RTHD and RTAC - Trane

Startup and Operation
Variable Speed Fan Control - RTAC Only
The Main Processor (MP) is configurable for operation with either 0, 1 or 2
variable speed fans per circuit. Variable speed fans are optional and included on
units when the low ambient operation is required. The variable speed fans are
standard fans and motors, used with the fixed speed fans, but are driven by the
small 1.5 HP inverters that create a variable voltage/frequency, 3 phase output
using Pulse Width Modulation (PWM) switching.
When either 1 or 2 of the variable speed fans are configured, the MP provides
startup and running control of the variable speed fans (as well as the other
constant speed fans).
Variable speed condenser fans provide start-up and operation down to an
ambient temperature of 0 o F without fan cycling, without safety trips, or other
instabilities that would otherwise occur with fixed speed fans only.
The MP provides an output, one per circuit, to control a contactor which delivers
3 phase power to the inverter(s) on that circuit. The output energizes the
contactor for the inverter(s) for that circuit at least 5 seconds prior to the first
compressor start of that circuit.
Once the compressor(s) on the circuit have started, the inverter(s) will be updated
with speed command signals (slaved to run at the same speed if 2 are present),
and are continuously monitored for a fault condition. The fan control algorithm
outputs a desired % fan capacity for the entire fan deck. This % fan deck capacity
is realized by staging on and off constant speed fans in discrete increments of
capacity, while the inverter is used to control fan capacity in-between steps.
20 RLC-SVD05A-EN