Variable frequency drives have a myriad of control inputs available for
use in the light commercial application. They can be used to meet
specific requirements or whatever unique control strategy you might be
trying to implement. The purpose of this article isn’t to discuss all
inputs and outputs on every drive, but to highlight the typical I/O needed for
converting a constant volume (CV) rooftop unit (RTU) into a variable air volume (VAV) RTU.
Digital or Binary Inputs
A typical implementation of
a VFD requires an enable signal as an absolute minimum. The drive will
require the presence of 24 volts DC at one of its inputs. When the drive
detects this high logic level, it begins controlling the motor speed to one of
several speeds depending upon other settings discussed below. This mode
of control is often described as two wire control. Quite often the drive
will have a macro to automatically set it
up for two or three wire control. This macro will save
time in adjusting all the parameters needed to implement two wire control.
A second digital input that
is often required when converting a CV unit to VAV is a High-Speed Index input. When a set of
contacts are closed across this input and the 24V source, the drive will begin
operating at a pre-programmed speed, typically
60 Hz, regardless of the
frequency reference input. This input is often used on electric heat and
gas heat conversions to operate the unit’s supply fan at full speed when a need
for heat is present.
Analog Inputs
A VFD operating in any sort of automatic mode will require a frequency
reference for determining the speed at which the connected motor needs to be
driven. This is typically a 0-10 VDC signal from a closed-loop PID
controller. This controller monitors duct static pressure and using a PID
algorithm, it increases the fan speed when duct static pressure is below setpoint
and decreases fan speed when duct static pressure is above setpoint. It
is very rare that any other analog input is used on a VAV conversion VFD.
Digital or Binary Outputs
Quite often the VFD is used as the over current protection device in the
motor branch circuit. While the drive will safely shut down the motor in
the event of an over current condition, or any other drive fault, the unit
control board needs to know the status of the drive to the control board can
lock out compressors or unit heat in the event of a drive malfunction.
This is usually accomplished by a relay output on the VFD. Normally open
contacts that are driven closed during normal operation are the preferred choice for these fault contacts because this configuration will also permit
the detection of a broken wire.
Because of the nature of a VAV system, a fan proving switch that
operates on velocity pressure or differential pressure is not always reliable.
A second relay output on the VFD can be used in lieu of a fan proving switch to
positively verify the operation of the supply fan motor for other control
algorithms.
Analog Outputs
While VFDs have AOs available to output drive current, hp, rpm, or one
of several other process variables, it is very rare when this would be needed
as part of a simple VAV
conversion.
