Choosing the right motor
Dec. 20, 2011
Fred J. Fairchild
Whether deciding to rewind or replace an existing motor, or specifying motors for new equipment, there are several things to consider including the initial cost and lifetime operating costs to own and operate each motor.
Most equipment used in an elevator or grain processing plant is powered by an electric motor using electrical power. These motors are sized, in horsepower (hp), by the required work necessary to operate the equipment.
James Watt, the inventor of the steam engine, defined what horsepower is. He concluded that the average horse of the time could lift a 550-pound weight one foot in one second, thereby performing work at the rate of 550 foot-pounds per second, or 33,000 pound-feet (also expressed as foot-pounds) per minute. He then published those findings, stating that 33,000 foot-pounds per minute of work was equivalent to the power of one horse or one horsepower. This is the value we use today to express the amount of work being done. Thus, when you calculate the amount of work being done by a machine, it is expressed in foot-pounds per minute. Dividing the amount of work to be done (ft. lbs/min) by 33,000 foot-pounds per minute, you get the amount of power required in horsepower.
When specifying or selecting a motor for a replacement or for new equipment, it is important to properly determine the horsepower required to operate the equipment. Once this is determined, then the proper size motor must be chosen to provide the needed power for the piece of equipment. Usually these are standard size and speed AC motors.
Two things are important in choosing a motor: its performance at various loadings and the operating efficiency of the motor. The more the load applied to a motor approaches the total load available from a motor, usually the better the performance of the motor and the better the power factor gained from using that motor. Consequently, motors operate more efficiently at higher loads. It is recommended that a motor be sized with enough horsepower to do the required work, but not powered by oversized motors.
There is a clear link between the motor’s efficiency and the load. Manufacturers design motors to operate at a 50% to 100% load and to be most efficient at a 75% load. But once the load drops below 50%, the efficiency decreases rapidly. Operating motors below 50% of rated loads has a similar but less significant impact on the power factor. High motor efficiencies and power factor close to 1 are desirable for an efficient operation and for keeping the entire plant’s costs down, not just the motor.
When considering the cost of a motor, one must consider not only the original purchase cost but the other costs of owning and operating the motor over its lifetime.
When considering a lifetime, the total costs are normally based on a 20-year lifecycle for a motor. When looking at these costs, the purchase of the motor is about 1% of the total cost. Add in 4% of the cost for rebuilding and an additional 5% for downtime due to various reasons.
These three items add up to only 10% of the 20-year lifecycle cost. Thus, 90% of the cycle cost is the cost of power to operate the motor. With this in mind, selecting and using motors with the highest efficiencies is important and cost saving.
Motor Efficiency Ratings
The National Electric Manufacturers Association (NEMA) publishes more than 500 standards, application guides, white papers, and technical papers. Many of these address construction and performance standards for electric motors. They establish the efficiency standards for motors of different construction and types.
Beginning with the energy crisis in the early 1970s, motor manufacturers began building and selling motors they called “high-efficiency” and “energy-efficient” motors, but these terms were not clearly defined. These motors, however, were more efficient than what were considered the “standard-efficiency” motors of that time, but also cost more to purchase.
The U. S. Congress enacted the Energy Policy Act of 1992 that gave the Department of Energy (DOE) the authority to set minimum efficiency standards for certain classes of motors, including many of the ones commonly used in the grain handling processing industries in the U.S. Any motors built after Oct. 27, 1997 had to meet these minimum standards. These motors became known as Epact motors. They range from 1% to 4% more efficient than earlier “standard-efficiency” motors.
Motor manufacturers continued to improve motor construction, performance and operation efficiencies that surpassed even the Epact efficiencies. In June 2001, NEMA established standards for these motors and designated them as “NEMA Premium” motors.
Let’s pick one motor size to do a comparison of original costs and annual savings installing Epact or NEMA Premium motors in place of a standard-efficiency motor (see chart above). We will use a 5-hp, 1,800-rpm motor at 75% loading and the electrical power to operate the motor costs $0.075 per kilowatt hour.
Any motor purchased, of the types covered by the Energy Policy Act of 1992, will be at least an Epact motor. It may be wise to look at existing “standard-efficiency” motors and determine if replacing them with an Epact motor may result in significant cost savings over the lifecycle of the motor.
In motor selection and/or replacement for new and existing equipment, consider the additional cost savings for purchasing a NEMA Premium motor.
NEMA Premium motors should be considered:
1. For new installations.
2. When purchasing new equipment packages.
3. When making major modifications to processes and facilities.
4. To replace oversized or underloaded motors.
5. As part of an Energy Conservation Program.
Choosing the proper motor for the application can result in significant electrical energy savings over the lifecycle of the motor.