Time to focus on improving and replacing fractional-horsepower devices, says John Morehead of Bison Gear & Engineering
With growing energy crisis concerns, the efficiency of electric motors has become an important, timely topic — the reason being that electric motors consume from 63 to 70 per cent of the electricity used in U.S. manufacturing, and they represent 23 per cent of total United States electricity consumption.
However, most motor energy efficiency efforts to date have focused on only about ten per cent of the total electric motor population. An opportunity exists for OEM machinery designers to significantly contribute further to overall energy conservation efforts by carefully selecting and applying appropriate new electric motor, gearmotor and drive solutions.
The 1992 Energy Policy Act (EPAct) and the Energy Independence and Security Act of 2007 (EISA) have done a lot to legislate new levels of increased motor efficiency. For good reason, these efforts have been primarily directed at integral (greater than one) horsepower electric motors. An EPA study found that more than 80 per cent of total electrical consumption for motors came from motors of 20hp or greater.
However, these large motors represent less than one per cent of all motors in use. It has been estimated that motors in the one to 19hp range represent an additional nine per cent of all motors in use. The remaining 90 per cent is made up of the billions of fractional (less than one) horsepower electric motors used literally everywhere: from kitchen and bath vent fans, ice dispensers, vacuums, furnace blowers, garage door operators, and thousands of other consumer applications. Many millions more fractional horsepower motors and gearmotors are also found in thousands of commercial and industrial OEM applications that involve pumping, dispensing, cooling, conveying, mixing, and every facet of automation.
To put fractional horsepower motor efficiency into perspective, it helps to understand that in general the efficiency of electric motors increases with their size. Also, the gap between standard and premium efficiency motors decreases as motor horsepower ratings increase. For example, a standard efficiency rating for a 1hp motor might be 78 per cent, while a premium efficiency for the same size would be 82.5 per cent. However, for a 250hp motor the standard efficiency increases to 94.1 per cent, with a premium efficiency of 95.8 per cent.
Increased fhp efficiency pays off
In the fractional horsepower (fhp) motor world, however, the nominal efficiencies are much lower because the smaller physical size of fhp motors does not readily nor economically permit the inclusion of a greater percentage of copper or other efficiency improvement techniques employed in integral horsepower motors. Furthermore, the efficiency gap between a conventional fhp motor and an increased efficiency solution can be as much as thirty percentage points or more.
Potential energy cost savings for the user can be significant, therefore, even with a small motor. For example, to operate a 50 per cent efficient 1/8hp (93W) gearmotor in California or Illinois, at a 10ยข/kw-hr commercial rate, the annual cost will be $164.25. By comparison, the energy efficient alternative at 80 per cent efficiency, would only incur $102.20 in operating expense yielding annual savings of $62.05 to the user over the life of the machine.
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 efficiency 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 energy efficient than their single-phase counterparts. For example, a single phase 1/20hp (37W) gearmotor can have an efficiency of 53 per cent, while the three-phase alternative offers 64 per cent 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 VFD
A logical solution is to power the thre-phase motor from a frequency inverter that takes the single phase 115V or 230V input voltage and delivers 230V three-phase output with adjustable frequency from typically 0 to 120Hz. Thanks to the benefits of increasingly cost-effective electronics, these Variable Frequency Drives (VFDs) have become more compact, offer more features and exceptional value, even when compared to low cost DC speed controls.
According to the ARC Advisory Group, the 2007 worldwide market for low power VFDs was over $7 billion and they forecast its growth to almost $11 billion by 2012. Despite a softening economy in the U.S. and Europe, the demand for low-power AC drives will remain strong due to an overdue modernization of the industrial infrastructure that is currently underway. Furthermore, additional automation projects will also be driven by a need to increase energy efficiency in manufacturing.
The U.S. Department of Energy has estimated that up to 18 per cent of total electric motor energy consumption could be saved by controlling speed with VFDs. In fan, blower and pump applications, VFDs can offer very significant energy savings. Rather than employing some type of mechanical system to reduce flow, by employing a pressure sensor feeding into a VFD one can adjust flow more precisely and efficiently. By using a VFD to operate a fan, blower or pump at 80 per cent of rated speed, energy costs can be reduced by half.
In fhp sizes, VFDs are somewhat more expensive than simple DC speed controls. However, they offer many more features such as soft starts, to reduce shock to mechanical components and increase equipment life. A VFD’s current limiting feature can also offer safe shutdown of equipment and reduced mechanical damage.
And don't forget gearing efficiency. If you’re trying to maximize the overall efficiency of your gearmotor system, avoid negating motor efficiency gains with inefficient gearing. While a nice solution for tight spaces, right-angle worm gear reducers can have efficiencies of 50 per cent or lower, while spur and helical gears used in parallel shaft reducers are more typically in the area of 98 per cent. Therefore, it pays to see if possibly offset parallel shaft designs are available that will fit one’s package size, while still offering high efficiency.
