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Question about single phase AC motors

  1. Apr 5, 2012 #1
    Single phase AC motors generate a rotating magnetic field using two inductors which are turned on and off sequentially. The second inductor is connected to a capacitor to change the phase angle of the electricity delivered to it. When the supply current is turned on, one inductor is turned on while the other is turned off because most of the supply current is used to charge the capacitor connected to the secondary inductor. When the supply current is turned off, the first inductor is also turned off while the capacitor discharges the electrical energy it has stored and this is used to power the second inductor. The constant on and off alternation between one inductor and another produces the changing magnetic field that rotates the rotor.

    Is there a single phase AC motor that only uses one inductor to rotate the rotor?
  2. jcsd
  3. Apr 8, 2012 #2


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    Possibly the reason you have received no responses is your reference to coils being "turned on and off sequentially." This implies some sort of continuous alternating switching arrangement, which is not correct.
    Single phase motors can operate with only one winding powered, it's only for starting that they require the auxiliary winding. Do you mean are there induction motors with no auxiliary winding? For low power applications there's the shaded pole motor. http://www.clrwtr.com/Single-Phase-Electric-Motors-Characteristics-Applications.htm

    I suppose you could always use a separate second motor to run any motor up to speed... :wink:
  4. Apr 9, 2012 #3
    The information about the two coils being switched on and off sequentially almost like a two phase motor is found in the following link:


    The information also came from a discovery channel documentary about electromechanical clocks that used motors incorporating two inductors being switched on and off sequentially to turn the rotor.

    Additionally, the motor that was described in the first post where the second winding is continuously on and out of phase with the main winding could be either a Permanent Split Capacitor Motor or Capacitor Start/Capacitor Run Motor.

    Source: http://www.clrwtr.com/Single-Phase-Electric-Motors-Characteristics-Applications.htm

    May it also be possible to incorporate a blocking diode into each of the two inductors so that they can be switched on and off sequentially? The diode of the first inductor may permit the passage of the positive cycle to keep it on during this cycle while the diode of the second inductor can do the opposite. The diagram can be found below:

    http://img407.imageshack.us/img407/9074/singlephasemotor.png [Broken]
    Last edited by a moderator: May 5, 2017
  5. Apr 9, 2012 #4

    jim hardy

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    I'm confused by the assertions above as to how motors work. I think stepper motor concepts have been mixed up with induction motor concepts. .

    I'd suggest reviewing this TI Motor Control Compendium to get vocabulary straightened out. it should answer most of the questions.

    click the link that says "click here to view presentation"

    it's a powerpoint slideshow so be sure you have microsoft's free viewer installed.

    oops i just noticed thay have a pdf but i didnt try that one.
  6. Apr 9, 2012 #5
    In post #3, a link was already posted to a website about single phase motors that have a secondary winding as a second phase through the use of a run capacitor that changes the winding's phase angle. This secondary winding is not merely a starter winding in these types of motors but is continuously turned on to assist the primary winding in generating the rotating magnetic field.

    But the new question is about a design that cannot even be found in the sources provided. Is it possible to use diodes to control two induction windings to make them out of phase instead of using a capacitor?
  7. Apr 9, 2012 #6
    The shaded pole motor has only a single coil; see
    Special copper busses on the stator poles produce a phase shift to provide a rotating magnetic field.
  8. Apr 9, 2012 #7
    Yes that is correct, but what about the use of diodes to control the alternate switching between two windings in a single phase induction motor? Is such a motor possible?
  9. Apr 9, 2012 #8
    No. You have to deal with the stored inductve energy.
  10. Apr 9, 2012 #9


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    Could you copy the few sentences and paste here, as I can't see it.
    I'll look closer at it, but diodes, probably not. A diode in series with an inductor is the way to produce a DC component. And the DC component won't do anything useful in an induction motor.
  11. Apr 9, 2012 #10
    The lines can be found below:

    "Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90 degrees from the single-phase supply and feeds it to the second motor winding."

    Here is the diagram for the diode controlled single phase motor so that it can be studied more carefully:

    http://img407.imageshack.us/img407/9074/singlephasemotor.png [Broken]
    Last edited by a moderator: May 5, 2017
  12. Apr 9, 2012 #11


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    There is nothing about switching on and off sequentially there. The auxiliary winding can be disconnected once the motor has reached speed, or it can be left connected, according to design.

    I'm not dismissing the possibility that someone may invent a new design for an AC motor.

    Any DC in the windings represents wasted energy; it generates heat but produces no torque.
  13. Apr 10, 2012 #12
    But the auxiliary winding is made out of phase with the primary winding because of the capacitor so that means that the auxiliary winding will not be on at the same time as the main winding if this definition of electronic components being out of phase is correct.

    And in the design using diodes to block cycles from the AC source, the inductors will still be powered on at the positive cycle for one inductor and the negative cycle for the other inductor, so each inductor would be running on a mono-cyclic AC current since the current entering each inductor is still fluctuating. After all it is only a DC current if the power level is continuous and constant with respect to time.
  14. Apr 10, 2012 #13

    AC induction motors have squirrel cages built into the rotor. Any magnetic field in the stator from residual dc currents will create huge eddy current braking.
    Last edited by a moderator: May 5, 2017
  15. Apr 10, 2012 #14
    Here is a picture of a single-phase induction motor with no "starting" coil. It is a repulsion-start motor, made in 1914, with a radial commutator used for shorting some special squirrel cage copper busses inside the rotor. It has a higher starting torque and lower surge current than modern split phase or capacitor start motors.

    Attached Files:

  16. Apr 10, 2012 #15
    Why has this motor become obsolete if it is a better design?

    And how does this motor work? Can a circuit diagram be posted in this thread?

    How can there be residual DC currents in the inductors if they are running on single cycles from an AC current which constantly fluctuates? After all the DC current has to stay on and remain on a constant power level for a given amount of time to be considered a DC current.
    Last edited: Apr 10, 2012
  17. Apr 10, 2012 #16
    It is about twice as heavy as a modern motor with same HP, and very expensive to build.
    Look up "Repulsion start electric motor" on web. There are articles discussing theory, and at least three videos showing Century Electric repulsion start electric motors in operation.
    The circuit diagrams in above posts showed half-cycle rectification, which will produce a dc current.
  18. Apr 11, 2012 #17


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    With a commutator it is going to be expensive to build, and require more maintenance than one with no sliding electrical contacts. (It will be noisy, too, but maybe they lift the brushes after it has run up to speed?)
    If it has an average value which is not = 0, then the voltage contains DC. An average-reading meter will reveal this.
  19. Apr 11, 2012 #18
    But direct current is supposed to remain completely constant over time. If the rectifiers are not connected to a voltage regulating capacitor the output will be either a half wave or full wave alternating current of a single polarity so it is still an alternating current. A list of electric power graphs will be posted below:

    http://img69.imageshack.us/img69/2018/electricpowergraphs.png [Broken]
    Last edited by a moderator: May 5, 2017
  20. Apr 11, 2012 #19


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    No, nothing of the sort. It is never changes "direction" then it is pure DC. It does not have to maintain a constant potential to be DC.
    If it never changes polarity, it cannot be alternating. A rectified AC is DC. You could call it fluctuating DC, but it's no longer AC if it never alternates polarity.
  21. Apr 11, 2012 #20
    What is eddy current braking? How is it caused by DC electricity? Why is it necessary to use an alternating current in the induction winding to avoid this phenomenon?
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