Why field weakening works

Discussion in 'General Electronics Chat' started by AidanCroft, Mar 13, 2015.

  1. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
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    Hello folks,

    I understand that two opposing fields causes rotation of a motor's rotor but as it speeds up back EMF from the stator (field coils) opposing the EMF in the rotor, so that a low Voltage and current keeps the rotor spinning at a balancing speed against friction.

    I also understand that weakening the field in the stator has the effect, as per above, of reducing back EMF so allowing Voltage and current in the rotor to rise - this causes a speed increase. Why is this so? If the stator field has now weakened then surely the two opposing fields aren't reacting as "powerfully" to rotate the rotor?

    Any guidance would be appreciated.

    Kind regards,

    Aidan.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,373
    In a shunt or PM motor the maximum rpm is derived from the applied voltage and BEMF voltage until both are essentially equal, in a field weakened motor the BEMF is weaker this reduces the magnetic flux in the air gap and because the speed is inversely proportional to the flux, the motor will speed up. But the torque is directly proportional to the magnetic flux so that the speed increase will also result in a reduction in torque.
    This is the reason that a series wound DC (and Universal) motor will tend to run away or reach excessive rpm if unloaded.
    Max.
     
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  3. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
    24
    2
    Thanks Max.

    If speed is inversely proportional to field strength between the rotor and stator then how does the motor even start spinning at all?

    Kind regards,

    Aidan.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The other factors to consider are current (torque), both motors have maximum torque at zero rpm, the two basic configurations are shunt and series connected, the first exhibits excellent constant rpm and rpm can be increased by field weakening at the expense of torque, the series motor has poor rpm control but very high torque, e.g. automotive starter, but essentially max rpm is controlled by load.
    This is an excellent reference that takes it further.
    Max.
     
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  5. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Excellent reference indeed... thanks!
     
  6. AidanCroft

    Thread Starter New Member

    Jan 7, 2015
    24
    2
    The PDF is very helpful, thank you.

    I know I might seem like a bit of a numpty but why does reducing flux increase speed? It makes sense that doing so reduces back EMF so allowing speed to increase but how can we reconcile this against reduced flux - I'd expect two strong and opposing fields to be best for highest RPM.

    Kind regards,

    Aidan.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    As you pointed out in your first post, the BEMF plays the part, if you imagine a DC shunt motor with a fixed voltage applied to both stator and armature, the rpm will increase to a point until the back EMF from generation is close to equal the applied voltage, so if the field strength is reduced at this point, the generated field drops in strength, the armature is now able to increase in RPM in order for the generated voltage to reach the same voltage as the applied voltage, but at a higher rpm, at the expense of torque.
    Max.
     
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