Is the Motor Rotor Held in Position or Free to Rotate When the BK Terminal is Grounded in a Brushless Drive?

The BK (Brake) terminal in a brushless drive serves a key function related to "brake release" or "enable control."

When the BK terminal is grounded (or receives a low-level signal), it sends a clear command to the drive: "Release the electromagnetic lock on the motor." The drive will cut off the output to the motor windings and disable its internal dynamic braking circuit.

At this point:

• The motor's three-phase windings enter an open-circuit or high-impedance state.
• No effective short-circuit current can be generated to produce braking torque.

As a result, the motor rotor loses all electromagnetic holding force provided by the drive and enters a "free state." It can be easily rotated by hand or moved by an external load. This function is essential for applications such as equipment debugging, manual homing, or emergency manual repositioning.

Important Implications of the "Free State":

• The motor cannot resist any external force.
• In vertically mounted applications, the load may descend due to its own weight.
• In conveyor applications, workpieces may shift if pushed.

This clearly indicates that the BK function alone cannot reliably hold a position.

The Key to Holding Position: An External Brake

If your application requires the motor to remain firmly in place when stopped — especially during power loss (e.g., for safety protection or to prevent a vertical axis from dropping) — using a physical brake is the only reliable solution.

Updated on: 28/11/2025