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Electric Motor Question

  1. Jul 26, 2013 #1
    So I've been working with variable frequency drives recently and am curious about what they're actually controlling: motors.

    So I'm curious how current and torque are related. So obviously torque is a force that causes something to rotate around an axis, therefore when the motor speed is controlled to increase or decrease, the torque should temporarily go up but then drop off to an extent once its settles on a speed?

    I'm also curious as to how voltage and frequency affect the speed of motor.

    Say its a 3 phase AC motor, voltage will have both a magnitude and frequency. I want someone to correct me if I'm wrong. Frequency in essence will control the speed of the motor to an extent. However, if you want the motor to go fast, you're also going to have to ramp up the voltage magnitude to keep the motor going to stay in sync with the frequency? Is that an incorrect assumption?

    And lastly, if the motor reaches some resistance, this will decrease the power outputted from the motor, but why does the drawn current also decrease?
  2. jcsd
  3. Jul 26, 2013 #2
    Not sure if I can answer all your questions but here is a try,

    Current and torque are related quite easily as current flowing through a conductor creates a magnetic field , the stronger the current the stronger the corresponding magnetic field, a motor is nothing more than a bunch of wires wired in a way so that each of their individual magnetic fields would line up altogether thus a iron core in the middle to amplify the magnetic field effect.
    So more current stringer field more torque to to more work.

    Whenever you want to increase rpm you have to get more power involved yet the current has to rise until more rpm is reached and it can take a steady state in which the current rise stops.Same happens if you don't rise the rpm but put more load on the axis , the magnetic field now has to do more work so it needs to be stronger so it takes more current to get a stronger field.Easy as that.

    Now another thing remember that voltage is proportional to current , you cannot just rise voltage and leave the current behind , when raising one the other will also come up.

    Yes frequency does influence the rpm, higher frequency will mean higher rpm , the magnetic field inside a motor is rotating and it has to do with the ac frequency the higher it is the more the field rotates the higher the rpm but for a given motor this cannot be infinite , there is a moment when more increase in frequency will not yield any more rpm , it has to do with the way the motor is wired and wound.

    I don't quite understand about what kind of decrease in resistance are you talking about but if the power output of a motor decreases so has the current , everything is related here , you cannot have a motor running with no load consuming maximum current etc. That would be illogical.
  4. Jul 26, 2013 #3
    I appreciate your response.

    Take this image for example. It has rated drive current and horsepower outputs.
    So if continuous torque is required (Heavy Duty), the power output goes down and so does the output current. I understand if there is more strain on the motor, then of course the power outputted will go down. Quite simply, not as much work is being done. However, why is the current going down? Common sense tells me that if there is a heavy load to keep "torque-ing", there should be more current required, not less.

    This normal duty - heavy duty stuff is confusing the hell out of me.
  5. Jul 26, 2013 #4
    A Vfd runs a motor via pulse width modulation schemes, There are different schemes to handle torque applications. Volt vs hertx vfd's have low break away torque for example as compared to a flux control vfd. In essence the scheme is a predictive current controller. If you are using any scheme other than volts vs hertz do not hook up more than one motor to that vfd. It will misinterpret the current requirement and blow the vfd. Though not right away.

    here is a couple of related articles the Rockwell knowledge base has numerous others for free download.


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