The unit's blower VFD speed is calculated and controlled by several temperature sensors located on the unit.
The VFD is located inside the blower compartment next to the blower housing. The unit has several sensors that are used to monitor the outside air and unit operating temperature ranges. With these sensors the unit will change the VFD staging accordingly.
The information below provides a detailed breakdown of the VFD blower speed staging illustrated in the diagram above.
- No Economizer Installed
• G / Fan only call – VFD 60 HZ all the time
• Y1+G call- 60 HZ for 20 seconds and then based off T4 OAS Sensor
o Decrease VFD to 40 HZ if T4 is below 86°F / 30°C
o Increase VFD to 60 HZ if T4 is above 89.6°F / 32°C
Note: Only T4 (OAS) is used for G / Fan call only
• Y1 + Y2 + G call- 60 Hz for 20 seconds and then based on T1 & TcA OR TcB Sensor
o Decrease VFD to 40 HZ if T1 is below 77°F and TcA is below 131°F / 28°C
o Decrease VFD to 40 HZ if T1 is below 77°F and TcB is below 131°F / 28°C
o Increase VFD to 60 HZ if T1 is above 82.4°F and TcA is above 143.6°F / 62°C
o Increase VFD to 60 HZ if T1 is above 82.4°F and TcB is above 143.6°F / 62°C
Note 1: The T4 (OAS) is not used for VFD control.
Note 2: The Tc/T3 sensors do not control the VFD alone.
The VFD Speed is calculated by the board with sensors T1, Tc/T3, T4, and T5 as specified above. There is a differential of 5.4°F for the Y1 and Y2 operation between the 40HZ and 60HZ adjustment.
2. Units with an economizer, the logic for the fan is the same. The only change is the logic from the economizer controller controlling the compressor operation based on ambient conditions.
Sensor information below.
| SENSOR | PLUG | LOCATION |
| T1 | CN-40 | MAS Return Air |
| T4 | CN-23 | OAS Outside control panel post |
| Tc-T3A | CN-22 | Circuit A Condenser Coil Temperature Sensor |
| Tc-T3B | CN-47 | Circuit B Condenser Coil Temperature Sensor |
| T5A | CN-51 | Circuit A Compressor Discharge Sensor |
| T5B | CN-52 | Circuit B Compressor Discharge Sensor |
How to adjust Medium Foundation blower RPM's click link below.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.