Shox Dr
Chief Engineer to Carlos Fandango
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Most will state that you can’t have any reactive load.
They do. But what is an induction motor if not a reactive load?
Have a look here, not sure if it’s much help as the guy never replied with what he did.How about powering it via a VFD would it harm the vfd
Have a look here, not sure if it’s much help as the guy never replied with what he did.
Some good points mentioned about the frequency and harmonics, if I were to try it I wouldn’t run it as a vfd by adjusting the speed but as a “fixed frequency” converter at 50 hz.
https://www.practicalmachinist.com/...rters-and-vfd/vfd-step-up-transformer-171638/
I think there is a common misconception about vfds - that they produce an ac waveform - i.e. Sinusoidal; this isn't the case!
Most provide a square wave of sorts, with two frequencies - the first is the positive and negative half-cycles that simulate the ac supply, frequency matched to the speed of the motor, the second is a variable switching on/off to average out to a sine wave on each of the half cycles - imagine that at the peak (centre) the voltage is switched on most of the time, around half way down it is on around half the time and so-on.
The voltage is therefore always a series of the same-height peaks (on both the positive and negative half cycles) - just more frequent to give a larger average voltage within each half cycle etc.
It is this average voltage, put through a motor (rotating magnetic field) which smooths the current to a fairly sinusoidal waveform, without the rotating field it doesn't produce such a smooth waveform.
Thus, a vfd can't be used for resistive type loads (welders etc) - it would just produce terrible harmonics and radio interference!
Disclaimer!
Some vfds use a three step version, which splits each half-cycle into three steps, each step following the same switching process as described above. - There is also a matrix version, it works without converting to DC first, and basically supplies the output pulse from whichever input phase is best at the time
Thanks, there’s no misconception on my side, I work for probably the largest manufacturer of vfd’s in the world.
Your theory is a little off as well, as there is generally no “square wave matched to the speed of the motor” - the dc bus is switched at a high frequency (usually adjustable but generally set at around 4 kHz , if you play with the frequency setting you can make the motor run smoother/quieter at the expense of increased heating of the motor at higher frequencies) and actually uses pulse width modulation to achieve the approximation of a sine wave - I’ve never heard of one using two frequencies, to achieve this you would need a duplicate set of switching transistors in the drive.
I’m not advocating that connecting a vfd to the transformer will work, as I’ve never done it myself but there’s a good chance it may.
Have a look here, not sure if it’s much help as the guy never replied with what he did.
Some good points mentioned about the frequency and harmonics, if I were to try it I wouldn’t run it as a vfd by adjusting the speed but as a “fixed frequency” converter at 50 hz.
https://www.practicalmachinist.com/...rters-and-vfd/vfd-step-up-transformer-171638/