Harmonics in VFD output 1

[edison123]

This is related to my other thread about changing the rated voltage of a VFD motor.

I have some basic questions about VFD outputs.

1. Does the output voltage of VFD below the rated voltage have more harmonics as compared to the rated voltage ? i.e. If the rated output voltage is 440 V, does operating the VFD at 220 V produce more harmonics ?

2. Do the harmonics of the VFD output change with the frequency ? For example, does a 25 hz output have more harmonics as compared to a 50 hz one ?


3. Do all VFD's have a parameter to take care of V/f issue ? For example, if a 380V, 60 hz motor is operated at 50 hz, how does the VFD take care of the V/f issue ?

 

[jraef]

To define the question a little better, you really need to consider the difference between voltage harmonics and current harmonics, the difference between problems on the line and load, and the difference between what a VFD would normally d and what you want to do in your situation.

First let's look at "normal" operation. As you lower the speed, the voltage THD (Total Harmonic Distortion) on your supply grid will stay the same because it is a function of how power is drawn by the rectifier, not how it is recreated by the inverter. Current harmonics will change however, especially what is referred to as the TDD, Total Demand Distortion. There will be more current distortion as a percentage of total current drawn, but at 1/2 speed, you will be using 1/2 the power, so the TDD as it will affect your grid will be lower because the overall current is lower.

On the load side, the only thing that matters is the current harmonics as far as the motor goes, so again, at 1/2 speed the total load on the motor will be less so the effect of current harmonics will be less even though the distortion percentage of demand current may be higher.

Now applied to your situation where (IIRC) you want to use a 440V supply to feed a VFD and have it deliver only 220V to the motor at full speed. Now you will have more current TDD, AND the motor will be fully loaded, so I would expect the motor to run extra hot.

As to your last question, when you initially program the drive to provide a maximum voltage at a given output frequency, that sets up the V/Hz pattern parameters and once set, it is maintained at the same ratio at all speeds (assuming Constant Torque here). So you will go in to programming and take the drive off of default settings, telling it instead that at full speed frequency (60Hz), you want the maximum voltage to be 220V. That creates a V/Hz ratio of 3.7:1, which would be automatically maintained throughout the speed range.

 

[edison123]

Thanks jraef. That was excellent. It answered many questions.