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
.
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.
: magnetic ballasts.
The starter
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.
Compensation
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.
Possible layouts
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 .
The compensation capacitors are therefore sized
so that the global power factor is greater than 0.85.
In the most common case, that 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.
c Assuming an assembly of 50 fluorescent tubes
of 36 W each:
v total active power: 1800 W,
v apparent power: 2 kVA,
v total rms current: 9 A,
v peak current: 13 A.
c With:
v a total capacity: C = 175 μF,
v a line inductance (corresponding to a shortcircuit
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.
There is therefore a risk of contact welding in
electromechanical control devices (remotecontrol
switch, contactor, circuit-breaker) or
destruction of solid state switches with semiconductors.
In reality, the constraints are usually less severe,
due to the impedance of the cables.
A technological development
The most recent magnetic ballasts are known as
“low-loss”. Their 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).
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