefore sized so that the global power factor is greater than 0.85. In the most common case of parallel compensation, its capacity is on average 1 µF for 10 W of active power, for any type of lamp. However, this compensation is incompatible with dimmer switches.
Constraints affecting compensation
The layout for parallel compensation creates constraints on ignition of the lamp Since the capacitor is initially discharged, switch-on produces an overcurrent. An overvoltage also appears, due to the oscillations in the circuit made up of the capacitor and the power supply inductance.
The following example can be used to determine the orders of magnitude.
Assuming an assembly of 50 fluorescent tubes of 36 W each:
- Total active power: 1,800 W
- Apparent power: 2 kVA
- Total rms current: 9 A
- Peak current: 13 A
- A total capacity: C = 175 µF
- A line inductance (corresponding to a short-circuit current of 5 kA): L = 150 µH
The maximum peak current at switch-on equals:
The theoretical peak current at switch-on can therefore reach 27 times the peak current during normal operation.
The shape of the voltage and current at ignition is given in Figure N42 for switch closing at the line supply voltage peak.
Fig. N42: Power supply voltage at switch-on and inrush current
There is therefore a risk of contact welding in electromechanical control devices (remote-control switch, contactor, circuit-breaker) or destruction of solid state switches with semi-conductors.
In reality, the constraints are usually less severe, due to the impedance of the cables.
Ignition of fluorescent tubes in groups implies one specific constraint. When a group of tubes is already switched on, the compensation capacitors in these tubes which are already energized participate in the inrush current at the moment of ignition of a second group of tubes: they “amplify” the current peak in the control switch at the moment of ignition of the second group.
The table in Figure N43, resulting from measurements, specifies the magnitude of the first current peak, for different values of prospective short-circuit current Isc. It is seen that the current peak can be multiplied by 2 or 3, depending on the number of tubes already in use at the moment of connection of the last group of tubes.
|Number of tubes already in use||Number of tubes connected||Inrush current peak (A)|
|Isc = 1,500 A||Isc = 3,000 A||Isc = 6,000 A|
Fig. N43: Magnitude of the current peak in the control switch of the moment of ignition of a second group of tubes
Nonetheless, sequential ignition of each group of tubes is recommended so as to reduce the current peak in the main switch.
The most recent magnetic ballasts are known as “low-loss”. The magnetic circuit has been optimized, but the operating principle remains the same. This new generation of ballasts is coming into widespread use, under the influence of new regulations (European Directive, Energy Policy Act - USA).
In these conditions, the use of electronic ballasts is likely to increase, to the detriment of magnetic ballasts.
Fluorescent and discharge lamps with electronic ballast
Electronic ballasts are used as a replacement for magnetic ballasts to supply power to fluorescent tubes (including compact fluorescent lamps) and discharge lamps. They also provide the “starter” function and do not need any compensation capacity.
The principle of the electronic ballast (see Fig. N44) consists of supplying the lamp arc via an electronic device that generates a rectangular form AC voltage with a frequency between 20 and 60 kHz.
Supplying the arc with a high-frequency voltage can totally eliminate the flicker phenomenon and strobe effects. The electronic ballast is totally silent.
During the preheating period of a discharge lamp, this ballast supplies the lamp with increasing voltage, imposing an almost constant current. In steady state, it regulates the voltage applied to the lamp independently of any fluctuations in the line voltage.
Since the arc is supplied in optimum voltage conditions, this results in energy savings of 5 to 10% and increased lamp service life. Moreover, the efficiency of the electronic ballast can exceed 93%, whereas the average efficiency of a magnetic device is only 85%.
The power factor is high (> 0.9).
The electronic ballast is also used to provide the light dimming function. Varying the frequency in fact varies the current magnitude in the arc and hence the luminous intensity.
The main constraint that electronic ballasts bring to line supplies is the high inrush current on switch-on linked to the initial load of the smoothing capacitors (see Fig. N45).
|Technology||Max. inrush current||Duration|
|Rectifier with PFC||30 to 100 In||≤ 1 ms|
|Rectifier with choke||10 to 30 In||≤ 5 ms|
|Magnetic ballast||≤ 13 In||5 to 10 ms|
Fig. N45: Orders of magnitude of the inrush current maximum values, depending on the technologies used
In reality, due to the wiring impedances, the inrush currents for an assembly of lamps is much lower than these values, in the order of 5 to 10 In for less than 5 ms.Unlike magnetic ballasts, this inrush current is not accompanied by an overvoltage.
For ballasts associated with high-power discharge lamps, the current drawn from the line supply has a low total harmonic distortion (< 20% in general and < 10% for the most sophisticated devices).
Conversely, devices associated with low-power lamps, in particular compact fluorescent lamps, draw a very distorted current (see Fig. N46). The total harmonic distortion can be as high as 150%. In these conditions, the rms current drawn from the line supply equals 1.8 times the current corresponding to the lamp active power, which corresponds to a power factor of 0.55.
In order to balance the load between the different phases, lighting circuits are usually connected between phases and neutral in a balanced way. In these conditions, the high level of third harmonic and harmonics that are multiple of 3 can cause an overload of the neutral conductor. The least favorable situation leads to a neutral current which may reach times the current in each phase.
Harmonic emission limits for electric or electronic systems are set by IEC standard 61000-3-2. For simplification, the limits for lighting equipment are given here only for harmonic orders 3 and 5 which are the most relevant (see Fig. N47).
|Harmonic order||Active input power > 25W||Active input power ≤ 25W one of the 2 sets of limits apply:|
|% of fundamental current||% of fundamental current||% Harmonic current relative to active power|
Fig. N47: Maximum permissible harmonic current
Electronic ballasts usually have capacitors placed between the power supply conductors and the earth. These interference-suppressing capacitors are responsible for the circulation of a permanent leakage current in the order of 0.5 to 1 mA per ballast. This therefore results in a limit being placed on the number of ballasts that can be supplied by a Residual Current Differential Safety Device (RCD).
At switch-on, the initial load of these capacitors can also cause the circulation of a current peak whose magnitude can reach several amps for 10 µs. This current peak may cause unwanted tripping of unsuitable devices.
Electronic ballasts are responsible for high-frequency conducted and radiated emissions.
The very steep rising edges applied to the ballast output conductors cause current pulses circulating in the stray capacities to earth. As a result, stray currents circulate in the earth conductor and the power supply conductors. Due to the high frequency of these currents, there is also electromagnetic radiation. To limit these HF emissions, the lamp should be placed in the immediate proximity of the ballast, thus reducing the length of the most strongly radiating conductors.
LED lamps & fixtures
The LED lighting technology presents the particularity of being the first technology to allow the development of appropriate and effective solutions for all applications of functional lighting, unlike earlier technologies.
To better understand why the use of LED lighting can result in these remarkable efficiency gains, basic terminology needs to first be explained. Listed below are definitions of the key terms in use :
- LED (Light Emitting Diode) - A diode type semiconductor which emits light when a current passes through it. LED semiconductor materials convert electrical energy into visible electromagnetic radiation (i.e., into light).
- LED component - The substrate and primary optical unit of the light assembly. The purpose of the LED component is to protect the semiconductor and to conduct the heat generated from LED to dissipation systems.
- LED module - The assembly of one or more LED components with optical, mechanical and thermal elements.
- LED luminaire - A complete system consisting of a LED module, a housing, an optical reflector, wiring, connectors, joints, heat dissipation system (heat sink or fan), and for most cases the driver
- Driver - An electronic device which can convert the electric power of a low-voltage AC electrical network into electric power appropriate for the LED luminaire (direct voltage and current). The driver may be external or internal to the luminaire. A driver can power one or more luminaires. Light dimming function can be embedded in this device (1-10 V control, DALI control, ...)
Inrush and steady-state currents
When a LED luminaire is first energized, a variable current is required by the luminaire during the first second time interval, and then the current stabilizes as soon as rated operating conditions are reached. Three transient fundamental events occur during the start up phase: the power supply of the luminaire, the start of the driver, and the powering of the LED module (light is on). Then the luminaire transitions to the steady state operating condition.
In the initial moments after a luminaire is energized, a significant transient current appears (can be up to 250 times the rated current depending of the characteristics of products) due to the capacitors used to perform the power factor correction (the power factor of LED luminaires is generally greater than 90%, since the luminaire driver includes a power factor correction stage). The duration of this transient current is less than 1 millisecond (ms). When the luminaire is powered on, the current will be at its highest level when the voltage angle is 90° (in that case, the voltage is at its peak value of 325 V for a 230 Volt AC network). When switching on at zero angle voltage, the inrush current is far smaller.
Once the inrush current has passed, a time range of between 100 ms and 1.5 seconds elapses. During this time, the driver is initialized (power supply for electronic control circuits, are energized, for example). The current consumed during this phase is less than the rated current.
Once the driver is initialized, the LED module is energized and light appears. An overload of about twice the rated current occurs during the initial period of power supply of the module containing the LEDs. Fig. N48b illustrates the various stages involved in energizing the luminaire. Note that state 4 in Fig. N48b represents the steady state operating condition.
In the steady-state condition, the current consumed by LED luminaires is not perfectly sinusoidal. The total harmonic distortion of current (THDI) ranges between 10% and 20%. Given that the rated currents of LED luminaires are low, the impact of these currents on network voltage is slight. Measurements in various industrial plants powered by the public low-voltage power supply system (on which the short-circuit impedance is low) show that the total voltage harmonic distortion (THDV) is generally less than 3%. According to the IEC 61000-2-4 standard relating to the compatibility levels of voltage tolerances, if the THDV is less than 5% (class 1 electromagnetic environment), the network is considered sound and compliant.
Common mode currents
Definition: when currents flow without close-by opposing currents, the unopposed portion of current is referred to as common mode current. Common mode currents can result in radiation which can then result in interference or distortion.
How LED technology deals with this challenge? In the following example, measurements were performed by first energizing 20 luminaires that were isolated from earth. Given the configuration, the leakage current could only be looped back via the protective earth (PE) conductor of the power cable. The current flowing in this conductor at the energizing stage is presented below (see Fig. N48c)
For switching on at zero voltage, the leakage current is practically zero.
The frequency of the transient current is high (about 100 kHz).
At the steady state stage, for the 20 luminaires isolated from earth, the leakage current value measured at 50 Hz was about 2 mA.
- ^ IEC 61000-2-4 standard: Electromagnetic compatibility (EMC) – Part 2: Environment – Section 4: Compatibility levels in industrial plants for low-frequency conducted disturbances