POWER CELL BYPASS VERSUS FULL VOLTAGE BYPASS
Jun 24, 2011
POWER CELL BYPASS VERSUS FULL VOLTAGE BYPASS
Power Cell Bypass in Perfect Harmony drives is a superior option to alternative drives utilizing full voltage bypass. Advantages include higher system availability, longer life for motors, simpler system that is easier to maintain. Siemens LD-A Perfect Harmony drives are the only drives which can offer this option. In the event of a failed power cell, the faulty cell is bypassed within less than ¼ seconds without interrupting system operation.
On the VFD input side, Cell Bypass provides the following advantages to the user.
Continuous operation of the system without interruption, in the event of cell failure – a failure in other drive systems will shut down the system and the process. The customer process must be restarted with the motor operating on the Full Voltage Contactor Bypass. System availability of the Full Voltage Contactor Bypass system is significantly lower than Cell Bypass option.
With Cell Bypass option variable speed operation is still available in the event of a power cell fault. System availability with Cell Bypass option is significantly higher than Full Voltage Contactor Bypass system.
Cell Bypass option allows the system to maintain a high input power factor across the entire speed range. With Full Voltage Contactor Bypass the system power factor is no better than the motor power factor.
Overall size of the Perfect Harmony VFD with Cell Bypass option is much smaller than alternative drives with Full Voltage Contactor Bypass. All Cell Bypass components fit inside the standard Perfect Harmony drive cabinets versus Full Voltage Contactor Bypass which requires one or more 916mm or 1018mm wide cabinets.
The Perfect Harmony VFD with Cell Bypass is a simpler system which requires fewer spare parts and is easier to maintain than drives with Full Voltage Contactor Bypass.
The Cell bypass option costs less than Full Voltage Contactor Bypass
On Motor Side, following will be the advantages of using Cell Bypass Option
The motor used with Perfect Harmony VFD does not need to be rated for line starting when. The cost and size of conductors between the VFD and the motor is lower than VFDs with Full Voltage Contactor Bypass.
The number of starts per hour is not limited when starting a motor on the Perfect Harmony drive. Even with a power cell bypassed, the motor voltage is fully balanced and the inrush current is fully controlled. There is far less mechanical stress and therefore the life of the motor is increased.
Inverter duty motors are not required for operation on Prefect Harmony VFDs. Overall less capital dollars will be spent on motors
On Process Side, Cell Bypass Option offers the following additional value
Perfect Harmony with Cell Bypass option is a simple system. A power cell fault will not require system shut down and re-configuration of the customer process for constant speed operation.
No dual protection system required for motor as required in Full Voltage Contactor Bypass systems. There is no need to engineer a process control strategies for constant fixed speed operation.
Less spares are required for the Perfect harmony drive with Cell bypass than with alternative drive with Full Voltage Contactor Bypass.
Bypass of a faulted power cell results in minimum degradation of input power quality. Perfect Harmony with Cell Bypass option provides better input power factor over the entire range of operation than Full Voltage Contactor Bypass systems.
VFD starting causes far less stress on the mechanical system. The Perfect Harmon system with cell bypass will have higher reliability and will require less maintenance over the l8ife of the system.
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