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Motor physics - Clever answers only

  1. Nov 17, 2004 #1
    Hello. I have a dilemma with a DC circuit containing a motor. The motor has a load attached, and i am varying power to the motor. I have a voltmeter connected across the motor in parrallel, and an ammeter connected in series to the wire, as i should. I expected that as i raised the power, voltage and current would increase, as P=IV. My load is constant and i make no changes to the motor, so I expected resistance to stay the same. However, i found that as the motor began to move, my current remained at 0.5A, but my voltage increased, violating V=IR. Either resistance is changing in a way i can't understand, or current is icreaseing but my ammeter is not picking up on this for some reason. Two theories i came up with are: 1-I thought maybe 0.5A was a sort of threshold for movement, and after that all of the extra current would be transferrend into movement energy and so the ammeter is only measuring the remaining current in the wire and not taking into account the current transferred to movement energy. 2-But perhaps as the load begins to move, it's resistance increases. I just want to know which theory would be right, if any. Cheers. John, A level student, 2nd year.
  2. jcsd
  3. Nov 17, 2004 #2


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    Resistance in eletrical motors increases with rpms.
    Last edited: Nov 17, 2004
  4. Nov 17, 2004 #3

    Your statements do not make a whole lot of sense -- how can you control input power ??? How and what did you change to begin with ??
    This is a simple DC motor -- if it is not moving then the voltage and current are related by the static resistance of the windings -- that's it and it will be a LOT of current.
    However when it moves there is a a back EMF generated which will change the impedance of the motor which will reduce the currents for a GIVEN input voltage and sttel according to load.
    To increase the speed you increase the VOLTS and the current will rise , the basic resisance will be constant or rise slightly due to heat , the impedance ( real and imaginary ) will depend on load -- if you load it the first reaction is to decrease speed -- this lowers the back EMF to compensate somewhat , but the static resistance will cause more loss as the current rises .

    There is no magic here even Faraday understood DC motors . However since normally there is a way of swithing the current into the windings as the motor rotates the actual waveforms are NOT smooth and the the inductance of the windings matter and in complex ways.
    You will not see this without an oscilloscope meters will not respond and just give you an average reading.

    Yours Ray
  5. Nov 19, 2004 #4


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    This depends if the "constant load" is a constant torque load (current, and power both increase), or is a constant power load (current decreases, power remains constant).

    There's also an efficiency factor involved. A typical motor will be most efficient at a certain rpm. Radio controlled aircraft use gearing, prop diameter and prop pitch to optimize the usage of electric motors.
  6. Nov 20, 2004 #5
    Small permanent magnet D.C. electric motors and generators are the same as each other. When an electric motor spins faster it generates a voltage and this opposes the applied one.

    If the motor was ultra efficient (no losses) and it was turning no load (except it's own inertia) then it wouldn't take any current at all once spinning at a constant speed. When you changed the applied voltage, it would consume some current as it spun up to speed, but it would eventually reach a speed where the generated voltage equalled the applied one, and then the current would be zero.

    Real motors have some losses, so they always consume some current. However, if they are running on a very light load, the current tends to be virtually constant, regardless of the applied voltage. Only the speed changes to match the volts.

    Larger non-permanent magnet motors behave differently, as the magnetic field of the stator also varies with current. There are two main varieties - shunt, where the rotor armature and stator windings are in parallel; or series, where the same current flows through both rotor and stator.

    Shunt motors behave much the same as permanent magnet motors, but series motors behave very differently.
  7. Nov 23, 2004 #6
    Thanks everyone, except ray - you were absolutely no help. I said 'clever answers only'. You control voltage by varying the power output. I used a lab pack.
  8. Nov 23, 2004 #7
    Good name seems to suit.

    Well thanks for picking me out for your barbed reply -- but you do not generally control power in that way ( does the 'lab pack' have a dial or display saying 'power' ).
    It would be extremely dangerous -- An Inductive load consumes lttle power so get get any power in requires potentially VERY high voltages -- what you control is either the voltage with current limit , or in special cases the current with voltage limits.
    To know the 'power' going into a DC motor is not a simple thing , when it's not moving the 'impedance ' is simple just the DC resistance , but when it is moving the ' impedance ' is a complex mix of reistance capacitance and inductance which are all being switched at the revolution rate
    It's good to require 'clever' answers the question is can you understand them ??
  9. Nov 23, 2004 #8


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    Just to back up Ray: You cannot control the amount of power going into a motor without at the same time controlling the amount of load. To go to a mechanical analogy: if your car is idling and has an engine capable of 100hp, you cannot do anything to make it run at 100hp unless you load it (say with a dyno). Simply stepping on the gas will just cause it to pass redline and blow up (if there is no rev limiter).
  10. Nov 24, 2004 #9
    Thanks Krab

    At least there is one person thinking here. Ray.
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