frequency inverter control

hi, please pardon a beginner's question.. i will be modifying a machine by adding sensors, motor & inverter..i have an inquiry regarding frequency inverter, pardon me because i am not familiar with control of this item on PLC..am i correct that voltage coming out from the inverter is discrete?in this case i do not need an analog reader to control the output of the inverter? if this is the case, when will i know that an analog reader is needed? what are the usual applications?

Read the VFD manual. Take a loook at the I/O on the inverter and analyze the functions.

In general you need a discrete output to start the VFD and an analog input to the VFD to tell it what speed to run. The discrete ouput on your PLC or panel should typically be a dry contact, since most VFDs supply the voltage for their inputs. The analog input on the drive can usually be configured to accept either 0-10 VDC or 4-20 mA and your analog output on the PLC should match.

The VFD status should also be monitored. Most will have a dry contact on the VFD to indiate run and fault status and at least one analog output. The analog signal is usually configurable to be 0-10 VDC or 4-20 mA, and can be set to reflect actual speed, amps, or kW. Your PLC should have an analog input to match. The VFD's discrete outputs are usually dry contacts, so your PLC input should be powered from your panel.

I'd like to help you, theripley, but I don't understand what you mean by an "analog reader". An inverter is normally configured to follow an input speed signal which varies from 0 to 10VDC or from 4 to 20 ma. Many inverters also provide a corresponding analog output signal which varies over the same voltage or current ranges to represent their actual output running frequency.

Oops! I see that Tom Jenkins got there first. Consider my post as duplication, then. Sorry!