All the "energy-efficient" continuously-variable-speed electric motors I've seen in consumer applications (e.g., HVAC, water pumps – all motors producing on the order of one horsepower) appear to convert incoming AC to DC, which is then modulated by an ECM to achieve the variable speed.

The variable speed AC electric motors in this power class that I know of just use multiple windings, and so the number of speeds equals the number windings that can be unenergized – typically 2 out of 3.

Are there any electric motor designs that can efficiently modulate AC to directly drive the motor over a wide range of speeds or torque, without converting to DC?

  • $\begingroup$ @DKNguyen Every time I've spec'd residential HVAC systems in the last decade, the higher-end ones use variable-speed ... drives? What's the difference from "motors"? They're also common for residential swimming pool pumps. $\endgroup$
    – feetwet
    Mar 28 at 0:48
  • $\begingroup$ @jsotola Touché; but I mean using line AC power without transforming/rectifying it to DC at any point before it hits the motor. $\endgroup$
    – feetwet
    Mar 28 at 0:50
  • $\begingroup$ @feetwet using line AC power without transforming/rectifying it to DC at any point before it hits the motor ... how would you change the frequency of the voltage? $\endgroup$
    – jsotola
    Mar 28 at 1:55

2 Answers 2


Are there AC input variable speed drives without a DC conversion (without a DC-link)? That answer is yes: Cycloconverters.

But that's not the motor. In your first paragraph, you are not describing motors; you are describing variable speed drives. A motor is just a dumb motor of coils and maybe magnets. A modern variable speed drive is a box of electronics that takes DC and modulates it to produce waveforms to properly commutate the motor at various speeds. If the power source is AC it rectifies the AC to DC first. It provides continuously variable speed and speed control.

The variable speed motors are variable speed but this does not mean continuously variable. No rectification or modulation occurs. The coils inside have taps so you can choose to energize fewer coils to run the motor as if it had fewer turns which changes the speed. There is no rectifying or modulating going on. It is still just a dumb motor so there is no speed control. So your first and last paragraph is muddling motors and variable speed drives with each other. Motors don't modulate and there are no motors that convert AC to DC.

  • $\begingroup$ a lot of newer fridges and A/C units use variable speed drives $\endgroup$
    – user253751
    Mar 28 at 10:09
  • $\begingroup$ @user253751 Do they? Interesting. $\endgroup$
    – DKNguyen
    Mar 28 at 13:17
  • $\begingroup$ often marketed with the keyword "inverter" $\endgroup$
    – user253751
    Mar 28 at 13:19
  • 1
    $\begingroup$ Q - "all motors producing on the order of one horsepower". A - "The competitive power rating span of standardized CCVs ranges from few megawatts up to many tens of megawatts." en.wikipedia.org/wiki/Cycloconverter $\endgroup$
    – Phil Sweet
    Mar 29 at 0:34

As others have said, this isn't about motors, it's about the VSD. Most work with a very simple and robust architecture on the power side. They convert the incoming AC to DC and then convert this back to AC at the desired frequency (at around 98% efficiency). The electronics to do this are small, efficient, and the design is quite elegant. The outgoing AC can be more or less true sine wave depending on details and need. There is no reason to not do this, so unless you have a better solution, Just run with this. This design accommodates all manner of control ranging from simple open loop operation to sophisticated PLC control.

Options that don't include DC are CVT variable speed belt drives and variable speed hydraulic drives. Both have their niches, but aren't common, are far less efficient, and have to be located near the end point. VSDs are normally located in an electrical equipment room and the variable frequency is distributed to the plant.

Bottom line is that the performance specifications are what is crucial. Just treat the thing as a black box and don't worry about how it works, just make sure it works for your task. That is plenty challenge enough since there are hundreds of different units with different characteristics.



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