Hi Gerry,
Also, the VFD stores power in capacitors, which the spindle draws from. It shouldn't be sending voltage back through the electrical system.
You are correct, the only way the charge stored in the capacitors to get back into the power supply would be a catastrophic
failure of the input rectifier bridge.
What that does not accommodate is the natural inductance of the supply.
If the load (VFD or whatever) suddenly increases its current draw the voltage will dip. Part of the dip will be the resistance
of the supply, we hope its small but we know its there. The other part of the voltage dip is caused by the inductance
of the power supply. Even a straight wire has an inductance and the secondary of the distribution transformer will
have both magnetizing reactance and leakage inductance. The net result of the wiring and the distribution transformer
is a residual inductance commonly of the order of 1mH.
Lets now imagine the load which had been drawing 20A from the supply sudden stops drawing current, lets say you threw
open the switch, then the inductance will mean that the current cannot stop immediately an a marked increase in voltage
in the supply will result.
In high power electronic circuits where large currents are switched on and off extremely quickly the residual inductance
of the power supply can cause very high voltage spikes.
Take a single phase VFD for instance. The incoming line voltage is 230VAC(rms), its rectified and capacitively
smoothed to 320VDC. At a given instant one of the MOSFET's is conducting 20A and it switches off. The current
will stop within, and ideally, less that 1 microsecond. The inductance of the input supply will cause a voltage spike
(without control or precaution) of another 300V or so. Your nice rugged industrial grade 500V MOSFET has now
a voltage spike of 620VDC to contend with. Whats more the same MOSFET will turn on and off rapidly, say 50,000
times a second, so it gets repeated hits and promptly blows up.
The voltage spikes due to residual inductance are very troublesome to the design engineer who has to design and
implement snubber circuits. Indeed the reliability of the equipment depends on it. Often the quality of the design
can be estimated by the effectiveness and ruggedness of the components used in the snubber circuits.
The upshot is that it is not impossible for the current drawn by a VFD to cause a very high voltage spike which
could easily damage other equipment like PC's connected to the same circuit. I like you have my VFD connected
to the same outlet as my PC and stepper supplies and have not encountered any difficulty. It is only a 750W VFD
however and I treat pretty lightly as a rule.
More recently I made another spindle based on a 1.8kW AC servo with a single phase 10A output/phase servo drive.
I give it hell......and found that I was getting conducted noise issues through the common supply. Nothing blew up but
I think I would have if I'd carried on. What I did was fit a line reactor and all those issues went away and seemed incidentally
to improve the servo drives overload capacity.
That OP managed not just have a voltage spike but a surge enough to take out all other stuff connected to the same circuit,
PC, stepper supplies etc suggests a rather bigger fault than a regular voltage spike. I don't recall ever having seen
a 110V VFD. What is it about them that causes them to be troublesome?
Craig
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