4-Wire Pulse Width Modulation (PWM) Controlled Fans Specification

Fan Speed Control
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3 Fan Speed Control
3.1 Maximum Fan Speed Requirements
The maximum fan speed shall be specified for the fan model by the fan vendor and correspond to
100% duty cycle PWM signal input.
3.2 Minimum Fan Speed Requirements
The vendor shall specify the minimum RPM and the corresponding PWM duty cycle. This
specified minimum RPM shall be 30% of maximum RPM or less. The fan shall be able to start
and run at this RPM. To allow a lower specified minimum RPM, it is acceptable to provide a
higher PWM duty cycle to the fan motor for a short period of time for startup conditions. This
pulse should not exceed 30% maximum RPM and should last no longer than 2 seconds. See
Figure 2.
Figure 2 Minimum Fan Speed Requirements: Start Pulse
Startup Pulse - Speed vs. Time
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Time (seconds)
% of full speed
Max % of full speed
RPM for startup
Max duration of
startup pulse