From: cr88192@gmail.com
On 9/27/2025 6:52 AM, David Brown wrote:
> On 27/09/2025 10:14, Thomas Koenig wrote:
>> Michael S schrieb:
>>> On Fri, 26 Sep 2025 12:10:41 -0000 (UTC)
>>> Thomas Koenig wrote:
>>>
>>>> BGB schrieb:
>>>>
>>>>> Brings up a thought: 960VDC is a semi-common voltage in industrial
>>>>> applications IIRC.
>>>>
>>>> I've never encountered that voltage. Direct current motors are
>>>> also mostly being phased out (pun intended) by asynchronous motors
>>>> with frequency inverters.
>>>>
>>>
>>> Are you sure?
>>> Indeed, in industry, outside of transportation, asynchronous AC motors
>>> were that most wide-spread motors by far up to 25-30 years ago. But my
>>> imressioon was that today various type of electric motors (DC, esp.
>>> brushlees, AC sync, AC async) enjoy similar popularity.
>>
>> I can only speak from poersonal experience about the industry I
>> work in (chemical). People used to use DC motors when they needed
>> variable motor speed, but have now switched to asynchronous (AC)
>> motors with frequency inverters, which usually have a 1:10 ratio
>> of speed. There are no DC network in chemical plants.
>>
>> If you have high-voltage DC system (like in an electric car) then
>> using DC motors makes more sense.
>>
>
> These are not "DC motors" in the traditional sense, like brushed DC
> motors. The motors you use in a car have (roughly) sine wave drive
> signals, generally 3 phases (but sometimes more). Even motors referred
> to as "Brushless DC motors" - "BLDC" - use AC inputs, though the
> waveforms are more trapezoidal than sinusoidal.
>
Yes.
Typically one needs to generate a 3-phase waveform at the speed they
want to spin the motor at.
I had noted in some experience when writing some code to spin motors
(typically on an MSP430, mostly experimentally) or similar:
Sine waves give low noise, but less power;
Square waves are noisier and only work well at low RPM,
but have higher torque.
Sawtooth waves seem to work well at higher RPMs.
Well, sorta, more like sawtooth with alternating sign.
Square-Root Sine: Intermediate between sign and square.
Gives torque more like a square wave, but quieter.
Trapezoid waves are similar to this, but more noise.
Seemingly, one "better" option might be to mutate the wave-shape between
Square-Root-Sine and sawtooth depending on the target RPM. Also dropping
the wave amplitude at lower RPMs (at low RPMs motors pull more amperage
and thus generate a lot of heat otherwise).
In this case, the sawtooth wave helps because the coils don't like
changing quickly, so in this case one hits them full power at the start
(to get them going) and then rapidly drop back down to zero, then hit
them the same way on the opposite sign for the next part of the wave.
When I was messing around with it at the time, input control signals
were typically one of:
ADC input connected to a POT (for direct control);
1-2ms RC style PWM.
Step/Dir signaling (typical for stepper drivers and servomotor
controllers) could also make sense.
One other option is dual-phase motors, which have the partial advantage
that one can use a repurposed stepper driver. In this case typically set
to micro-stepping. A lot of the dual-phase motors in this case though
were built from repurposed capacitor-run split-phase motors.
Say, for example, one can be like, "Yeah, this AC split phase motor is
close enough to being a NEMA34 stepper...".
Typically need to partly rewire it as typically the split phase motors
have 3 wires, but need 4 wire in this case. Some other motors are easily
modified into 3-phase though (with the same coils as a 3-phase motor
internally, just wired into a split-phase configuration with a 60-degree
phase offset; vs 90 degrees in some other motors).
One can get different properties if they machine a new rotor, as these
motors invariably come with squirrel-cage rotors. Easier/cheaper to
machine here being a reluctance rotor.
Main annoyance mostly being that this can be a pretty big chunk of steel
for any non-trivial motor (also heavy). Could likely reduce weight by
making the base rotor by layering multiple sizes of steel tubing, then
brazing or welding it all together with some steel end-caps (drilled out
for the motor shaft, probably also brazed in place). Then turn it to the
target diameter, and mill the side grooves.
Well, or find something with sufficiently thick walls (say, a chunk of
5" OD, schedule 120 or 180 steel pipe). This would simplify the process,
and be cheaper (and lighter) than, say, a chunk of 5" bar stock.
Haven't done much in this area for a while, was mostly messing around a
lot more with this when I was a little younger.
> And whenever you have a frequency inverter, the input to the frequency
> is first rectified to DC, then new AC waveforms are generated using PWM
> controlled semiconductor switches.
>
Yes:
Dual-phase: may use a "Dual H-Bridge" configuration
Where, the H-bridge is built using power transistors;
Three-phase: "Triple Half-Bridge"
Needs fewer transistors than dual phase.
It is slightly easier to build these drivers with BJTs or Darlington
transistors, but these tend to handle less power and generate more heat,
but are more fault tolerant.
MOSFETs can handle more power, but one needs to be very careful not to
exceed the Gain-Source voltage limit, otherwise they are insta-dead (and
will behave as if they are shorted).
So, one needs a more complex circuit, say:
MOSFET power transistor (typically NMOS);
NPN or PNP control transistor (such as a 2N3904 or similar);
Pull up/down resistors;
Zener diode.
In which case the control transistors can be driven as in a typical
H-Bridge.
Say, for example:
Pull down resistor pulls Gate to Source, keeping it off by default;
Zener diode in parallel with resistor, to impose VGS limit;
Pull-up transistor connects to Drain via a resistor
(via emitter or collector, depending on PNP or NPN).
Base on control transistor used for control.
Then can control the MOSFETs as-if they were BJTs. Not sure why they
can't have this stuff built-in (sort of like with a Darlington), but alas.
One typically also needs flyback diodes, and a main DC rail capacitor,
and a DC rail zener diode, ...
Though, at this stage, more preferable to buy these things than build
them, as the hand-built ones tend to have a bad habit of exploding.
> Really, the distinction between "DC motor" and "AC motor" is mostly
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