Induction Motors – Mechanical Design and Installation Considerations

Motor manufacturers go to great lengths to design, build and test their motors to comply with applicable NEMA and IEC guidelines. In nearly all cases, customers can be assured that a motor provided by a name-brand manufacturer will meet their needs IF properly specified, installed, and maintained. Because applications vary so widely, sometimes that “IF” can be a big one. There are many design and installation considerations, mechanical as well as electrical, which must be clearly understood and adhered to for a successful application. Let’s examine some of the more typical mechanical considerations and how they might affect your motor application. By the way, much of which follows is referenced against NEMA guidelines; however, the same general concepts apply whether US domestic or international standards apply.

• Ambient considerations: NEMA MG 1, American National Standard for Motors and Generators, specifies usual service conditions under which motors are designed to operate. These include an upper ambient temperature rating of 40C, above which adequate cooling is difficult to provide and the motor internal components are at greater risk of exceeding operating temperature design limits. This is important because it has been estimated that for every 10C above design temperature the motor operates, the life of the insulation system is cut in half. An additional ambient consideration is altitude; motors are rated for use up to 1000 meters above sea level. Again this is related to cooling; less dense air is not as effective at heat transfer.

• Bearing loading:It is important to understand the forces placed on the drive end (DE) bearings – those supporting the end of the motor shaft which drives the load. NEMA MG 1 provides maximum permissible loading values, assuming specific connection conditions. Two types of forces can be applied depending on the motor’s method of connection to its load and its mounting position – axial and radial.
  • Axial forces attempt to “push” or “pull” the shaft into or out of the motor. These forces are rarely an issue for motors directly coupled to their loads, unless the motor is mounted vertically or at some angle from the horizontal so that the shaft is carrying more than just the equivalent weight of the rotor. Manufacturers can compensate for axial loads by fitting the motor bearing assemblies with pre-load washers or springs. Flexible couplings area also recommended, to absorb any forces caused by misalignment and prevent them from being transmitted to the bearings.
  • Radial forces pull the shaft to one side, and are typically encountered when connected to belt/pulley driven loads. In such cases, roller bearings are sometimes used. There are very specific loading calculations which should be performed in order to properly understand radial loading, so that motors, sheaves, and belts can be properly sized and installed.
  • Manufacturers typically publish nominal bearing life as an “L10″ (or “L10h”) value, which represents the number of hours that 90% of the bearings tested under maximum specified loads will operate without failure. Note that these values assume very specific test conditions; less than optimal conditions of loading, lubrication, and temperature can reduce expected bearing life.

• Lubrication: closely related to the bearing considerations noted above, manufacturers publish very specific lubricant specifications and schedules to help ensure bearing life is not reduced. In order to get the most out of your motor, these guidelines should be followed. Note that under adverse operating conditions, such as heavy bearing loading, high ambient temperatures, or high dust/dirt locations, lubricants will break down more quickly and lubrication should be done more frequently. But more is not necessarily better; exceeding the amount of lubricant specified by the motor manufacturer can damage bearing seals and affect cooling, thereby reducing bearing life.

• Vibration: Although manufacturers typically test vibration levels under no-load conditions (i.e. with no load connected), this can provide only an approximation of real-world conditions. This is because most vibration is highly dependent on the load and its method of connection to the motor. Manufacturers attempt to compensate for this inherent limitation by dynamically balancing their motors to ensure they meet or exceed NEMA guidelines. When published, values will usually be expressed in inches/second. Several motor manufacturers offer precision balanced motors with lower vibration levels by special order. Also, most motors of with cast iron frames of size 143 and larger are provided with vibration instrument mounting pads on the frame to facilitate vibration testing in the field. Also keep in mind that the use of a variable speed drive to control the motor can sometimes induce vibration, either due to mechanical resonance or torque ripple. Mechanical resonance may arise when motor vibration levels increase dramatically at certain operating speeds, due to the physical construction of the motor. Most drives include a “skip (or jump) frequency” setting which allows 2 – 3 suspect frequencies to be skipped during acceleration. Torque ripple, on the other hand, is induced as a function of the drive’s harmonic frequency output content. It sometimes manifests itself as cogging (speed/torque fluctuation or pulsation) at low motor speeds. This can be an issue for sensitive low-speed/high-torque applications and can be reduced through improved drive design or external output filtering.

If you have concerns or questions about any of the above, please let us know by visiting our Comments section, or contact us at simon.fan@vtdrive.com or visit us at www.vtdrive.com. We’ll be glad to assist. And please join me in a couple of weeks for another column.