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TIDA-00281: How max current decision is made

Part Number: TIDA-00281


Could you please highlight as to how the 30A rating was selected? 
Also, is there a way to estimate the rms winding current ? What all parameters does this depend on

Thank you!

  • A 1 kW motor finds applications in various automotive equipment, such as pumps and blowers.  At 48V, a current of about 20 amps gives the operating power of 1kW.  The board design considered also short-term conditions such as stall, requiring higher currents, thus the 30A figure was used.

    In terms of estimating the rms winding current, this will depend primarily on the motor parameters and loading conditions.  In a stall condition, the maximum current will be limited by the resistance of the motor winding.  In normal operation, when rms or average current might be more of interest, the back emf of the motor reduces the winding current.  Since the back emf is proportional to the speed of the motor, if the motor slows due to heavy loading, the back emf decreases and the winding current increases.  Typically you can get torque-speed curves from motor manufacturers which give guidance on this relationship.  Motor specifications may also provide the torque constant and back-emf constant.  Each different motor will have a different set of parameters; these typically will determine the operating current if the load torque is known.

  • Okay, thank you. 
    So, for a 48V, 3kW Motor driver , will ~ 90A board design suffice?



  • Probably; a 3 kW motor running at 48V would have a current of about 60A at rated power.  Depending on the stall current and worst-case use conditions, designing for 90A peak sounds reasonable.  There is a discussion of current limiting at this post: