Mabman

Well Sanyo has had the patent on OJ Birkestrands AC motor (https://www.rabbittool.com/frames/frelcwhl.html) for about 5 years and it looks like they are at least doing something with it for the Japanese market. https://www.sanyo.com/news/2008/12/01-1en.html They are really unwilling to bring anything to market here but I think this hubmotor could be the best one going if they at least would do a 500w version here where out laws are a bit more lax wattage wise. The whole pedelec thing looks like it is getting a slight revamp also with the new 1:2 law on the books there in Japan. But in reality I don't think pedelec is any real benefit. It has always felt jerky and counterintuitive when I have tried it out on bikes.

They are really playing up the regen card too which is pretty minimal in reality but it does make for a nice front brake assist. Also interesting how they are marketing the 2 wheel drive aspect as a safety feature, which it actually can be according to my experience. I would like to see the 500w motor here for sure though.

misslexi

Interesting. I wonder if any of the more knowledgeable members could explain the advantages of an AC motor approach over a DC one? It seems counter-intuitive to me that starting with a DC power source (battery), then converting it to AC with some sort of inverter would be more efficient than directly driving a DC motor.

unime

A brushless DC motor is essentially a 3-phase permanent magnet AC motor. In both cases, the motor's three phases are driven by alternating electric power sources that are 120 degrees out of phase. The difference is that an AC motor is powered by smoothly varying (sinusoidal) voltages on the phases, while a brushless controller abruptly switches the voltages (with one high, one low, and the third off at any point in time).

AC motors are implicitly brushless so I read the phrase "3 phase a.c. brushless 24 volt motor - generator" to means they are using a brushless dc motor. It is conceivable that they have a controller that generates a more complex waveform than usual, optimizing the efficiency of their motor (thus the patent).

By the way, all electric motors require current to alternate direction. This can be obtained from an AC source, by the use of brushes, or with the sort of controllers used for brushless motors. A high efficiency controller will have a resistance of a milliohm or two, wasting only a fraction of a watt, so there is relatively little power lost in the generation of the power signal for the motor.

cerewa

Huh?

Sure, I can't imagine you would have any kind of "AC motor" powered by batteries unless it was brushless, but in fact brushed (not brushless) AC motors are very common in household appliances.

unime

Those appliances use universal motors, which run on either AC or DC. Notice that their RPM does not depend on the AC frequency. AC motors are synchronous, and do not have brushes.

Hope that helps.

misslexi

Thanks unime, very clear explanation. One question I'm left with is what are the instantaneous voltages with a DC controller at high, low and off? Lets say it's a nominal 36V battery, are the voltages +18, -18 and 0?

unime

The negative voltage is the result of a connection to the battery's negative terminal, through a transistor switch in the controller. Similarly for positive. Both transistors are turned off during transitions (1/6 of the cycle time, twice per cycle), disconnecting the phase from any external power source. Interestingly, the internal wiring and magnetic circuitry of the motor result in a signal on the unconnected phase, used for timing by "sensorless" controllers.

If you were to measure a phase with an oscilloscope (with battery negative terminal at zero), you would see a line at 0V (for 1/3 cycle), then a somewhat irregular ramp ending with a jump to 36V (1/6 cycle), 36V (1/3 cycle), irregular ramp descending from 36V ending with a jump to 0V.

misslexi

Thanks again unime, so each pole sees the full voltage potential for it's period of time, 1/3 cycle, is what I think you said. I'm still trying to wrap my mind around what's going on the two other poles though that same phase. Once any one pole sees the full voltage, I can grasp another pole at zero, it's the third pole I'm having a hard time with. I may need to consider that remedial course in multi-phase AC I've been putting off

wernmax

Trying to watch 3 phase motors go round, makes me dizzy.

Just use the "step through animation" arrows under the motor. Course most of our bike motors have the permanent magnets on the outside of the motor instead of the inside.

https://users.tinyworld.co.uk/flecc/4...otor031102.swf

misslexi

Thanks wernmax, once again, a picture is worth 360 words

unime

Maybe it will help to discuss a three phase permanent magnet motor. Inside the motor are three windings (a, b and c) that act as electromagnets when current flows through them. Assuming wye configuration, one lead of each winding will exiting the motor (call these wires A, B and C) and the other leads will connect to each other internally.

Now, if you connect A to 36V and B to 0V, current will flow through a and b creating, say, a magnetic north pole on the working end of a and a south pole on b (the polarity follows the direction of current flow). C is not connected, and (ignoring any rotor generated magnetic flux for the moment) it will see the voltage at the common connection, which is 18V because coils a and b will have equal voltage drops. The corresponding case for an AC motor is when A is positive, B negative (and of equal magnitude) and the voltage at C crosses zero. Since both ends of C see the same voltage (zero) no current flows through C at this moment.

The next step for the DC motor is to disconnect B and connect C to 0V instead, leaving A at 36V. This allows current to continue to flow through a (north) and start flowing through c (south). The changing magnetic fields lead the motion of the rotor, push/pulling it in the desired direction. The process continues, with connections B->C (notice B is north now), then B->A, C->A, C->B, and back to the beginning with A->B.

The actual current flow is not as simple as this because it takes time for the applied voltage to induce current in the windings. Similarly, current cannot stop flowing instantly when windings are disconnected, and an electrical path exists for this discharge within the controller. In practice this means the current flow is somewhat rounded off, not abruptly stepped like voltage. In the AC case, all of the waveforms are smooth, but follow essentially the same sequence.

Real motors typically have multiple coils for each phase, with magnetic orientations chosen to work with the rotor, which has a different number of poles than the stator. Additionally, the magnetic flux generated by the motion of the stator induces currents in all of the windings. Visualizing the operation of these motors is not an easy exercise.

I hope this helps!

wernmax

Man...It's like we have an in-house engineer.

It would be cool to pop an oscilloscope into the mix so we could watch the waveforms.

misslexi

I hear that, I'm starting to think AC is really just DC on vacation! I'm going to try and piece this together tomorrow with a fresh mind...