Problems with VFD Flying Start at low speeds
Technical documentation, contact your local Allen-Bradley distributor or Rockwell Automation sales representative. For Allen-Bradley® drives technical support, see Rockwell Automation Support on the back cover of this manual. Resource Description PowerFlex 70 Adjustable Frequency AC Drive Installation Instructions, publication 20A-IN009. Flying Cut-Off (Flying Shear) DESCRIPTION The flying shear is a typical motion control application in which material or product must be processed or cut while in motion. The flying shear control must track a point on the product, accelerate to match the speed, and send an output to operate the cutter while.
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![Flying Flying](https://inductothermhw.com/wp-content/uploads/sites/35/2015/01/Alpha-Controller-Cutout.jpg)
Problems with VFD Flying Start at low speeds
I have a 350Hp motor spinning a seperator (type of fan application) that is controlled with an AB Powerflex 700S 480V VFD. We had an issue where a fan upstream of this seperator was pulling a draft which was causing this seperator to freespin. When this seperator was freespinning the drive was given a command to start and when started appeard to bring the motor to a stop at 0Hz before it began its ramp. This sudden stop with the seperator spinning caused mechanical damage of the coupling between the motor and gearbox.
I went ahead and enabled the flying start feature of the drive so that when the drive was given a start command, it would start at the same frequency at which the motor was already spinning. To test this we ran the drive up to full speed (60Hz output) and gave the drive a stop command which stopped the output to the motor and allowed the motor to coast to a stop. While the motor was coasting we attempted to restart the drive, and we saw that indeed the drive recognized the speed at which the motor was spinning and matched that speed when it began its ouput to the motor. After syncing the speed with the spinning motor the drive began its ramp up to setpoint. We repeated this several times and saw each time that the drive synced up to speed with the spinning motor.
When we attempted to restart the drive with the motor spinning at low speeds around 300-400rmp (10-15Hz) we noticed that the drive would not sync and start to the speed of the spinning motor, but would rather bring the motor to a stop before it began its ramp. We tried this several times at low speeds and saw that each time, it would not match the low speed of the spinning motor but would bring it to a suddent stop. This sudden stop at low speeds could have casued the mechanical coupling damage I mentioned earlier.
In speaking to Rockwell they mentioned that at low speeds the current ouput tha the drive uses to find the spinning frequency may be enough to bring the motor to a stop, where it will not for higher speeds. They recommended disabling the 'fast flux ramp' paramater in the drive however this made no change, and the drive is still stopping the motor each time at low speeds. My understanding is that the fast flux is used to build the flux field in the rotor which may be causing the drive to stop?
Does anyone have any ideas why I am having problems with this flying start feature at low speeds?
I should mention that during these tests the motor was uncoupled from the load.