Switched reluctance motors have exceptionally high starting torque, reaching 3-6 times their rated torque, which is an advantage. However, improper design can also lead to shaft breakage. Around 2005, when switched reluctance motors were first widely used in China, shaft breakage accidents were frequent. In recent years, thanks to continuous improvements by researchers, the shaft breakage problem has been largely resolved.
This article summarizes and analyzes the causes and solutions for motor shaft breakage, so as to serve as a warning for the future. The locations of shaft breakage in switched reluctance motors are mainly concentrated at the root of the shaft extension, the root of the bearing seat, and the end of the web plate of the shaft with web plate. Different locations and different fracture patterns may have different causes.
I. Improper motor assembly
When the motor and the driven equipment are not concentric, the motor bears excessive radial load, eventually leading to metal fatigue. When the radial load on the motor shaft extension is too large, it will cause the motor shaft to bend and deform radially. When the motor rotates, the shaft is subjected to torque in all directions and deforms, eventually causing the motor shaft to break, usually near the bearing.
For motors using pulley connections, some customers, when equipping the motor output shaft with pulleys that are too heavy or the belt installed too tightly, cause the motor output shaft to be continuously subjected to alternating stress during operation. This stress generates a maximum bending moment near the bearing support point of the output shaft. Repeated impacts cause fatigue, gradually leading to cracks in the shaft and eventually complete breakage, resulting in excessive vibration of the equipment and motor during operation. If the motor is not securely fixed, such as when running on a frame, the unstable foundation causes shaking during operation, resulting in unstable belt tension, which can also damage the shaft.
Figure 1 Motor shaft
II. The stress groove in the motor shaft is not up to standard.
This problem often occurs at the root of the shaft extension. Numerous case analyses have revealed that improper machining of the radius (R) at the root of the shaft extension leads to stress concentration at this location. When the motor is running, it is subjected to alternating stresses from the shaft diameter and radial direction, resulting in fracture.
Figure 2 Motor shaft
III. Defects in the Shaft Design Itself
A sudden change in the diameter of the shaft can also cause breakage, but this problem is relatively rare, and there are relevant requirements in motor design specifications.
Figure 3 Motor shaft
Excessive load, such as a small motor pulling a heavy load, can cause the motor shaft to be in a state of fatigue for a long time. External impacts can also cause the shaft to break. Shaft breakage is a relatively complex problem, and both motor manufacturers and customers must pay attention to this issue in order to avoid similar problems from occurring.
