We have a 350hp motor connected to a 480V VFD. We recently have found that due to the location of the motor a fan upstream is pulling a draft and causing the load on this 350hp motor to spin and thus causing the motor to spin. This motor and load spinning is causing mechanical problems (suprisingly no electrical problems yet) when this motor is spinning and then started while spinning.
We are trying to determine a way to detect if this load and motor is rotating (due to upstream draft) prior to starting the motor with the vfd. The only thing that I can think of is putting a shaft encoder on the shaft of the motor? Witout a shaft encoder, and with the motor not energized is there a way for the drive to be able to detect that the motor is turning? For instance is there enough residual magnetisim remaining in the motor when it is de-energized for the drive to detect a forward or reverse rotation? Maybe with some sort of sensorless vector setup??
What happens if this load is spinning forwards due to the draft and is then started while spinning in the direction of rotation? Will the current drawn be less since the motor is already running at a higher rpm point on the motor speed/torque curve?
What about if the motor was spinning in the reverse direction when started? I have heard that the motor would draw more than LRC due to the fact that it would be operating with a negative slip. Would the current seen be much larger than LRC? This is what I would think you would see with a DOL start but have no idea what you would expect to see with a VFD start.
Is there a function on the drive similar to DC braking that would lock the motor in place when not running.
Flychatching is for another purpose and would not solve the problem. it would speed up your drive in the reverse direction.
There is a function in some drives that is specifically designed for cooling tower fans and they detect the backfeed from the motor. They continuously inject a variable current to counteract the airmovement accross the blades and that is sufficient to stop the fan from moving in either direction.
A shaft sensor may not be able to tell the direction, only that there is movement.