ADB Stagelight EURODIM Twin Tech, Руководство по установке

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EURODIM TWIN TECH
User Manual - page 57
Issue 2.1 www.adbstagelight.com
12 Appendix C– Power Supply for Thyristor Dimmers – Basic Principles
for Safe Electrical Design
The global performance of a dimmer system depends on the dimmers and also on its
electrical supply system: supply transformer, supply cabling to the dimmers. Planners and
electrical contractors may not be fully aware that some rules which are valid for resistive
loads and sinusoidal currents (heating systems, incandescent lighting) do not apply to a
phase-control dimmer system. With this Technical Information we try to provide some
basic technical information about lesser-known aspects of a dimmer system.
12.1 Phase-control dimmers
The broad principles of phase-control dimmers are as follows.
• In an ideal world, the input voltage of a dimmer is a pure sine wave, with a frequency of
50Hz or 60Hz. A phase-control dimmer reduces the voltage by switching off part of the
sine wave. So the shape of the dimmer output voltage is no longer sinusoidal: it
contains the fundamental frequency (50Hz) and also harmonics (odd multiples of the
fundamental: 150Hz, 250Hz, 350Hz ...).
• The output voltage of the dimmer is applied to the lamp. A lamp is comparable to a
resistor, so the current has the same shape as the dimmer output voltage: part of a sine
wave. So the current contains the same harmonics as the dimmer output voltage.
Note : this current flows through all parts of the system: lamp, dimmer, supply cables,
switchgear, supply transformer.
12.2 Use ‘true rms’ voltmeters only
For a correct measurement of the output voltage of a dimmer, you need a ‘true rms’
voltmeter. Other – cheaper – voltmeters are useless. They give a false reading when used
on the output of a dimmer. They assume that the voltage is a sine wave.
12.3 Current in the Neutral – Sine Wave
Equipment requiring more than say 10 kW is usually supplied from a ‘star’ (wye) three-
phase system: three live Lines and a Neutral (and Protective Earth PE). The Neutral is the
common return conductor for the three Lines. The symbols for the Lines are L1, L2, L3;
Lines are sometimes called Phases.
The following rules are well-known
1. When you use only one phase of a three-phase system, then the current in the Neutral
is identical to the current in the Phase.
2. When an identical linear load is connected to all three phases, we speak of ‘a balanced
load’. In that case the actual current in the Neutral is 0. Indeed the sum of three sine
wave currents, equal in amplitude, 120
°
out of phase, is zero.
3. for sine wave currents, the max. current in the Neutral is equal to - or lower than - the
max. current in any Line. For sine wave currents it is never larger.
IMPORTANT
Rule No. 1 is also valid for dimmers. It is a sufficient reason to forbid the use of supply
cables with a reduced-size Neutral.