That's a lot of requests and the concepts are intermingled with others. You may want to look for a local basics training class to get a deeper dive. I teach a class like this called VFD-101, it's a 6 hour class. That would be a lot of typing... But I'll try to give you the highlights.
Thanks, for all the good information definitely helpful, so this has got me interested more now in the actual construction of the VFD itself….I assume with the input you have the diodes with a a dc bus then the IGBT inverter part. Can anyone point me to any good reads on how each part is constructed / operates to achieve the desired output.
I'll get back to that or others may have some for you.
Also when looking at the basic wiring schematic what is the difference between the source and sink control inputs?
On small DC inputs, it just denotes whether the input is looking for a change on the DC positive side or the negative side. It makes a difference based on the type of electronic output field device or PLC output you are using to signal the drive inputs. You only really care in that they must match, so the drives are usually configurable to match whatever you have. If your field devices are real contact closures, it makes no difference.
To add to that what is the typical carrier frequency / switching frequency used in common drive construction?
It's a complex story, hang tight.
The output going to the motor is a series of DC pulses of a uniform potential, then the motor voltage is varied by the timing of successive On pulses and no pulses forming blocks of On time and Off time, referred to as Pulse Width Modulation (PWM). The inductive circuit that is the motor stator coils makes it so that current from those progressively wider then narrower pulse widths cannot change rapidly, called the "inductive time constant", and the net result is that although the peaks of all of the individual pulses are identical, the RMS (Root Mean Squared, a form of moving average) of voltage is controlled by altering the timing of the On pulses and gaps between them. At the same time, the VFD output has two DC pulse trains for each phase; one going from 0V to maximin positive, the other going from 0V to maximum negative. By changing how often you switch back and forth between + & -, you change the frequency getting to the motor. Combining that with the PWM voltage control, you create a full sine wave of RMS voltage at controllable frequency going to the motor. So by controlling the frequency and the RMS voltage, you can maintain the ratio of voltage and frequency that the motor was designed for so that the motor can produce full torque at any speed (in theory).
Within all of that stuff going on, the carrier frequency (CF) is the basic pulse rate of those high speed DC pulses making up the PWM output. It's not related to the output frequency going to the motor, it's the internal pulse firing speed of the transistors themselves, usually in the area of 1 through 16kHZ. The higher the CF, the smoother the sine wave looks to the motor, so the motor runs quieter. That's because inside of the motor, the steel parts of the stator core are made of stacked plates that are bolted/riveted together (on small motors, welded together on larger ones). The DC pulses cause what is called "magnetostrictive" vibrations in those plates that become a high pitched whining sound. The higher the CF, the higher the pitch and if you get the pitch to above 10kHz, most humans can no longer hear it (5kHZ if you are over 50 I've discovered...). However there is no free lunch as they say, so increasing the CF also increases the switching losses in the transistors of the drive, making the drive itself run hotter. Therefore if you increase the CF, you must also de-rate the drive. Every mfr has a different formula on how you do this, some more accurate than others, some almost impossible to understand as well because this is, as you can see, a complex subject that tends to not translate well from Chinese, so the cheap junk drives on FleaBay may not help you understand this well.
And who or what determines the final carrier frequency setting? from some additional reading it appears this influences the recommend conductor length from the drive to the motor as well, it appears there may be advantages to lower vs higher, but when one installs a drive “out of the box” what would the default be and who or why would one change it?
YOU decide the CF setting based on what you need to do, IF anything. The best practice is to leave it as low as you can live with, it's better for the drive and better for the circuit. If someone insists on putting the whining sound into the dogs hearing range, you must explain to them that it means buying a bigger drive for that motor and de-rating it. Besides the noise issue, if you are trying to get extremely precise torque control of that motor for a very specialized machine, increasing the CF increases the accuracy of that torque control.
As to the conductor length, that has to do with a phenomenon called "standing wave generation" that takes place in the conductors between the drive and motor. The high speed DC pulses in the conductors have the effect of making them act like capacitors, storing a charge. That charge represents a new voltage and it travels along the conductors until it meets a change in impedance, usually the terminations, at which point it "reflects" and goes back the other way. But on its way, it hits another wave and adds to it. Think of ripples in a pond after throwing in a rock, then throw in a second rock and watch how the ripples meet and amplify on one another in some spots. That's what happens in the conductors. That becomes a wave that moves along the conductors and the cross points can reach voltages that are as much as 3x the DC voltage. So on a 480V drive, you can see voltage spikes from those waves exceeding 2000V under certain conditions. Standard (old) motor insulation was barely 2x the AC voltage, so 1000V on old motors. That's why old motors rarely last long on VFDs. The longer your cables are, the more capacitance there can be, so the worse this gets. But if you increase the CF, then it takes even LESS distance before you get into trouble. That's why you saw the references to shortening the motor conductor length with increasing the CF and it's important to pay heed to that.
