Re: Ps, Re: [emcoV10lathe] vfd load reactor suggestions?
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Please note this explanation works on concept level, and is not always what is actually happening. VFD on-off output is then smoothed by the winding which resists current change, effectively making a low pass current filter - higher frequency is blocked and lower one is allowed through. When you switch the power off on the winding (or any inductor) it still opposes any current change and it starts dumping stored magnetic energy to keep the current going, but if you just disconnected the power the current has nowhere to go. This results in a voltage rise (spike) over the winding and this voltage can be significantly larger then what was originally supplied by the VFD. This is similar to the water hammer that occurs when you close off the tap quickly. Spikes are very short and they don't have a whole lot of energy, but you get one each time the VFD switches, which is usually 4-20 thousand times a second (4-20Hz). Maximum voltage the spike will hit depends on the speed the VFD transistor switches off. You could make the transistor turn off slower, but that results in heat (in the transistor) and overall inefficiency which is not good.
VFDs (good ones at least) include some sort of a freewheeling diode variant that
allows the inductor current to keep flowing after the power is switched
off.
Simplest way is to include a diode in parallel to the inductor so when power is switched off, current can flow the diode instead of through the VFD and "freewheel" between the winding and the diode (VFDs don't actually use diodes for this, but the principle is the same). Bicycle analogy works for this, bad idea circuit is like a fixie - if you try to stop pedalling, the energy in the system will still force your legs up and down. A regular freewheeling bike will allow for the pedals to remain stationary even when the bike is still moving. So far so good, but each wire is an inductor, capacitor and resistor in one, so there are higher order effects because of that and other reasons. For instance, diode needs a bit od time to start conducting. It means it won't get the first (and the worst) of the voltage spike. When you add filters to the motor, you're either slowing down the switching speed, thus making the spikes smaller or trying to arrest the voltage spikes once they occur (or both).? Regards Vedran On Sun, Mar 23, 2025 at 12:13?AM Sierra Tunafish Monkeymind via <barkingcarpet=[email protected]> wrote:
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