optimal PWM frequency

Discussion in 'The Projects Forum' started by ammar87, Jan 10, 2013.

  1. ammar87

    Thread Starter New Member

    Jan 7, 2013
    hi everyone
    i have this qustion:
    what is the best PWM frequency to control brushes dc motor ?
    can anyone lead me to an answer please
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Usually in the ~20kHz range. This is done to keep it out of the hearing/audio range of humans.
  3. Readro

    New Member

    Jan 8, 2013
    So it'll annoy plenty of dogs then. :D
    shortbus likes this.
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    As shortbus pointed out, you usually put it into the >20kHz range to avoid audible noise, however, you should also check your driver as I have seen some that refused to be pushed that fast.
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
    ~20khz is normal for noiseless operation as mentioned, but I believe that lower frequency optimizes efficiency. Reduction in switching = reduction is switching losses. Most industrial drives that I'm aware of switch at around 2khz; probably because in a plant, noise reduction at this level is not much of a consideration, but saving on massive utility bills is.
  6. Papabravo


    Feb 24, 2006
    In order to select an optimal solution (aka "best") you need to specify one or more criteria which need to be met. Once criteria are specified a unique solution may exist, the solution set may include more than one solution, or a countable infinity of solutions, or no feasible solution at all.

    Outside the range of human hearing is probably not the criteria you were expecting. You might have expected something like maximum torque per watt of power or maximum velocity per watt or something specific to your particular motor.

    Tell us about your motor and why you believe in the existence of a unique solution to this problem. I'm curious.
    Sensacell likes this.
  7. ammar87

    Thread Starter New Member

    Jan 7, 2013
    well i have this motor :

    i read that if the motor has low inductance and low resistance i should use high PWM frequency , i don't know what is the criteria should i follow , and what this could effect the efficiency of the control , i want to control the position of this motor , the motor intended to switch it's direction rapidly since the position is not constant
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    A lot of commercial DC motor drivers for traction etc use low freq PWM often in a few hundred Hz. It gives much better drive for low speed high load situations like traction motors.

    High freq PWM is filtered by the motor inductance to become the equivalent of a constant low motor voltage, which just keeps the motor in stall condition if the load is very high.
    ammar87 likes this.
  9. Papabravo


    Feb 24, 2006
    OK..you say you "don't know what is the criteria should i follow", then you say "i want to control the position of this motor,...switch it's direction rapidly..."

    That is a start. In order to stop a rotating shaft with any appreciable inertia you need a power supply that can supply enough current to put on the brakes, bring the rotor and the load to a stop, and then accelerate in the reverse direction. Accelerating from a dead stop also requires a power supply with extra capacity.

    So read the data sheet and ask yourself if you can find out how much current is required to accelerate the rotor and the load. That is where I would start.
    ammar87 likes this.
  10. takao21203

    Distinguished Member

    Apr 28, 2012
    Depends on the motor armature inductance.

    Higher frequency for small motors, and lower freq. for large motors.

    It also depends on the type of the motor. There are specialized motors which actually have a disc as rotor. They can start/stop very quickly.

    Older locomotives actually used a diesel engine + generator, since some decades ago, it was not that easy to control these large currents.

    At Megawatt power levels, you would not want to deal with the harmonics from high frequency drive, so you keep it as low as possible. The motor will produce a lot of noise anyway.
    ammar87 likes this.