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Brushless motors - open loop all the way

shai kon
Expert 1960 points

Hello,

I know that brushless sensorless motor controllers establish initial back EMF by spinning the motor in an open loop.

Once the BEMF is high enough - they power the coils accordingly to the position derived from the BEMF.

2 Questions:

1. I my understanding of the process accurate?

2. Why not continue spinning the motors in an open loop?.

  • 0
    TI__Guru** 114905 points
    Hi Shai,

    Your understanding of the basic open loop startup is correct. Prior to spinning the motor position is either detected or set to a known position.

    At slow speeds, you can continue to run in open loop. Running in open loop is not an efficient method of running the motor.
    Running in closed loop also provides feedback such as locked rotor (no BEMF signal).
  • 0 in reply to Rick Duncan
    Expert 1960 points
    Thanks for your help.

    "Running in closed loop also provides feedback such as locked rotor (no BEMF signal)."
    But if there's no BEMF signal and we have a sensorless configuration, how can the closed loop be maintained?

    Are most of the Hobby ESCs open loop or closed loop ?
  • 0 in reply to shai kon
    TI__Guru** 114905 points
    Hi Shai,

    Running in closed loop implies the BEMF or current is being monitored. When the motor is running properly, the BEMF provides a signal for commutating. The signal can be used to determine the motor speed.

    If the motor stalls, the BEMF signal will collapse. The motor will no longer run in closed loop, and will have to go through the startup routine.

    A quick web search shows that most ESC are closed loop.