Electric motor power efficiency has taken centre stage. Individuals, corporations, and governments are increasingly interested in saving power, now that technology can make it possible and economy demands it.
Advances in motor control algorithms and cost-effective electronic components for implementing motor drives are creating a revolution in virtually every electric motor market. Control of the power factor in an efficient manner also means less lost energy, both in the motor and drive electronics, and in the power grids supplying the electricity to the homes, offices and factories where the motors are used.
Potential savings
The potential energy savings are staggering. Over 40 million electric motors are used in the United States alone.1 Electric motors account for 65 to 70% of industrial electrical energy consumption and approximately 57% of all electrical consumption worldwide.2 Saving even a few percent of the world’s estimated 16 000-plus terawatt-hours (TWh) annual consumption of electricity3 amounts to several hundreds of trillions of watt-hours per year. Currently, the average motor in use today has an efficiency of 88% in converting electrical into mechanical energy. Figures on the order of 96% conversion efficiency are technically feasible for larger motors.
By comparison, the electrical generation capacity of photovoltaic solar cells in all of Europe, where both Germany and Spain currently lead the US in installed base, is projected to be only 15 TWh/yr in 2010.4 In the UK alone, with an annual total electrical consumption of approximately 350 TWh, the Institute of Engineering and Technology estimated that 5 TWh could be saved annually through the use of more efficient electric motors.5
Furthermore, many motors are not used in an efficient manner. For example, the motor may be oversized for the job at hand, or much of its mechanical output power may be wasted, meaning that additional savings may come from how the motor is used, on top of the savings from the motor itself. In 1996, the United States Department of Energy speculated on savings of 5 TWh per year by 2000, and a 100 TWh per year savings potential by 2010,6 considering both motor and related system-level savings.
The potential is there to make significant advances in the next few years as older motors and drives are replaced by newer, more efficient ones. Because of the cost savings in electricity, many industries are voluntarily accelerating the turn-over of their installed motor base, even replacing motors before they wear out. This is because the payback for the newer, more efficient motors and drives can be realised in less than a year and usually less than two years.
Great strides are already being made. In the UK, for instance, sales of the least efficient motors, grade Eff3, dropped from 68 to 8% between 1997 and 2004. During the same period, sales of the most efficient grade (Eff1) increased from 2 to 7%,7 and further jumped to 17% in 20068, with the middle grade (Eff2) making up the balance of sales.
Regulatory influences on motor efficiency
Governments around the world are providing regulatory pressure to use more efficient motors. Starting with the Environmental Protection Act of 1992, which mandated motor efficiency standards and took effect in 1997, the United States government has been steadily increasing regulations. There are other voluntary incentives as well, such as National Electrical Manufacturers Association’s (NEMA) premium efficiency labelling standard (2001).
Australia implemented standards on motors ranging from 0,73 kW to 185 kW in 2001 and tightened efficiency requirements in 2006. Very recently (March 2009), the European Union passed mandatory minimum efficiency performance standards (MEPS), which will be phased in from 2011 to 2017. Brazil (2002) and China (starting in 2010) also have current or planned mandatory standards. Figure 1 shows a comparison of efficiency requirements for various-sized motors in several jurisdictions, including the voluntary NEMA and Consortium for Energy Efficiency (CEE) standards, versus the wide range of efficiencies of available motors.9,10
Motor controllers
Electric motor savings are achieved in several ways. The first is in the motor design itself, through the use of better materials, design and construction.
Another is by optimising the mechanical angle between the various rotating magnetic fields inside the motor. This is done using the newer family of motor control algorithms, generally referred to together as space vector control, flux vector control or field-oriented control. By keeping the magnetic fields of the rotor and stator oriented with the optimal angles between them under various speed and torque conditions (typically near 90°), the motor can always be operated at peak efficiency.
As a side benefit, other characteristics can also be optimised, such as fast and stable dynamic response to load changes, precise control of speed or torque, soft starting and braking, prevention of stalling at low speeds, high starting torques and fault detection, often without sacrificing much in the way of overall energy efficiency. Some of these features were once obtainable only from a more expensive motor type, but can be achieved with the now ubiquitous, low-cost, and reliable AC induction motor, which comprises 90% of US motor sales. One of the most significant advantages of the newer control algorithms is efficient variable speed operation.
A very large opportunity for system-level energy savings comes from using variable speed motor drives. A well-designed pump or fan motor running at half the speed consumes only one-eighth the energy compared to running at full speed. Many older pump and fan installations used fixed-speed motors connected directly to the power mains, and controlled the liquid or air flow using throttling valves or air dampers. The valves or dampers create a back pressure, reducing the flow, but at the expense of efficiency. This is probably how the HVAC forced-air system works in your office building; dampers control the airflow into each workspace while the central fan, which is sized for peak requirements, runs at full speed all the time—even if the combined airflow requirements of the building are currently very low. Replacing these motors with variable speed drives and eliminating or controlling the dampers more intelligently can save up to two-thirds their overall energy consumption.11
Power factor
One often overlooked aspect of overall motor drive efficiency is the power factor. The power factor relates the shape of the current waveform drawn by a load to the sinusoidal voltage waveform supplied by the power company. If a load looks purely resistive, then the current drawn by the load is a sinusoid exactly in phase with the voltage waveform, and the power factor is unity, or 1. This is the most efficient condition.
If the load appears to be inductive, as many motors do, then the current will lag behind the voltage in phase, and the power factor will be less than one, according to the cosine of the phase angle. Capacitive loads, which cause the current to lead the voltage, also reduce the power factor below one. In either case, the energy supplied to the motor will not be used optimally. Since the peak (and shape) of the current sine wave does not line up with the peak of the voltage sine wave, the instantaneous product of voltage times current averaged over a full cycle is lower. This is called the true power and is measured in watts.
Since the mains voltage is fixed, a higher current is required from the power company to compensate for the phase shift and deliver the same useable power to the motor, bringing the useable power (in Watts) back up to that required to do the desired mechanical work (in horsepower, for example).The product of this higher RMS current and the RMS voltage (measured in volt-amps) is called the apparent power. In many respects the power company has to build the infrastructure and pay for the higher apparent power, even though only the true power is doing useful work for the end user.
This higher current means that there will be more losses by the power company generating and distributing the power. Power-line transformers can heat up and fail. Power losses go up as the square of the apparent power. A power factor of 0,7 means an apparent power of 1,4 times the true power, with nearly double the losses compared to a power factor of one. Higher capacity circuit breakers will be needed on the branch circuit where the motor is used. Voltage drops on power distribution wiring will as much as double, necessitating an even higher current for the same delivered power. There are higher resistive losses in the motor as well, creating more heat and a shorter motor life. Alternatively, heavier wire must be used for the windings, reducing the number of turns and hence the efficiency of the motor.
The reactive component of the current, which is out of phase with the voltage, is accomplishing no useful work, yet it creates additional losses in the overall system above and beyond those of the in-phase component that is doing all the real work. In cases where inductive or capacitive loads are linear, the power factor is often expressed as the true power divided by the apparent power.
Because of the extra capital and operating costs imposed upon them, it is very common in industrial settings for power companies to add surcharges for power factors below 0,95, though this is rarer in residential settings where the price of power reflects average residential power factors and the associated costs.
Why worry about power factor?
In an industrial setting, you can reduce your electrical bill by cutting power factor surcharges from the power company. You will be able to put more true load on your branch circuit, since reduced reactive load currents will flow through your circuit-breaker junction box. Efficiency-sapping voltage drops in your branch circuits may also be reduced.
Finally, you might be required to worry about power factor by government regulations. European countries require power factor correction for power supplies rated over 75 W (IEC 555) and limit the harmonic distortion a power supply can inject into the mains though IEC/EN61000-3-2. These regulations require controlling the input current distortion up to the 40th harmonic of the line frequency.
Combating low power factor
Fixed-speed AC induction motors connected directly to the mains voltage look primarily inductive from the point of view of the power plant and distribution grid. To combat the inefficiency this causes (and the corresponding surcharges from the power company), industrial concerns will often add a compensating capacitive load to the power line. This shifts the phase of the power line current so it is back in phase with the voltage. Since the added capacitive load is mainly reactive, it dissipates almost no power itself, except due to non-idealities such as non-zero series resistance and leakage.
Fixed-value capacitors can be applied or removed automatically by a centralised power factor controller, based upon measurements of reactive currents as factory motors are turned on and off. Another scheme is to use an unloaded motor-generator as a sort of synthetic capacitor called a synchronous condenser; usually one such machine for a whole factory full of motors. The closer the compensation capacitors are to the motor(s), the better, as there are still reactive currents flowing back and forth between the inductive and capacitive reactive loads. Note that the current component supplied by the power company can be made to look almost purely resistive with the right compensation, localising the reactive part of the load current so it does not have to go over the long transmission lines from the power company to the factory – and keeping it off your electric bill.
This article will be continued in a future edition of Dataweek.
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