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Industry, Wastewater

5 Reasons Why TEFC Motors Fail

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Totally enclosed fan-cooled (TEFC) motors are great for harsh environments such as petroleum, chemical and wastewater industries. But just because they’re made to work hard in tough settings doesn’t mean they can’t wear down, too. Without proper installation, operation and maintenance, TEFC motors can fail. Below are five reasons TEFC motors typically fail, and how you can prevent that.

  1. Improper installation resulting in excessive heat. 

If TEFC motors are installed in a poorly ventilated area, winding temperatures can exceed the design temperature rise rating, leading to premature failure. Make sure the motor has is in an adequately ventilated area to prevent overheating.

  1. Improper installation resulting in excessive vibration.

In wastewater treatment plants, many motors are belt-driven or mounted to equipment via a flexible coupling. In both instances, laser alignment of the shafts is necessary to ensure proper performance. If motor shafts are not properly aligned, excessive vibration will result in premature bearing failure.

  1. Improper installation of motors with variable frequency drives (VFDs) resulting in electrical discharge machining (EDM) and bearing fluting.

Pumps and blowers account for up to 70 percent of the total electrical load of a wastewater treatment plant. As a result, most pump and blower motors are operated on VFDs in order to keep starting current low and allow operators to maintain better effluent quality during diurnal loading. Motors that are not equipped with shaft current mitigating devices will be subjected to stray shaft currents that cause bearing failure. 

On motors under 25 HP, shaft grounding rings are recommended as a low-cost solution to mitigate shaft currents and protect bearings from EDM. However, these devices require regular maintenance to ensure that particles do not accumulate on the brushes and reduce the current mitigating abilities of the shaft grounding ring.

On motors larger than 25 HP, insulated motor bearings are recommended as a lower maintenance option for current mitigation.

  1. Improper operation of motors with variable frequency drives (VFDs) resulting in excessive motor winding temperatures.

It is important to understand the frequency limitations of equipment. Typical TEFC motors are designed to operate between 20 Hz and 60 Hz. Operating outside of these limits can cause the windings to overheat and the motor to fail. It is also important to note that many pumps and blowers have limited turndown at frequencies greater than 20 Hz. If frequencies exceed 20 Hz, different technologies can experience a range of issues:

  • Centrifugal pumps can encounter cavitation.
  • Centrifugal blowers can encounter surge.
  • Positive displacement lobe blowers can encounter thermal expansion from rotor slip.

Manufacturers should supply process equipment (pumps, blowers, etc.) with the correct motor and VFD combination. Improper programming of a VFD can result in premature motor or equipment failure.

  1. Improper maintenance of motor bearing lubrication and shaft grounding rings resulting in premature failure.

As with any equipment, you should always follow manufacturers recommended service intervals for bearing regreasing and shaft grounding ring cleaning or replacement.

If you’re having trouble with a TEFC motor, one of these problems may be the cause. Always ensure that motors are designed for the correct duty cycle, are outfitted with current mitigating devices and are installed and maintained in accordance with all manufacturer’s instructions. If you have more questions about this topic, contact one of our experts or leave a comment below.
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