Efficient Motor Controls Save Terawatt-hours/Year
Various types of motors now have sophisticated controls to drive them to their maximum potential
By Joe Roy, Fairchild Semiconductor
Designers of appliances and control systems are simultaneously challenged with numerous inflexible targets. On top of that, they need to meet these demands in an environmentally friendly manner consistent with multiple energy policies.
In the U.S., energy policies are driven by the Department of Energy (DOE), the Environmental Protection Agency (EPA), and the state of California—which, as the twelfth largest consumer of electricity in the world, has spent decades poising itself as the U.S. leader in energy efficiency. The California Energy Commission (CEC) strives to produce cutting-edge legislation that pushes for advanced technologies with energy savings and lower emissions. Their forward-thinking policies are commonly a template for national U.S. policy.
Although these policies and constraints often appear as barriers to the design and release of products, they are also necessary catalysts that promote the innovation that will help keep our planet healthy and energy available. Governments constantly struggle with ways to reduce greenhouse gas emissions, decrease the need for more power-generation facilities, and keep electrical power affordable. The most cost-effective and timely way to achieve these goals is to require that common residential and industrial products perform with higher efficiencies.
Due to the U.S. Energy Policy and Conservation Act and the European Union’s Energy Label Directive, 92/75/EEC, it is easy for businesses and consumers to compare electrical efficiencies of many appliances, as well as other resources such as natural gas and water. The labels illustrate how much an appliance, used in a typical fashion, will cost the owner to operate. The EPA’s Energy Star qualified appliances use between 10% and 66% less energy and/or water than standard models. The DOE believes that Energy Star appliances can save 20% to 30% of annual energy costs.
Fig. 1. Projected 2010 household motor energy consumption in terawatt-hours.
Figure 1 shows the distribution of annual electricity consumption by U.S. households. Approximately 57% of the total energy produced worldwide is used to drive electric motors. In the U.S., electric motors account for 20% of electrical energy consumption.
The typical American household consumes nearly 11,000 kWh/year and costs the homeowner about $1,000. A substantial portion of this is expended spinning motors in HVAC systems, well pumps, clothes washers, and so on—a total of approximately 100 billion kWh of electricity annually. At this level, enhancements in efficiency of just 5% make a big difference.
Averaging just a few horsepower each, the ac induction motor (ACIM) has been the workhorse in most residential, commercial, and industrial applications for many decades. Its inexpensive construction, minimal maintenance, and excellent reliability have resulted in worldwide popularity; 90% of installed motors worldwide are induction motors.
With the advent of new motor technologies, engineers have many new colors in their pallet—and yes, they are all shades of green. These include brushless dc (BLDC), switch reluctance (SR), and even versions of the steadfast ACIM. Although many of these motor topologies have been around for decades, they now have sophisticated controls to drive them to their maximum potential.
Most ACIMs perform at their highest efficiencies at 75% to 90% of their rated load. For applications that routinely use motors at a fraction of their peak load, it is possible to save 50% of the motor/control’s purchase price in energy savings per year by optimizing efficiency over their load range. The DOE estimates that 44% of all industrial motors are consistently operating at less that 40% of their rated load. Today’s intelligent variable-speed drives (VSDs) can adapt to an application’s needs by providing peak torque or speed only when required.
When designing VSDs, careful attention is needed to ensure that the control does not suffer unnecessary losses causing more heat to dissipate into heat sinks, enclosures, and nearby circuitry, leading to higher costs, larger footprints, and reduced lifespan. The VSD will suffer its largest losses in the switches that drive the motor’s windings and in the recovery diodes that carry the phase currents for a short time after the switches turn off.
For three-phase ACIM and BLDC drives, there will be six IGBTs or MOSFETs driving three motor phases. For SR drives, as little as two IGBTs and diodes are required, depending on the number of phases used.
Most line-voltage controls are based on IGBTs instead of MOSFETs because of their improved conduction performance at higher operating temperatures. At lower voltages, the MOSFET is the preferred device. Conduction losses are dc losses in the turned-on switch. Switching losses occur at both the turn-on and turn-off phases and are proportional to the switching frequency.
The drive designer can do little to improve conduction losses other than provide an adequate gate voltage, with a nominal VGE of about 15 V. Switching losses can be mitigated by selecting IGBTs or modules that use either NPT or Field-Stop technology to decrease turn-on and turn-off losses, decreasing switching frequency, and/or carefully choosing drivers and resistances to optimally drive the IGBT’s gate.
Optimizing the gate’s driver can be quite challenging with two objectives to satisfy simultaneously—minimizing switching losses and keeping EMI in check. In integrated motor-control modules, the optimization is fixed and a permanent feature of the module. Semiconductor companies have invested much effort in the development of technologies that allow makers of appliances and industrial machines to implement high-efficiency electronic controls with a minimum of R&D expense and significantly decreased time to market. One example of a cost-competitive power bridge with high efficiency and EMI optimized drivers, diodes, and overcurrent protection features is Fairchild Semiconductor’s Smart Power Module (SPM). Figure 2 shows the simple layout of a high-efficiency drive.
Fig. 2. A simple ac induction motor control can use advanced technology such as Fairchild’s Smart Power Module.
The variable-frequency drive (VFD) is the simplest and most common three-phase ACIM. Costing just tens of dollars per kilowatt, these controls are being used in applications where a motor drive was not cost-effective even a decade ago.
Field-oriented control (FOC) algorithms allow the VFD to precisely control the motor’s stator field such that it always leads the rotor field by 90, providing the optimal magnetic configuration for efficiency. Flux-vector is another name for this algorithm. In pump and ventilation drives up to 50% annual energy saving can be realized by using VSDs with ac induction motors. VSDs also often allow the elimination of mechanical gearboxes, pulleys, belts, and other hardware.
BLDC controls are available with both the trapezoidal and newer sinusoidal modulation methods. The need for magnetic Hall sensors to resolve the rotor’s position for electronic commutation has been removed by sensorless algorithms which approximate the rotor’s position by measuring the back-EMF on the unexcited motor windings between conducting phases. The common availability of these drives has lead to decreased brushless motor cost.
Switch reluctance motors provide excellent starting torque and high-reliability, and are simpler to construct than even the ACIM. Unfortunately, the widespread lack of understanding of this motor topology has led to it being misapplied and it has had a low adoption rate.
Many existing SR systems suffer from excessive torque ripple and are not acceptable for use in residential or industrial applications. Sensorless control techniques and dedicated power modules are now available that will allow these motors to be commutated in a fashion that will allow their cost/performance ratio to be appreciated.
Although VSDs can simplify a mechanical system and enhance a system’s reliability and long-term efficiency, some characteristics of the traditional drives prevented them from being considered as effective solutions in fixed-speed applications in the past. Advanced control algorithms combined with state-of-the-art electronics are now able to overcome most of these concerns. For example, Direct Torque Control algorithms allow an ACIM to perform tasks that were previously in the brush dc motor’s domain due to their improved response to dynamic changes in load and improved low-speed performance. Recently, the cost of an ACIM and control has begun to approach that of a simple brush dc motor and control.