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Homework Help: Do motors have some voltage drop across them

  1. Oct 23, 2015 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    Hello, i have been thinking, what couses a dc motor to have lets say 6volts drop across him. I was thinking that since it is just a coil it should have a verry little voltage drop, but afted testing on breadboard i figured out that it have near 6 volts.

    3. The attempt at a solution
    So i assume that since it is rotating continuesly in a magnetic stator, that stator field inducec counter emf in to the coil and in order to pass, the motor voltage have to overcome that counter emf. Is that right?
  2. jcsd
  3. Oct 23, 2015 #2


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    Isn't the voltage drop across the motor going to be related to the load the motor is driving?
  4. Oct 23, 2015 #3


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    Pretty close. Many DC permanent magnet motors also work as generators. When used as a motor they generate a "back EMF" much as you describe. The back emf is proportional to the speed that the motor is running at. So with no load they tend to accelerate until the back emf is roughly equal to the supply voltage and then maintain that speed until a load is applied.

    This leads to some interesting effects... For example the back emf is also proportional to the strength of the magnets. So all things being equal.. cheap ferrite magnet motors tend to run faster than motors that use high power rare earth magnets. In practice it's not that simple. For most applications you want a motor that turns at a particular rpm. That means a ferrite magnet motor needs more turns (to generate the same back emf) than a rare earth magnet motor. Having fewer turns means you can use thicker wire with lower resistance and less resistive loss (less heat). This makes rare earth magnet motors more efficient than ferrite magnet motors.
  5. Oct 23, 2015 #4
    Thank you, CWatters!
  6. Oct 23, 2015 #5

    rude man

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    That's correct. And, incidentally, every motor is also a generator. But if the load stalls the motor there is no back emf and the drop across the windings would be zero except for winding resistance which of course is always present.
  7. Oct 24, 2015 #6
    Thank you for the answer Rude man
  8. Oct 24, 2015 #7
    Yes it is related. If you hold the load is increased the rotation per minute is decreased and now the coil rotates in the stator magnetic field slower. And from what i consider to be truth: from the formula of em induction E= ^f/^t x n; V it seems that when u decrease the rate of rotation that ^f/^t becomes smaller and so is the induced emf. And if u stop the motor from rotating u aint gonna get back emf and the only resistance will be that of the coil wire
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