Variable frequency drives have a myriad of control inputs available for use in the light commercial application. They can be used to meet specification 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 is closed across this input and the 24V source, the drive will begin operating at a preprogrammed 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.
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 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 so 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.
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 pard of a simple VAV conversion.