In most circumstances, electric motor problems manifest themselves electrically, and electrical testing equipment is necessary for proper troubleshooting. In some cases, however, motor failure is mechanical, and can be assessed without analyzing any electrical components. Mechanical failure can often be diagnosed with your ears, and sense of touch. Loud sounds and excessive heat coming from a motor are often indicative of a mechanical problem. Bearing failure and inadequate ventilation are the leading causes of mechanical failure.
Video Synopsis
Mechanical Failure: There are few moving parts in an electric motor, so there are only a handful of mechanical points of failure. Usually, if a motor has a mechanical failure the cause is one of the bearings. It is also possible for the rotor bar to break, though this is much less common than bearing failure. Testing for rotor bar failure usually requires the motor to be disassembled and to test the voltage at low speeds to check for problems in the magnetic field. Beyond basic mechanical failure, other issues inside the motor (the coil and magnetic drive) are considered electrical, and are covered in another video.
Motor Applications: The application in which an electric motor is running can often be at fault, rather than the motor itself. Problems with the system that a motor is driving can cause undue stress on the electric motor, and can cause both electrical and mechanical failure. Due to the versatility of electric motors and the wide variety of applications in which they are used, troubleshooting the applications of electric motors for can’t possibly be covered in a single video. This video series does include a section about pumps. Follow this link for more details.
Physical Conditions: The operating environment for a motor is essential. Electric motors generate heat; some are designed to radiate heat passively, while others require cooling fans. In either case, blocking the flow of air to the fans or the metal fins on the casing can cause the motor to overheat and subsequently fail. Please note that even under normal conditions, a motor may be too hot to touch; 120 to 150 degrees Fahrenheit externally is usually acceptable. Totally enclosed motors are more prone to overheating than their ventilated counterparts, but environmental problems are applicable to either type.
Types of Bearings: Some of the bearings used in motors include tapered roller bearings, angular contact bearings (or thrust bearing), ball bearings, and double row angular contact bearings. Angular contact bearings are used in vertical motors. Double row angular contact bearings are essentially two angular contact bearings in a single housing. Use of double row bearings instead of two separate bearings is not standardized, and varies between manufacturers and engineering firms.
Greasing Motors/Bearings: A question was asked about adding grease to bearings, and David recommended using any grease with a polyurea base (David specifically recommended Mobil Polyrex EM in the video). He also recommends to change out grease entirely when adding grease to a motor. Do not mix different types of grease. Insufficient grease in a bearing can create a risk of mechanical failure; without proper lubrication, a bearing can overheat.