Controlling a VFD Frequency

Can someone point me in the right direction as how to accomplish controlling the frequency of a VFD when the frequency is a variable supplied by line of ladder logic?

Depends on how you interface. Simplest way is a 0-10 volt output card going to the frequency drive speed refrence. The ladder will be fairly straightforward, as in a few condtions with an out instruction to the output card.

One way is fieldbus. I have use it in last years. DeviceNet with AllenBradley and Omron brands and ProfibusDP with Siemens.

Usually there are 16-bit status-word and 16-bit integer for act-speed from VFD and 16-bit control-word and 16-bit integer for ref-speed or ref frequence to VFD. With Vacon VFD (http://ww.vacon.com and with many others) you can do identification drive 1'st when VFD 'studies' motor properties and can compensate motor speed when speed-referense have used. Depending of VFD manuf. ref/act signal is +/- 10.000 or +/-32.000, -sign means reversing.

With different PPO? (Parameter/process data object) you can even parametrisize VFD via Profibus and get more information from VFD as Torgue, Current, Power, Fault-ID etc.
You can 'standarisize' you program-blocks and not need any digital or analoque I/O.

br. Seppo