I know that in the variable voltage variable frequency you are able to keep the torque constant in various speeds , but i have also read something about them not requiring commutators but i couldn't really understand this part , can someone explain or provide me with some useful sources . thanks

  • \$\begingroup\$ There are some applications where the AC motor can simply be connected to the mains power directly and run at its rated capacity continuously for years. No other motor can do that. Induction motors do not use commutation(switching). The stator is energized with AC power. This causes a rotating magnetic field which energizes the rotor by induction. The rotor tries to keep up with the rotating field. There is no electrical connection between rotor and stator. \$\endgroup\$
    – user57037
    Nov 19, 2016 at 4:03
  • 1
    \$\begingroup\$ @mkeith I know what you meant BUT "no electrical connection" might be better put as "no electromechanical connection" or even "no mechanical connection" as there is indeed very much an "electrical" one in the form of the magnetic field that the power is transferred through (as I know you know). \$\endgroup\$
    – Russell McMahon
    Nov 19, 2016 at 6:28
  • \$\begingroup\$ @RussellMcMahon, noted. But can't edit the comment. \$\endgroup\$
    – user57037
    Nov 19, 2016 at 7:02
  • \$\begingroup\$ @mkeith If you care: copy comment to editor -> delete comment -> repost edited comment -> ping RusselMcMahon so that he deletes his now obsolete comment. In this way we can keep comments clean. \$\endgroup\$ Nov 19, 2016 at 9:12

2 Answers 2


DC motors require commutators. All of the mechanical output power must be transferred to the rotor through the commutator. The maximum voltage and current for which a commutator can be designed limits the maximum power for which DC motors can be designed. AC motors don't have that limitation, so they are used at power levels that DC motors are either impossible to design or very difficult to build.

Even at power levels for which DC motors have commonly been used in the past, the commutator poses a maintenance and reliability problem. As economically competitive variable frequency induction motor drives have matched and exceeded the performance and reliability of DC motors and controllers, they have become the preferred alternative for most uses.

Above some power level, wound-rotor synchronous motors are used rather than induction motors. That may have to do with some design issue, efficiency, manufacturing cost or some combination of factors.

Product line descriptions of various manufacturers indicate that induction motors are available with ratings up to at least 10,000 horsepower (7.5 MW). Wound-rotor synchronous motors are available with ratings up to 135,000 Hp (100 MW). Synchronous generators are available with ratings up to at least 2000 MW. Synchronous generators have been operated up to 380 MW as motors in a pumped storage hydroelectric plant.

  • 1
    \$\begingroup\$ I guess real engineers design stuff like that. I am feeling kind of puny at the moment. \$\endgroup\$
    – user57037
    Nov 19, 2016 at 4:04

Your question is different than what the subject line says.
There is a truly vast amount on web re the how and why of AC induction motors.

See references below, but:

The induction motor was invented by Nikola Tesla - so it obviously uses magic as part of its operating principle. The magic Niccolai discovered here is probably his most useful invention.

A 3 phase induction motor is effectively a combination of a rotating transformer plus an application of magnet to magnet attraction and repulsion.

The stator of a 3pIM consists of 3 sets of coils that produce a rotating magnetic field at AC mains frequency.

The rotor of the motor consists of a series of windings in the form of conducting "bars" which act as the secondary of a transformer in conjunction with the stator windings. When the rotor spins the effective frequency "seen" by the rotor windings is the difference between the rotating field frequency (usually 50 Hz or 60 Hz depending on country) and the rotational frequency of the rotor. If the rotor frequency equalled the mains frequency there would be 0 frequency difference so no transformer action. As the rotor lags behind the stator field frequency the difference frequency induces currents in the rotor bars and produces magnetic fields.
The motor is designed such that the induced rotor magnetic fields serve to attract the rotor magneic poles towards the rotating magnetic field poles causing the induced currents, so the rotor chases" the field causing its currents. If there is minimal loading on the rotor then it will spin close to mains frequency. As the rotor is loaded it is "pulled away from" the rotating fields, causing the "slip" frequency to rise, causing more current to be induced, causing the available powerto increase, causing the rotor t pull back towards lock, causing the slip frequency to decrease, causing .... !.
So that a stable point is reached where the rotor trails the rotating field by a sli frequency which allows the power to be transferred to cause stable operation.

Look at this specific, detailed and very well illustrated tutorial followed if then needed by a slow and careful reading of this section on this page and a look at the animated diagram, plus a look at these many images each linked to a web page , and if still needed as many of these videos as you can tolerate and these as requisite should make you a world expert, if desired.


To answer your subject line question:

Why are AC induction motors used for high power applications?

High power AC induction motors are almost invariably 3 phase powered motors. Low power motors may be single phase operated due to the mote common use of songle phase feeds in domestic and small commercial and small industrial use, but for power above a kilowatt or few 3 phase motors are the norm.

3 phase AC induction motors tend to have the best overall cost-benefit ratio of any motor type available in many applications. While in some specific applications other types may be more suitable the 3 phase AC induction motors features of

  • No commutator or brushes

  • No permanent magnets

  • ONE moving part (plus bearings)

  • Speed defined by frequency of power supply (less a small amount),

  • Relatively simple construction

  • Good power density

  • Relatively available and low cost materials (copper or aluminium for windings, steel for laminations, + housing material (steel or cast iron usually).

  • Low cost compared with many alternatives

and a few more factors, make it 'hard to beat' in many cases.


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