So you may have seen me yelling about power line harmonics… here’s what I was looking at earlier this morning. This is the power at a facility I was doing some work at earlier today. The same power has laid waste to two variable frequency drive units and an Omron 24v power supply used to run a Programmable Logic Controller (PLC).
I used the FFT mode on my Tektronix DPO 2012 oscilloscope to better detail what’s going on. The yellow trace is the waveform coming in (you can see it’s not a perfect sine wave). The red trace is a plot of the frequency vs. amplitude after a Fourier transform. This same kind of plot may already be familiar to you: it’s displayed on many audio players and stereo systems as a spectrum analyzer visualization.
If this were perfect clean 60 hz power, I would have only the tall peak at the left which is 60 hz, then it would roll off to the baseline. What I got, however, is this (and a realization I accidentally set my scope’s clock 12 hours off *before* DST kicked in…)
The FOUR other peaks are the harmonics. Initially I thought I was looking at a second, third, fourth, and fifth harmonic, which would put the last one at 300 hz– reading this again though, it’s scaled 500 hz/division so what I’m looking at is the odd order harmonics at up to 540 hz– a NINTH harmonic.
Now let’s look at that third harmonic, the most prominent. That one’s about 20dB down from the fundamental (the scale is 20dB per vertical division). That’s a voltage ratio of 0.1… so the third harmonic, if you were to isolate it, would be 12 volts at 180 hz. That just plain doesn’t belong!!!
A textbook square wave is the fundamental frequency plus all of its odd-order harmonics– 3, 5, 7, 9…. I suppose this makes sense, as if you smashed this AC waveform very very badly you would get a square wave.
Power line harmonics are an annoying effect of loads with a poor power factor. Want an example of a load with a poor power factor? You’re looking at it. The wall chargers for smartphones, power supplies for computers, and even LED, florescent, and compact florescent light bulbs are guilty due to the nature of their power supplies. Without getting too much into the theory I’ll say this: the way they work is that they draw power right at the peaks of the usual sine wave power. This is why the peaks in the above screenshot are getting smashed: that’s when EVERYTHING draws its juice.
Some *nicer* devices incorporate power factor correction circuitry to mitigate this. This usually takes the form of a low pass filter that acts on the amount of current drawn by the device, in an effort to keep it from just suddenly grabbing only the peak. Note that I say— “some”—
The harmonics tend to overstress everything by causing high currents to flow in wires, transformers, power supplies, etc– they are not only harsh to the equipment but they are a waste of energy.
And here, boy, do we ever have ’em.
The solution, ultimately, will probably be to have the local power company install a bank of capacitors for power factor correction on their poles.
Then, hopefully, the power will stop being quite so…….. hungry for electronic snacks……
Fine stranded 16 gauge electrical wire (as opposed to the thick stranded THHN type stuff you’d run through conduits in a wall)
I had a fun time calling around this morning to find this out– Grainger only has two locations left around here, and even the sales lady on the phone was surprised by this. She was used to South Florida having a ton of them a few years back. I first learned of the Richmond Heights one having closed up shop when my last brush with high energy explosion happened… Luckily the one remaining location happened to have a VFD in stock I could use. Phew.
Grainger were the only ones who would be able to get the fuse holder here next day because they had them in a warehouse in Orlando (they also have their own trucks, so I’m confident it didn’t get melted down in Hialeah in the UPS warehouse fire). They had TWO in stock, dashing my plans to get three matching ones for the 3-phase VFD— oh well, different fuse holders but same fuses, whatever works for now. A 3 pole fuse holder would have had to ride the slow boat from China, just like if I’d wanted three of the single pole ones.
What bugs me is I USED to have the exact fuse holders I would have needed but threw them out last year, trying to reduce the size of my collection of parts I thought would be easy to buy again if needed.
Anyway, here’s the beautiful mess that inspired me to get very mad this morning while standing in front of a voltageless robot.
The problem: VFD controller smoked out. I didn’t take any pictures of the outside of the controller for it looked unremarkable save the “MOFF” error flashing. It looked like “ПOFF” due to the limitation of it having only a 7-segment readout, more rambling on that later. It might as well have said “F.OFF” though. The error meant it wasn’t getting sufficient voltage on the internal DC buss.
A quick primer on how VFD drives work:
On this inverter, the high energy semiconductor devices for the rectifier and the inverter were neatly built into one easy to deploy little module by Infineon, an EasyPIM series module.
I’ve seen similar modules in several VFD drive products. They contain multiple transistors and diodes coupled thermally (but not electrically) to a backing plate for heatsinking purposes. To avoid undue stress to the semiconductor junctions, they are filled with a curious sort of…. snot.
Sorry if I’m grossing you out, but touch it sometime, it’s really reminiscent of a sticky booger.
A similar product to that is Raychem’s “GelGuard” which I’ve seen used in outdoor telephone connectors. Very weird stuff.
Here’s… what was left of the module… This is 180 degrees to the layout of the diagram above, more or less, and I picked away the blackened snot to reveal what…. DID NOT remain…
The obliterated parts are the rectifier diodes. Even though only four of the six were used in circuit due to this being a single phase drive, the remaining two were blasted to smithereens by excessive reverse voltage. The IGBTs at left (right in the diagram) that control the outputs look pristine and would probably even test good. Sorry, too lazy to whip out the ESR meter. (Does that even work on high voltage IGBTs?)
The CPU block in the above diagram is the timekeeper of the operation. It reads in whatever form of commands the unit is set to accept as to which direction and speed to run the motor. It also monitors several analog values throughout the unit – voltages, current levels, and temperatures – and can adjust its operation to protect itself and the motor if necessary by throttling back the amount of power it sends to the motor or shutting down if necessary. In this case, it was successfully detecting the lack of high voltage DC to run the drive and just wouldn’t do nuffin’.
Now if the damage had been confined to the module – that’s actually a standard part from Infineon and could probably just be ordered, buuuut—-
Yeah, this let it all right. It almost looks like the conformal coating tried to contain the Magic Smoke, and maybe even flashed over? The two line fuses for the drive were blown so something fun must have happened on the board itself. If the rectifier had just failed short, I would have expected ONE of the two to have gone, not both simultaneously. Flash to ground probably got the other one.
The client has been having a drive blow up every three months or so!! I’m going to retrofit the machine for him with some BIG HONKIN’ SURGE PROTECTION. After his first VFD blowout he called in Florida Plunder & Loot who did *something* to adjust his line voltage, but it’s still mega nasty. I’ll put the scope across it before installing the new drive and likely cringe so hard my face freezes that way for life.
Amazingly the drive still powered up, scrolled “HELLO” before going to “MOFF”, and I was able to get all the settings from it via the front panel user interface. See the small transformer at top left on the board? I suspect there’s a separate switcher power supply just for the CPU and logic that allowed for this even though the HVDC system was blown to bits.
And herein is another bit of rambling: The user interface on VFDs, for the most part, royally sucks. You’re presented with a menu system on the front panel which gives you anywhere from about a dozen to *HUNDREDS* of parameters, identified only by number. You have to break open the manual and go through dozens if not hundreds of pages of crap if your setup is in any way complicated. Fortunately THIS ONE IS NOT. This one was simply being driven by a +24vdc trigger to turn the motor on, and 0-10 volts DC to control the frequency between 0 and 60 hz. But still— Nice features I’ve seen in some units: Danfoss and Cutler-Hammer VFDs have a small control module that speaks plain English to you. Some other controllers (I’m unsure what brands off the top of my head) save the parameters to a small serial EEPROM or even an industry standard memory card, so if your drive does— this— you just swap the card into the new one and you’re ready to spin again.
An important word on the “Death Caps”:
THIS NAME IS NOT UNDESERVED
The capacitors inside a VFD drive will retain potentially lethal voltages FOR A VERY LONG TIME after power off. Even if they are discharged, they may exhibit a peculiar self-recharging phenomena where a stored charge in the electrolyte partially recharges the cap to dangerous levels again. Always discharge caps and test before touching! On a small self contained drive you cannot get in contact with the caps without disassembling the unit; however, I’m aware of there being larger units where the DC section is a separate unit and those can probably really get you. TEST FIRST. Also, if you are storing a spare unit, be sure to check the manufacturer’s directions on maintaining the capacitors!! Most say to power the drive up every year or so to keep the capacitors “formed”. If you suddenly connect power to a drive that’s been stored too long, EXPLOSIVE capacitor failure may occur. If you’ve got one that’s been sitting, consult the manufacturer’s directions for reforming. If unsure, use a Variac to slowly bring the voltage up, or put a small incandescent light bulb in series to limit the current. (The drive should draw almost nothing if it’s just powered up and not doing anything.)
Just got my first package from HackerBoxes and I’d originally planned to make an unboxing video but that just wasn’t gonna happen unfortunately–
So I’ll spill the secrets of what was inside…
Waaait up is that
Is that…. A PENGUIN?
Clearly I have made a very good decision here.
Holy moly I love this already.
This is the “Cellular Metal” box. Next to the breadboard is a small GSM(?) cellular modem. I haven’t looked up the info just yet. The black cable is a u.fl to SMA pigtail to connect it to the small antenna. There’s also a SIM card.
A pack of breadboard jumpers and components are present, as well as an Arduino compatible board, a USB to TTL level serial adapter, an Atmega328 (as used on the Arduino) and an ATTiny.
The card, meanwhile, does double duty. Flip it over and…
This is just as cool as the business cards from Marlin P. Jones Associates that have an electronics color code/resistor code guide on the back.
Soon I shall be looking at their online documentation to see what possibilities are in store for all this.
And…. Finding a suitably awesome spot to put that penguin in. I mean— it’s Tux with “Hack The Planet” written on it—!!!!! Soooo perfect.
From the MOTOTRBO mailing list. Names omitted to protect the innocent.
While I understand the meaning of RSSI in the WiFi world, I am curious if a LOW (i.e. -60 DBm) means a better signal being received from the DMR repeater I am connect to, or is a -106 DBm a better signal.
While I think this might sound like a remedial question, I am trying to get clarification.
Appreciate any information you can provide
The first reply:
LOWER IS BETTER
And the light clicks on
-106 is lower than -60. Are you guys sure you’re explaining yourselves well?
Such is just basic number theory.
(Negative numbers) Weaker signal <- 0dBm -> Stronger signal (Positive)
The 0dBm point is the reference – one milliwatt. Numbers below or above are that many dB above or below one milliwatt. In the case of receivers, the signal strength is usually very well below one milliwatt. In the case of transmitter output power, it is often above. In either case a higher number is a better receive signal, and -60 is higher than -106.
It always just bothers me when I see this as someone who thinks a growing absolute value of a negative number in this field will be chasing their tail FOREVER trying to figure out reception problems. 😉
This Tektronix 1705A L-band spectrum analyzer has temporarily borrowed a garment from an old waveform monitor.
Because as I had it on my bench powered up naked after running through a calibration on the CRT power supply, an operator comes walking in and rests his arm riiiiight here. Luckily I got him to move before any BANG occurred.
How do you even guard against awful human error like this?!
Now if you’ll excuse me— I need to make a sign for him. 😉
Bonus: The resistor the other tech never bothered to replace. Amazingly it still works, but I wasn’t gonna leave it like this!
Today’s— uh, victim— JBL LSR2325P active studio monitor. It’s a nice sounding biamplified monitor with an active crossover system and suspicious “Imagine” brand capacitors. Hmmmmm. 😉
Our music producer came to me with this loudspeaker he uses to play his creations for our news director, among other things, because it was crackling and popping ferociously when the input gain knob was touched. I found the input gain knob loose on the rear panel and guessed I’d also find cracked solder joints. But where?
Input gain control is below the inverted plastic bathtub under that board. So how do you remove this plastic bathtub? Desolder the shitty thermoplastic power switch—- which will melt and eject its metal parts. WTF??!!
You can see the switch on the panel here – it’s a snap in flange mount – the only way to get around doing this would be to cut away the plastic flange and back it out, I guess. The tub it’s in is sealed so this wouldn’t create an air leak. But still— AARGH!!! Also, WHY THERMOPLASTIC? I have a problem with this. See, if the switch starts warming up, the plastic will soften, removing pressure from the contacts, creating more heat. Eventually the fault will only clear when the switch either totally loses contact or the thermoplastic erupts into flames.
Proper electronic assemblies use thermoSETTING resins. Glass reinforced polyesters/epoxies are nice. These are resins that set either when two parts (a resin and a hardener) are mixed, or enough heat+pressure are applied to kick over a curing reaction. This reaction is a one way process and the resulting product WILL NOT MELT and soften. It may eventually be flammable, but most thermosetting resins, especially glass fibre filled ones, have a very good track record of self extinguishing.
Phenolics are very common in solderable connectors. You can always tell when you’re dealing with a phenolic resin because it will not soften and allow the connector to deform with extended heating during soldering. These resins are often colored teal blue/green, or a tan color on Amphenol products. Ever wondered what the name “Amphenol” is about? 😉
I don’t even want to think too hard about what that AC power inlet fixture is made of, all things considered.
The header pins leading to both the boards inside this tub were also graced with total shit-grade soldering and I reworked them. There’s one board below with the three jacks and one board above with the amplitude pad and the HF/LF trim filters. I resoldered the input pot and tightened the nut around it with some Loctite purple on it. In theory, I probably should have used blue, but I can’t find the blue, and red is right out of the question. Whatever works, right? I’ve had just as good luck with things like this using nail polish on them.
That’s the fate that befalls any nail polish I buy that looked GREAT in the store but when I put it upon my claws it turned out all watery looking or otherwise unsatisfying. (“NYC Color”, this means you. Well– some of their shades. Some of their newer ones are actually formulated with, well, color, in them.)
After this– I can’t wait for my assistant here to show up so I can run my fingers through his thermal insulation and hear him make silly happy squeaky meows.