Unit installation

102 ACZ / AGZ-BM IOMM ACZ/AGZ-3
Optional VFD Low Ambient Fan Control
Low ambient air temperature fan control is accomplished by using the Optional Low Ambient VFD
to control the speed of the first fan on each circuit. This VFD control uses a proportional integral
function to drive the saturated condenser temperature to a target value by changing the fan speed.
The target value is normally the same as the saturated condenser temperature target setpoint.
The fan VFD always starts when the saturated condenser temperature rises higher than the target.
What is an Inverter?
The term inverter and variable-frequency drive are related and somewhat interchangeable. An
electronic motor drive for an AC motor controls the motor’s speed by varying the frequency of the
power sent to the motor.
An inverter, in general, is a device that converts DC power to AC power. The figure below shows
how the variable-frequency drive employs an internal inverter. The drive first converts incoming AC
power to DC through a rectifier bridge, creating an internal DC bus voltage. Then the inverter circuit
converts the DC back to AC again to power the motor. The special inverter can vary its output
frequency and voltage according to the desired motor speed.
Inverter Output to the Motor
The AC motor must be connected only to the inverter’s output
terminals. The output terminals are uniquely labeled (to
differentiate them from the input terminals) with the designations
U/T1, V/T2, and W/T3. This corresponds to typical motor lead
connection designations T1, T2, and T3. The consequence of
swapping any two of the three connections is the reversal of the
motor direction. This must not be done. In applications where
reversed rotation could cause equipment damage or personnel
injury, be sure to verify direction of rotation before attempting
full-speed operation. For safety to personnel, the motor chassis
ground must be connected to the ground connection at the bottom of the inverter housing.
Notice the three connections to the motor do not include one marked “Neutral” or “Return.” The
motor represents a balanced “Y” impedance to the inverter, so there is no need for a separate
return. In other words, each of the three “Hot” connections serves also as a return for the other
connections, because of their phase relationship.
Do not to switch off power to the inverter while the motor is running (unless it is an emergency
stop). Also, do not install or use disconnect switches in the wiring from the inverter to the motor
(except thermal disconnect).