in particular that the AC motor frequency determines the wheel rpm and therefore limits/prevents slippage. not sure if this completely explains why AC traction provides more tractive effort because it can operate closer to theoretical limits.
it was also interesting to read how power is adjusted between lead and trailing trucks depending on weight distribution.
but i’m not sure what type of AC motor design is typical. and specifically how the directions of rotation is determined with that design? is there some mechanical change to the motor structure or is it purely electrical?
By the computer-controlled sequence of the PHASING of the variable-frequency AC to the multiple field coils around the motor. The rotating armature consists of slanted aluminum or copper bars. The inverters that convert the rectified-to-DC electricity bsck to AC at variable frequencies produce different phases to each of the multiple field coiis, and the sequence deternines direction.
In an inside-out wheel-motor, the “armiture” is a fixed. non-rotating circle of field coils, and the rotating bars are on the outside. All railroad locomotives (diesel and straight electric) and nearly all MUs use the first type of AC motor. The wheel-motor is in use mainly in low-floor European trolley buses, dual-mode buses, battery buses, and airport plane-side buses and in several experimental ultra-light light-rail cars.
Rotation direction in all three phase AC motors is determined by the ELECTRICAL rotation. If one swaps two (of the three) phases, that reverses the electrical rotation & hence the motor rotation.
Dave is correct about the inverter managing this function in inverter fed motors, which is true for all AC motors, not just AC traction motors. Instead of physically reconnecting the power leads, the inverter software controls the power electronics to perform the reversal.
Why would following what is essentially a rotating magnetic field require any active current through the rotor?
Granted, if you don’t see how magnetism is induced in the rotor ‘bars’ you may wonder how the rotating field acts on them, let alone produces very high torque. But any good motor textbook will explain this… or ought to.
I take it you understand how the arrangement of stator windings produces the ‘rotating field’ effect. A variant of this can be used for BLDC motor control, where the effective magnetic field producing rotary torque is controlled many times a second to be at the best angle between stator and rotor poles – here the stator-induced field does not ‘rotate’ but is switched to remain effectively ahead of the (magnetic) rotor in the direction of rotation…
I suspect if you want a real headache, look up control algorithms for torque-producing switched-reluctance motors… but when this is done right, some of the real-world operating results can be frankly surprising.
the article said “The variable frequency drive creates a rotating magnetic field which spins about 1% faster than the motor is turning”. not sure they didn’t mean it’s a few degrees ahead of the rotor.
the amount of lag affects torque
but presumably there’s a phase (angular location) of the stator field and the armature that must be synchronized such that if the armature increases rpm, wheel slip, torque drops or even changes direction.
i wonder how the phase of the armature is measured to adjust the field rotation, mechanically or electrically.
If you look up ‘slip’ in a discussion of synchronous three-phase motors you’ll find an explanation. The point at which the rotor spontaneously ‘locks in sync’ (I.e. runs as a synchronous motor with stable peak torque’ is with about that 1% slip rather than perfect following.
This is with self-induced magnetism in the rotor, which is essentially passive; no active control of the rotor is required.
The speed control is regulated with respect to the feedback signal from rotor speed, not phase-angle change; if you were to measure the rate of ‘rotation’ of the effective stator field you would find it faster by the slip…
1% slip (phase difference) is not the same as “1% faster” and of course not the same a wheel slip. i think i remember that day in motors class where we used a strobe light to see the phase of the rotor and watched as sync broke when we exceeded the load limit.
and 1% ahead, presumable 3.6deg, in phase requires knowing the exact phase of the rotor, not just the rpm.
what’s the frequency and phase(!) of an AC traction motor when starting at 0.000 rpm?
i’m guessing when starting, an AC traction motor is like a stepper motor.
Having had a couple of power systems labs as well as an electric machinery lab…
The pulling out of synchronism is for a synchronous motor, where the torque for a non-salient pole machine is proportional to the cosine of the angle between where the motor is operating with respect to the rotating field and where the rotor produces zero torque. Large synchronous motors using have field windings on the rotor, fed though slip rings in the olden days and now an AC generator and rectifiers mounted on the rotor. The advantage of a controllable field excitation is being able to adjust power factor, along with increased field for when the machine is producing higher torque as either a motor or generator.
Induction motors make use of the slip between the rotating field and the bars on the rotor to induce currents in the rotor bars that interact with the rotating field to produce torque. I got a visceral demonstration of that when putting a thick wall aluminum tube between the pole faces of a 0.25T magnet. With the axis of the tube parallel with the magnetic field, I could move the tube up and down with little resistance. I got a great deal of resistance when trying to rotate the axis of the away from being parallel with the magnetic field due to the eddy currents excited in the aluminum tube. The faster I tried roatating the tube, the stronger the resistance to rotation.
The limiting factor for induction motor torque is where the combination of skin effect and inductance gangs up to limit the in-phase induced current. Thick copper rotor bars will develop maximum torque at a small slip, but very little torque at a large slip. High resistance rotors will generate large torque at high slip, but much less torque at low slip. This also leads to lower motor effiecincy.
A compromise was the wound rotor motor, where the winding connections were brought out via slip rings. For starting, a high resistance would be put in series with the windings, which would then be cut out as
one thing i miss from working at Bell Labs is how some people were able to explain what i thought were fairly complex concepts so clearly. some lunch conversations were so much better than some college lectures.
one person would often start a discussion by saying “I don’t know if you know this …” to discuss basics without implying ignorance so that the listener could better understand the topic of discussion.
what are salient pole motors, what is their benefit over non-salient pole motors and are AC traction motors that type of motor?
Remember that a magnetic field only induces current when it is changing. In a typical ‘synchronous’ motor two things are happening: the magnetic field change in the stator induces a current in elements of the rotor (and this current develops a magnetic field as it flows) and the changing magnetic field in the stator then reacts with the current and magnetic field it has induced. The ‘slip’ is necessary because it represents the relative motion that induces the currents in the rotor elements.
In the wound-rotor induction motor the windings act just like the shorted bars in the ‘regular’ type as far as having current (and flux) induced in them. The slip rings are a convenience to get the induced-current path outside the rotating element to where variable resistance to the induced current can be imposed. The higher the resistance to current flow, the less actual current flow occurs in induction, therefore the less magnetic field to interact with, hence starting control.
Note that were the rotating field and the rotor poles to run in sync, there could be no induced current in the rotor; it would then cease to show magnetism and hence torque. Of course that would cause it to start slowing down… more so if it is driving some sort of load… and when it slows there starts to be mutual change again, and current is induced in the rotor again, and the magnetic fields can react again…
In a permanently-magnetized rotor, the ‘reacting’ magnetic field is always present, hence such a motor could be operated at a speed synchronous to the field the rotor ‘sees’ – since relative motion to induce rotor magnetism is not necessary.
(Of course the rotating permanent magnet structure tries to induce back EMF in the stator architecture, so to get faster rotation you need field weakening, paradoxical as that might seem.)
You may find interest in the explanation of introductory material on motor operati
AC traction motors are INDUCTION motors. As such they rely on the slip FREQUENCY to INDUCE a current in the rotor bars, which produces a magnetic field. This magnetic field IS SLOWER than the rotating magnetic field of the 3 phase AC winding. Slip definition:
It might be more helpful to him to start with the equation that shows the magnetic field induction as a proportion of relative speed between ‘field rotation’ and rotor face (or, more appropriate, effective rotor-bar) rotation…
(e.g. E = BL (Vsyn - Vr) or variant as appropriate; Vr being the same ‘thing’ as “Vm” defined as motion of the conductor relative to the field and measured accordingly…)
so he can see what’s producing the numbers in the slip formula. He is an EE so most of the principles are well known to him.
(This reminds me all of a sudden of a guy at Lockheed, a brilliant engineer, to whom eddy currents were so magical but inscrutable that he attempted to get a classified patent on them…)
that’s a very good point. how does it work at startup when the rotor is barely moving? do the stator fields get pulsed?
that’s helpful. so torque can be controlled/reduced by adding resistance in the rotor current path instead of reducing field current. is this a benefit of salient pole design?
yes. my understanding is this is why wheel slip is less, if not prevented with AC traction motors.
now i’m interested in startup, when a rotor is still and just starting to move
i’m not going to understand something quoted out of a text book without some pre-text
i’ve tried to say that I believe the rpm of the rotating magnetic field and the rotor must be the same, but that the rotating field is slightly ahead of the rotor phase.
if this isn’t correct, what are relative the phases of the magnetic fields in the rotor and stator?
It is important to recognize that the slip is continuous; the rotor always runs that little bit slower than ‘synchrony’ with the rotating field visualized as a thing. Think of it as the amount of “turning power” that is consumed in keeping the rotor bars developing their magnetic field. They cannot BE magnetic without current, and that current is only produced when magnetic-field strength at the bar is changing…
An important thing to realize, though, is that this is an ‘automatic action’; the slip changes in proportion to ‘the speed difference that induces whatever magnetic field to produce output torque balancing load at speed’. That’s a lot of word salad, but you see the point that the action is self-correcting based only on relative change between the ‘rotating field’ and the magnetic field generated by the current that was induced in the shorted (or resistively loaded) rotor element by that same changing magnetic field … in the house that Jack built. [:)]
Step back a bit to understand what is happening here. The field itself is not physically ‘rotating’; it is the resultant of the fields induced by the three phases in their respective windings, and only has the net effect of handed rotation as seen by a rotor pole. The alternation of stator current will produce induction in an adjacent rotor pole, and the resultant magnetic field interaction between stator and pole will produce attraction or repulsion and, since the rotor pole bar is built into a rotor, this will produce torque on the rotor and this will be ‘handed’ in the direction the overall stator field acts.
Now, the stator fields are changing in the same general sense that marquee lights induce perception to ‘follow’ them, even though each light considered by itself is just turning on or off. So if you had pre-magnetized rotor poles they would be attracted to the resultant, and try to ‘lock in’ to where the attraction pulling them around is greatest. But the shorted poles of these motors aren’t pre-magnetized; in fact they’re designed not&n
i believe i understood the concept of a rotating field
each pole on the stator is connected to one of the 3 phases of power. so the magentic field builds on one pole, the next in sequence and then decreases. yes like the marqui lights. with 3 phases there is a definite direction
you did not answer this question and i believe it would be very insightful.
you couldn’t use a few more technical terms.
again, those Bell Lab discussions made things simple to understand