In short, applying rectification to either the armature or the field alone converts the uniuversal motor to a DC-only moior.
Absolutely not. It’s the “direction” of the a.c. cycles against the rotor; in engineering technology, the control system “sends” the three phase a.c. wave form in either a-b-c or c-b-a direction, depending on whether forward or reverse is selected. Simple basic a.c. engineering stuff.
You are not discussiung the same subject that Overmod and I diverted to discussing: The universal comutator motors used in toy trains running on DC or single-phase house current. He and I had already covered the control of multi-phase non-synchronous hysteresis-effect induction motors used in both diesel-electric and straight-electric railway and transit (including buses) equipment.
To finish up the toy-train reversal matter, with universal comutator motors that run on AC and DC. my memory says that both Lionel and AC Gilbert American Flyer used sequence reveral relays, changing the polarity of the commutor with respect to the field. Activate your “train-set,” and the train goes forward. Stop it, and when you start it the second time, it goes backwards. So every time you make a station stop with your Lionel “train set.” you have to “take slack!”
Is the control more realistic and sophisticated with toy trains today?
Oh, ususlly the sewquence-reversal relay was in the “coal car.” (Most toy train buyers did not use the word “tender.”)
Any complaints that this belongs on the Toy Trains Forum can be answered that Overmod and I are simply trying to be as complete as possible in answering the questions posed by this thread’s originator.
And Al, the 3-phase AC motor that you refer to, NOT the AC-DC universal motor that Overmod correctly pointed could be made current-supply potarity reversel directional reversal (but for DC operation only), any application of any rectifier between the inverters under computer control and the field coils woild obviously make the motor inoperable, defeating the purpose of the inverters to begin with. I suspect you simiply read the post previous to yours, without reading the whole thread, and assumed it was the 3-phase motor I was discussing. I think there may be a lesson there. And yes, I have made that kind of mistake on a Kalmbach forum, possibly more than once.
I note that the video explained very well how the non-synchronous induction motor worked but did not explain the synchronous. The explanaition is quite similar except that it is simply the lag between the fields produced in the rotating bars with respect to those in the stator that produces the magnetic fields and torque. Instead of slipping and falling behind in speed, the rotary never catches up to be exactly in step, but lags behind at the same rotating speed. The video showed the slanted bars of the non-synchronous motor. But Tesler’s original invention (if memory is correct) was for the synchronous type with bars parallel to the shaft.
While a Senior at MIT, I had a part-time job as a transformer designer at Mystic Transformers in Winchester, at the same time as I was testing load-regulator-control performance on B&M GP-7s 1567 and 1568 for my SB thesis. So commuting to Winchester wasn’t a problem, with my B&M engine pass, and my Raleigh bike on the front platform of the B&M wood open-platorm commuter train. I got to know the properties of various grades of iron used in transformers. Obviously, a permanent magnet uses different iron aditives than an audio transformer. Too much hysterises effect, and the audio trainsformer won’t pass high-frequency signals, first distorting them, and then being useless as the frequency is raised. At the same time, a high-frequency power-transfer transformer requires much less iron and fewer windings than a low-frequency power transformer. Iron with a greater hysterises effect is not useful at high-frequencies. So, I suspect that the iron used in the variable-frequency-driven motors now in use has to have a more limited and controlled hysterises effect than comutator motors, used for single-phase AC, 60, 50. 25, and 16-2/3 Hz and DC. (And locomotives and rail cars that must draw power from 25 Hz as well as 60 Hz require transformers far heavier and larger than those running on
This is an interesting point which I had not carefully considered in design detail. Of course the armature bars are intentionally designed to have zero remanent magnetism, and permag armatures designed to resist transient changes in magnetism right up to the effective Curie point with any transient hysteresis effects accounted for empirically, so the question relates to the stator architecture. A relatively easy way to check is to see if the stators use some transformer-grade alloy (for example including silicon); another would be to see if they use a multiplicity of thin laminations.
I have a funny story about how the ‘better’ alloys for large transformers led to my predicting the 1987 market crash, in writing, to a former head of Morgan Stanley. It would have made a good episode of the show ‘Connections’… [8-|]
The stator on any rotating field machine needs to be made of laminations to prevent eddy current loss. Possible exceeption would be using ferrite or some other powdered high frequency magnetic material. The rotor on high speed synchronous machines is often made from a solid cylinder of steel with slots milled in for the armature windings.
One advantage of a synchronous traction motor is higher efficiency and lower rotor heating at vey low speeds. Disadvantages are requiring an angular position sensor for the rotor and a dedicated inverter per motor.
This thread fails to mention several weak points of the AC motor as it applies to railcars operated by VVVF inverters. The VVVF inverters essentially create sine waves from square waves. The fast on switch-off switch of the square waves pound the stator insulation requiring continuous hipot testing for breakdown. Additionally the eddy currents between the rotor shaft and the rotor bearing casing result in bearing asperity, pitting and early bearing wear.
Correct, the high frequency currents from the switching frequency (and harmonics from the rapid rise/fall times), typically dictate the use of inverter rated wire. OTOH, the higher switching frequencies possible with SiC and GaN FET’s should allow for some filtering, which would greatly reduce the high frequency currents.
Armature windings? Synchronous and non-synchronous INDUCTION motors do not have armature windings. But there are other types of AC motors that do have windings. Which type are you discussing?
One point about synthesis is that the inverters can generate a pretty good shaped waveform when switched at ‘modern’ frequencies (even at power currents, this can be high frequency corresponding to good waveform in the ‘rotating field’). Conventional methods of smoothing will also work, albeit adding expense and packaging complications to the final drive.
Smoothing this would be almost childishly simple at each winding if it is an observed concern. On the other hand, since this isn’t an application where the square waveform has to be imposed on the power (as, amusingly, it is in NMRA/Lenz DCC model railroading!) using higher frequency combined with the natural inductance of the stator windings pretty well solves any risetime problems the modern insulation I’m familiar with would have difficulty with long-term.
This is as childishly simple as proper third-brush installation – I can say with great pride that I solved this problem for the Precor treadmill company in the early Nineties when they started experiencing disintegrating bearings in their early large AC-drive treadmill offerings.[:)] A simple low-impedance connection around the bearings solves the issue definitively, as do certain other methods of neutralizing current across intalled bearing structure.
One point about synthesis is that the inverters can generate a pretty good shaped waveform when switched at ‘modern’ frequencies (even at power currents, this can be high frequency corresponding to good waveform in the ‘rotating field’). Conventional methods of smoothing will also work, albeit adding expense and packaging complications to the final drive.
Smoothing this would be almost childishly simple at each winding if it is an observed concern. On the other hand, since this isn’t an application where the square waveform has to be imposed on the power (as, amusingly, it is in NMRA/Lenz DCC model railroading!) using higher frequency combined with the natural inductance of the stator windings pretty well solves any risetime problems the modern insulation I’m familiar with would have difficulty with long-term.
This is as childishly simple as proper third-brush installation – I can say with great pride that I solved this problem for the Precor treadmill company in the early Nineties when they started experiencing disintegrating bearings in their early large AC-drive treadmill offerings.[:)] A simple low-impedance connection around the bearings solves the issue definitively, as do certain other methods of neutralizing current across installed bearing structure.
My railcar maintenance experience in both primary and secondary repair, showed the DC motor brush replacement, flash overs, and commutator cutting was soon replaced by AC motor stator replacement, rotor bar separation and bearing asperity. An attempt to replace the bearings with ceramic coated races lessened the asperity but not completely. Stator breakdowns were diffcult to detect with the standard hipot tester. The leading edges of the VVVF inverter square waves could overshoot with 5kv spikes appearing across the stator. A purchase of a stator signature analysis tool…some kind of a magnetic sniffer…helped pinpoint shady stators…a stator removal process requiring dry ice…just sayin…
If you were getting that level of spiking something a bit more agile might be needed to avoid ‘bearing asperity’; I admit I’m surprised good ceramic race coatings wouldn’t increase dielectric strength to where “achievable” current flow across the bearings would not produce sufficient plasma for microarcing.
I take it there was insufficient adhesion or excessive loading to use actual ceramic rolling elements. In my experience it was asperities on those, more than on the races, that led to the greater problems.
The problem with the spikes is inherent in your description. That’s a synthesis artifact, and a truly nasty one to be of that magnitude. Sometimes very fast current risetime capability is NOT a good thing!
Likely you had someone build the ‘megger’ equivalent of a logic probe to be able to read those spikes at what might have been microsecond or even nanosecond duration… but with voltage high enough, and current following good enough… there would be your sparks across the bearing contacts…
Be glad you didn’t have to use something like liquid nitrogen on the shaft… and preheating the stator assembly. [;)]
The AC motor stator tester I described was only used after the rotor was removed while in the traction motor repair shop. It was a non destructive test method. There was never a hipot test placed across the bearings…but all in all the MTBF of AC motor railcars were 8 times or more than that of DC motor rail cars…
You have accurately put your finger on the likely issue, though, which indicates you knew more about both AC powertrain design and likely fault analysis than some of the ‘engineers’ from wherever those cars came from.
The ideal thing to do this testing today might even be one of the portable recording oscilloscope replacements, with an accurate time base reference so you could identify spikes even in the nanosecond range and associate them with various external conditions or control actions. Be fun to identify what was causing the actual spiking and even more fun seeing what the builder would do 'bout it…
Ah yes…and many hours spent cutting wheel flats…
You have my admiration. Thankless out of all proportion to its necessity…
The overshoot sound like the product of a high frequency resonance in the stator windings being excited by the high dV/dt of the inverter. These resonances are due to the distributed winding capacitance interacting with the various ways inductance expresses itself in the windings. Having built a number of inductors and then measuring impedance as a function of frequency, it doesn’t surprise that large motors would have these issues. Other fun part of designing/building/measuring inductors was running into proximity effect, which is one reason why special wire is needed for inverter driven motors.
Putting a filter between the inverter and motor can help, though you probably want at least a decade between the highest VVVF frequency and the filter cut-off frequency as well as another decade between the filter cut-off frequency and the pwm switching frequency. Large IGBT’s tend to like witching at relatively low frequencies, which make filters impractical. The other issues is designing a high power high efficiency filter is non-trivial.