How Green Is Your Gearmotor?
Electric motor efficiency is important because motor-driven systems use 39% of all electrical energy consumed in Canada. Most motor efficiency efforts to date, however, have focused on only 10% of the...
February 1, 2009 | By John Morehead
Electric motor efficiency is important because motor-driven systems use 39% of all electrical energy consumed in Canada. Most motor efficiency efforts to date, however, have focused on only 10% of the motor population.
The opportunity exists for machinery and equipment designers — as well as those responsible for maintenance, repair and operations (MRO) — to signifi cantly contribute to energy conservation by applying appropriate new electric motor, gearmotor and drive solutions.
The U. S. Environmental Protection Agency found that 80% of the total electrical consumption for motors came from those greater than 20 hp. That’s less than 1% of all motors in use. Those in the 1 hp to 19 hp range represent an additional 9%.
The remaining 90% of motors are made up of the billions of fractional horsepower (FHP) models used everywhere one turns: vent fans, ice dispensers, vacuums, furnace blowers, garage door operators, and thousands of other consumer applications. Millions more FHP motors and gearmotors are found in thousands of commercial and industrial OEM applications that involve pumping, dispensing, cooling, conveying, mixing and every facet of automation.
With regard to FHP motors, the effi- ciency of motors increases with size. And, the gap between standard and premium efficiency motors decreases as horsepower ratings increase. For example, the standard efficiency rating for a 1 hp motor might be 78%, while premium effi- ciency for the same size would be 82.5%. However, for a 250 hp motor, the standard efficiency increases to 94.1%, with a premium efficiency of 95.8%.
Increased efficiency pays off
In the FHP world, nominal efficiencies are much lower because their smaller physical size does not readily nor economically permit the inclusion of a greater percentage of copper or other efficiency- improvement techniques employed in integral horsepower motors.
The gap between conventional and increased-efficiency FHP solutions can be 30 percentage points or more. For example, in commercial refrigeration applications, a shaded-pole AC motor may have an efficiency as low as 32%, compared to 49% for a comparable permanent split-capacitor motor, and ranging all the way to 70% or more for an electronically commutated motor (ECM).
The savings for the user, therefore, can be significant, even with a small motor. Operating a 50%-efficient, 1/8-hp (93- watt) gearmotor would, for example, at a 10¢/kWh commercial rate, result in an annual cost of $164.25. The 80% efficiency alternative incurs $102.20 in expense, yielding annual savings of $62.05.
PMDC vs. PSC
In some cases, it is possible to achieve significant efficiency increases by replacing a permanent split capacitor (PSC) AC motor with a permanent magnet DC (PMDC) motor. While the PSC motor may have greater maximum effi- ciency than the PMDC motor, the PSC motor’s efficiency can be much lower at the operating load point.
Three-phase trumps single-phase
Three-phase electric motors are typically more efficient than single-phase models. For example, a single-phase, 1/20-hp (37- watt) gearmotor can have an efficiency of 53%, while the three-phase alternative offers 64% efficiency.
In addition to greater efficiency, the three-phase motor is more reliable. By itself, a single-phase motor is not self-starting. Therefore, different starting mechanisms have been developed, all of which add to component count and cost, while becoming potential weak points in terms of maximized motor life.
So, if three-phase motors are so much better than single-phase, how come they’re not everywhere? The catch is that for many commercial equipment installations, three-phase power is not readily available.
Enter the variable-frequency drive
The logical solution is to power the three-phase motor from a frequency inverter that takes the single-phase 115V or 230V input voltage and delivers 230V three-phase output, with an adjustable frequency typically from 0 to 120 Hz.
These variable frequency drives (VFDs) have become more compact, offer more features and exceptional value, even when compared to low-cost DC speed controls. And, they offer features such as soft starts to reduce shock to mechanical components and increase equipment life. A VFD’s current-limiting feature also offers safe shutdown of equipment and reduced mechanical damage.
The U. S. Department of Energy has estimated that up to 18% of total electric motor energy consumption could be saved by controlling speed with VFDs.
Don’t Forget Gearing Efficiency
If you’re trying to maximize the overall efficiency of your gearmotor system, avoid cancelling motor efficiency gains with inefficient gearing. While a nice solution for tight spaces, right-angle worm gear reducers can have efficiencies of 50% or lower, while spur and helical gears used in parallel-shaft reducers are more typically in the area of 98%. Therefore, it pays to see if offset parallel-shaft designs are available that will fit one’s package size, while still offering high efficiency.
Machinery and equipment designers, as well as buyers, have many possibilities today for increasing energy efficiency in the motor systems employed. To tap into those possibilities, make the most of the application engineering assistance available from electric motor, gearmotor and drives suppliers.
John Morehead is vice-president of strategic planning and marketing for Bison Gear & Engineering Corp. For more information, visit www.bisongear.com.
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