Safest way to verify AC motor speed controller waveforms on oscilloscope - Page 1
Forum section: electronics / beginners

author: amaschas
author_rank: Regular Contributor
timestamp: February 23, 2023, 04:58:13 pm
content: Hell all, I'm posting this in the beginners forums because even though I have a fair amount of experience with DC electronics, I'm relatively new to working with AC mains and I'm understandably nervous about doing so. I'm currently in the process of trying to debug an issue with an AC motor speed controller. I believe that the controller is designed to set the motor speed via phase control, and I want to check to see the actual waveform it produces. I'd also like to evaluate some motor speed controllers I purchased to see if they utilize phase control in order to determine if they can act as an adequate replacement in case repair is impossible. I'm in the US and so the mains voltage is 110-120VAC.I have few tools at my disposal to help me in this investigation. I have an SDS1104X-E bench-top scope, a Tektronix THS720A, which is Cat. II and comes with a set of probes which are also Cat. II and rated to 300V RMS. Additionally, I have a bare transformer with a 24V secondary.While the Tektronix is designed (from what I understand) for safely probing mains voltages, at least in the US, I'd prefer to work with a lower voltage if I can, but I'm unsure if the speed controller will even do anything at 24VAC. I'm also a little uncertain whether the distinction between the neutral and line matters on the secondary winding, and if I'm better off using the Tektronix even when using the transformer, (after verifying that the primary and secondary are in fact totally isolated), or if it's safe to use the Siglent. Any advice would be appreciated!

author: Jwillis
author_rank: Super Contributor
timestamp: February 24, 2023, 12:18:00 am
content: Never connect an oscilloscope directly to mains power. EVER!Best way is with an differential probe. But they are very expensive. So an isolation transformer is the next best solution.Your 24V transformer will work , provided the secondary and primary are completely isolated from each other. Do not use an auto transformer since the secondary is not isolated from the primary.Keep in mind that it won't be 100% accurate because of inherent looses in the transformer. Also you will need to calculate the voltage ratio between the primary side and secondary side. This is important in determining the values your are seeing on the scope.Are you using a SCR motor driver or a VFD (Variable Frequency Driver)? Just asking to clarify..

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 01:24:48 am
content: Differential probes are not really all that expensive these days, I bought both of my Tek HV diff probes for around $100 each, I think there are some budget new ones for around $200.

author: amaschas
author_rank: Regular Contributor
timestamp: February 24, 2023, 02:12:39 am
content: Quote from: Jwillis on February 24, 2023, 12:18:00 amI'm actually trying to verify whether it's an SCR or a VFD. The listing didn't specify, and I would prefer a VFD for my use case since I want to retain the motor torque as much as possible across the entire speed range.I tried both devices with the 24VAC transformers and while I could see the AC waveform fine, neither device seems to do anything at 24VAC.So even though the Tektronix is battery powered, the device and the probes are Cat. II and rated to 300V RMS, it's not safe to use it with mains? Would it make a difference if I used a 120V isolation transformer?

author: nimish
author_rank: Regular Contributor
timestamp: February 24, 2023, 02:16:14 am
content: Use differential probes certified for the voltage and/or isolated probes/instruments. Can never be too careful.Note that using an isolation transformer effectively disables GFCI/RCD detection. Defeating grounds by removing them is also not a great way to go.See:https://www.cbtricks.com/miscellaneous/tech_publications/scope/floating.pdfDifferential probes are pretty cheap, optically isolated probes are not. Isolated scopes aren't cheap either, unfortunately:https://cleverscope.com/products/CS548

author: EPAIII
author_rank: Super Contributor
timestamp: February 24, 2023, 09:49:17 am
content: Oh, give me a break! I have put oscilloscopes on AC power mains a million times without the least amount of hesitation. I have worked with Tektronix scopes for over 60 years and every one of them, all the way back to tube types, was perfectly capable of having 120 or even 240 VAC input with even a 1X probe that was rated for those Voltages. That would be a rating of 336 VDC for the 240 VAC RMS. 10X probes would reduce the scope input by that factor and be even safer.In over 45 years of professional electronic work and more in college and now in my retirement, I have never once even been in a shop with a differential probe. The simple workaround is a two channel scope, the summing of the two channels, and reversing the phase on one of them. BINGO! You have a differential display on the scope. This worked on every Teketonix scope I ever worked with. It also worked with a Heathkit scope I had. And with a USB scope I now have. And my Hantek, two channel scope. In fact it would probably work with 99.9% of the two channel scopes made in the past 75 years or more.DO use probes that are rated for the DC Voltages needed (multiply AC, RMS values by 1.4X to get that DC value). And your go-to probes should be the 10X models. I only used 1X probes when absolutely necessary. A 10X probe will drop the circuit's Voltage by a factor of 10 at the input jack of the scope. So a 10X probe rated for 500 VDC will drop 500 VDC at the point being probed down to 50 VDC at the scope's input jack. That's all the scope ever sees, 50 V.Having worked for an entire career in electronics, I find it astonishing that people today are so hung up on things like differential probes and true RMS multimeters. The actual need for such niceties, outside of RnD labs, is so small as to be almost totally negligible. I have trouble shot and repaired and SET UP some of the most sophisticated equipment in the business with just basic VOMs and two channel scopes rated at 25 or 50 Mhz. bandwidth. I dare say that around 90% of the troubleshooting I have ever done could have been performed with a $15 Radio Shack, analog VOM. And YES, I did have one which I used for many years.An isolation transformer is NOT a good way to examine waveforms. It will distort the signal and confuse you more than help. FORGET IT!In today's market you can get two, 10X probes with 400 or 500 Volt ratings for under $50. Those and a two channel scope will show you any waveforms in a 120/240 VAC motor circuit you need to see. I am not aware of any scopes with less than two channels so that should not be a limit if you have or can buy almost any scope made in the past 50+ years.One more thing. All this CAT this and CAT that stuff is more about the ability of an input circuit shutting down when a catastrophic short is created. Every scope and VOM probe I have ever worked with has had enough insulation to protect against Voltages up to at least 500 V. Those catastrophic shorts can melt metal and plastic parts and thereby eliminating the protection against shocks. BUT, such melting will only occur when AC power mains are involved and the fuses/circuit breakers do not interrupt the current. I am not saying that CAT xxx protection is worthless. I am only saying it comes out of the POWER industry, not from electronics. Your motor control is in both areas, but your building power should be properly protected to make the CAT rating of a device at least less important.Quote from: Jwillis on February 24, 2023, 12:18:00 am

author: EPAIII
author_rank: Super Contributor
timestamp: February 24, 2023, 10:11:21 am
content: Voltage is Voltage. It does not matter which side of an isolation transformer it comes from, if you get ACROSS 115 VAC, you are going to feel it. Been there! Done that! NOT FUN. 230 VAC would, of course, be even worse and more dangerous.The only thing an isolation transformer does is makes it harder to "get across it". In other words, you can grip a solidly grounded water pipe with one hand and EITHER, but only one secondary terminal of the isolation transformer and you should not get a shock. The isolation transformer removes the possibility of being accidentally grounded and therefore completing the circuit without knowing it. Please do not try this without an ambulance and a pair of paramedics with a defibrillator standing by.But if you get across the two secondary terminals of the isolation transformer, you will be exposed to the full 115 VAC EXACTLY as if you had held that water pipe in one hand and the HOT side of the AC power with the other. All the effects will be EXACTLY the same in both of those cases. So an isolation transformer is a precaution, a worthwhile precaution, but not a guarantee. I can count all the times that I used an isolation transformer and all the times that I have received shocks on the fingers of one hand and have fingers left over.IT IS BETTER TO KNOW WHAT YOU ARE DOING AND TO DO IT IN A SAFE MANNER than to use an isolation transformer, which may give you a false sense of safety.The same thing applies to safe use of a scope.If you do not feel safe working with AC power, please do not. At the very, very least, have someone else with first aid training present. Most testing with AC circuits can be performed in a hands off manner. Hook up a test circuit and then apply power. Remove power with the wall plug before changing things.One good rule is to keep one hand in your pocket and use just the other one to do things with the circuit.And ALWAYS THINK FIRST.Quote from: amaschas on February 24, 2023, 02:12:39 am

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 24, 2023, 11:30:43 am
content: Hi amaschas,For your need, powering your device with a sensitive RCD/GFCI (ideally 10 mA trip current) and using a differential probe will keep you on the safe side.The isolation transformer should not be used because it gives a false security feeling.Quote from: EPAIII on February 24, 2023, 10:11:21 amI would even say that using an isolation transformer is actually a worse solution : if current goes through your body, the RCD/GFCI of your mains installation won't be able to detect it and won't trip, because the isolation transformer hinders the fault current detection on its' secondary side.The most adequate solution for probing on VFD/motor controllers output side is to use atruly isolated oscilloscopelike theone you already own, with compliantfully isolated 600V CAT IV probes. However it's really expensive stuff.If the DUT has a metal enclosure, you may use a strong alligator clip to make an additional link from chassis to earth.Even if it's not practical, wearingclass 00 or class 0 insulated glovesprovides an additional level of safety.Quote from: amaschas on February 24, 2023, 02:12:39 amNot surprising at all : a well designed motor drive always checks for adequate input voltage before doing anything. Unless fiddling the device to override this protection and provide external power to all of the control electronics, you won't be able to use your unit at another voltage that the one it's specified for.[update #1 : typo corrected]

author: TomKatt
author_rank: Frequent Contributor
timestamp: February 24, 2023, 12:39:26 pm
content: Quote from: EPAIII on February 24, 2023, 09:49:17 amThe thing to keep in mind is that many folks here are hobbyists with varying levels of test gear and experience. While the majority of scopes will indeed handle mains level probing, there are those that have much lower voltage ratings. Also, while you understand the principle and would never connect your ground leads probing mains, not everyone has that knowledge and experience and might connect the ground out of habit because they've never used 2 channel differential methods. A differential probe eliminates the earth grounding risk.So, are differential probesmandatoryin this case? No. Are they the safer suggestion? I think so. I picked up a Micsig differential probe for around $150 for motor drive tests. Because even if my inputs can withstand the mains, there may be inductive elements at play that raise voltages even higher.

author: shapirus
author_rank: Super Contributor
timestamp: February 24, 2023, 12:43:24 pm
content: Quote from: EPAIII on February 24, 2023, 10:11:21 amThis should be printed as a poster to be put on a wall in every lab.

author: py-bb
author_rank: Regular Contributor
timestamp: February 24, 2023, 01:50:08 pm
content: Quote from: amaschas on February 23, 2023, 04:58:13 pmYou can measure 300v + safely, just use a x10 probe. You can get a cheap 100x off Ebay rated to 2.5kv - I've used this at a little over 2kv and it's got good frequency response (which you probably wont ever use)If you have an analogue oscilloscope you can have a "poor man's differential probe" - use two probes and put it in subtraction mode (A+(-B)) - it's noisier than a true differential but better than a software subtraction.Your oscilloscope may have a few couplings, you need to be wary of high voltages with your 50ohm couplings, but I've never seen an oscilloscope that doesn't trip the probe coupling to ground (floating probe - grounded electronics) when over say 5vRMS is applied briefly (I've used LeCroy oscilloscopes that do this and I don't think twice about it)If your coupling is say 1Mohm it's going to draw so little current (unless the voltage is so stupidly high stuff breaks down) you don't need to worry about any sort of overload.What you do need to worry about is where you put your ground strap. You can create a circuit then!Make sure you understand this.I have a bipolar power supply that can sink current, I once turned it off and rather than disconnecting the output the battery I was charging in it discharged through barely half an ohm - if that until I saw the smoke and stopped it. I put a fuse in that (you can buy a fuseholder with wires dirt cheap in packs off 5 off of ebay) because I'm likely to forget and do it again - my first instinct is to switch something off (which is bad here)Use caution with the ground clip, you can stick the probe tip anywhere it's rated for just fine. If you're using a coupling that isn't 1Mohm then you may have to worry.To check simply connect a [battery powered] multimeter (acting as an ohmeter on the 2M range) across the probe tip and the earth clip - if you see 1M or too highyou only need to worry about arcing and other breakdown- if the scope is rated for say 300v and your probe is x100 - you'd need 30,000v at the probe tip to see that - if your x10 probe is rated for 600v (any higher and something might arc/breakdown in the probe) the scope will never see more than 60v and everything is fine.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 24, 2023, 04:02:32 pm
content: Hi amaschas,What TomKatt underscores is correct : not everybody is familiar with measurements on mains/live circuits.Act carefully and take no risks. Always double-check before setting the power to ON and use only top-notch TE and accessories. Reserve the cheap crap to probe on Arduino circuits...Let me be more specific on some oscilloscope features.A) Non-isolated oscilloscopes98% of the oscilloscopes available on Earth, like your SDS1104X-E, have non-isolated inputs, so :- the metal rings of each BNC input areconnected together and also connected to the earth pinof the power cable.- for that reason, they are not suitable for live/mains probing- should you need to make live/mains probing with such a vanilla scope, you'll need to use adifferential probefor each input that has to be hooked to a live circuit.Quote from: py-bb on February 24, 2023, 01:50:08 pmpy-bb is absolutly right : this is essential to understand when using a non-isolated scope.Dave plainly explains why such a vanilla scope cannot be connected to live circuits without specific accessories :B) Isolated oscilloscopes1% of the oscilloscopes aretruly isolated, like theTHS720Ayou already own.- The 'truly isolated' feature has to be clearly written in the documentation, as one can see for theTHS720A:"Independently isolated channels to allow improved safety for measurements to 1000 VRMS while floating up to 600 V RMS using P5102 probes"- These devicesalready have differential inputs, so you don't need a differential probe(but acurrent clampmight still be useful as it is safer than wiring a current shunt)- TheBNC connector rings are made of plastic: they are non-conductive. So where's the ground connection ? It'sburied inside the plastic ring: you can see it but you can not touch it.- The 'ground' connexion of each input is isolated (= those 'gounds' are not tied together and not connected to the earth plug of the scopes' power supply). To make valid measurements, you must hook two connexions to each probe : the signal AND the ground connexion. You can connect the 'grounds' wherever you want : it will never create a circuit. Here resides the immense advantage of the isolated inputs.- when probing into a live circuit, do not use standard oscilloscope probes having metal parts you can touch. You need dedicated oscilloscope probes where the 'ground' connection is enclosed in plastic. Tek and other renowned brands, of course, sell thosekinds of accessories.- if you use the suitablebanana to BNC adapter, you dont even need a (10x) probe : set the input range to 50 V/div and connect the oscilloscope input directly to the mains, by using twoIEC1010 compliant banana cables + accessories. Even if it seems uncanny, it's safe. Of course, there's a voltage limit at the scopes' inputs and a frequency derating to consider. For this information, theuser manual again tells the truth: it is"300 VAC rms in a CAT II environment"and the frequency derating begins at 100kHz. For normal cases, it is nevertheless wise to use a suitable 10x scope probe : bandwidth is much higher and probe capacitance is lower. The coaxial cable of the probe also avoids that those wires act as an antenna.- the fact that the ground connexions of each input are isolated has some side effects (crosstalk) due to parasitic/stray capacitance.You won't be able to avoid that one completely, but there aresome good practices.-> For those who need an isolated scope on short notice at a bearable price, check outthese devices. For professional users, nothing compares to Tek, Fluke, R&S, Keysight and LeCroy.-> What other tools are required to probe into mains/live circuits, when using an isolated oscilloscope ? :- power the DUT via a sensitive RCD/GFCI (rated at 10mA) and wearclass 00 or class 0 insulated gloves.- If the DUT has a metal enclosure, you may use a strong alligator clip to make an additional link from chassis to earth.- If your DUT tends to be nervous and is willing to release some 'magic smoke', wearing safety goggles is also wise.- As already stated above (see previous post), it is not wise to use an isolation transformer, nor for the DUT, nor for the TE.C) Battery powered (cheap) oscilloscopesThe remaining 1%...Unless explicitly specified, it's not possible to assume that a battery powered oscilloscope is truly isolated. Most of the time, the input ground connexions are actually tied together and the devices' case and isolation barrier are not manufactured to protect the user against high voltages (>= 42VAC).Don't take any risk : assume that they are non-isolated and use them accordingly.[update #1 : missing word 'those'; up -> out]

author: shapirus
author_rank: Super Contributor
timestamp: February 24, 2023, 05:14:38 pm
content: Quote from: timeandfrequency on February 24, 2023, 04:02:32 pmshouldn't the following be added to this: "...if the probe's ground clip is connected anywhere but mains earth"?

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2023, 05:30:17 pm
content: The one issue that hasn't been hilighted above is that a VFD will probably be using a H-bridge to output PWM from a DC bus of at least 170V for a 120V motor, and up around 340V for a 230V motor. The waveform will be a variable duty cycle squarewave with fairly fast edges, so you may need to consider the probe's voltage derating with respect to frequency (considering the edge risetime as a quarter-wave of a higher frequency), and with an inductive load there is a risk of transients of several KV if the load becomes disconnected while running.IMHO you'd *always* want something better than a CAT II 300V probe in that environment. A 100:1 >600V CAT II probe on an isolated (by design) scope, or a matched pair of them with ground clips removed as a poor-man's differential probe, using waveform maths on two channels of a grounded scope is the *LEAST* I would consider using as I strongly prefer both myself and my test equipment to survive! Budget permitting a CAT II >600V HV differential probe should be used for *EACH* voltage you need to monitor. For three phase you'll need at least two, assuming a Delta connected load, or three if its star connected and you need waveforms with respect to neutral. That rapidly gets pretty expensive, so assuming you've got enough scope channels, the poor-mans differential probe by waveform maths becomes fairly attractive.*NEVER* use a x1/x10 switchable probe when probing mains - even if it, and your scope, claim an adequate voltage rating!

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2023, 05:46:29 pm
content: Quote from: shapirus on February 24, 2023, 12:43:24 pmIt's really easy to confuse one plug for another and unplug the wrong thing if you are in a rush. You may think it will never happen to you, until it does.

author: shapirus
author_rank: Super Contributor
timestamp: February 24, 2023, 05:53:30 pm
content: Quote from: Ian.M on February 24, 2023, 05:30:17 pmis it because of them being switchable or the division ratio being too low?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 24, 2023, 06:03:38 pm
content: Put the 24V transformer across the motor. Put 10k across the 24V and a 10k in series with both XFMR leads (so you don't blow your 50ohm input by mistake). Set scope to 5V per.Dont do this with an auto transformer!Dont float the scope or put the scope on an isolation transformer!Dont put your ground clip on the motor, a mistake here will be dangerous and/or costly!Probe safely- dont use a probe!

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 24, 2023, 06:05:48 pm
content: Quote from: shapirus on February 24, 2023, 05:14:38 pmHi shapirus,You are right : if the ground clips are hooked to a grounded pin, you can do live probing.In that situation, the actual risk shifts to an OP error: connecting one ground clip elsewhere than earth, or if the ground clip jumps off and touches any other track or pin.With much care, it is possible to proceed, but any issue or error will lead to a mishap.[update # 1 : semantic improvement]

author: Ian.M
author_rank: Super Contributor
timestamp: February 24, 2023, 06:27:32 pm
content: The problem with switchable probes and mains circuits is you are one slip of the finger from applying full mains voltage + any transients present direct to your scope input. Most scopes don't have inputs rated to withstand 1KV transients, which are far too common on 'raw' mains circuits.

author: nimish
author_rank: Regular Contributor
timestamp: February 24, 2023, 06:42:48 pm
content: Quote from: shapirus on February 24, 2023, 12:43:24 pmNo, it should be enforced by having an interlock between human access to HV equipment and energizing that equipment. It's super stupid the shit that I've seen on _manufacturer demo videos_ of PFC gear: 400VDC, live, with no real safety procedures. Dumb way to die.No amount of first aid will do shit if you're dead before you hit the floor. Best way to stay safe is to prevent the possibility of accidents at all.

author: amaschas
author_rank: Regular Contributor
timestamp: February 24, 2023, 07:04:30 pm
content: Thank you all for replies, this is all really helpful. Just to clarify, I have some experience with AC mains, I have what I believe is a good safety practice: I have a cliff quick test that I use with any mains powered test, and I always unplug the device entirely when not testing. I received enough jolts over my life to be extremely cautious working with mains power, which is why I'm soliciting advice here before diving in.I think I will proceed with the tektronix and a good amount of caution. I also now understand why the device comes with probes that only work at 10x! I was always a little confused by that. I'm also going to add some insulated gloves to the mix because I don't really trust these cheap devices.To provide some context for what I'm doing, I'm repairing an old photographic print processor which uses a geared shade pole induction motor to drive the rollers, and what I suspect is a single phase VFD to control the speed for different photographic processes. I'm starting to realize that single phase VFDs are actually kind of unusual and the commercial offerings tend to be expensive, and the cheaper set of offerings tend to be less than explicit with regard to what they actually do, which is what sent me down this path. Is a "phase control dimmer" the same as a single phase VFD? I have no idea, so I want to verify what the actual waveform coming out of these devices looks like.The more I look into this, the more I'm starting to feel like pulling the current motor out and replacing it with a geared DC motor with a more widely available PWM speed controller is worth the work it will take to adapt the mounting.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 25, 2023, 12:08:32 am
content: Hi amaschas,Quote from: amaschas on February 24, 2023, 07:04:30 pmThat's a really interesting subject which also drives my curiosity.My guess is as follows : I think it is possible to design a single phase VFD like a basic 'phase control dimmer' if the torque = f(speed) is a monotonic increasing function. So it should work for fans, turbines and pumps because of their centrifugal torque function.In all of those cases, we have the torque T = k × ω² (k = constant depending on the equipment and process) and the power is P = k × ω^3Some torque and power functions vs rotating speed for various processes(in French) :C = TorqueP = Powern = rotation speedA phase dimmer can only control/change the amount of power supplied to the motor. It does not change the frequency of the rotating magnetic field.Thanks to the monotonic centrifugal torque function, the actual speed of the motor stabilizes at a value which depends on the power received by the motor and there is no oscillation.Vanilla single phase asynchronous motorsbuilt with a single turn Frager coil (= shaded-pole motors) that drive fan blades can probably be speed controlled by a basic 'phase control dimmer'.Perhaps some advanced control techniques also probe motor current to improve loop stability and/or speed span.For some years, VFDs suitable for single-phase asynchronous motors have been available for purchase.These devices actually output a power sinewave with variable frequency and use a basic 'V/f = constant' control law.https://www.ato.com/1-2hp-vfd-single-phase-input-outputhttps://www.ato.com/1-4hp-non-enclosure-vfdOnce you have been able to investigate your Photographic Print Processor, let us know your findings on how the speed of the motors is controlled.How to speed down a shaded pole motor with a kind of magnetic diverter/brake :https://maker.pro/forums/threads/shaded-pole-fan-motor-speed-control-question.282849/

author: py-bb
author_rank: Regular Contributor
timestamp: February 26, 2023, 10:56:47 am
content: Quote from: Ian.M on February 24, 2023, 05:30:17 pmI don't agree with this - this is like saying "don't use a screwdriver incase you stab yourself in the face" - yes there is a risk but it's incredibly minor. I /like/ switchable probes and I've never done this (you'd notice)A far far better bit of advice which everyone here will have actually encountered isbe careful you don't short stuff with the hook or probe tip- eg you're probing the legs of an IC, probe slips and ends up jamming itself neatly across two legs. Current is shorted between the legs (via the probe tip, not the probe).I don't want to drag it out but yeah it's not a significant risk.Another risk is "what if your proibe's scale switch is broken?" where does it end.

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2023, 09:51:03 pm
content: I will never understand why so many probes are switchable. I usually glue the switch in the 10X position as accidentally switching it to 1X can result in a damaged scope. It is extremely rare that I have ever wanted to use a 1X probe.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 28, 2023, 10:36:27 pm
content: Quote from: amaschas on February 24, 2023, 07:04:30 pmI have installed literally thousands of VFDs and have never seen one for a single phase motor. The ones that mention single phase are usually referring to the input side of the VFD. If what you are looking at is in fact a VFD, I would suspect it is custom for the application and likely linear instead of PWM.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 01, 2023, 03:01:11 pm
content: Hi Ground_Loop,You are absolutely right : VFD for single phase motors are not common at all. But things are changing and new products become available.So we could spot one manufacturer for such an uncanny gear :https://www.ato.com/1-2hp-vfd-single-phase-input-outputhttps://www.ato.com/1-4hp-non-enclosure-vfdIt's pretty normal that the offer is so sparse because a 'single phase' is not sufficient to spin an asynchronous motor.We are actually talking about 'shaded pole' motors where a single shorted wire in the stator shifts the magnetic field to create an additionnal magnetic pôle.For some years, there are also VFDs' for two phase motors on the market.Those motors have 2 windings and 3 connections. They are also called "2/3 - 1/3" motors and require anexternal capacitorto start/run.

author: amaschas
author_rank: Regular Contributor
timestamp: March 02, 2023, 06:05:04 am
content: Quote from: Ground_Loop on February 28, 2023, 10:36:27 pmSo as I mentioned, this is a shaded pole motor, and the driver board that it comes with employs a U111B phase control circuit IC to drive a triac. From what I understand, phase control usually implies a dimmer, one with a combination of leader and trailing edge control that centers the waveform.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2023, 08:04:10 am
content: A single TRIAC can only do leading edge control as once triggered, it wont commutate (turn off) till the load current through it drops to near zero.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 02, 2023, 08:38:28 am
content: What Ian.M says is correct : with a triac, sole the leading edges' position is controllable.Il you want to control the trailing edge, you need a combination of a diode full wave rectifier and a high voltage mosfet.As the 'VFD' is built around aTemic/Telefunken U111B phase control circuit IC, this confirms that it's possible to variate the speed of a shaded pole motor by using a phase control dimmer.

author: Ian.M
author_rank: Super Contributor
timestamp: March 02, 2023, 09:26:43 am
content: However 'back in the day' there were methods to force a TRIAC or SCR off by diverting the load current, e.g. by triggering another TRIAC or SCR capacitively coupled to the first TRIAC or SCR's load. The extra TRIAC or SCR turns off when the capacitor runs out of charge. This is calledforced commutation.Forced commutation circuits are fairly rare nowadays as IGBTs and power MOSFETs replacing TRIACs and SCRS in many applications, have removed most of the need for them.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 02, 2023, 11:47:48 am
content: Quote from: james_s on February 24, 2023, 05:46:29 pmThere is never any need to be in a rush if you are dealing with Mains supply voltages.

author: amaschas
author_rank: Regular Contributor
timestamp: March 02, 2023, 11:51:31 pm
content: Just for context, here a photo of the motor. Note that is includes an encoder. I believe that the control board has a feature to use the encoder input to shut the motor off after a certain point. This is actually sort of annoying in a print processor, and leads me to believe that I have an early version of this processor that has potentially pre-production parts.And here is a photo of the controller board. The black connector on the right is for the speed control pot, and also includes a few unused pins connected to wires that don't go anywhere. I'm guessing this was for the timer controls, and the motor is set to the default maximum time.AC power connects on the left of the orange connector, and the motor and encoder connections are on the right of the orange connector.Here is a backlit photo of board to illuminate the traces.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 03, 2023, 11:17:19 am
content: Hi amaschas,Such an opto encoder provides a pulse signal which can be used to perform a position and/or speed control/regulation.As none of the ICs on the PCB (U111B, LF353) has any digital counting feature, i would say that the encoder pulses are solely used as speed feedback.

author: amaschas
author_rank: Regular Contributor
timestamp: March 03, 2023, 06:00:18 pm
content: Quote from: timeandfrequency on March 03, 2023, 11:17:19 amInteresting. So here's the thing: the motor definitely does turn off after a certain amount of time. Now, this could be mechanical, since I noticed that the motor was rather hot at the point it turned off, and I think it's probably a little underpowered for what it does. However it seems to turn off after a specific interval, and it even turns off when not under load when driven by that controller.My assumption was that there was some variety of simple analog timing circuit that I didn't understand at work.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: March 03, 2023, 10:02:17 pm
content: Hi amaschas,Quote from: amaschas on March 03, 2023, 06:00:18 pmYour assumption gives me a hint After thinking twice, let me add a new hypothesis.There's a 'free for use' OPAMP in the U111B (at pins 3, 4 & 5) and the LF353 is a dual OPAMP. So it's definitely possible to built a delay cell with those and even use the encoder pulses - converted to current pulses - to charge or discharge a capacitor (C11 ? C12 ? C14 ?) to a determined voltage value, so that the motor would stop after a certain number of revolutions.This would be significantely less accurate than digitally counting the pulses, but might be sufficient for that particular application.D4, in the TO92 package, could be a voltage reference, maybe a TL431. U111B actually includes a threshold switch (pin 9), but its' toggle value has much dispersion, so it might not be the best choice for a delay cell. U111B is also able to provide a stable voltage reference of 8,6-8,7 VDC at pin 10 (*) that could be used as voltage threshold reference for the timing capacitor / timing cell.As the PCB is just single layer, reverse engeneer the schematic can be done in a couple of hours.With the full schematic, it would be easyer to determine all of the functions packed into that board.The fact that the motor gets pretty hot does not really surprise me. Even at nominal speed, the shaded pole motors have an ugly efficency. I would say not more than 30-35%. So at reduced speed, beacause of to the phase dimmer, it will even be worse.Even on theATO VFD Internet page, there's a warning about low speed :Note: The single phase motor should Not be adjusted to run based on a lower frequency than 80% of its rated frequency for along time, otherwise the motor may fail.(*) I did not immediately notice it, because of the wrong unit shown in the datasheet at page 2 (µA is printed instead of V for pin 10 in the table at page 120 of theTemic/Telefunken U111B phase control circuit IC datasheet)[update #1] : D4 is not a TO92 part but is hidden behind C12. Not sure if it is a vanilla diode or a zener diode. The TO92 part is named T3. Hypothesis provided above is still valid because U1111B also contains a reference voltage.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2023, 11:40:34 pm
content: That's definitely a shaded pole motor, so the speed control will be a very similar to a phase control light dimmer.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 04, 2023, 01:16:35 am
content: Quote from: james_s on March 03, 2023, 11:40:34 pmIf it is, there are very few "nuances" which need to be considered, as the action is at 50 or 60Hz Mains frequency, so it can be observed with little error using a X10 probe (X100 if you are using a modern 'scope of unknown provenance) with the ground lead removed, so the return path is via the Neutral/PE connection at the building entry.Of course, as the OP has a Tektronix THS720A, "all bets are off" as it can safely observe Mains connections using a quite conventional probing method.My old work bought one to look at the 3ph input & output of a "Sola" brand power conditioner--an amazing device which could, if it lost one input phase, synthesise it for long enough for the EPP to take over, keeping the whole TV Studio up! ---Basically, 'the daddy of all UPS".Unfortunately, this conditioner had a "bug", which the EE & "Sparky" spent some time finding.After that job, the TH720A was "put away", but slowly found its way into the mainstream.It wasn't a great general purpose 'scope but was considerably better for such use than the rest of the early generation DSOs we were offered.