What happens when motor stalls?

Discussion in 'General Electronics Chat' started by raghuvarranvh, Feb 12, 2013.

  1. raghuvarranvh

    Thread Starter New Member

    Feb 12, 2013
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    Hi,

    What will happen when a DC motor is not able to drive its load? For example if the attached load is far beyond its maximum capacity, but somehow motor is switched ON? What happens when the motor tries to rotate but it can't.

    Regards,
    Raghuvarran
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
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    In a stalled brush type DC motor, the windings produce no back-EMF voltage, the current is limited only by the impedance of the driving voltage source and the inherent resistance of the armature windings. The motor will produce a static torque proportional to the armature current, unless the current is so large it causes magnetic saturation.

    This current can be very high and cause the windings to rapidly overheat unless the current is limited by some means.
     
  3. raghuvarranvh

    Thread Starter New Member

    Feb 12, 2013
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    Thanks. Is there any way this scenario can be sensed and the supply to the motor cutoff? What about brushless DC motors?
     
  4. Sensacell

    Well-Known Member

    Jun 19, 2012
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    One solution is to add a PTC thermistor in series, this acts as a self-resetting "fuse" limiting the current during overload conditions.

    Automotive power windows and locks usually have this type of burnout protection incorporated in the circuit.

    Brushless motors can have protective measures built into the controller chips themselves.
     
  5. raghuvarranvh

    Thread Starter New Member

    Feb 12, 2013
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    But, how can the chip understand that the motor has stalled? Is there any such motors that has this capacity in the market?
     
  6. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
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    most motor installations have some type of overload protection which sense current. Single phase motors often are equipment with a thermal trip. Larger DC motors will often have a thermistor embedded within.
     
  7. Jaguarjoe

    Active Member

    Apr 7, 2010
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    90
    Non-self resetting circuit breakers are often used in auto applications.

    Some motors (like an uber expensive servo motor I once dealt with) lift their commutator segments when stalled.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    3,730
    Motors draw much more current when they stall. You can put a small value resistor (0.1 - 1.0 ohm) in series with the motor power supply. Use an op amp or comparitor or even a pass transistor with a current sense transistor to limit current. For example, a small motor that draws 250 mA will draw close to two amps when stalled. Two amps over your 0.33 ohm resistor will generate an easy-to-measure 0.66 volts. This 0.66 volts is enough voltage drop to turn on you transistor and limit current from the pass transistor. See link below for pictures and more.


    http://www.radio-electronics.com/in...urrent_limiter/transistor_current_limiter.php
     
  9. perfo

    New Member

    Aug 15, 2012
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    Detecting over current is the norm as it is fairly easy to do and you should be able to get the max current data from the motor specs. However if you are going to control this with some sort of MP then you can look at the back EMF (depends on motor) or use the rotating bit to give you a pulse every rev. This could be done in numerous ways. Magnet and reed switch, reflective target and RX/TX pair or hall effext and disk.
     
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