VFDs, Noise, and RS-485

Ahh, interesting. Do you know of any papers on this? One would expect that 'over modulating' would have on adverse affect on the performance of the motor, ie heating. I guess this is not the case though if it is common practice.

I am building my first VFD for my father who is a retired engineer. So far so good. I am running motors, albeit, unloaded atm. Using the new IR iMotion IGBT module and a pic18f4431. I often wondered about the third harmonic theory. I guess for driving a 3phase lathe motor from single phase its probably not worth the effort. :)

hundreds, if not thousands.

Space Vector Modulation is by far the easiest to do digitally, and automagically takes care of overmodulation. Retter's Matrix and Space Phasor Theory of Electrical Machines is a darned good reference, and a lot cheaper than anything by Peter Vas.

The basic idea is to convert the 3-phase rotating vectors into an equivalent single rotating vector (can do because of fixed 120 degree relationships). This vector is broken into x- and y- components (called a,b). The observer then rotates at the same frequency, generating a stationary (wrt obs) pair of coordinates d,q. Align to one axis, the other is by def. zero. I'll see if I cant dig up a decent SVM paper...

Cheers Terry

I did do some reading on this concept. There is a good note on Microchips website.

If you can find that paper that would be great.

Thanks:

Andy

I'd also be interested in that paper. If you wish to email it, levy(underscore)jeff(at)hotmail(dot)com is a vailid email address, after replacing the obvious in brackets.

Jeffg

You've already brushed on the subject. It seems to be a question of understanding that all devices generate earth currents, the obvious and the not-so obvious, (eg, power transistors on a heatsink, or a switcher, or the field coils of a motor that you mentioned). It is important to plot a path to ensure that all such earth currents are explicitly returned to where they came from, not allowed to find their way home via some other device. That is an obvious thing to say, but sometimes not so easy to see and implement in practice.

Bitter experience results in automatic design habits that help to avoid earthing troubles in the completed system. Stupid little things, such as.......

Make twisted pairs, (triples, quads), your baseline wiring habit. This forces you to consider and make the explicit return between source and load, and twisted pairs reduces differential pickup or radiation. Keep the twisted pairs going, even through such things as fuses or CT-assemblies.

Screen each functional set if possible, with the screen normally Earthed at one end only.

With heavyweight source-loads, Earth at the source, and have a low impedance Earth-Earth connection between the source and the load.... already mentioned is the circumstance where this may be via the screen, although I would put in a separate Earth wire.

Low level devices which have an 'earthy' 0v and which are to be connected to an earthed device are always a source of ambush, because of the possibility of A.N Other's earth currents coming back through them. This would be things like scopes, counter-timers, comms from PC's, etc.

Isolation would be nice, but often not practical. The secondary solution is low-value resistors in the 0v lines, use differential signals if possible and/or try to have some sort of CMRR capability.

eg. Use RS485 not RS232 if possible, and try to check the CMV at the receiver end.

eg. DC to moderate freqency signals can be transmitted as currents, with a burden resistor at the receiving end. The c-c source does the CMRR and the transmitted signal is converted to a voltage at the receiver, with respect to the receiver's 0v. This can be applied to dc control signals, RS232 comms (if you are stuck with it) and even scope or counter-timer pickups.

Note: Run RS232 comms as two single-screened cables. This avoids Txd/Rxd crosstalk over long distances.

Simple tricks really.................