Understanding The Vibration Forces In Induction Motors.
M. Costello
1990
Citations
21
Citations
Journal
Journal name not available for this finding
Full text
Semantic Scholar
Key Takeaway Key Takeaway: Improvements in insulation materials and manufacturing techniques allowed motors to run at higher temperatures, leading to increased reliability and lower prices.
Abstract
Squirrel cage induction motors have been used extensively in industry for over 50 years. While it appears that vibration prob lems are more pronounced nowadays, certain basic construction features have always existed and have created considerable diffi culty from the initial stages of motor development. As induction motor theory has never changed, the electromechanical forcing functions have always existed and created vibration problems. In fact, some of the most complete and best references on vibra tion in induction motors were written 30 to 40 years ago. It ap pears, however, that only recently has the induction motor been critically reviewed by mechanical engineers and rotating machinery specialists. Motors are now being treated for what they are-extremely complex rotating machines having not only the associated mechanical forces, but electromagnetic and elec tromechanical forces as well. The basic operating principles of motors are discussed as well as the lateral vibration forcing func tions encountered when troubleshooting motor vibration prob lems. All motors described herein are squirrel cage, polyphase, 60Hz design. INTRODUCTION The present trend in industry is towards long term reliability on all major equipment. In order to accomplish this, more and more motors are being outfitted for vibration and temperature monitoring systems. \Vhile proximity probes have been in serv ice to measure vibration for over 20 years on turbines and com pressors, most motor manufacturers have not used them until thl;l last five to seven years. It was only five years ago when a manufacturer stated he knew a motor's mechanical performance 67 was acceptable when he could stand a nickel on end on the bear ing housing. There is no doubt that the induction motor has evolved con siderably over the past 20 years; however, this evolution was dic tated primarily from an electrical standpoint. Insulation mate rials were developed which allowed manufacturers to build larger horsepower machines, and run them at progressively higher and higher temperatures. As an aftermath of government legal actions in the 1950s, the "White Sale" eliminated price fix ing between the manufacturers. This brought competition and effectively lowered motor prices drastically. During the 1960s and 1970s, material improvements and new manufacturing pro cedures resulted in significantly more efficient machines. Motor base prices continued to drop and even now are lower than they were 15 years ago. During this period, the mechanical aspect of the motor became altered significantly. Motor frames were re duced in physical size, weight, and structural strength for a given horsepower. However, they were still to contain the same forces as their larger and more robust predecessors. As a result of recent problems, the need for equipment relia bility, more knowledge in rotordynamics and more stringent user specifications, motor manufacturers are presently being forced to evaluate their product's mechanical performance. INDUCTION MOTOR OPERATING PRINCIPLES To understand the operation of an induction motor, it is impor tant to become familiar with its major components. A cutaway of a typical large motor is shown in Figure 1. A squirrel cage in duction motor consists of the following major components: • Stator-The stator consists of an electrical winding and a cylindrical laminated steel core in which the winding coils are inserted. After insertion, the coils are connected in a manner to produce alternate pole polarity, the number of which dictates the speed of the motor. • RotorThe rotor is made up of a shaft, and a cylindrical laminated steel core in which the rotor winding is inserted. In a squirrel cage design, the rotor winding consists of nonmagne tic bars which are inserted through slots in the core. The bar ends connect to end rings which short circuit the bars. The bars and end rings together make up the rotor "squirrel cage." • Frame-The frame of the machine is either a fabricated or cast structure in which the stator is inserted. This frame must be strong enough to withstand mechanical and electromechani cal forces along with providing air passages employed to cool the motor. • Enclosure-Various enclosures can be specified such as DP (drip proof), WPI (weather protected I), WPII (weather pro tected II), TEWAC (totally enclosed, water-air-cooled), etc. These enclosures are either integral vdth or are installed on top, bottom, or sides of the frame. The basic theory of the induction motor is actually very sim ple. As an alternating polyphase voltage is applied to the ends of the stator windings, currents flowing in coil groups produce a multipole alternating magnetic field which rotates around the 68 PROCEEDINGS O F THE NINETEENTH TURBO MACHINERY SYMPOSIUM Figure 1. Cutaway of a Typical Large Squirrel Cage Induction Motor Outlining the Various Major Components. stator ID. The number of alternate polarity magnetic poles set up by the winding connections dictate the speed of the rotating magnetic field. The motor synchronous speed is as follows: motor synchronous speed == 120 X frequency of applied voltage (Hz)