From: david.brown@hesbynett.no
On 27/09/2025 21:23, BGB wrote:
> 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.
>
Details of motor drives is perhaps getting a bit OT for this group - but
there are people here interested in all sorts of things. If you want to
have more discussions on motor drives, comp.arch.embedded might be a
nice place for a new thread - the group appears fairly empty, but
experts crawl out of the woodwork whenever an interesting new thread is
started!
>
>
> I had noted in some experience when writing some code to spin motors
> (typically on an MSP430, mostly experimentally) or similar:
Experiments are always good, but it is also helpful to combine them with
a bit of theory so that you don't generalise too much from a small
number of tests. In particular, the motor windings in a three phase AC
motor can be done in several different ways, optimised for different
kinds of controlling waves. The two main ones for small and medium
permanent magnet motors are for sinusoidal waves (aiming for smoothest
and most controlled driving - often called "PMSM - permanent magnet
synchronous motors") and for trapezoidal driving (for simpler driving,
often referred to as "BLDC - Brushless DC").
Then there are different ways to track the position of the motor. You
can have hall effect sensors, which are simple and cheap, giving 6
positions per electrical rotation (motors can have multiple sets of
windings and magnets, giving two or more electrical rotations per
mechanical rotation). These are good for trapezoidal BLDC control. It
is also possible to use sensorless control, where the hall effect
signals are calculated by measuring the back EMF from the motor windings
during the off periods of the driver half bridges. This avoids the
sensors and can make cabling easier, but can't be used at low speed - it
is only suitable for continuously running motors rather than positioning
motors.
Or you can have encoders, which give the more precise position needed
for sine wave or PMSM waves. These are usually quadrature encoders,
which are accurate and reliable but need to pass through an index
position to get their absolute position. Sometimes absolute encoders
are used - these are either cheaper but less precise using analogue hall
effect sensors, or much more expensive using multiple Grey code rings
with optical or inductive sensing.
For trapezoidal drives, you usually have a simple 6-step switching
sequence, with each of the three half-bridges driving high for 2 steps,
off for 1 step, low for 2 steps, off for one step. You can control the
speed of the motor by the speed of the steps, and the power by using PWM
modulation when driving high or low (or by using a single PWM control
for the common DC bus voltage).
For sine wave driving, you need fast PWM for each of the three half
bridges to generate three sine waves at 120° phase differences. The PWM
frequency has to be high enough so that after the filtering in the motor
windings, you have little in the way of harmonics.
Generally, however, instead of actively producing sine waves, you do
what is known as "vector control" - you measure the currents in the
three branches, and use the angle data to convert these to currents
perpendicular to and aligned to the motor position. You then regulate
the PWM values to control these two currents - aiming to get the desired
current in the active direction, and zero current perpendicular to it
(since that is just wasted effort). The resulting waveforms are
somewhat distorted sine waves.
An msp430 is fine for trapezoidal control and hall effect sensors, but a
bit underpowered for serious sine wave or vector control. You are
better with a Cortex-M4 for motors.
> Sine waves give low noise, but less power;
Sine waves are closer to the ideal for many motors, but you'll get even
lower noise with good vector control.
You can also try adding some third harmonic - use sin(x) + 1/9 sin(3x).
The third harmonic disappears in the motor, since it affects all three
phases equally. But it flattens out the peaks of the sine wave and lets
you then increase the amplitude by about 12.5% before hitting 100% of
your DC bus voltage.
> Square waves are noisier and only work well at low RPM,
> but have higher torque.
Square waves are a really bad idea - you jump between high torque and
low torque, and will regularly be pulling the motor back a bit rather
than forwards. Prefer trapezoidal control - it is just as easy, and
works vastly better. You of course get more torque ripple than with
sine waves or vector control.
> Sawtooth waves seem to work well at higher RPMs.
> Well, sorta, more like sawtooth with alternating sign.
Do you mean trapezoidal control?
> Square-Root Sine: Intermediate between sign and square.
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