Influence of brake shocks on safety components of electric motors

Brake shocks, which occur on electric motors when the brake is applied, have a damaging effect on the immediate surrounding components. Safety-relevant rotary encoders, used to detect angular position and speed, are particularly affected. The influence of brake shocks can be either reversible, in the form of temporary signal distortion, or lead to permanent mechanical damage. In the past, these types of shocks were insufficiently considered in the standardization of tests. Shock tests for encoders were specified only with half-sine excitation, whereas real brake shocks are high-frequency, transient events. In 2021, a draft of the IEC 61800-5-3 standard was published, proposing an extended shock-testing procedure that considers the characteristics of real brake shocks. Based on this testing procedure, this publication presents extensive lifetime tests on safety encoders and their results, including a simplified qualification procedure.
First, the pseudo-velocity shock response spectrum, used to describe brake shocks, is introduced. After that, the testing procedure is explained, and an overview of the parameterization of the developed testing machine is given. It was observed that brake shocks cause both reversible impairments and irreversible damage to safety encoders. A wide variability of failure times and damage patterns was observed. The results of the endurance tests were statistically evaluated using Weibull analysis. Additional evaluation was carried out using the Weibayes approach because the testing procedure is time-consuming and the available basic population for statistical evaluation may be small. Finally, a newly developed shock measurement system is presented, suitable for practical brake shock measurements on the motor shaft. Additionally, a qualification procedure for safety-relevant encoders is introduced, based on a proven-in-use approach. This procedure offers a simple solution to fulfill the requirements of the new draft of IEC 61800-5-3.

Keywords: Brake shocks, Reliability, Encoders, Electric motors.