Understadning Voltage Drop and the Current Theroy

Discussion in 'General Electronics Chat' started by Dhaval Trivedi, Aug 20, 2015.

  1. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    Question :

    If there is voltage drop in a line then why the motor would burn or you have to do the rewinding of the motor.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Presumably, you are talking about an AC induction motor. If so, then the current that the motor draws from the line increases dramatically as the input (line) voltage to it decreases. Heating losses in the motor windings are proportional to I squared, so if you run the motor on a line voltage which is 10% below its rating, the current might increase to the point where the motor overheats.
     
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  3. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    Thank you mike for the reply.

    Now voltage is the charge that current carries
    So if the voltage drops then current would loose the charge

    So to get the required power the motor would draw more current

    But then why it overheats then.
    I am not able to understand this.

    Because the current now has less charge so the motor is only taking the power required for its function right

    No current is wasted so that it converts to heat.
     
  4. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    One more thing

    This one is not the induction motor It is a capacitance motor
    It requires the start capacitor and running capacitor

    the motor capacity is 6 amp

    it is for my submersible pump.

    it is not working

    either I have to change the capacitor or the motor winding

    buti am not sure what has went wrong
     
  5. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The magnetic fields in a motor at full speed limit the amount of current it requires to operate. If the motor is not turning, like it is when you first turn it on, the only thing which limits current is resistance of the wire.
    You should be able to 'smell' if the motor windings have overheated and burned. Absent the strong burning smell, try replacing the capacitors.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The motor windings are much more reliable than the capacitors of the switches that control them.

    ak
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It is still a induction motor, just split phase.
    If the start capacitor or centrifugal switch has gone then it should start with a quick flick of the shaft or impeller when you apply power.
    If the motor is Chinese origin, change the capacitors, as long as the winding's are still OK.
    At the point of switch on, an induction motor is a transformer with a shorted turn secondary, it is important for it to start revolving ASAP.
    Max.
     
  8. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    "At the point of switch on, an induction motor is a transformer with a shorted turn secondary, it is important for it to start revolving ASAP.
    Max."


    Can you explain this is more detail ?
     
  9. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    As you said that if the motor is not turning then there is no magnetic field in the motor to limit the current

    So now the entire current flow to the wire and the only limiting thing is the resistance of wire so as the current increase so wire will start burning !
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In a nutshell!
    The stator represents the primary winding, the secondary is the rotor which is iron with embedded coils in the form or copper or aluminum bars connected together essentially forming shorted turns.
    These bars or turns are used to create (by induction) the required magnetic field in the rotor, the rotor field immediately tries to 'follow' the revolving stator field, the induced frequency in the rotor gradually comes close to the stator frequency, this is called the slip, as the slip frequency decreases, so does the stator current.
    If a load is placed on the motor, this causes a increase in slip frequency and so the current increases.
    The rotor can never catch up completely with the revolving field, it gets as close as around 5-6 cyles.
    A motor that is designed to allow the rotor to catch up with the revolving field is a Synchronous motor.
    Max.
     
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  11. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You need to troubleshoot the motor.
    Remove it from the load by taking off the belt, or unbolt it and disengage it from the gears if it drives a gear box. Turn the shaft by hand a verify that you do not have any rotor bearing problems.
    If the motor has a connection cover, remove the cover and inspect the wiring for overheating and verify the screw terminals are tight.
    If that all checks out and nothing "smells" burned , proceed with an ohm meter and check for low ohm readings on your stator winding. If it is not "open" then replace the capacitors and hope that is it.
    If the centrifugal switch is bad then you will need expert help. Most motors can be repaired if the switch is bad, but it is not a DIY project for the average person.
     
  12. MaxHeadRoom

    Expert

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    If the motor shaft is accessible it is usually possible to eliminate the start cap or switch by spinning the shaft at turn on, and the motor should run up to speed at this point if everything else is OK.
    Max.
     
  13. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    How the Slip frequency would decrease : Initially when no load is connected then the slip frequency would be the minimum right. and as an when we connect the load the slip frequency would increase.

    Also by placing the load on the motor : is like connecting the pump to the motor. So indirectly the load on the rotor increase So it lags even more to the stator magnetic field.

    So going by this if there is a voltage drop in the line then more current will flow thru the stator which will generate a more bigger magnetic field in stator. Now as the load (pump) is connected to the rotor so the rotor tries to match the stator magnetic field but more current is flowing so the slip frequency becomes very high.

    Then how my wires in stator would burn. leading to cost of rewinding.
     
  14. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    If there is a voltage drop in a line then there will high current flow thru stator winding right.
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Torque in the motor is directly related to current, Torque is directly proportional to the square of the supply voltage.
    When the supply voltage V decreases, then the torque T also decreases. But there is a load connected to the motor and it requires a constant torque all the time. Thus in order to maintain a constant torque during reduced supply voltage, as a compensation, the speed of the motor decreases.
    The torque equation, “T α s * V^2”, as the supply voltage is reduced, the slip increases to maintain a constant torque. As the slip increases the speed of the motor reduces.
    T = Torque
    s = Slip
    V = Voltage.
    A point is reached where the relative torque is not able to sustain the fixed load.
    Max.
     
  16. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    When voltage goes lower, the motor slows down. This means less magnetic field is opposing the current flow into the stator so the stator draws more current and attempts to bring the rotor back up to speed. The increased current at low voltage levels burns out the stator.
    It is more complicated than that, but it gives the general idea of why low voltage is bad for a motor designed for a higher voltage.
     
  17. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    Q : First If there is less magnetic field generated by the rotor then less current is induced in the stator then why the stator is trying to pull more current from the supply. because here the stator also has low magnetic field because of low current then the slip frequency is also less right. In other words why does the stator wants to bring the rotor back to speed. it would simply stop or the rotor of the motor would rotate very slowly (not generating enough head for the water to reach heights in submersible pump). or is there any microprocessor or microcontroller which is trying to give the rates speed of the motor all the time. Because if the system is only electrical it cannot sense at what speed the motor needs to run.

    thanks
    Dhaval

     
  18. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    Q :
    "Thus in order to maintain a constant torque during reduced supply voltage, as a compensation, the speed of the motor decreases "
    As I Understood : reducing the voltage would reduce the torque but why the speed of the motor would decrease. Like how the rotor would know to maintain the constant torque.


    Here is the main thing which is confusing my head:

    how is the slip frequency , voltage, current, torque , magnetic field inside the winding , faraday law related to each other.

    Also the biggest of all who is drawing the current from the main supply the stator or the rotor
     
  19. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    You don't seem to understand the electrical principles so it might help to review.
     
  20. Dhaval Trivedi

    Thread Starter Member

    Jun 10, 2015
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    Sir your content is blocked I can't see it
     
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