Solve these Common Electric Motor Problems and Get Great Electric Motor Reliability

Solve these Common Electric Motor Problems and Get Great Electric Motor Reliability

Abstract:

• Test the continuity and resistance of new and rewound electric motors. How do you know that the electric motors in your store are in good operating condition and ready to change-out for a failed motor? Even new motors, and especially rewound motors, have crimping problems, internal wiring faults, insulation damage, poor terminations, etc. What testing do you do before they go into store to prove they will be reliable out in the plant?
• Specify exactly what you want in the electric motors on your site. Write a ‘Motor Purchasing Specification’ and a ‘Motor Overhaul Specification’ that suits your working environment and your site’s experiences.
• Use the right electric motor storage and handling methods. Write an ‘Electric Motor Site Storage, Handling And Management Procedure’ and train the right people in how to store and handle motors well. Simple things like facing the shafts outward so you can turn the bearings to a new spot each week or two, to prevent the rolling elements indenting the bearing race; sitting the frames on rubber mats or wood to isolate the bearings from ground and building vibration; identifying and marking the motors permanently and clearly; etc.
• Use motor covers that make maintenance easy. Design one-piece electric motor covers that are light weight, are well ventilated and can be lifted-off by one person. If you make multi-piece motor covers with screws for assembly, then you are adding another hour to the job. And you can bet that the cover will not be screwed back into place properly. It will end up on the floor and the motor will be without protection.
• Quick, easy, clear identification. Mark the number of poles, the power and frame size on the motor in big text. You will help your people get it right every time. For example, when they see ‘4P – 11kW – 132’ in big letters on the motor they know exactly what they are looking at.
• Show people the history of the motor. One site paints motors different colours to reflect the service history. The first rewind after a burn-out and the frame is painted red, the second rewind it’s painted black. There is no third rewind. They had found that motors with a poor service life are never repaired successfully. Two motor burn-outs and it’s replaced with a new unit.
• Match the motor to the environment and the hazards. You are asking for trouble if you use open frame motors in wet or dusty environments. The motor internals need protection from contamination. Watch that the motor can handle the operating hazards. Is it suitably rated for flammables or explosive vapours and dusts if that is its service requirement?
• Use the right bearings for the environment. Are you sure the standard bearings in the motor are right to provide a long, trouble-free life? In wet environments, water can run along a motor shaft and into the front bearing. If the bearing is not suitably sealed against water ingress the motor will fail within months.One user in the beverage industry went as far as sending brand new motors to the rebuilders. They removed the standard ZZ shielded bearings that were filled with lithium grease and replaced them with 2RS rubber shielded bearings and filled with polyurea grease. This modification lifted motor life from 12 months to 36 months.
• Improve the shaft to housing sealing. A weak point that fails quickly is the shaft seal. Failure can be due to vibration, rubbing, contamination working its way between shaft and seal. Installing a better seal design with a slinger improves bearing life. One such seal is the Stefa Seal.
• Put key motors on a vibration analysis round. Record the new bearing signature and then trend changes in bearing behaviour to forewarn you of coming problems.
• Do preventative maintenance testing on the entire motor circuit. Equipment is available to test motor integrity when the motor is off-line. The test unit sends a low voltage signal through the circuit and indicates faults with resistance, impedance, insulation, internal wiring and more. Over crimping of connectors to the cable core is a common problem, even with new motors, and produces high resistance.
• Thermography the isolators, connectors and drives. Thermal imaging will identify hot spots in operating circuits fast.
• Is the motor compatible with the Variable Speed Drive? A VSD can alter the power characteristics but the motor may not be designed for it. There have been winding insulation failures and internal short circuits across wires and cores due to frequency cycling.
• Motor terminal connections can vibrate loose over time. The nuts on the connection studs inside a terminal box are often not torqued down well. One site found that 70% of the lower terminal nuts were loose – they had never been torqued down properly.The electricians had only ever tightened the top nuts. They didn’t know any better because no one had taught them the right way to connect motor terminals. They had copied what someone had incorrectly shown them years ago.The answer to this problem is to write a thorough terminal connection procedure and train the electricians to follow it. Use detailed explanations and pictures of the right way to terminate and make it standard practice to train every electrician in the right practices.
• Use non-corroding silicon sealant for sealing. There is a special grade of silicone sealant to use on electrical equipment. Do not use any other grade of silicone sealant because the vapours given off corrode metal electrical components. The ‘electrical grade’ silicone sealant does not corrode metal.

Disclaimer: Because the authors, publisher and resellers do not know the context in which the information presented in this article is to be used they accept no responsibility for the consequences of using the information contained or implied in any articles.

P.S. If you have maintenance engineering advice on industrial equipment maintenance, especially defect elimination and failure prevention of plant and equipment, or have made successful equipment reliability improvements, please feel free to send me your articles to post on this website. You can contact me by email at info@lifetime-reliability.com