How to get a GE Yokogawa AB40 Sync Scope to rotate without a powerplant.

[Bobakman]

HI folks. I am a retired power plant Operator,Electrician and Mechanic 39+years.
As a hobby I make lamps out of just about any item and my current project is to make one using a GE Yokogawa AB40 Catalog#106452AAAA7 120V 60HZ Sync Scope. I have placed as an operator many units on-line syncing with this very model sync scope.
I have called and spoke with an engineer at weschler evidently they are the new owner and was told that it was a very difficult thing to do and requires a complex circuit. My intent is to if it can be done have a switch and a rheostat so I can speed up and slow down and stop the needle rotation for display purposes only.
If you have any ideas on how to do this as is  without disassembling the meter and installing a separate reversible speed controlled motor to drive the needle  it will be much appreciated.
Meter bulletin is attached.

Bob

[MRMILSTAR]

I'll offer a guess with the caveat that I've never used one. According to the data sheet this device accepts two nominal 60(50) Hz, 120(240) volt inputs. The phase can differ between the two. This is what causes the rotation. The data sheet doesn't say but I'm guessing that it requires a few amps of current to operate. The basic problem then is that you need two power supplies. One is fixed at 60 Hz and provides the reference phase. That one is simple, its just your standard wall power. The other power supply is the more difficult one. It needs to deliver 60 Hz at 120 volts with some level of phase adjustment. The simplest method that I can think of which may work is to use what is called a "John Freau phase shifter". This is something that is used with some spark gap Tesla coils to perform phase adjustment for the rotary spark gap motor. Basically, it is a bandpass filter consisting of some motor run capacitors connected to a small variac. The nominal center frequency is at 60 Hz. The input is the wall power. The variable inductance provided by the variac provides the phase adjustment. Depending on the variac inductance you will have to experiment to determine the proper capacitance to achieve the nominal 60 Hz center frequency. I built one of these for my spark gap Tesla coil to adjust the phase powering the rotary spark gap motor. Do a search for "John Freau phase shifter" to see a simple schematic.

[Bobakman]

I just started researching this as you recommended. Is it possible to make one small enough for the lamp project? When I think of a Variac I think of a large Toroidal coil transformer with a large knob on it.
The sync scope does have what looks like two large power resistors in the end portion of its housing this is probably where the “few amps” that you mentioned are dissipated? Typically, when Syncing the AC generator to the grid, the scope is only in service for no more than a few minutes, then the Sync plug switch is turned off and the meter is deenergized. BTW the Sync Plug switch which I currently lack would be a nice addition to the lamp project.
Thanks for your post and your help.
Bob

[klugesmith]

Hi Bob. Nice project you got there.

Here are some suggested baby steps, inspired by looking at the datasheet you sent.
Can you connect a regular 120-volt 60-Hz source to both the A and B inputs?
That should make the pointer go to 12:00, or to 6:00 when one pair is swapped.

If that's successful, the next step is to determine the A and B input currents.
If they are really more than an amp, you can measure them with a clip-on ammeter.
Otherwise use a multimeter on AC-amps mode, in series.   Or install a current-sensing resistor and measure its AC voltage drop.

With or without the current measurement, I bet you can make pointer go to a deterministic intermediate phase by putting a suitable capacitor in series with A or B input.  From observed angle with some different capacitors you can learn about the instrument's input resistance and inductance.

Active circuit design can wait till those exercises are successful.
As an alternative to active circuits, one might be able to move your 'scope pointer arbitrarily by properly connecting an AC synchro (selsyn) motor like the demonstration set on a shelf here.

[edit] this reminds me of unfinished project to generate 120 volts AC at slightly nonstandard frequencies, to demonstrate vibrating-reed frequency meters.

[MRMILSTAR]

I just started researching this as you recommended. Is it possible to make one small enough for the lamp project? When I think of a Variac I think of a large Toroidal coil transformer with a large knob on it.

There are some very small variacs. I have one that is only about 3 inches in diameter. Consult E-Bay.

[klugesmith]

Have you seen this video, about what's inside a similar AC sync indicator?
It includes current measurement, and a demonstration of fixed 12:00 and 6:00 as I'd suggested.

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[Bobakman]

 Thank you all for the help. I have increased confidence now that I’ll be able to accomplish the task. Got a few tests to run like the needle movement verification. I’m still a bit off from completing the project still acquiring parts I’ve got a buddy looking for a sync scope plug, and switch. I now have a lot more tools in my toolbox to help me be successful thanks to all of you!
If I have any more questions I know where to ask.  Once I complete the project you guys will be the first to see the fruits of “our” labor.

Bob

[klugesmith]

Bob, your OP says you want a knob to control the continuous rotation of pointer, adjustable from running a little bit slow to a little bit fast.
I think that rules out a "Freau phase adjuster", which would need to have 360 degrees of adjustment range, and a control knob turning continuously to make Sync Scope pointer turn continuously.   As I understand it, you really do need a source of 120 volts AC with frequency that can be dialed between, say, 58 and 62 Hz.  No big deal to get a digital square wave with frequency range like that, e.g. with a 555 timer circuit.   The challenge is to get the signal amplified to appropriate voltage and power for one input of your Sync Scope.

How about starting with an inexpensive DC-to-AC inverter, as used to run small AC appliances from battery power in vehicles?   Depending on accuracy of the inverter's built-in frequency reference (before modification), your scope might rotate in fast or slow direction to begin with.   These days some of those have battery built in.  Another way to get fast or slow rotation is with throttle regulation of an engine-powered generator.

[Bobakman]

I do have a couple of 12VDC to 120VAC square wave inverters. I was wondering though would a small 120VAC VFD work as the incoming source?
Bob

[klugesmith]

I was wondering though would a small 120VAC VFD work as the incoming source?
Bob

Why the heck not, if you've got such a thing.   I'd beware of accidentally setting the frequency too low.  120 volts DC might immediately blow up your sync scope (or any AC motor driven by the VFD).  For protection you could put a fuse or a capacitor in series with the variable frequency source.

Square wave inverter sounds like a good place to start.   Aren't most of those "modified square wave inverters", with some intervals of low impedance 0 volts in between the +V and -V phases?  That greatly reduces harmonic distortion without adding much complexity to the output bridge.  Even if the inverter frequency is crystal controlled, I bet it differs from your grid frequency enough that your sync scope pointer will move.

[Bobakman]

I don't have one but I have been looking they are not cheap. I'll try my inverters the voltage though on the ones I have are usually well below my utility voltage of 120-122VAC at about 100-115VAC modified sine wave AKA Square wave.  There are more expensive sine wave inverters though.

 Bob

[Bobakman]

Here is the waveform of my Dewalt Battery powered inverter the frequency in the image is 60HZ but it was actually bouncing around usually in the 80HZ range then back to 60HZ I guess frequency control or lack of is what it is in such an inexpensive device.
Bob

[MRMILSTAR]

I thought that you just wanted the ability to change the output needle position according to some user-adjustable control. For that case, the "John Freau phase shifter" may work because you only need an adjustable phase shift with a 60 Hz frequency. However, if you want to see the needle rotate then you need some small frequency adjustment as others have stated. And as others have stated, some sort of inverter with 120 volt output is probably the most logical way to do it. I have seen "true sine wave" inverters for sale but they aren't cheap. I'm not sure if a conventional PWM (square wave) output inverter will work. You would just have to try one. In either case, you would then need a way to slightly adjust the output frequency of the inverter since they are designed to be fixed at 60 Hz. Depending on the method used to generate the 60 Hz output perhaps you could possibly hack into the circuitry to provide a way to adjust the output frequency slightly. I am not hopeful of that because I think that the output frequency is tied directly to the input frequency.

If the meter will operate properly with a PWM output inverter, you could use a single phase of the 3-phase output of a VFD. These commonly provide a way for the user to adjust the output frequency with a knob, either built-in or externally supplied. 240 volt single phase input VFDs with 240 volt 3-phase output are commonly available. I use one to control my lathe. I can adjust the output frequency with a knob. There may be a 120 volt VFD available but I don't know. If this would work I have seen small cheap Chinese VFDs on E-Bay for less than $100.

[Bobakman]

This video shows exactly what I am hoping to accomplish by rotation of the needle. I want to be able to rotate it in both directions Fast/Slow as in the video.
He is using rather large oscillators in the video though.


Bob

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[klugesmith]

Applause for that video of sync-scope demonstration using benchtop frequency generators.

Now back to Bob's oscilloscope screen capture with battery inverter output waveform.  The annotation says trigger is on channel 1.
See what happens with trigger set to Line, if that mode is available.   Then when inverter output is close to 60 Hz, the difference from local AC grid frequency will be obvious.
Here's an online blurb for the Ecarke inverter, which says 60 Hz with no tolerance given.  80 Hz would be pretty disappointing, but maybe it's got some self-oscillating drive circuit that would not be easy to take control of.
https://www.amazon.com/Portable-Inverter-Ecarke-Converter-110V-120V/dp/B0CQ4Q6WB9?th=1

[Bobakman]

Applause for that video of sync-scope demonstration using benchtop frequency generators.

Now back to Bob's oscilloscope screen capture with Dewalt battery inverter waveform.  The annotation says trigger is on channel 1.
See what happens with trigger set to Line, if that mode is available.   Then when inverter output is close to 60 Hz, the difference from local AC grid frequency will be obvious.  Does the product come with any specification of output frequency?

No and the scope I have is internal battery powered so it is isolated from the AC mains. If the frequency deviates more than a few HZ it probably won't
like it and not be a smooth slow rotation.  When we synced the generator to the grid we always came in with the 2 pole machine RPM  at around 3605 slightly higher than the grid our sync scopes were only a backup the final decision for generator breaker close was the Sync Relay. The few generators that I have synced manually I closed the generator breaker in the fast rotation direction going slowly and contact close for the breaker was just as it approached near dead on  12 O'Clock.
Bob