Trane RTWA 70-125 TON - Manual (Page 21)

RLC-SVD03A-EN 21
Variable Speed
Inverter/Condenser Fan Control
When Fan Control and Variable Speed Fan (VSF) are set to Enable in the
Machine Configuration Menu, the UCM will control both the variable speed
fan and the remaining constant speed fans per the VSF Control Algorithm. If
VSF Control is disabled for a given circuit but Fan Control is enabled for the
machine, the circuit will perform normal constant speed fan control. The VSF
is enabled and operational, the control attempts to provide a 70 ± 5 psid
between the Condenser Pressure and the Evaporator Pressure (as derived
from the temperature sensor measurements).
The VSF Inverter is commanded to a given speed by the UCM, using a PWM
(Pulse Width Modulated) signal (10V, 15mA, 10 Hz Fundamental) with a duty
cycle proportional to the desired voltage and frequency from the Inverter. The
UCM also controls power to the Inverter through a contactor. The Inverter
Contactor for the respective circuit is energized approximately 20 seconds
prior to compressor start on that circuit. The VSF Control algorithm runs on a
5-second interval and is limited to a commanded rate of change of no greater
than 40% of full speed per interval. The same algorithm that controls the
Figure 4 Variable Speed Fan (VSF) and Fan Staging Control
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