We retrofitted a furnace a few months ago by adding two 125 HP forced draft fans. One fan is driven by a 125# steam turbine and the other has a 1200 RPM IEEE 841 motor with an AB Powerflex 755 drive. They are redundant - only one fan is required to run at around 60% speed to maintain proper airflow into the furnace.
We had an AB field engineer out to start up the drive and check out the control I/O and parameter settings. All drive commands are either hardwired contacts or 4-20 mA - no communications protocols. A 4-20 mA input is set to control the speed with Volt/Hz control - no fancy stuff programmed in the drive.
When we were test running the fan at startup the drive current was surging from about 60 to 85 amps, swinging back and forth once per second or so. The speed command was constant and the RPM reading from the drive stayed fairly constant. The field engineer said the amps surging was due to actual mechanical load changes in the fan, furnace backpressure, etc.
Now that the FD fans have been running a few months, when the turbine driven fan is running the turbine speed is constant, the feedback signal to the turbine controller is constant, airflow is constant, etc. - just what you would expect. The VFD driven fan is still surging like it was during startup. Since the fans are identical within manufacturing variances, all vibration checked out OK, and they are blowing into the same furnace with the same firing conditions, we can no longer buy the story that the driven load is changing once per second causing the drive current to swing around substantially. I don't think the drive cares if the curring is surging, but surging current means surging motor torque which will tear up the motor and coupling prematurely.
Any ideas?
We had an AB field engineer out to start up the drive and check out the control I/O and parameter settings. All drive commands are either hardwired contacts or 4-20 mA - no communications protocols. A 4-20 mA input is set to control the speed with Volt/Hz control - no fancy stuff programmed in the drive.
When we were test running the fan at startup the drive current was surging from about 60 to 85 amps, swinging back and forth once per second or so. The speed command was constant and the RPM reading from the drive stayed fairly constant. The field engineer said the amps surging was due to actual mechanical load changes in the fan, furnace backpressure, etc.
Now that the FD fans have been running a few months, when the turbine driven fan is running the turbine speed is constant, the feedback signal to the turbine controller is constant, airflow is constant, etc. - just what you would expect. The VFD driven fan is still surging like it was during startup. Since the fans are identical within manufacturing variances, all vibration checked out OK, and they are blowing into the same furnace with the same firing conditions, we can no longer buy the story that the driven load is changing once per second causing the drive current to swing around substantially. I don't think the drive cares if the curring is surging, but surging current means surging motor torque which will tear up the motor and coupling prematurely.
Any ideas?