Filter Question! How to get rid of SPIKES!

I have a 60Hz AC voltage comming from a transformer 120/220V that=20
powers some cooling fans, but it's got some spikes at the output, and=20
Im having problems with that. The spikes are burning the fans.
Here is the link of the wave form:
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What the main problem is at 2Khz, am I right?
I want to make a filter to attenuate those spikes, but I=B4ve never done=20
a filter for voltages above 12V. I know just how to use op-amps, but=20
for 220V Its got to be a passive filter.
Does anybody have an idea of what type of filter would be more=20
appropriate? Low pass, tuned filters....
I have JUST some 3,9mH inductors to make it, the other components can=20
be anything to match the configuration.
Anything will be helpful!!!
Thanks a lot!
Reply to
Victor ippolito
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really? what size fans? little computer chassis fans or big building ventilators?
the dog is named 'Spike" ?
A; use a constant voltage transformer from Sola or some other mfr.
B: use an appropriatly sized MOV.
transformers mostly do not generate "spikes". it is likely that whatever you are seeing is caused by the load.
Reply to
Do NOT use a constant voltage transformer. A constant voltage transformer is a special type of transformer that is tuned to give the correct voltage at 60Hz, which means that they don't really filter out anything CV xfmrs are also designed to run with a magnetically saturated iron core. This means that they run HOT, which wastes electricity and costs $$$.
An MOV will also NOT solve the problem, only make it a little bit better. An MOV will keep the voltage from getting too high, which means it will clip off the top & bottom peaks, but the spikes in the middle of the waveform will still get through.
You should add an MOV, but mostly you need LC filters. Put all components at the primary side of the transformer. The MOV connects to hot & neutral. Then put one inductor from hot to one side of the primary, and another inductor from neutral to the other side of the primary. Put the capacitor directly across the primary.
Values can only be determined by experimentation. Use a voltage rating for the C which is much higher than the RMS primary voltage. E.g. 120VAC = 120 x 1.414 =169V peak = 340V p-p so you need 400V minimum. The MOV will keep any input spikes from exceeding the capacitor voltage rating.
Capacitor value can only be determined by trial and error. Start with 0.001 uF and increase.
Also put an MOV rated at the transformer output voltage across the secondary, in case there are any turn-on spikes.
As another check, is it possible to montor the transformer secondary voltage with nothing connected to it? Maybe something in your gear is generating the spikes, so you need to put a filter between the fans and that stuff. ------------
Reply to
No Spam
The advise sounds OK, if you have the surges on the primary side of the transformer, except I would also suggest an MOV on the primary side of the transformer. The MOVs need rating at the line voltage plus 10% to allow for variations in the supply voltage, plus another 10% to allow for the tolerance on the MOV itself, otherwise you could end up very short MOV life. As I have read your posting you have a primary voltage of 120VAC, therefore plus 10% = 132, plus another 10% = 145VAC. The nearest standard MOV rating is 150VAC. On the secondary side the voltage is 220V, therefore plus 10% = 242, plus 10% =262V rms. The nearest standard MOV voltage easily available is 275V rms. To protect the MOVs form going off with a bang when it fails you should fit a fuse in series rated at the full load current of the fan(s). To size the MOV is no easy, but if you can post the fan's full load current(s) I will try and advise you.
If the surges are actually been generated by the fan, this will need a better suppression circuit on the secondary side of the transformer. Do you have a on/off contactor or relay in the circuit where you switch the fans on and off? If the surges are due to the fans I would suggest MOVs line to line and line to ground, (three MOVs). This offers the best protection. again the sizing of the MOVs depend on the full load current.
On similar systems I have measured surge voltages of 5000 volts, but with a supply of 480V, these were reduced to less than 700 volts peak once the MOVs were installed. you can also add a simple RC network to help reduce the peak value.
MOV can be dangerous so please read the Littlefuse publications regarding there installation for more detailed information,
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have look at:
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Do not try installing MOVs in parallel as it does not increase the ratings due to the tolerances on the MOV's voltage, fit a larger MOV.
Hope this helps
Reply to
Since there are 6 spikes per waveform, with two of them at the peaks, I'll _guess_ that a cause may be lots of computer equipment with switching power supplies plus poor (high impedance) wiring. If it is, the equipment (whatever it is) is on all three phases of a 3 phase supply.
Maybe a separate circuit is all you need.
I'm somewhat surprised that a motor (fan) would be bothered by those spikes, even though they are large.
Reply to
Michael Moroney
MOVs would not even appreciably affect those larger spikes. MOVs=20 for 120 VAC typically set with a let-through voltage of 330 volts. =20 Anything less is mostly ignored. The many recommendations for MOVs=20 simply do not understand how MOVs operate; would not solve your=20 problem.
Consider a line filter. Of course filtering must be sized to handle=20 the load (current). Some examples:
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Meanwhile, a better solution identifies source of that noise. If it=20 is adversely affecting fans, then AC electric noise is doing same to=20 other building electrics. Furthermore that noise may be the symptom=20 of something else about to fail - a suspect list also includes utility=20 power transformer. IOW solve the problem - the source of that noise. =20 Filtering the fan would only cure a symptom.
Reply to

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