Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 154)

sections should be selected and rated correspondingly to comply with loading limits and to keep
the voltage drop within the permissible limits.
5.2.1.1 Control transformers for contactor controls
In accordance with IEC 60947-4-1 contactors have a normal control voltage range of 85 % –
110 % the rated control supply voltage, i.e. they reliably close and stay closed within these
voltage limits. Often contactors are available with an extended control voltage range, for
example 80 % – 110 % or 115 %. Because of the high pick-up currents of AC magnets –
especially when using large contactors and when several contactors are switched simultane-
ously – it should be ensured that the voltage does not fall below the lower limit. This could result
in contactors not completely closing and the high pick-up current flowing for an extended period.
The consequence can be burning of the coil and / or welding of the main contacts.
For control transformers supplying electromagnetic loads such as contactors, an important
selection criterion is the short-term power P
S(S6)
. The drop in secondary voltage in comparison to
the rated voltage may be no more than 5 % at this power rating. The peak load of the control
transformer should be determined in each individual case and the control transformer should be
selected considering the prospective supply voltage variations. Often the worst case scenario is
to assume that the largest contactor must reliably close at a certain basic load of the trans-
former.
5.2.1.2 Frequencies < 50 Hz and > 60 Hz
Normal AC magnets are rated for 50 Hz or 60 Hz. Usually the voltage specifications for both
frequencies are provided in the technical documentation. Often dual frequency versions are also
available, i.e. the devices can be used at the same rated voltage with 50 Hz and 60 Hz supplies.
Dual frequency designs are advantageous for the export industry, thus the same devices can be
used in all markets – subject to adjustment of the voltage magnitude (tapping of the control
transformer). For controls that do not require this flexibility, it is recommended to choose
contactors for one (1) rated frequency as the tolerance range of the control voltage and the
mechanical life span are then optimized.
For applications in railway transport systems at 16 2/3 Hz and applications at 400 Hz (airports,
military), devices with direct current magnets must be used and – in case that a direct current
supply is not available – the alternating voltage rectified.
5.2.2 Direct voltage
Direct voltage is used as a control voltage in a large and growing range of applications. Typical
applications are installations on vehicles (for example refrigeration systems), stationary battery-
powered systems (power utility installations) and constantly growing areas of electronic controls
in industry and building technology.
Preferred direct voltages are 24 V (industry, vehicles), 48 V (vehicles) and 110 V, 220 V …
250 V (power utilities, high-voltage batteries, energy regeneration).
Given the wide range of battery voltages on vehicles (overcharging and complete discharge)
contactors with an extended voltage range are offered (for example 0.7 … 1.25 U
N
).
Quality of direct voltage
When direct voltage is used as a control voltage, in addition to the tolerance range of the
voltage attention should be paid to its harmonic content (ripple content). Battery powered
systems are in this regard an ideal voltage source without harmonics. Also switched power
supplies produce well smoothed direct voltages.
If the direct voltage is obtained from alternating voltage by rectification, the harmonic content
depending on the circuit may be relatively large and must be considered. In these cases it
should always be remembered that the arithmetical average voltage is the critical quantity for
the pull-in performance of conventional electromagnets. Two-way rectification (Graetz rectifier)
is usually permissible for supplying conventional contactor coils; also 3-phase bridge circuits
with a ripple of around 5 %.
LVSAM-WP001A-EN-P - April 2009 5-2