Basics of an HVAC bypass system02/05/2018In the industrial and commercial HVAC market, three of the most used manufacturers are Carrier, Trane, and York. Each of these typically use a branded version of variable frequency drives specifically made for them. Often a Carrier HVAC drive will be a Lenze or AC Tech brand. Trane often uses Danfoss VLT series drives. York often uses ABB ACH series AC drives. These drives have specialized programming and can be integrated into a bypass system.The bypass option allows a motor to be operated across line power in the event that the drive is unavailable, has malfunctioned, or if continued operation at full speed is required. The bypass consists of contactors. In drive mode, the VFD input and output contactors are closed, and the bypass contactor is open. In bypass mode, the VFD input and output contactors open to fully isolate the drive, while the bypass contactor closes to apply line power directly to the motor. Some drive suppliers only use two contactors in their bypass. They do not utilize a VFD input contactor, which means that even in bypass mode, power is still applied to the drive. If a short circuit were to occur in the drive portion of the unit, attempting to apply main power would result in clearing input fuses or tripping the input breaker. This renders the bypass useless, which means the motor cannot be operated (thus defeating the purpose of the bypass). In the event of a short circuit, the VFD must be physically disconnected to run in bypass mode. By having the third contactor to completely isolate the drive, the AC bypass will still operate, insuring critical equipment remains on line. Voltage is completely removed from and routed around the AC drive so that it can be removed while the motor remains in operation. Typical application of the variable frequency drives in the HVAC system will include supply & return fans, condensing fans, and cooling tower fans. These may or may not be configured with a bypass unit. If the system is configured with a bypass, the system can remain in operation while the variable speed drive is removed for troubleshooting or maintenance. With the drive removed, the system fans will run at top speed with no other speed control, but it does provide the means to continue running.If you HVAC system’s variable speed AC drive is in need of repair, call on the drive experts at Precision Electronic Services, Inc. today at 800-732-4695. Back To Blog
Basics of an HVAC bypass system02/05/2018In the industrial and commercial HVAC market, three of the most used manufacturers are Carrier, Trane, and York. Each of these typically use a branded version of variable frequency drives specifically made for them. Often a Carrier HVAC drive will be a Lenze or AC Tech brand. Trane often uses Danfoss VLT series drives. York often uses ABB ACH series AC drives. These drives have specialized programming and can be integrated into a bypass system.The bypass option allows a motor to be operated across line power in the event that the drive is unavailable, has malfunctioned, or if continued operation at full speed is required. The bypass consists of contactors. In drive mode, the VFD input and output contactors are closed, and the bypass contactor is open. In bypass mode, the VFD input and output contactors open to fully isolate the drive, while the bypass contactor closes to apply line power directly to the motor. Some drive suppliers only use two contactors in their bypass. They do not utilize a VFD input contactor, which means that even in bypass mode, power is still applied to the drive. If a short circuit were to occur in the drive portion of the unit, attempting to apply main power would result in clearing input fuses or tripping the input breaker. This renders the bypass useless, which means the motor cannot be operated (thus defeating the purpose of the bypass). In the event of a short circuit, the VFD must be physically disconnected to run in bypass mode. By having the third contactor to completely isolate the drive, the AC bypass will still operate, insuring critical equipment remains on line. Voltage is completely removed from and routed around the AC drive so that it can be removed while the motor remains in operation. Typical application of the variable frequency drives in the HVAC system will include supply & return fans, condensing fans, and cooling tower fans. These may or may not be configured with a bypass unit. If the system is configured with a bypass, the system can remain in operation while the variable speed drive is removed for troubleshooting or maintenance. With the drive removed, the system fans will run at top speed with no other speed control, but it does provide the means to continue running.If you HVAC system’s variable speed AC drive is in need of repair, call on the drive experts at Precision Electronic Services, Inc. today at 800-732-4695.