Introduction
The Safety Brake Switch (SBS) is a motor drive subsystem designed for rapid and safe motor braking in high-inertia applications, such as mobile robots. When the motor drive enters the STO state, the SBS initiates motor braking by executing a 3-phase short across the motor phases. This method reliably and effectively reduces deceleration time. Following this initial braking action, the automatic engagement of the motor brake ensures the motor comes to a complete stop.
A three-phase short is initiated a few milliseconds after the safety control signals reach the STO inputs of the motor drive. This delay ensures that the drive's power stage is fully disabled before applying the short. The motor’s back electromotive force (back EMF) then induces a current in the windings, generating a magnetic field that counteracts the motor’s initial rotation, effectively slowing and stopping the motor.
It is crucial to ensure that the current generated by the three-phase short does not exceed the maximum current that the motor and the Roboteq controller can handle for 30 seconds. Therefore, the Braking-Short current must be calculated.
Calculation of Braking-Short current
The equation for calculating the braking-short current error is given below:
where:
Example
We will calculate the short current for the Nidec142LDA motor. The Specifications of the motor are as follows:
