Quick stepper motor question

Discussion in 'General Electronics Chat' started by Rake32, Jan 17, 2016.

  1. Rake32

    Thread Starter New Member

    Jan 17, 2016
    As the title says, just a quick one I could use some help on.

    I'm looking through my uni notes and the lecturer has said the power supply needs to be turned off when measuring the winding resistance of a motor. He the put a (why??) after, I'm just not sure why it does? Will the motor draw some current even if it's not turning?

  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    Your meter measures resistance by supplying a known constant current across the resistor and then measuring the voltage across the unknown resistor. Current * resistance = voltage

    So, how would adding a current flow from your power supply screw things up?
  3. Rake32

    Thread Starter New Member

    Jan 17, 2016
    Well It'd give a lower resistance reading. Is that it then? The motor will draw even when it's not stepping?
  4. MaxHeadRoom


    Jul 18, 2013
    You should always remove power when measuring resistance, otherwise the reading will be affected, as per @GopherT the meter supplies the current needed.
    A stepper motor will draw current with steady DC and remain stationary, this is known as the Holding torque, and if the DC is the motor rated value, the torque will be the rated torque of the motor.
  5. Roderick Young


    Feb 22, 2015
    Depending on how the motor is driven, there may be power flowing to it even if the rotor is not moving. If there are 4 sets of windings, A, B, C, and D, the drive could be

    A - B - C - D - A - ...

    and when the rotor is not moving, all the windings could be turned off to save power, or one could be energized to give extra holding strength and make sure the rotor does not slip out of positions.

    Or it could be driven

    A - AB - B - BC - C - CD - D - DA - A - ...

    The above gives half-steps between the individual winding positions, and more power. Again, when the rotor is stopped, all windings could be turned off to save power, but in that case, the rotor will slip to one of the natural positions. To hold the rotor in a position between windings requires continuous power to a pair of windings.

    And besides that, it's always a good idea to turn power off before probing something.
  6. Rake32

    Thread Starter New Member

    Jan 17, 2016
    Ok that makes sense.
    Thanks for the replies guys!