I'm trying to remove a 60Hz hum from the outputs of a 3phase amplifier used to control an electro-dynamic shaker. I am guessing that the hum is being caused by a ground loop between the amplifier controller and the amplifier itself (coax connection).
Are there many differences between the brand/styles of ground loop isolators? How do these devices work?
I had a Radio Shack isolator which recently seemed to be introducing more noise than it removed from the system. I'm hoping the isolator is broke and not something else in the system.
I would like to find something of industrial durability to use; if anyone has recommendations please post them!
I've been told that the noise could be introduced by other devices leaking current to ground, or noisy current leaked between AC mains transformers through capacitance. I don't quite have an understanding of the last reason there, but if the problem is with the building wiring what methods could I use to check/fix this?
Hi, David. Try to be concise, and provide complete information when posting with problems. It makes it easier when you've taken a minute to reread your post and made sure all of the information you think someone might need to answer your problem is there. It would have been very helpful to know the equipment you're working with.
If it's any consolation, though, this is not an uncommon problem. Shaker tables seem to want to be bad as far as hum. Here's a list of checkables off the top of my head and limited experience:
It _did_ work right before -- something changed. It's basically working right now -- there's just been a change of some kind. Look at the changes first, particularly those which have been introduced from outside the system.
Check with the house electrician/maintenance person for any recent house wiring changes. That can cause this kind of problem.
I suppose the signal generator operates off 120VAC, and the shaker amplifier off 240 or 3-phase -- they're usually high power beasties. Look for yourself that they have a common GND, and that no GND connections are resistive. Look to see that L2 has not been swapped with L1 on the 120VAC line, and that it's not floating. If you don't know, or don't know how to check, ask the house electrician/maintenance person.
Try to determine what changes the problem. That's frequently a good indicator of where to look. Try giving your amplifier a 0V signal input by replacing the signal generator with a signal terminator, and see what happens (using proper terminating resistors, of course). This will tell you something very important.
Check _all_ GND straps, connectors, &c., on everything. Reseat all connectors in their sockets, check all nut/lockwasher connections by giving them a twist. If you see anything loose, fix it, then retest. A shaker table can be a high vibration environment for electronics.
The coax between your signal generator and the amp could be bad. Flexing the cable and checking with an ohmmeter isn't the best check, but might tell you something. Try swapping in a known good cable, and see if it helps. Make sure you've got the right coax.
If the chassis of the signal generator is not bolted to a common rack or otherwise tied into the system, try physically and electrically isolating the signal generator chassis from the rest of the system. (This is where your power isolation transformer comes in.) Test. Does it eliminate the problem, do nothing, make it better, make it worse?
Remove all unnecessary test equipment. Test. If something is only required for setup, use it and then disconnect it. Does that make a difference? (Note: I once worked on a shaker table where an old DuMont scope permanently installed in the amplifier system and used solely for monitoring the input signal and accelerometer feedback had gone flaky. The scope itself was injecting hum into the system! I'd be embarassed to say how many other things I checked first! ;-) )
While you're on the subject, check any coax going to an accelerometer or other system stuff, too.
A couple of other points. The RS audio isolator (270-054) probably won't help you. A signal isolator might, though. If you gave more specific information about what you've got, you might get a more specific answer. A signal isolator provides transformer isolation of the signal. It's very dependent on the input and output impedances (which you didn't mention). They are also made to work over a given frequency range (which you also didn't mention).
Power transformers have many turns, both on the primary and the secondary. These windings are in close proximity, and separated by an insulator. The definition of a capacitor is two conductors separated by an insulator. The value of the capacitance is dependent on the details. Look up transformer capacitance to find out more. The thing is, it's possible that this capacitance is injecting a 60Hz AC component into the power supply of the signal generator or the amplifier.
Oh, yes. When all else fails, RTFM (Read The Fine Manual). It's a free education, and the answer you need just might be in there.
The classic loop killer is just a lot of turns of coax wound on a high-mu transformer core, with response down to DC and no actual electrical isolation. This adds a lot of common-mode impedance but doesn't affect the normal-mode signal path.
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