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issue: October 2003 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices

Motors & Air-Moving Devices
Integrating Brushless D.C. Motors Into Plastic Fans for Efficient, Quiet, and Cost-Effective Appliance Solutions


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Fans are often viewed as commodity products when, in fact, they are the respiratory system of the appliance. If the fan under-performs, the appliance under-performs. If the fan fails, the appliance fails.

&zoneFan designs must be optimized to produce the desired air, power, and sound quality performance required to differentiate the appliance in the market place. The objective is to design the fan close to its peak efficiency. There are two reasons for doing this. First is to minimize the size of the motor. The benefit in doing this is to minimize the production cost, size, and weight of the fan. Second is to minimize energy losses. The losses result in heat, and heat accelerates failures. An efficient fan is a longer lasting one as well.

There are many different types of air movers including axial, centrifugal, transverse, and mixed flow. Each fan type has a different peak efficiency point relative to its performance expectation (Figure 1). A high-pressure low-flow fan behaves differently than a low-pressure high-flow device. The initial selection step as part of the design concept phase is important because it establishes the air mover type quickly without model building, iteration, or testing. It also sets the stage for the rest of the work.

Optimization of Aerodynamic Design


Figure 1. CLICK for larger graphic.
Air is a compressible gas. However, in a household appliance application, air can be considered incompressible. Its density is directly proportional to absolute pressure. For most household appliances, the absolute pressure varies by less than 0.5 percent. The HVAC industry defines standard air density as 0.075 lbs/ft3. Fan performance is typically reported in this standard air density [1].

The energy in moving air is characterized by two forms of energy - kinetic energy and potential energy. Kinetic energy is found in airflow velocity, known as velocity pressure. Potential energy is found in air stream compressive forces associated with static pressure.

A properly designed airflow system optimizes three crucial system characteristics; flow rate expressed in cu ft per min (CFM), static pressure (inches H2O), and velocity of the air stream. The objective is to minimize the total energy losses in the system as much as possible. How to accomplish this depends upon the design priorities. A leaf blower would require high velocity to move leaves; a computer system fan requires high flow to remove heat; high pressure may be required to provide a load-bearing or force-transmitting medium. The successful design achieves the desired results using a minimum amount of flow, pressure, and velocity. Adherence to these fan design principles minimizes energy losses, reduces noise, and optimizes the package size. More often then not, these design efficiencies translate into cost savings.

Audible Noise Reduction

Low noise has become an increasingly important product feature in consumer products and home appliances. The sources of fan noise can be reduced to three categories: aerodynamic noise, mechanical noise, and magnetic noise.

Aerodynamic noise - There are many issues that affect fan noise, and selected ones are highlighted below:

The primary factor is the rotational speed of the fan. This is a fundamental generator of noise.

Rotational noise (also known as blade noise) occurs each time the blade passes a given point; the air at that point receives an impulse. The repetition rate of the impulse, or blade-passing frequency, determines the basic tone of this type of noise.

Vortex noise occurs when the fan blade moves through the air; a pressure gradient builds up across the blade. If the fan blade is not correctly designed, the airflow separates from the convex side of the blade creating large eddies. The point of separation varies, so the pressure pattern and eddy formation fluctuate rapidly, causing significant noise.

Mechanical noise

Fan unbalance, runout, and concentricity variations result in a complete vibration once per revolution. This can be a significant source of noise.

Bearing noise can occur when bearings are misaligned, inadequately preloaded, not adequately lubricated, or are damaged.

Structural resonance can occur when the energy in a given band is high and corresponds to the natural frequency of a part of the fan. The resulting noise can be radiated efficiently throughout the fan structure.

Motor noise can be the result of magnetic noise radiated by the fan when the impeller is mounted directly to the motor shaft.

Magnetic noise

Magnetic fields in the motor stator vary at twice the frequency of the electric power supply. Waveform distortion, originating either in the power supply or motor, can introduce harmonics of this variation, mostly the second and the third. Particular motor designs may generate other frequencies through interaction of the power supply and rotational frequencies. This type of noise occurs because of irregularities in the magnetic fields, as determined by lamination shapes, slots, and tolerances [3].

Electronics related noise

This report highlights electronically commutated or Brushless d.c. motors. They are powered by an electronic control that generates a waveform much like the a.c. power from the grid. This waveform, if different from what the motor needs, generates inefficiencies and noise. The noise is concentrated on the frequency and looks very much like the 60 cycle tones from common motor designs.

In an efficient fan, airflow is likely to be relatively smooth with little aerodynamic disturbance. Since air energy is lost as a result of turbulence, it follows that noisy fans and air moving systems are operating inefficiently. It is difficult to directly relate fan efficiency to noise level since sound power is one-hundredth to one-thousandth the level of shaft power. As an example, guide vanes, which can be used to make significant improvements in fan efficiency and allow the designer to use a lower operating speed, may actually increase fan noise if not done correctly.


Figure 2. CLICK for larger graphic.
Reducing the air horsepower requirement may also have the effect of lowering noise. A careful evaluation of the airflow path should be conducted to insure that no unnecessary flow work is required. For example, a conservatively designed heat exchanger may result in an excessively high system pressure and airflow requirements, adding to system noise.

Minimizing vibration is another step towards noise reduction. Other benefits include lower likelihood of structural failure due to stress fatigue, loosening of fasteners, and premature wear of bearings.

Some basic steps to design for low noise and vibration are:

  • Specify the minimum airflow rate that will get the job accomplished.
  • Design the system airflow path for minimum flow resistance (Figure 2).
  • Design the fan to run at the minimum tip speed.
  • Design the fan and motor for maximum efficiency (Figure 3).
  • Designing the motor to maximize power factor will minimize torque pulsation.
  • Make sure the rotating system is adequately balanced.
  • Mount the fan to avoid exciting resonance in the structural elements.

Overview of Brushless D.C. Motors

Brushless d.c. motors are highly efficient, have long life, are compact, have a very wide speed control, low noise, and EMI/RFI emissions. These motors have advantages over shaded pole a.c., split phase a.c., permanent split-capacior a.c., and brush (d.c., a.c., or Universal) motors, but have had the tradition of being more costly due to the need for electronics to drive the motor.

Performance Characteristics


Figure 3. CLICK for larger graphic.
The speed vs. torque curve of a brushless d.c. motor represents the steady-state capability of the motor in driving different types of loads. The speed/torque curve of the motor should be compatible with the speed/torque characteristic of the load. Some loads, such as compressors, have a constant torque that does not vary with speed. Other loads, such as those found in fans, have a torque that increases in proportion to the speed squared. Loads found in traction type loads (electric vehicles is one example) require a constant torque drive up to a certain base and a constant power drive at faster speeds. The most basic function of the speed/torque curve is to ensure that the motor has enough torque at all speeds from zero speed up to full speed, to accelerate the load from standstill, and maintain full speed without exceeding any thermal or electrical limits [4].

Brushless d.c. motors have a linear torque/current characteristic, low torque ripple, and fast response. This makes them highly suitable for controlling speed, position, or torque, either in a single-quadrant variable control mode or as four quadrant servomotors with reverse speed capability and dynamic braking [4].

The perfectly smooth torque of the ideal brushless d.c. motor cannot be obtained in a practical motor, although it can be closely approached. Torque variations during one revolution arise from imperfect commutation of the phase currents; from ripple in the current waveform caused by chopping; and from variations in the reluctance of the magnetic circuit, due to slotting, as the rotor rotates. The last effect is sometimes called cogging [4].

Audible noise in appliances and electronics is a problem that continues to proliferate, as more airflow is required for enhanced performance of the appliance or cooling of the electronics. Brushless d.c. motors compare very favorably with other types of motors. Three-phase motors with sine wave drives greatly reduce the switching noise in a brushless d.c. motor application.

Motor Control Electronics

A brushless d.c. motor has permanent magnets in the rotor (the rotating part) and electrical windings in the stator (the fixed part). As an electromagnetic field rotates around the stator and attracts the permanent-magnetic rotor, the rotor follows this field and thus rotates. A BL d.c. motor can't generate this rotating stator field by itself, however. Additional electronic components must produce field rotation by appropriately sequencing the flow of current into the different phase windings around the stator. In most cases, the key component for controlling this sequence is a microprocessor or a Digital Signal Processor (DSP). Because microprocessors and DSPs are programmable, they offer design flexibility that hardwired control components can't [5].

MOSFET or IGBT bridges amplify motor control signals from the microprocessor. Digital controllers are more accurate, offer flexible programming, and are less susceptible to noise. A wide range of features is available with digital controllers including diagnostic information, self-calibration, communications, and protective functions.

A digital Proportional, Integral, Derivative (PID) controller can have features like anti-windup integrated action, derivative filtering, direct encoder input, and summing junction with analog output all on one chip [4]. Today's microprocessors and DSPs provide powerful on-board functions like PWM output signals, high speed floating point computing facilities, and sine/cosine lookup tables for vector control.

Motor Life and Reliability

The fan module life is a function of the brush wear in the motor. While some work to extend brush life, a better solution is to eliminate the brushes altogether and extend life significantly. Because brushless d.c. motors have no commutator, they are more efficient, need less maintenance, and can operate at higher speeds than conventional d.c. motors. High efficiency and small size are especially important for military, aircraft, and automotive applications, and for portable instruments and communications equipment.

Integration of Plastic Fan and Motor - Shrinking the Envelope

Traditional motors, with an attached fan as an add-on component, require housings to protect it from the environment, a bracket, and many fasteners to attach the motor and fan subassembly to the end unit. An alternative approach is the integration of a highly efficient and variable-speed brushless d.c. motor into a fan and shroud that is efficient, quiet, and cost effective. The rotor of a brushless d.c. motor includes a magnet and backup iron. These elements are integrated into the hub of the axial fan and, therefore, do not add to the axial length of the package. The stator windings are often mounted to the PCB and use very little additional space in the package. This arrangement can save significant axial space.

The significant efficiency improvements of the fan and motor reduce the mass needed to meet the performance requirements. Furthermore, the integration process that is employed reduces use of materials. The fan hub and fan shroud double to hold the motor components and eliminate the steel motor shell.

Energy Conservation

Brushless motors are inherently more efficient than a.c. induction motors because their rotors contain permanent magnets, not electromagnets. Also, variable speed operations result in system efficiency improvements. In a refrigerator or an air-conditioner, for example, a constantly running, variable-speed compressor that supplies just the right amount of cooling typically uses 30-percent less energy than a compressor that runs intermittently at a higher speed. In a washing machine, a variable-speed motor can provide a high-speed spin-cycle that extracts extra water from clothing and thus reduces the amount of energy needed for subsequent drying [5].

The difference between a traditional fan with a brush motor and an efficient aerodynamic fan design with an efficient, integrated brushless d.c. solution represents a 20-percent reduction of power. The motor electronics accepts control inputs that regulate the motor speed. At reduced speed, the power reduces by the cube of the speed ratio. For example, a 20 percent reduction of speed equals a halving of the power. Brushless d.c. motors provide benefit without adding additional losses.

The fan and motor includes features that reduce noise, minimize the axial length of the design, reduce weight, and power consumption while increasing reliability and maintaining competitive cost structures.

Part Count Reduction

Several fundamental issues are at play. First, due to the higher efficiency, there is less material used to generate the performance required. Second, a "design for manufacturing" approach greatly reduces part count and production labor. Elimination of screws, fasteners, and brackets further reduces the total part count and, therefore, the total cost of the fan solution. Third, the low part counts from the integration process yields a large reduction of labor. These cost savings are offset by new, low cost electronics to run the motor. The result is a cost-effective solution for the industry.

Design for Manufacturing and Assembly

The advantages of integrating brushless d.c. motors into plastic fans are numerous, but what about the cost? Using design for manufacturing and design for assembly principles can significantly reduce the cost of production. This step, strategically, generates a cost reduction of the fan and motor combination, thus allowing room for the cost of the electronics. The ultimate goal is to design a brushless d.c. motorized solution economically. Successful mechanical design and engineering is environment and process dependent. There are many factors that affect the design. The following are major factors:

  • Product scope, intent, and complexity
  • Time to market
  • Cost
  • Product competitive environment
  • Organization infrastructure
  • Design, engineering, and manufacturing tools
  • Staff experience [7]

One development approach broadly applied involves design for manufacturing/assembly (DFM/DFA) methods where products are systematically designed and evaluated to minimize set-ups during production operations, component counts, and overall product complexity (Boothryod, et. al. 1994). By simplifying a product's design to enhance component manufacturing and product assembly, production costs are reduced and unit quality improved [6]. Lean manufacturing principles are employed to simplify the elements crucial to the assembly process including individual operations, equipment used, staging of raw components, and a holistic view of how all of this comes together.

Concurrent Engineering

Concurrently designing an integrated brushless d.c. plastic fan and the appliance it will go into is integral to achieving the predicted performance of the final product. Likewise, concurrently designing for manufacturing will have the desired result of reducing fabrication costs, improving product quality, reducing part count, minimizing documentation, decreasing labor, and lowering inventory costs. Repeatable tolerances, standardized parts, modular components, ease of assembly, end of line customization (postponement of product differentiation until the latest possible stage of the manufacturing process), and selective automation also serves to keep the cost of the fan down.

Simplifying Supply Chain Logistics and Reducing Costs

By adhering to the design for manufacturing principles outlined here, it is possible to simplify supply chain logistics by significantly reducing the number of suppliers and inventory carrying costs through part count reduction. In addition, reducing the overall labor required in assembly eliminates costs associated with freight, duty, transportation, handling, and warehousing costs incurred when shuttling assemblies back and forth from the pacific rim or other low cost labor sources.

Conclusion

By utilizing highly efficient aerodynamic fan designs with integrated, variable speed brushless d.c. motor technology and incorporating a design for manufacturing and assembly methodology to manufacture plastic fans, it is now possible to have quiet, cost-effective, and reduced axial length brushless d.c. fan solutions. Benefits to the appliance industry include cost reductions through smaller package size facilitated by a higher efficiency and integrated motor solution, coupled with reduced part count and production labor.

Higher efficiencies and the variable speed will substantially reduce the power used by the fan for improved energy ratings.

Appliances can be designed to run at lower speeds to reduce noise levels.

This information is provided by Patrick A. Sandefur, vice president of Sales and Marketing for Torrington Research (Torrington, CT, U.S.).


BIBLIOGRAPHY

"Development of Air Moving Systems for Household Appliances," AMCE Conference, 1999, John O'Connor, Jr., Ann M. Casper.

Handbook of Noise Control, Cyril M. Harris, Ph.D., 1957, McGraw-Hill, Inc. "Fan Noise," R.J. Wells, General Electric Company and R.D. Madison, Buffalo Forge Company. Pg 25-5 to 25-9

"Designing with Moving Air," Torin HVAC

Design of Brushless Permanent-Magnet Motors, 1994, J.R. Hendershot, Jr. and TJE Miller, Pg 1-12, 1-18, 14-1

"Save your energy," Design News, Nov. 18, 2002, Gary Legg.

"A Coordinated Product and Process Development Environment for Design for Assembly," David E. Lee and Thomas Hahn, 1996, Integrated Manufacturing Engineering Program for Advanced Transportation Systems, School of Engineering and Applied Science, University of California, Los Angeles. Pg 2

"Design for Manufacturing (DFM)," Engineer's Edge, 2000.


October 2003 Special Product Supplement
Motors & Air-Moving Devices
by Peter Simmons, Assistant Editor

1 - The Art of Providing Savings
2 - Electric Motor Testing on the Production Line
3 - Integrating Brushless D.C. Motors Into Plastic Fans for Efficient, Quiet, and Cost-Effective Appliance Solutions
4 - Design Platforms
5 - Discovering the Use of Tangential Blowers in Low-Noise Applications
6 - Automated and Data-Reliant

7 - Improving IAQ in the Home
8 - Floor Care Appliance Boasts 5,000-HR Life
9 - Getting Pumped Up

 

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