Why Do Electric Motors Shake During Startup or Power-Off?

Why Do Motors Shake When Starting or Powering Off?

Anyone who has participated in motor testing or used electric motors in practice may have encountered a situation where the motor experiences noticeable shaking during startup or when power is suddenly cut off.

During motor testing, if a voltage regulation or frequency modulation method is used for starting, the motor start-up process is relatively smooth. In contrast, when starting the motor at rated voltage, noticeable shaking may occur. Similarly, if a motor running under normal conditions suddenly loses power, a similar phenomenon may occur, sometimes accompanied by loud noise.

1. Mechanical Resonance

When the excitation frequency acting on the mechanical system of a running motor is close to the natural frequency of one of the system’s stages, significant resonance may occur. This results in pronounced displacement vibrations and sometimes audible noise caused by the vibration. The "shaking" observed during motor startup or power-off is a specific manifestation of this resonance.

This issue is also common with variable frequency motors. A variable frequency motor may operate smoothly at certain frequency ranges, but in other ranges, it may exhibit obvious electromagnetic noise. The root cause lies in electromagnetic resonance.

2. Manifestation of Inertia Change

The processes of motor starting and stopping both involve changes in inertia. Changing inertia means the motor system is undergoing a change in the forces acting on it. The shorter the time it takes to make this change, and the greater the force involved, the more obvious the change in state becomes. Physically, this is often perceived as vibration. In motors, this can manifest as changes in sound or displacement.

Therefore, secure installation of the motor is essential. This includes fixing the motor through its mounting feet or flange to a stable base. When necessary, additional limiting measures should be taken. During operation, vibrations can cause the motor to loosen from its fixed position, so regular inspection and maintenance are particularly important.