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VFD control in action: Temperature control   |   Pressure control   |   Zone control   |   Flow control  

Variable frequency drive (VFD) control in HVAC systems

The importance of maintaining static pressure

A certain level of static pressure is necessary to maintain a proper amount of airflow in a ventilation system. If the pressure is not strong enough, a suitable amount of air will not be supplied to the vents and diffusers throughout the building. 

If the pressure is too excessive, the system will become noisy, and lead to issues such as leaks and an inability to maintain room conditions after excessive wear and tear. Additionally, incorrect static pressure also has a negative impact on the efficiency of the system.

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Nano programmable logic controllers (PLCs) with analog expansion units like the easyE4 have outputs that can send a signal to a variable frequency drive (VFD) as a speed reference to control temperature.

For example, in a variable speed air conditioner where the speed of the blower is controlled by a VFD, a nano PLC can read the temperature of the thermal sensor and input it into a PID along with the setpoint.

The output of the PID block can then be used to control the speed of the blower to supply less or more cool air to the lab until the desired setpoint is achieved.

Intelligent control optimizes energy efficiency

The easyE4 nano-programmable logic controller (PLC) analog expansion unit has outputs that can send a signal to a VFD as a speed reference to control the speed of a motor.

For example, in a variable speed air conditioner where the speed of the blower is controlled by a VFD, the easyE4 nano-PLC can read the pressure from sensor feedback and input it into a Proportional, Integral, Derivative (PID) along with the setpoint. The output of the PID block can then be used to control the speed of the fan to supply less or more cool air to the building until the setpoint is reached.

DF = Diffuser

SPS = Static Pressure Sensor

VFD = Variable Frequency Drive

Diagram depicting static pressure control in an HVAC system using variable frequency drives
Figure 1: Diagram depicting static pressure control in an HVAC system using variable frequency drives
VFD and Nano PLC driven HVAC system
Figure 2: VFD and Nano PLC driven HVAC system

easyE4's software simplifies the PLC programming process

One can easily select the gains needed to enable the device by setting EP, EI, or ED and simply has to set I1 to be the setpoint and I2 to be the temperature feedback from the system.

Additionally, the proportional gain KP is configured directly on the faceplate. This value will be determined empirically based on the desired response.

The device calculates the integral and derivative gain slightly differently. The integral gain, KI=KP*TC/TN. Where KP is the proportional gain, TC is the scan time of the function block, and TN is the integration time.

Likewise, the derivative gain KD=KP*TV/TC, where KP and TC are the same parameters and TV is the differential time. Like the proportional gain, the integral and derivative gains need to be determined empirically.

The summation of the gains, QV can then be set directly as the speed reference to the drive.

QA05 speed reference chart
Figure 2: QA05 speed reference chart
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Explore PID control capabilities of the easyE4 Nano PLC

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