Measuring R, L and K for PMDC Motor

Discussion in 'General Electronics Chat' started by jegues, Sep 20, 2013.

  1. jegues

    Thread Starter Well-Known Member

    Sep 13, 2010
    Evening gents,

    We are trying to measure the resistance, inductance and back emf constant for our PMDC motor.

    To measure the resistance we plan on simply placing an ohm meter across the terminals of the motor and recording the resistance value.

    To measure the inductance are either going to,

    a) Use an inductance meter and simply measure it across the terminals of the motor, or

    b) Excite the motor with a certain voltage (either a step input or periodic square wave) while locking the shaft of the motor in place(thus the back EMF is fixed to 0V) and by measuring the time for the exponential rise and/or decay of the current waveform on an oscilloscope, determine L via the time constant.

    To determine the back EMF constant K, we were planning on exciting the motor with a 12V lead acid battery and a current limiting carbon disk resistor. Once the motor reaches a steady state speed, we will use a handheld tachometer turning with the shaft of the motor to measure the speed of the motor. Knowing the speed of the motor, the voltage at the terminals of the motor and the current flowing into the motor we can back solve for the back emf.

    Using the ratio of the back emf with the speed of the motor (in rad/s) we will determine the back emf constant K.

    Does this seem like a reasonable and valid way to determine the parameters of our PMDC motor?

    Thanks again!
  2. #12


    Nov 30, 2010
    The contact resistance of the brushes will give you problems getting a good reading. Wiggle the shaft to get the lowest reading. You might have to clean the commutator to get the best reading. Or you might do a DC 1 amp supply with locked rotor measurement. It depends on how dirty the connection seems to be.
  3. MaxHeadRoom


    Jul 18, 2013
    That is not considered the best method, measure the current when a small voltage is applied to the locked rotor, and calculate accordingly.
    You can also back drive the motor at a known rpm and measure the generated voltage to determine the V/rpm.
    Last edited: Sep 20, 2013
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    The locked rotor method is not a great way to measure the motor resistance. Depending on the position of the commutator it may be engaging one armature coil or two, or even give a higher value reading if the commutator has burnt segments or brush issues etc.

    I prefer to measure "stall" current, by applying a regulated fixed voltage (like 3v for a 12v motor) and allowing the motor to turn very slowly (say 1 rev per second) braked by my fingers etc and read the average "stall" current on an ammeter while the motor is JUST turning.

    Obviously you can calc total armature resistance from the regulated voltage and average current.