Then you have no difficulty seeing how that field induces current in the shorted stator bars. And you have a right hand if the effects of the rotating field on that induction might seem confusing…
I did answer it… I told you to think about it. Since you say you understand the rotating field, it is trivial to map its effect in the rotor bars, and only slightly less trivial to do so with the rotor in relative motion.
You will stop asking the same kinda pointy-headed questions over and over again if you think a moment about what you say you already know. [;)] Capisce?
I probably could, but the reference I provided does that far better. I am not a motor engineer – they would be the ones with the best technical-terms-matching-action. Perhaps one of more of them will comment here if necessary. But as you say you already understand what’s going on, jargon or correct technical reference is incidental to understanding.
And if we had other Bell Labs-level people here in this thread, I’m quite certain they would make this simple to understand.
I’ve never seen the answer to that – how many RPMs does the field rotate to get maximum TE at a standstill?
(Overmod says the field doesn’t rotate, which sounds like a semantic quibble. If nothing rotates, effectively at least, how does the motor know which way to start turning?)
He seemed to be having trouble with the idea of a rotating field that continually ‘rotated’ faster than the rotor it was driving, so I tried a different way to look at the magnetic physics … including an analogy to marquee lights which he ‘got’.
In a sense the ‘rotating’ field is only a resultant; it does not detach from the stator and spin on its own dragging the rotor along. But it is an attractive and intuitive way to explain the effect. I have no difficulty imagining how a rotating field produces slip, either, but gregc seemed to, so I tried another way.
Obviously as with Balaam’s-ass metastability, the rotor will tend to turn in the direction it interacts with the stator magnetic fields more strongly … moment by moment. This is not something like an unshaded-pole clock motor that needs to be turning to ‘know’ which way to run… or, really, that needs to have special start windings cut in.
Didn’t Tesla lecture fairly lucidly on how this kind of polyphase induction motor worked?
The answer is not that immediate, as the idea is to modulate the change ‘at the bar’ to keep current flowing but not so much that the bar melts, then have the resultant that pulls on each bar STATIONARY at closest approach, just as the locked rotor is stationary. In practice you can’t separate the two functions.
In practice you will likely fire the inverters so the field rotates around the slip differential speed, and presumably design the motor cooling so neither the stator current nor the induced rotor-bar circuit paths overheat. I would expect the ‘computer’ would carefully watch this and derate as necessary based on what the motor sensors tell it.
I would be surprised if Dave Goding doesn’t know the answer in detail for EMDs, including any special provisions when you use one inverter per truck. He will certainly know where to go for the answer if he needs more data to give a full enough reply.
my understanding is there are two or three different things occuring simultaneously.
the rotating magnetic field is cutting thru the rotor “bars” inducing a current. presumably that current is flowing in the opposite direction on a bar on the opposite side of the rotor as that bar moves in the opposite direction thru the same magnetic field
that current creates a magnetic field in the rotor. the rotor magnetic field rotates both due to the changing stator field (is this correct?) as well as the rotation of the rotor
the rotor magnetic field interacts with the rotating stator magnetic field generating a torque. (???)
so my difficulting is seeing how the rotating fields both induce a current and remains (oppositely) aligned with the rotor magnetic field to produce torque.
and then there’s the rotor turning inside both magnetic fields producing BEMF resisting the flow of current.
again, while this may be easier (for some) to abstractly visualize at speed, i believe a clear explanation of what happens when the motor is barely turning at startup would explain a lot!
perhaps at 0 rpm, a rotating stator magnetic field induces a rotating magnetic field in the rotor which remain appropriately aligned to produce torque
but then if the frequency of the AC current thru the stator field depends on the rotor rpm (preventing wheel slip), then that frequency at 0 rpm can’t be arbitrary, otherwise, wouldn’t the torque be be changing direction?
Not necessarily. As I understand the construction, all the bars in the rotor are massively shorted together at both ends. The rotating field can be thought of as essentially annular, with the active part in the air gap between stator pole faces and rotor, so there is very little magnetic effect from the field affecting a given bar on the bars diametrically across from it. I do not have a reference expressly proving this but have always assumed that the ‘return current’ in the rotor is through the bars and structure not affected by close magnetic-field induction. This should be easy to confirm with reference to a good text.
Technically true, but the only part of the rotor you want to consider at this point is the particular part of the bar structure in which the current of interest, and hence the associated magnetic field of interest, is being induced. Only the magnetic fields associated with the currents actually being induced in excited bars will contribute to the motor action…
(???) is so right! Remember I said look only at the bars? Think of the rotor ONLY as the magnetic fields produced by the bars. You could almost treat this like the fields in magnetic gears, with the ‘lobes’ of the rotating magnetic field then interacting with them. Do not bother with some &qu
If your familiar with the small DC motors used in model trains, the armature windings are on what would be called salient poles in a AC synchronous motor or alternator. These are almost invariably used on machines with greater than 4 poles, but are rarely used on large machines running at 1800 or 3600 RPM in a 60Hz world due to the high centrifugal force (centripetal acceleration). The salient pole itself will provide some torque without any field excitation, but the torque is not a simple sine of the ‘angle’ between the field and the center of the pole. Angle is in quotes as the electrical ‘angle’ is the physical angle multiplied by the number of pole pairs on the machine (60 Hz, 3600rpm 1 pole pair, 1800rpm 2 pole pairs, 1200rpm 3 pole pairs…).
In a synchronous machine, the rotor spins at the same speed and same direction as the rotating field, with the angle between the rotor varying with the torque produced. If the rotor position lags the rotating field, the machine will be acting as a motor generating mechanical work, if the rotor is leading the field, the machine will be acting as a generator absorbing mechanical work and generating electrical power.
The modern AC motors used in railway service are non-sychronous “induction” motors. They do not have slip rings or commutators and do operate on the induction principle by use of the hystoresis effect. In a synchronous induction motor, the rotating aluminum or copper bars are parallel to the motor shaft. In railway motors, the bars are slanted. There is always slip between the rotating field and the rotating bars, with the slip greater with increasing load. The rotating field is slow at start-up, so the frequency of the AC current is low.
thanks. that certainly cleared up my understanding.
i found the following, which explained how multiphase sinusoidal currents are re-generated.
however, still curious about startup. i think in particular if wheel slip can be prevented based on the frequency or if some other mechanism is needed at ~0 speed. (there must be a youtube video that describes this)
Don’t call it wheel slip. It’s magnetic-field-rotatitonal slip. And although the staring frquency, cycles-per-second or Herz, is low, it is not as low as you might think. Because in a three-phase non-synchronous, hystorises-induction motor, there are more than just three field coils acound the motor, as many as twelve or twenty-four. So one cycle takes the magnetic field only one-fourth one-eighth of a complete rotation.
AC Traction vs DC Traction describes an advantage of AC motors is that if the wheels slip the magnetic field slip (the angle between the stator and rotor fields) diminishes along with torque (thanks erik).
i think it’s become clear to me that this can’t work at 0 rpm, hence there must be some other mechanism (after all there is a computer)
the fact that there may be more than 3 poles has been confusing me. this is why i’ve been thinking of an AC motor as a stepper motor. with such a motor, does the rotor also develop multiple magnetic fields?
No, what he’s talking about when he says that is wheelslip, in the sense of preventing it at starting. It would be better to distinguish the railroad importance of prompt slip detection and remediation within a small effective degree of wheel rotation from the magnetic forces creating torque in the traction-motor rotor.
When I was younger, I assumed that a slip system would involve some sort of high-resolution means of detecting both angular rotation and absolute position (e.g. relative to fixed elements like poles in the motor structure). There turn out to be ways to implement ‘creep control’ that don’t depend on feedback from this (I believe the original Super Series ‘creep control’ is one such) and in fact something like a combination of monitoring true ground speed vs. individual motor (or inverter) power consumption could be used to determine the onset of wheelslip and compensate quickly for it.
What I think is being implicitly asked is how an induction traction motor using a rotating magnetic field inducing bars in the rotor handles developing wheelslip. This might be done by quickly slowing the ‘speed of rotation’ of the field (which as noted could actually provide braking torque on a spinning motor and the wheel it is geared to) or by reducing the developed torque by reducing the peak stator current inducing the field. In something like IGBT architecture these could be done very quickly and positively. I would be interested to learn the actual practical methods used in locomotive practice, and what experience may have guided and informed that development.
There is some inherent wheel-slip control in the modern ac traction motor, in that the wheel through the gear-power-transfer system cannot cause the motor to rotate faster than the fields are rotating without, as has been described in a previous post, the motor acting as a breaking force. Whether this is sufficieint wheel-slip control for all sitiuations is doubtful.
And yes, a modern AC railway (or bus) motor has multiple magnetic fields.
It is not, and for essentially one of the reasons gregc was invoking.
If an inverter-synthesized drive is right at its adhesion limit and then slips under conditions that constitute a relatively persistent decrease in physical adhesion, something interesting happens. The wheels will not wildly spin faster and faster – but they will continue to rotate with considerable torque at the speed corresponding to ‘field rotation’ less slip. As even under ideal traction conditions the coefficient of ‘sliding’ friction is lower than engaged, the wheel will happily keep turning until sufficient power is cut or field ‘rotation’ is slowed to where induced current in the bars drops or disappears.
I look at this as analogous at least to a vehicle like a '70s Eldorado with its automatic transmission. The engine has a required idle speed, and with the transmission in even low gear there is a corresponding final-drive speed (absent torque multiplication this would determine how fast the car would move on the level with brakes released). Now if you were to think that at idle and at rest the drive wheels won’t turn – put them on ice and they happily turn, not very fast but enough to utterly destroy steering integrity. Switching to equivalent of neutral will quickly, and dramatically restore steering integrity…
Overmod, thanks for you excellent explanaition and analogy.
And practical DC motors (and AC-comutator motors, now obsolete) also have multiple magentic fields. Count the number of field-coils and the identical number of armature coils on a typical EMD or GE locomotive motor.
Don’t be confused by there being only two brushes contacting the commutator. The armature coils are connected in parallel or series-parallel to the same pair.
Direction of rotation reversal of a DC motor requires reversal of armature polarity with respect to field polarity. With an electrically-produced field, reversal of the whole motor’s polarity will not reverse rotation direction. Thus, scale-model trains usually use DC motors with permanent-magnet fields.
Dave’s comment explains why “universal” motors work on both AC and DC. Lionel trains used used a motor with field windings instead of a permanent magnet, thus requiring a reversing circuit. Placing a bridge rectifier on either the field or armature (but not both) would allow polarity to control direction.
Note wrt variable frequency drive:
The use of variable frequency drive allows for getting good starting torque with a high efficiency (low slip) rotor. The low slip means that for a given drive frequency, the motor torque will drop rapidly with any increase in wheel speed. The experience with three phase traction motors on the GN, N&W and VGN showed that it was possible to get significantly higher levels of adhesion than what was possible with steam locomotives.
How such a rectifier works is illustrated. The arrows represent diodes, formerly a kind of vacuum tube, today a solid-state device, passing current in only one direction.
Using such a device for the field and not the armature or the converse csn permit polarity revesal for a “universal” motor, if the source is DC. If it is AC. the motor will try to reverse itself 120 times a second with 60 Hz house current. Lionel understood that.
