I have a client with the following question on stop modes for machinery.
I have a VFD which has a RUN input which when closed is configured to command the VFD to follow the DAC speed reference. When open, the VFD is configured to ramp stop. I have this input tied to my overload circuit such that a mechanical overload will open the circuit and cause the motor to ramp stop. I have a sharp deceleration such that the motor will stop in the shortest distance possible. When I E-Stop the machine, power is removed from the VFD. The motor (due to inertia) will coast some distance when power is removed from the drive in and E-Stop condition. The drive control is lost in this mode thereby eliminating the possibility of ramp stopping the drive. I want to design in a delay so that I can open the RUN input before removing power from the VFD in an E-Stop or overload condition. My thinking is that I can then ramp the drive down faster than just removing power from the drive. I have a time delay module which can be fitted to the main drive contactor. This module gives me a .3 - 3 sec adjustable time delay. My thinking is that when the safety or overload string drops out, the control power (switched 24 vdc) is lost thereby dropping out the power to the run relay and power contactor. Since the run relay drops out first, the drive will ramp down before the power contactor drops out. My question to you is: Is this allowable in CE and U/L? I believe that this approach is acceptable as long as a risk assessment is performed in accordance with EN 1050. Does anyone out there have a perspective on this issue? RCIC - http://www.rcic.com Regulatory Compliance Information Center
