Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Teacher/ physics forum conundrum

  1. Jan 22, 2013 #1
    I was talking to my physics teacher today about electric motors and used some information I learned here. I'm not calling you guys wrong by any means, I just would like to know what is real. He claimed two things: 1. As load on an electric motor goes up, current goes down (I know this is wrong but he wouldn't provide any proof)
    As I was trying to explain this to him, I started talking about how an electric motor generates electricity, as it spins, making the potential difference across the coils lower, and according to ohm's law, reducing current. According to him, electric motors do not generate any electrical energy, I again can't see this as true because of the law of conservation of energy. Some of the energy is used to accelerate the motor, but after that, the only energy actually being used is turned into thermal from the friction of the bearings (assuming no load).
    So can anyone explain what he was on about?
  2. jcsd
  3. Jan 22, 2013 #2


    User Avatar
    2017 Award

    Staff: Mentor

    A motor converts electric energy to mechanic energy, it does not generate electric energy.

    Electric motors with constant voltage basically operate at constant torque. Power is torque times angular velocity - if you add a load to the motor, angular velocity goes down, so power can go down (unless you increase voltage applied to the motor). With constant voltage, this leads to a lower current.
  4. Jan 22, 2013 #3

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Needs some definition of terms.

    What you said he said does sound backward.
    Induction motors draw less current as they approach operating speed, perhaps he's speaking of that phenomenon?

    And it needs some rigorous adherence to terminology:
    I assume you mean "counter emf" ?

    Now i assume you refer to potential difference across the impedance of the coils, as distinct from the counter emf? It is common to represent them separately on textbook drawings when teaching motors.
    In reality - the counter emf is produced in the windings -
    so potential difference between ends of a winding is sum of I X Z and counter emf;
    so teacher is rightly confused by your assertion that the pd across windings decreases.

    You seem to have a grasp of motors
    and presumably teacher does too
    so i think you're both mis-communicating.

    Today's hurry-scurry world does not encourage us to take the time to formulate our words with precision.
  5. Jan 22, 2013 #4
    Sorry, terminology wise I come from the model boat world where it's all just squished together and we all understand somehow.
    I do mean the counter EMF.
    If you put a voltmeter across the terminals of a brushed dc motor, you would not have the same potential difference that you have from the source, correct?
  6. Jan 22, 2013 #5

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    yes, there's a synergy.

    why wouldn't you have the same pd?

    Only loss should be in interconnecting wires, which loss ought to be very small else the wires would get hot and melt.

    Unless you have a speed controller between "the source" and motor - in that case put yourself in motor's shoes... your source is the speed controller.
  7. Jan 23, 2013 #6
    Okay, so back to the original question that started this whole thing.
    Reading real life current values off of motors, they don't follow ohm's law, they draw much less current than they should for the resistance they have. the original answer was that the counter EMF reduced the effective voltage across each coil, and when using that voltage, ohms law is true. In that case, what happens to the extra pd?
  8. Jan 27, 2013 #7


    User Avatar

    Staff: Mentor

    The current that flows into that counter emf represents the mechanical power that the motor is producing. (current x voltage = power)
  9. Jan 27, 2013 #8

    Yes, he is wrong. Try clamping the shaft of a motor so it cannot turn and watch the fuse or breaker blow or trip. The rotating coils produce the back-voltage that reduces the applied voltage to the motor, so it does not receive an excessive amount of current.

    What do you mean by "generates electricity"? What is electricity? That word means several things which have to be defined or assumed. You should have said an electric motor produces a back-voltage, etc.

    He is correct about that. Unless you hook the motor up to a rotating shaft, and run it like a dynamo, an electric motor consumes energy.

    What has that law got to do with a motor generating electricity (electrical energy)?

    More power is consumed to start the motor and bring it up to speed than is used when it is idling at its no-load speed.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook