Topics

Scope grounding and generators


Carsten Bormann
 

On 2020-10-25, at 19:07, sdturne@q.com wrote:

Sure, if you have one of those it is probably suitable as long as it's in good condition.

The point I was making was that the OP really should get the right tool, be it a scopemeter, a 222/222PS, etc. A laboratory oscilloscope is not the right tool for scoping high energy circuits. Just because you can doesn't mean you should!
Surprised that nobody has given the obvious answer yet:

Get a high-voltage differential probe.

These can be expensive, but if it does not have a to be a Tek, the original question would scream for a Micsig DP10013 or DP20003 (depending on your budget and the voltages you need to look at).

These come from Shenzhen, but are not bottom-drawer garbage.

The main limitation of these is the noise: ≤ 40 mV rms in 50x, ≤ 230 mV rms in 500x (DP10013 specs).
So you can’t measure a dynamic microphone that is stuck at 300 V from earth ground :-)
(These are 100 MHz probes, so setting a bandwidth limit on the scope is going to help some.)

https://images-na.ssl-images-amazon.com/images/I/91h8jF1eMFL.pdf
https://www.amazon.com/dp/B074K4XPW3 (a bit expensive)

Grüße, Carsten


Greg Muir
 

As many have stated on this list a high amount of respect for dealing with AC mains work is paramount. I think Chuck Harris said I best. I double his comments regarding using a standard oscilloscope. If one must there are ways to do so as to protect the scope and the operator. But that also comes with the operator carefully defining and knowing how to go about it. The better approach is to utilize equipment that is designed for the purpose.

I have seen others who have used their best effort only to end up with damage to instruments because of either wrong connections (probe ground lead to the high side of the AC circuit as one example) or simply not taking into account the maximum input rating of the instrument. And this issue has also extended over to those who decided to try ignition system work on their vehicles with a scope unsuspectingly connecting the probe to the secondary side of the coil. Not as dangerous to the operator but can be sudden death to the scope.

I have not had any bad experiences but did do work for a client who had a captive engineer who didn’t watch what he was doing on a high-power klystron transmitter. Not being careful he managed to get across the 440 volt 3-phase circuit in the contactor cabinet. To put it simply, the replacement engineer commented that he had spent some time picking pieces of the previous engineer’s jeans out of the rack that was located behind him.

I had the luck of purchasing an old BWD Instruments 880 PowerScope at an auction many years ago that had been owned by the Bonneville Power Administration (https://www.radiomuseum.org/dsp_multipage_pdf.cfm?pdf=bwd_880_pamplet.pdf). It is a special purpose oscilloscope specifically designed for use on AC power circuits. To allow proper connection to the circuit under test each of the 4 inputs feed high impedance, high voltage differential amplifiers that allow both the high side and common (ground lead) for each channel to work at voltages above AC ground. The test leads are also designed to protect the operator from contacting any voltages under test. It is a relatively old instrument but the only unit that I use when working with any form of AC mains voltage (120-440V, single or 3-phase).

There is plenty of discussion about use of conventional oscilloscopes on AC circuits if one Googles something akin to “ac power line oscilloscope” or similar.

Greg


Roy Thistle
 

On Sat, Oct 24, 2020 at 11:49 PM, Jean-Paul wrote:


Most generators have a distorted sine output, depends on rating, and design.
Yes... even the best motor generators with have an imperfect distribution of the magnetic fields, and pole alignment. The newer inverter generators (Generac et. al.) process the alternator output to get a sine wave with low THD (below what the power company can usually supply to the consumer.)... but they are more expensive than the older sets. The OP did not mention what manufacturer/type of generator he was using... or intended to use.
I have a couple of old...but good UPS units... they are good ones. If they don't trip out when connected to the generator I'm testing (under no load, under heavy load, and under a surge draw)... it's a good generator.
You can also use a distortion analyzer... if you really want to know the THD.


Roy Thistle
 

On Sat, Oct 24, 2020 at 06:39 PM, Robert Simpson wrote:


I want to see the shape of the power, the frequency and peak voltage.
I dunno. What you want to know (or what I would want to know)... is what the THD is. That might be tough to interpret with a 456M.
Anyway the frequency and the peak voltage change in the power provided by all power companies.
What's the generator you plan to use?


Roy Thistle
 

On Sat, Oct 24, 2020 at 07:33 PM, Mike Dinolfo wrote:


don't under any circumstances interconnect the neutral and/or "hot" wires
between the house and the generator.
That's called a backfeed... unless you have a transfer switch, or have cut the power at the distribution panel... if not...and it's a blackout... and if there is a lineman working on the wrong end of a line transformer... there might be lethal high voltage present, due to your backfeed... the linemen are not expecting that (it's a blackout). Anyway, most power companies will seriously terminate your account, if they find you have potential for a backfeed.


Roy Thistle
 

On Sun, Oct 25, 2020 at 11:07 AM, @0culus wrote:


The point I was making was that the OP really should get the right tool
What's the OP trying to do?
If he's only got a cheap/er generator... Iike a harber fright... there is going to be high THD, and poor regulation under heavy load, and surges. So the right tool would be an inverter generator, or to stick to running incandescents, from a cheap one.


Roy Thistle
 

On Sun, Oct 25, 2020 at 07:42 AM, - wrote:


Since then I've picked up a largish 440 or 220
to 220 or 110 volt transformer
They will act to diminish the harmonics; but, maybe not in the way you might like.
Every day... somewhere... in some building... distorted sine wave power kills and expensive transformer. Usually, its heat damage... but, sometimes there are fires too.


Roy Thistle
 

Yes...I see the OP is using a Briggs & Stratton Powermate 5000, and running a fridge, fans, lights (incandescent?)
Anyway... B&S make the engine...maybe the generator is a Generac?
The AVR seems like a standard one... so nothing great. (Unless someone knows better?)
I'd say okay for dumb appliances... maybe short term use, for fancier digital ones... but, long term may fry expensive digital controls.
Put a UPS on one end of it, and use that to power more power quality sensitive devices.


Robert Simpson
 

Yes, it is for rare temporary use. We have had several 12 hour or so blackouts since I have lived here. The reason for the scope test, I want to understand what the generator is producing. As for electronics ( a secondary use) I was looking into power conditioners. I saw a 600 watt Trip-Lite that might work. Again, rare occasional use.
Bob


Jean-Paul
 

Robert if you live on West Coast USA expect longer and more frequent blackouts in future years.

The UPS, and other power conditioners may not clean up a high thd generation effectively

Easier to buy a low THD generator rather than clean up a poor quality waveform.

Electronics with SMPS will not care about the line waveform.

Chuck Harris note re use of a filament transformer to sample and isolate the 120V is the simplest way to see the waveform safely

Just the ramblings of an old retired EE

Jon


Harrison
 

I've been following this thread from the beginning. Unfortunately I am a bit confused. I am familiar with transfer switches and not back feeding into a panel with out pulling mains, etc. But is the issue the ability/desire to monitor the sine wave with a scope or is the prevailing wisdom that using a portable or stand by generator is detrimental to modern electronic devices, be they a TV, Refer, or whatever. Sorry if the answer is there and just going over my head. Thanks and stay safe. Harrison N1FAM


Chuck Harris
 

The desire of the OP was to use his bench scope to look at the
waveform from the output of a 5000W power source (aka generator).

A perfectly reasonable thing to wish to do...

The concern from myself, and others on the group, was in how to
safely and easily measure the output of a power source that can
easily vaporize parts of the measurement equipment, or kill the
operator, should a wiring mistake be made, or a fault occur.

As to wisdom and generators and powering modern electronic devices,
there are generators, and then, there are generators.

When you are using a conventional small engine powered contractor's
generator (2.5-5KW) to power multiple sensitive loads, the generator
will create a deep brown out condition on attachment of a load, and
a large voltage surge on detachment of a load.

The larger the generator in comparison to the loads, and the more
loads, the cleaner the power will be from these brownouts and surges.

Standby generators meant for residential use are very much more
expensive than contractor's generators, and have better regulators
for both frequency and voltage, and filters to protect the load
devices.... all at a cost.

-Chuck Harris



Harrison wrote:

I've been following this thread from the beginning. Unfortunately I am a bit confused. I am familiar with transfer switches and not back feeding into a panel with out pulling mains, etc. But is the issue the ability/desire to monitor the sine wave with a scope or is the prevailing wisdom that using a portable or stand by generator is detrimental to modern electronic devices, be they a TV, Refer, or whatever. Sorry if the answer is there and just going over my head. Thanks and stay safe. Harrison N1FAM






Greg Muir
 

There are safe ways to observe outputs from AC generators with generic oscilloscopes but I hesitate to offer hints given the unknown level of knowledge and experience of the members reading these comments.

If you want more than a simple observation of the generator AC out waveform (which I will guarantee will show significant distortion) you then get into utilization of a distortion analyzer which opens up a whole new ballgame and I feel is too complex for this discussion.

The generator AC waveform will also vary given the loads applied so if you decide to undertake this project application of varying loads is also a way to get a better idea about what you are dealing with. And remember various loads can also dramatically affect the distortion of the supplied power depending upon how they deal with the incoming AC in their power supplies.

I have dealt with a few small client in-house IT operations where they decided to take the cheap way out and purchase a small generator to back up their data operation. Some of these generators were even of the Generac brand. Some of them experienced intermittent or full operation of their bigger UPS systems when on generator power due to the UPS units seeing the AC distortion present and reverting to the backup mode because the incoming AC from the generator did not meet the UPS qualifications for reliable power. In the case of Generac products, I would call the local Generac tech and he would adjust the generator so that the UPS units would rely on the generator power. I could never get him to tell me what he did to achieve that. It apparently took more than one adjustment.

I have sites utilizing diesel generators up to 200 kW that produce very pristine AC waveforms. But as you go to smaller sizes, your AC waveform starts to suffer. It is more-or-less an axiom of the bigger the generator, the better quality of AC power.

Greg


Roy Thistle
 

On Sun, Oct 25, 2020 at 11:33 PM, Jean-Paul wrote:


Easier to buy a low THD generator
Well... here in the colonies... many of local wiener guys use Honda EUs... which use inverter technology... over 1500 W (2.2 KW peak)...basically a robust Honda engine that drives an inverter. THD was about 1% at 1000W.
It has a 12V 100W output.
I just told them, get the Honda, and a UPS... use the generator for the wieners...charge the UPS battery with the generator... and use the UPS for your TV. But, the Honda was certainly good enough... better than the local power provider specs.
Drawback... in terms of real money... the Honda is more than 1000 USD.
But, that's what we used for Field Day... so we wouldn't pop anyone's fancy SDR.
By the way, some old rack mount UPS (I think APC?) measured less than 3% THD... some coming in better than less than 2% THD.


Ed Breya
 

This thread sure got pretty big, so I had to look. It's not all that complicated to make such measurements. I'd recommend as Chuck did, to just use a small power transformer to isolate the voltage signal. I have done this many times in test setups, and built such into equipment for line monitoring. One thing that will provide less distortion is to use say, a 240V primary for measuring 120V. If you use a small, cheap 120V one like from a wall wart or simple linear supply, you will find the flux level is pretty high, and will give higher distortion. These are optimized for cheapness, not efficiency or cleanliness. If you have a 240V or dual primary type hooked up for 240V, the flux level will be much lower, and far from saturation, and it will have good linearity and dynamic range in the 120V region. If you only have 120V ones, then go with that - it should be plenty good enough for this particular case.

The main thing is to figure out the turns ratio and proper scaling for the scope signal. Note that most transformers are rated at full-load, to accommodate the losses, so the actual turns ratio is not simply the ratio of the marked voltages - the secondary turns are necessarily higher. As a signal transformer with no load but a scope input, it will provide close to the ideal and true ratio. There are lots of ways to assess the ratio, and you can scale it with a medium impedance resistive divider, to a convenient factor to go into a 1 meg scope input, to get say 50V per div equivalent. The scope has very wide range, so pretty much any low voltage secondary can be scaled to fit.

Also, it's wise to provide fuse protection on the primary, for just in case, especially for experimenting. It can be much smaller than would be used in a normal power circuit associated with the transformer's size. The only loads will be the magnetizing current (maybe 5-10 % of VA rating) plus a slight increase in loss if it's a 120V unit with 120 applied, plus the scope scaling circuit load, which should be tiny.

Ed


Roy Thistle
 

On Mon, Oct 26, 2020 at 02:13 PM, Ed Breya wrote:


It's not all that complicated to make such measurements.
I dunno... what's this thread about anyway?
It seems to me that a cheap Chinese multi-meter with TRMS, Peak/Max/Min, and Freq measurements will tell someone more about this generator units performance. (That's what the small engine mechs use.) But, really it's already well known that it's not going to supply power line quality power, especially concerning THD.
However, if you just got to open a can of soup with a hammer, because you've got a hammer... and a can of soup...


greenboxmaven
 

Most simpler generators used on construction sites are self excited by use of a winding that is oriented differently from the load winding. The magnetic fields are "bent" by the load current, and increase excitation as the load increases, and decrease it when the load is reduced. There is a definite and easily observed time lag as the excitation adjusts. Frequency stability depends on the engine governor and the inertia of all the rotating components. It is desirable to make the generator as lightweight as possible, but this means there is less rotating mass to smooth out mechanical speed variations. Small spark ignition engines with carburetors are also less able to respond to a sudden load increase than a fuel injected one or a diesel. Newer generators often use an inverter circuit to produce the output power, the engine and generator are only to produce the needed DC for the inverter. The engine throttle is controlled by the inverter to maintain a suitable DC voltage at the input of the inverter. If you enjoy working on mechanical things like engines, there is a series of gasoline powered military generators that can often be obtained for modest prices, find a few military vehicle enthusiasts and you will find the generators. They are in two engine classes, the smaller is 500 watt, 1.5 KW, and 3kw. The larger is 5 and 10 KW. They all have very good voltage regulation, and the engines are able to pick up sudden loads fairly well. They are complex and can be challenging to work on, but this IS a group about Tektronix scopes, which are a bit more complex than an EICO tv shop scope.

Bruce Gentry, KA2IVY

On 10/26/20 9:57 AM, Chuck Harris wrote:
When you are using a conventional small engine powered contractor's
generator (2.5-5KW) to power multiple sensitive loads, the generator
will create a deep brown out condition on attachment of a load, and
a large voltage surge on detachment of a load.

The larger the generator in comparison to the loads, and the more
loads, the cleaner the power will be from these brownouts and surges.

Standby generators meant for residential use are very much more
expensive than contractor's generators, and have better regulators
for both frequency and voltage, and filters to protect the load
devices.... all at a cost.

-Chuck Harris









Robert Simpson
 

First, a question came up about my background.
With a year of military training I was a radio repair tech in the Army, both tube and transistor equipment. I worked my way through college (business degree) working with the university electric shop under the direction of certified electricians. We did ever thing from wall outlets, motors to building main panels. I still have my half inch conduit bender. I have been repairing Tek scopes, computer monitors, TVs etc. as a hobby for over ten years now including HV supplies in 7603s and a 7904. I have appropriate HV probes. My professional life was in Information Technology from early DEC mini computers to finally leading development teams building custom web data base business applications as a PMI certified project manager. Generators are new to me.

Moving on,
A little bit of progress. I have good ground wire set up. After the comment about the ground being 3/16”, I found a short piece of copper tubing with a 3/16 OD. By soldering the end of an insulated wire into a short section of tubing, I can push the tubing into the grounding hole part of a socket with no open wire exposed. As the tubing fits snugly, I have a protected secure connection. The other end of my wire is screwed into the ground lug of the generator.

Since the generator puts out distorted AC, I am curious about max volts. I have a Fluke DMM, model 8024B (the one with buttons along the left side) that has a max reading save option. So I am thinking of some experiments. Such as what is max volts when first starting the generator. What is the max volts when running with no load. What is the max volts when under some load.

The suggestion about isolating the scope through a transformer got me thinking about a voltage spike damaging the scope. I did probe my house power using a connected socket the scope is plugged into at my bench. I used a P6006 with 6 foot cable. Checking the P6006 on the calibrator signal showed a very clean square wave. Then using the 50V setting I checked house power. Not surprising, there is a very clean sine wave of about 350V P-P and a 60Herz frequency.

Also, would an isolation transformer (which I have somewhere around the house, Haven't used it in awhile) be just as good as a filament transformer (which I don't have)?

Note: All this effort is only meant for a couple of test runs. I have no plans of ongoing monitoring. I think my generator is running a little fast as my DMM shows 132V as compared to my house voltage of about 121V. Would a wonky sine wave of the generator affect the DMM reading?

So the scope can show me the frequency. If the generator is running fast, I will need to adjust the governor. If the frequency is correct then maybe the voltage regulator is off. And of course I am curious about just what kind of sine wave the generator puts out. I don't have a frequency meter as I haven't needed one until now.

Bob


Roy Thistle
 

On Tue, Oct 27, 2020 at 06:39 PM, Robert Simpson wrote:


Would a wonky sine wave of the generator affect the DMM reading?
Your model fluke meter measures the average value of a pure sine wave and displays the RMS value. Measuring anything but a pure sine wave is going to give you an erroneous reading (The design expects you are only measuring a very clean sine wave). So either the generator has a pure sine wave, and the meter is telling you the generator is outputting 131 volts RMS... or the generator has a distorted sine wave and your your meter is giving you an erroneous reading. (It's no guess what I think!)
Try using a cheap TRMS meter (or borrow one)... and see what it gives you. (Measure your home power line voltage, for a reference, and then measure the generators.)


Roy Thistle
 

On Tue, Oct 27, 2020 at 06:39 PM, Robert Simpson wrote:


And of course I am curious about just what kind of sine wave the generator
puts out.
It doesn't put out a sine wave at all. It outputs an A.C. waveform that is composed of harmonics.
IMO, the 465, being an analogue oscilloscope, is only going to act as an indicator, to show that you have a distorted sine wave, at the output of your generator.
But as many have said that's what's going to be there, given the generator that you've got. By the way is the generator used or new?