Energy Efficient Motor Systems
Discussion
Motor systems can be made more energy efficient by replacing motors with smaller, high-efficiency motors, running them at full load, and turning them off when not needed.
Improving the energy efficiency of motor systems can help conserve energy and save money, even where energy costs are low.
Readings
Motor Systems
Motor Systems. Energy Saving Manual. EnergySmart
Half the world's electrical energy goes through electric motors.
Save up to 40% of your energy costs by improving your motor system.
The key to improving efficiency is to consider:
- the efficiency of the actual motor.
- its suitability for a particular task.
The best way to maximise energy savings is to work backwards from the task to be performed to the power input of the motor, because large savings are the aggregation of many small savings made.
Due to energy losses from the power plant to end use, efficiency gains multiply back through the process. For example, in an average system in NSW, for every kilowatt of energy saved at a pipe (by reducing friction), 2.4 kilowatts are saved at the motor and 8 kilowatts at the power plant.
Therefore it is important to take a systems approach when looking at motor efficiency.
Motor Function
Do you need the motor?
Reassessing the motor's function sometimes means you can do without it
Motor Size
Is the motor too big?
Often a smaller motor can provide the necessary power requirements.
It is common practice in Australia to oversize motors in an attempt to insure against unexpected peak loads or to allow for process expansion in the future. As a result, hundreds of motors used in industry are grossly oversized.
Motors are most efficient when operating at full load. By installing a smaller motor, which operates at full load, the operating effciency will be greatly improved.
Variable Speed Drive
Is the motor running close to peak load?
A variable speed drive matches motor speed to task, [keeping the motor running at full load].
A VSD is an electrical device which controls power to the motor and is suitable if the loads are not always constant. Instead of continuously running at full speed, regardless of the loads, the VSD varies the speed of the motor to accurately match the loads.
VSDs can be 2-5 times the cost of the motor, but can achieve energy savings of up to 50% per year. Savings are most dramatic when a VSD is applied to a motor driving a centrifugal pump.
A no-variable-speed motor runs too fast most of the time. This means that the motor speed is often controlled by throttling valves.
Where VSDs are too expensive, cheaper alternatives include:
- Multi-speed motors
- Two or more motors running parallel systems, with one motor running for base load, and the second only used at times of peak load. For example, two chillers in a building air conditioning system, or two exhaust fans in a carpark. This approach also offers improved reliability, as a back up is available if one motor or component fails.
Time Switches
Can motor use be reduced?
A simple time switch restricts the time that the motor runs.
A calendar time switch prevents a motor from running on days it is not needed.
Energy Efficient Motors
By investing in a more efficient motor, the reduced operating costs achieved will far exceed the additional capital cost needed to buy one.
An efficient motor is 96% efficient at full load. An inefficient motor is only 90% efficient at full load.
High Efficiency Motors
"High Efficiency Motors" (HEM) are about 2-4% more efficient and offer lower operating costs and reduced energy consumption when compared to standard motors. The reduction in electricity costs will usually recoup the extra money paid for a HEM in about two years.
HEMs have a cooler running temperature, thus reducing space cooling costs and increasing grease life.
HEMs maintain high efficiency over a wider range of loads and have a greater thermal tolerance (i.e. don’t heat up so quickly when overloaded).
It is important to match the HEM to the application because it operates at a slightly higher full-load speed than standard motors. Centrifugal loads, such as pumps, fans and compressors, can be affected by this higher speed, delivering more fluid and consuming more energy.
To avoid negating your energy savings, be sure to match the motor’s operating speed with the load requirements. This may mean replacing an existing motor with a smaller one, trimming impellers on pumps or changing gear or pulley ratios.
Air Ducts
With undersized ducts fan energy is wasted due to high pressure drop.
Maintenance
Is the motor system well maintained?
- Check that drive belts, chains and coupling are in good condition and are adjusted in accordance with the equipment supplier’s recommendations.
- Check for motors which are running hot.
Running hot is a sign that energy is being lost. Use an infrared non-contact thermometer in preference to touching the casing. A motor casing temperature of 60°C or higher should be investigated.
- Check motor alignment.
If a motor is misaligned, some parts will wear more quickly, reducing the motor’s service life.
- Record data for each motor to determine if any are running unnecessarily.
Record information such as the motor’s application, the nominal power (in kW) from the motor identification plate and how the motor is controlled (float switch, time switch, etc.)
- Replace rather than rewind.
It is common practice to rewind an electric motor several times throughout its operational life. Each time a motor is rewound the efficiency is reduced by up to 3%. As a result, a large proportion of motors in Australia operate at a lower efficiency than their nameplate efficiency.
A 3% efficiency reduction in a 30kW motor would add $150 to the operating cost each year (based on an 80% load, 3,000 operating hours per year, 10 cents per kWh).
The best solution if your motor burns out may be to look at your loading needs and the purchase of a new high efficiency motor to match that load.
Electric Motor Systems
Electric Motor Systems. University of Massachusetts
Motors can consume excess amounts of energy if they are improperly mounted, if they are not connected to their load, or in the case of three phase motors, if the voltages of the opposing leads are different.
Manufacturers of three phase motors recommend that they should not be operated when this imbalance (of the opposing leads) is greater than 1%.
Bearing wear can also contribute a motor's reduction in efficiency.
Recommendations
- Optimize motor size with load; size motors for peak operating efficiency.
All motors run more efficiently at full load than at part load; most motors operate near peak efficiency from 75% to 110% of their rated load.
- Use multiple speed motors or variable speed drives (VSD) for variable pump, blower and compressor loads.
A variable or adjustable speed drive (ASD or VSD) will reduce the speed of a motor by adjusting the frequency, voltage, or current of the motor input so that the motor performance just matches the present load.
- Replace existing motors with energy efficient motors.
Motor losses occur from five major areas: core losses, stator losses, rotor losses, stray load losses, winding losses, and friction. High efficiency motors are designed to reduce these losses by 2 to 10 percent.
- Replace oil in gear motors with [high viscosity] synthetic oil to reduce friction losses.
The high viscosity of the oil results in less power loss in the gears. One manufacturer of synthetic oil claims a 5-10% decrease in average power consumption of gear motors when their product is used.
- Install energy efficient power transmission belts. Replace conventional V-belts with cogged V-belts.
Cogged V-belts have slots perpendicular to the belt length and so bend more easily. They are more energy efficient (around 2%), run cooler due to lower energy absorption, and last 20-30 percent longer than standard V-belts.
Links
Australian Greenhouse Office - Motor Solutions
2006.5.6