staco 3pn1020b messing with my CT sensors - Page 1
Forum section: electronics / projects

author: rush2112
author_rank: Newbie
timestamp: September 06, 2014, 12:36:32 am
content: Hello all,I am measuring electrical current using clamp on CT sensors (YHDC 50A sensors). I was using a variable load that consists of a bank of 100w incandescents with one of them on a variable autotransformer (the staco 3pn1020b). It worked great for getting the average amperages that I was looking for, but when I opened up the second by second CSV data that my device records, I noticed that there were some ridiculous looking spikes. e.g. if a normal reading is 86, all of a sudden it would spike to 1900, or even drop to 24...the readings were very erratic, and consistently erratic across multiple devices and multiple sensors.After some head scratching (read: panic) and wondering if my devices are all inherently flawed, I used an oscilloscope to look at the waveform coming out of the variable autotransformer (hereon referred to "incorrectly" as a variac) and noticed that at certain settings the waveform coming from the variac would go totally bananas. For instance, at 87.5% on the variac (which is 100% of normal power) the oscilloscope can no longer determine the frequency of the signal. Since I haven't experienced erratic behavior from my device anywhere else, and since I was able to reproduce the erratic behavior using the variac, I feel comfortable that the variac is the culprit.What I don't know, is why.This is where you guys come in. I have very little experience with variacs, and while I've learned a great deal about them in the last couple weeks or so, I still don't know why this would be happening. I've narrowed it down to several things:1) All variacs do this.2) I'm not using this variac properly.3) This variac is defective somehow.Regarding 2): This particular variac says "Input: 240V" but appears to work fine with our 120V US mains power. Could this affect it?What do you guys think?-Josiah

author: MK
author_rank: Regular Contributor
timestamp: September 06, 2014, 08:09:06 am
content: Perhaps you have worn brushes on it? it should cope just fine at lower input volts than nominal. It is just an autotransformer with a carbon brush providing a sliding contact to a suitable turn.

author: denelec
author_rank: Regular Contributor
timestamp: September 06, 2014, 12:06:18 pm
content: Maybe you have arcing in your variac. It would explain the spikes and the crazy waveforms.Like MK said, a worn brush could be the culprit.

author: saturation
author_rank: Super Contributor
timestamp: September 06, 2014, 01:00:01 pm
content: I think too, its the variac. If this is not a new one, or not stored properly it could also just be dirt/debris on the brush surfaces. Just rotate the knob back and forth 0-100% repeatedly to try and brush off anything that could be on it if it improves, then you have a diagnosis. Those variacs have vent slits to ambient, so they can pick up stuff it not stored properly.

author: coppice
author_rank: Super Contributor
timestamp: September 07, 2014, 05:50:37 am
content: What you are experiencing it pretty common with old Variacs. Few of them feel smooth as you turn them. If you take off the cover you'll probably see all sorts of gunk, and maybe a worn brush, showing you why. The are often quite easy to renovate. However, it sounds like you are hitting spots where the brush is actually arcing against the winding. Arc damage might be harder to repair, if it has been going on for a while.