Paper: The Cause, Analysis Prevention of Electrical mechanical Failure In Three Phase Electric Motors

The three phase squirrel cage induction electric motor built today is the most efficient electric motor that the industry has ever produced. It is the product of over one hundred years of evolution in materials and design of rotating electrical apparatus. Technology has allowed the manufacturer to reduce the physical size of the electric motor while increasing its efficiency. Concurrent with this reduction in frame size per horse power, we have experienced a product that is more sensitive to aberrant line, load and operating conditions than the more massively built product of previous generations. This paper will explore the conditions that compromise the electrical and mechanical life expectancy of the modern state-of-the-art electric motor. To illustrate the changes in NEMA frame sizes of electric motors, let us evaluate the evolution of a 5 HP 1800 RPM squirrel cage open drip motor over the last four decades. In 1950, this motor was in a 254 original NEMA frame and weighed approximately 195 lbs. With the NEMA redesign of 1952, this same 5 HP motor was condensed into a NEMA 215 frame and weighed 127 lbs. With the NEMA redesign of 1965, the 5 HP motor was further condensed to the smaller and current 184 frame weighing 88 lbs. The same 254 frame that was able to accommodate a 5 HP 4 pole motor in 1950, today accommodates a 15 HP 4 pole electric motor. The larger mass of the older apparatus represented a considerable heat sink for all the conditions that cause motor thermal failures. Consequently, the original NEMA and 1952 rerated motors were more forgiving of all the detrimental conditions that result in motor failures and particularly premature failure. The apparatus we use today is quite a different piece of equipment from that of yesteryear and a higher level of concern for its application and aberrant operational conditions must be considered if we are to maximize the design life and our return on capital investment. In order to preserve our investment in our rotating apparatus, it is incumbent that we understand the conditions that impact the electrical and mechanical life expectancy of the current vintage state-of-the-art electric motor. We will focus on five critical areas: 1.Electrical 2.Mechanical 3.Thermal 4.Environmental 5.Application