Inverter

How an Inverter Functions, How to Repair Inverters ...
Do you know neiljayr?

Isn't that how it works?

http://en.wikipedia.org/wiki/Power_inverter

http://www.circuitstoday.com/how-an-inverter-works

how is the frequency changing?

A moderate inverter frequency increase is necessary to compensate for the increased slip of loaded motor. Inverter fault or errors in programming it could make that increase bigger. How high does it go in your case at full load? S.M.

...so why is it perceived as a problem worthy of CR4's attention?

Assuming that by "inverter" you mean a variable frequency drive (VFD); "inverter" can mean any number of things now, but the context looks to be a motor.

A common feature in VFDs is what is called "slip compensation" for when you are using an open loop (scalar) output control algorithm (as opposed to vector control of some sort). When slip comp is used, the VFD is attempting to maintain a constant MOTOR speed regardless of load on the motor. Speed (RPM) and frequency (Hz), although related, are not the same. Speed = Hz - slip. When the load increases the slip, the VFD tweaks the Hz. How it does that is that under load, the motor slip increases, the VFD monitors the current draw, compares it to the Full Load Current you entered, estimates the new slip percentage then adjust the output FREQUENCY it sends to the motor in order to try to keep the actual RPMs constant. So if all you are doing is watching the display on the VFD, it appears to you that the "speed" is changing, but it is actually the opposite.

The down side of Sip Comp is that if you have widely fluctuating line voltage that dips below the programmed voltage, the maximum voltage that the VFD can output is artificially limited. The VFD might see a resulting change in current, ASSUME that the slip has increased, and the actual RPMs may change unwittingly. You should not use Slip Comp if that is a possibility.

Most modern VFDs now offer what is called Sensorless Vector Control. Good quality versions of this will have what is called an "Autotune" feature where the VFD tests the motor and creates a mathematical model of it in memory. It then uses that model to much more accurately control the final speed of the motor with precision of a magnitude 10X better than Slip Comp or greater. If your drive has that you may want to consider utilizing it instead, it is much more stable. Just be forewarned that several cheap brands of VFD state that they have "Sensorless Vector Control" when in fact all they really have is automatic slip comp, it is not the same.