Reg. variable frequency drives


Thread Starter


Can anyone explain to me what is meant by "fly to catch" option in a VSD. How to set the parameter and how far it influences the drive performance. Suppliers say it is used in textile applications only. Can anyone clarify with this doubt please. With thanks, Veda, India.
I have seen the feature used in Textile applications. However that is not what the feature was designed for. It was designed to catch fans while rotating. An example application would be a fan that is operating across the line at 60Hz and you want to switch the fan from Utility power to the Variable Frequency Drive (VFD) without stopping the fan. I’ve also seen the feature used in applications where you have a stand-by VFD. If for some reason the main VFD fails the load can be switched to a stand-by VFD without shutting down the process. Fly Catch, if it is the feature that I am thinking of will sense the frequency that the load is rotating at and then output that frequency with no voltage, it will then ramp the voltage up. How the feature works may vary slightly from manufacturer to manufacturer.

Hakan Ozevin

"Fly to catch" or "restart on the fly" means VSD is catching the speed of the motor when it is still rotating after an interruption. A fan after a short power interruption is a good example. Because of its high inertia and low friction, the fan will rotate long time after the power failure. When the power is back, the VSD will try to ramp it up from zero speed and zero voltage, however the motor is generating voltage at this moment. Under normal conditions, the VSD would fail but, when enabled, "Fly to catch" facility observes current drawn from the inverter, and if it is negative (means that motor is rotating) increases the output frequency and voltage to make this current zero. At this point, it continues the ramp up. This prevents a failure. Since it is a software solution, nearly all of the VSD's have this facility now. When parametering, you usually just enable or disable it and choose if the VSD will check the direction of rotation. To connect a not-rotating motor to a VSD after ramp up is something different, and difficult, unless the load demands very small starting torque, as in fans. Hakan Ozevin