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Building Technologies Basic documentation LMV37.4... CC1P7546en
12 Fan control 17.12.2018
12.14.2 Acquisition of speed PWM fan
Due to design-related restrictions, it is usually not possible to mount a sensor disk for
generating an independent, asymmetrical (direction of rotation sensitive) speed signal
on a PWM fan. Instead, the symmetrical speed signal of the PWM fan based on the Hall
effect is used. No direction of rotation recording is possible due to the symmetrical
structure.
The PWM function in the electronic fuel-air ratio control system is only permitted in
conjunction with a fail-safe feedback signal of the fan with error analysis (in accordance
with DIN EN 60730-1 class C).
For a plausibility check with regard to a fault-free Hall feedback, an additional speed-
dependent air pressure switch can therefore be used for a comparison between the
speed signal and the resulting air pressure (see chapter Additional speed-dependent air
pressure switch).
With a correspondingly high ON threshold of the air pressure switch, this also includes
verification of the correct direction of rotation and the required air quantity for prepurging
and high-fire.
The system manufacturer / system operator must determine and guarantee that
adequate fail-safe properties are achieved in accordance with standards.
The LMV37.4 fulfills the requirement of the UL standard, EN 12067-2 and ISO 23552-1
(fuel-air ratio control) if the following basic principles are met:
Directly connected PWM fan
Directly fail-safe feedback signal of the fan according to EN 60730-1 class C
If the basic principles outlined above are not met, the local safety regulations for the
application must be checked by the system manufacturer / system operator.
The safety of the entire LMV37.4 must be guaranteed by the system manufacturer /
system operator.
Attention!
To avoid personal injury or damage to property, the following note must be
observed.
If the feedback signal of the fan does not correspond to EN 60730-1 class C, there is a
risk of unclean combustion, which can lead to the following:
CO emissions and poisoning
Explosion
Damage to property
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