Motors consume 65 percent of the electricity used by industry in the U.S. That equals more than 20 percent of all the electricity generated in the U.S. every year, and a similar level of carbon dioxide emissions. Making a dent in this level of energy consumption requires looking beyond high-efficiency motors and into approaches that involve the entire power train.
With energy costs impacting every business, and growing concerns about how burning fossil fuels is affecting the world's climate, companies of all sizes are exploring ways to make their operations more energy efficient. The use of high-efficiency motors would appear to be a logical solution. But motor efficiency does not always equal energy savings.
The true potential for saving energy involves a much bigger story, one that requires taking into account the entire power train. That's because an energy efficient motor can reduce energy use by only 10 percent, even under optimum operating conditions. Far more important in saving significant amounts of energy are the use of electronic speed controls, which can reduce energy use by 30 percent, and optimizing mechanical systems, which can reduce energy use 60 percent.
Understanding Energy Loss
Energy is defined as the work stored in a system or a system's ability to do work. If any of the electrical energy that flows through a system is not actually applied in doing work, then it is wasted. In other words: the more efficient a system is, the less energy is lost.
To calculate how much energy is being lost through system inefficiencies, engineers use the following formula to determine the amount of power (P) introduced into a system compared to the energy output: Pin - Pout = Ploss
So how is energy lost? A typical power train consists of a motor, drive, gearing and wiring. Energy can be lost from each component. In an asynchronous motor, for example, there are friction, magnetization and ohmic (resistance) losses. Both the design of a motor and the quality of materials used to construct it can contribute to energy losses.
Since each component can contribute to energy loss, let's consider each one individually.
Motors - Major sources of energy loss in motors result from friction in the bearings and seals, wind resistance of the motor fan, iron losses in the forms of hysteresis and eddy currents (based on the quality of sheet metal used to construct the stator and rotor laminations), copper losses (I2R losses), and harmonics.
Improving the energy efficiency of motors can be achieved by using better materials with lower resistance, such as rotor cages of copper instead of aluminum. Motor efficiency can also be improved by increasing the size of the active components of the motor. This is why premium-efficiency motors have higher inertia rotors. But be careful here - while it may be tempting to use motors with higher horsepower ratings than your application calls for simply because their nameplate efficiency is higher, that efficiency is based on full-load operation.
A 10 hp motor may be listed as being more efficient than a 5 hp motor, but it will use a lot more current. And a partially loaded motor is terribly inefficient. A good rule of thumb is to operate motors in the 80 to 90 percent utilization range for optimal efficiency. The use of VFDs (variable frequency drives) allows you to slow the motor down to keep the utilization at its optimum.
Wiring - Electric cables produce both ohmic and capacitive (energy storage) losses. Ohmic losses in the conductors are inversely proportional to the diameter but are proportional to the length. With standard designs, total power losses on the cable can amount to as much as 5 percent of the transferred power. Shorter cables with larger diameter conductors minimize energy losses.
Gearing - Losses in gearing are due primarily to friction caused by movement between the teeth. Worm gears, for example, are the least efficient type of gearing because they experience a great deal of sliding and therefore generate significant friction.
Lack of sufficient lubrication is another source of friction and resulting energy loss. Incorrect mounting positions, temperature, and immersion depths are the key factors here. The flow of the oil is determined by its temperature and viscosity. The thicker the oil, the more torque is required to move the gear. The higher the oil temperature, the thinner the medium will be and the lower the power loss. The design of the housing also determines the arrangement of components, and therefore the oil flow.
Bearings and oil seals also have a part to play in energy-efficient operation. Gear unit efficiency is influenced by the seal between the motor and gear unit as well as by losses in the gear unit. The higher the input speed, the higher the losses in the bearings and churning losses in the oil.
Inverters - While inverters or variable speed drives are often added to a power train to reduce energy consumption, they also consume electricity in operation. A drive's contribution to energy savings lies in its ability to allow you to manage motor operations to reduce output power. Managing motor speeds, ramps, and available torque translates directly into managing power consumption.
Improve System Efficiency
In designing for energy efficiency, it is critical to take a holistic view of the system or process. When several machines or components work in series as a system, then their individual efficiency ratings must be multiplied to arrive at the overall efficiency rating.
For example, while a premium-efficiency motor has an efficiency rating of roughly 95 percent, the efficiency of a worm gear is between 50 and 80 percent, depending on the make and model. Using a premium-efficiency motor with worm gearing would therefore be counterproductive, since the system's overall efficiency rating would be closer to the gearing than that of the motor.
An understanding of the application is equally important in component selection. High-efficiency motors are designed to reduce power consumption in continuously running operations, such as fans or pumps. Therefore, they usually have heavier rotors than standard motors, which enable them to take advantage of inertia once they are started. But in applications where motors run intermittently, with frequent starts and stops, the heavier (often larger diameter), high inertia rotors can become real energy burners, as it requires more energy to start these high-efficiency motors.
Now think about the equipment at your site, such as sorters, pushers, indexers that are typically found in airports and parcel handling systems. Or applications such as conveyors in automotive plants, out-feed conveyors on packaging machines, or large packaging machines like palletizers and pallet wrappers. Motors powering high-cycling applications like these may only run at full speed for a few seconds, but they are in start-up mode much of their lives. For these applications, motors with lower inertia rotors will use much less energy.
Different applications require different engineering solutions to reduce power losses and consumption. The following examples represent just two approaches to saving energy.
Conveyors - Many conveyor systems, like those used in automotive assembly, packaging or baggage handling are powered by a combination of components that include a NEMA-frame motor, an external clutch brake, reducer, V-belt, and pulleys. The high inertial factors involved in this design approach and the repeated shaking of the equipment every time the clutch is engaged place a severe strain on components. Reducers and clutch brakes must be replaced frequently, resulting in downtime and energy consumption rates that are unacceptable.
While a single start/stop cycle might use only a fraction of a second of energy, some of these applications may involve thousands of cycles per hour. When compounded monthly or yearly, the impact on energy usage and heat damage to components is significant.
A more efficient alternative to this cumbersome drive train design is an IEC-type gearmotor with an integral brake, a shaft-mounted helical bevel gear reducer and a VFD. This streamlined system reduces maintenance, parts and energy usage, while equipment uptime is increased.
Storage and retrieval - In a vertical storage and retrieval system, like those used in a manufacturing plant to manage parts inventory, one drive handles the travel axis and another controls the hoist axis. Since energy is proportional to mass x height, items stored in the rack have associated potential energy. This energy is released when the load is lowered.
Also consider the kinetic energy in the motion of the horizontal carrier. What happens to this energy when this high-inertia horizontal load is decelerated? With conventional control, the released energy of both axes is dissipated using braking resistors, resulting in significant power loss. A more energy-efficient solution is to tightly coordinate the timing of the motions of the two drives using a programmable controller. This allows the regenerative energy produced during the braking operation of one drive to be diverted to power the motion operation of the second drive.
By taking advantage of this regenerative power sharing, total energy consumption falls dramatically, in the range of 25 to 40 percent depending on the system, without reducing system dynamics or cycle time.
Focus Factors for Efficiency
Designing for energy savings requires reducing both power losses and power consumption.
Following are the five key factors to focus on to reduce power losses:
- Increase gear unit efficiency;
- Increase motor efficiency;
- Eliminate unnecessary functions;
- Use/recycle released energy by direct utilization, regeneration of braking energy and energy storage; and
- Size components according to demand.
In addition to reducing power loss, engineers should also focus on reducing power consumption to address both portion of the power loss equation. Here are the four critical power consumption factors:
- Reduce/control output speeds;
- Reduce load torque through rigid transmission components, counterweights and minimizing friction;
- Use of energy-saving modes; and
- Turn the device off.
The bottom line is that there is no single solution for saving energy when it comes to motorized systems. Each component of the drive train offers opportunities to improve efficiency and reduce power losses. These opportunities vary by application, as does the engineering solution required. Being energy smart will require rethinking old assumptions and practices. Above all, it will mean engineering for energy savings in every system, every machine, and every process.
This story was published in the July 2009 issue of Control Engineering, www.controleng.com.