Electric Motors

Electric motors have long provided a reliable form of motive power for industrial applications.  Motors can typically last 15 to 25 years and require relatively little maintenance.

Electric motors use over 50% of all electrical energy used in New Zealand industry.  As the annual electricity cost of running an electric motor can be up to 10 times more than the cost of the motor, care must be taken to choose an enhanced efficiency motor of the right size.

Motor Replacement

• Ensure you specify the correctly sized motor for the application.  Motors operate at peak efficiency when fully loaded.  Select the motor size as close to full load as possible.  In some cases motors can be undersized by up to 10%.  Refer to Motor Replacement Flow Chart below.
• Set up a motor schedule with motor details, maximum load, size etc.  When a motor needs replacing all the details are on hand and a decision can be made promptly.
• Replace oversized motors with smaller ones.  The motor schedule should include the load on the motor, not just the motor size.  An oversized motor uses more energy than a correctly sized motor.
• When old motors fail, replace them with enhanced efficiency motors that operate at a lower annual cost. 
• Do not rewind motors unless essential.  Rewound motors generally lose efficiency by as much as 5%.

Motor Replacement Flow Chart

Motor Drives
• Use variable speed drives (VSDs) for speed control of motors.  Pump and Fan speeds can be adjusted as required.  The reduction in speed follows the “cube law factor” where energy saved is the cube of the percentage of speed reduction e.g. a 25% reduction in speed produces an energy reduction of 58% (75% of full speed uses 42% of full speed energy).
• VSDs make full torque available at all speeds and provide for current limiting in fault or overload conditions.
• Use electronic soft starters for fixed speed machines.  They reduce the voltage during start-up and avoid inrush currents.  Soft starters are not designed for speed control, however they provide smooth acceleration.  Due to low starting torque, soft starters are not suitable for on-load applications.
• Speed reduction doesn’t always require a VSD to be installed.  If the reduction in speed is permanent and is not required to be variable, the reduction can be achieved by changing pulley sizes, which will be cheaper than installing a VSD.

A case study outlining the savings a VSD can make can be downloaded here: VSD on Fan Motor Case Study (VSD case study )

Drive Systems

• Replace V-belts with toothed belts, raw edged cog belts or new generation flat belts.  A soft starter or variable speed drive will reduce belt wear.
• Make sure alignment, tension and operation of motors are correct.  Noise and heat can indicate energy losses.  V-belts at temperatures over 60°C need attention.  The optimum is 40°C.
• Hard or glazed V-belt sidewalls indicate slipping of the belt
• Excessive wear on the sidewall indicate misalignment of the pulleys, either due to the shafts not being parallel, or the pulleys are not lined up correctly on the shaft.  It is very likely that excessive noise is also created by misaligned V-belts.
• Don’t let oil, grease, dust, chemicals, sunlight etc. contaminate the V-belt.  The belt may slip more, and fail early due to the foreign materials wearing out the belt.
• Don’t force or pry a new belt over a pulley, the internal reinforced cord may be stretched and damaged resulting in early failure and unnecessary slip.
• Most new belts require a run in period; ensure the belt tension is checked after a few hours of operation.

Please note that toothed belts cannot be used in every application, download this Motor Technical Guide  produced by the U.S. Department of Energy for more information on V-belts replacement.  V-belt replacement 

Motor Maintenance

• Maintain all drives to manufacturer’s recommendations.
• Regularly check cooling fins and vents and clear any obstructions.
• Check bearings for noise, grease if required.
• Use the simple methods of observation on a daily basis when checking your plant: look, listen, feel, smell.  Most maintenance issues can be indentified before having to resort to vibration analysis, Amp readings, infrared temperature measurements, ultrasonics, oil analysis etc.
• Examples of maintenance prevention are: lubrication, alignment, balancing, operating procedures, cleaning, adjustment, and much more. Condition Monitoring is often looked upon as mostly being vibration analysis.

A booklet on Electric Motor Management has been produced by the MOC and ABB and can be downloaded in pdf format (24 pages, 4MB) by clicking here (Electric Motor Management ).  This booklet provides more details of motors and drives, energy efficient motors, variable speed drives and motor starters.  This is essential information for anyone specifying, installing and maintaining electric motors.  Please note that the data is of a technical nature, and professional advice may be required before purchasing any motor or motor drive to ensure correct operation of the plant and equipment.