From: david.brown@hesbynett.no   
      
   On 27/09/2025 14:38, Thomas Koenig wrote:   
   > David Brown  schrieb:   
   >   
   >> 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.   
   >   
   > If you have three phases (required for high-power industrial motors)   
   > I believe people use the three phases directly to convert from three   
   > phases to three phases.   
   >   
   > The resulting waveforms are not pretty, and contribute to the   
   > difficulty of measuing power input.   
   >   
      
   That used to be how it was done - using thyristors, and powering   
   induction motors.  But it is not how it has been done in new motors for   
   a long time.  (In industrial use, some motors can be very big, very   
   expensive, and very difficult to replace - thus factories can have the   
   same motors for decades, even though better and more efficient ones are   
   available.)   
      
   Using thyristors to regulate the power out from your three phase input   
   is relatively simple, but as you say, the waveforms are not pretty.   
   This leads to significant noise (electrical and audible), vibrations,   
   torque ripple, and wear and tear on the motor.  And it makes a mess of   
   the input supply, giving harmonics and phase differences between the   
   current and voltage input - which leads to significant loses in the   
   power delivery.  These loses are between the generation and the   
   customer, meaning the electricity supplier sees it but the customer does   
   not see it on their bill - thus electricity suppliers greatly dislike   
   it.  The effect is less with thyristors on three phase power than   
   thyristors on two phase power, but it is still very bad.   
      
   So these days, the AC power - two phase or three phase - is invariably   
   converted to DC first, using power factor correction rectification (so   
   that the instantaneous current draw is proportional to the voltage at   
   the time, keeping current and voltage in phase and nicely sinusoidal).   
   After that, the AC drive to the motor is generated using PWM signals -   
   from perhaps 2 or 4 kHz for old IGBT systems to at least 20 kHz for   
   newer systems (avoiding audible noise) or up to maybe 160 kHz using GaN   
   or SiC FETs - higher frequencies mean smaller and lighter inductors and   
   capacitors.   
      
   These kinds of motor control are smaller, more efficient, and much more   
   controllable than old thyristor-based drives.   
      
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