Magnetics Business & Technology - Spring 2016 - (Page 18)

CASE STUDY The Design of a Permanent Magnet Motor for a Direct Drive Fan Application By Lowell Christensen, Consultant | Lowell Christensen LLC In this case study we will compare two types of permanent magnet brushless motors for a medium size overhead fan used in agricultural types of applications. The comparison will be done on the magnetic circuit portion of the motors only so the only volumes and weights compared will be the magnetic circuit parts including the magnet, back iron, lamination and copper wire. After reviewing the application, it was determined that the parameters of the motor for this application would need to be for a motor voltage constant of 1,000 volts per KRPM and a maximum resistance of 5 ohms. The power requirement of the fan is 1.0HP at a maximum speed of 600 rpm. The outer and inner rotation motors will start with a 12 inch OD design and then decrease the outer diameter to an 8 inch diameter. The length of the motor and motor winding will be increased as the diameter decreases to maintain a constant voltage constant and a constant winding resistance. This will result in a constant power capability in the motor as the diameter and length changes. The ratio of the lamination inner diameter to the outer diameter was held constant at 0.7 to set a constant parameter for varying the motor sizes in diameter and length. This ratio of 0.7 will keep a good slot depth thru the diameter range used and keep the ID of the magnetic circuit large enough to get the size of bearings needed in the ID of the magnetic circuit. Other parameters that will stay the same in both designs are that the airgap flux density which will stay at 8.5 KG. The wire fill and iron flux density will also stay the same for both designs. The procedure of changing both types of designs was to change OD, then determine the ID using the ratio of the lamination ID to OD. The motor length was then increased until the resistance reached the maximum resistance value at a voltage constant value of 1000. This was done for the outer diameter range of 12 inches to 8 inches in diameter in 1 inch increments. The results were plotted and are discussed below. while the inner rotation version has it near the ID of the motor and is lower on the curve. The length increase is exponential as the diameter decreases. This would be expected since the resistance was kept constant and as the length increases a higher percentage of the copper wire will be in the slots. This means a higher percentage of the copper is producing toque in the longer motor lengths. The length curve also shows that at the larger diameters, the outer rotation motor length is less than the inner rotation motor length but as the length becomes longer, the curves come together and if the length was longer they would cross over. Figure 2. Figure 2 shows the magnetic circuit component weights for the outer rotation style motor. The highest weight component in this style is the laminattion weight. The next highest component was the copper in the winding. The magnet and rotor backiron weights were very small compared to the stator parts and the magnet had the least weight of the components. This would be explained by the fact that the stator is outside the airgap and will have a larger volume in this motor. The rotor is inside the airgap and will have a small volume. The lamination weight started flat but increased exponentially. The copper weight started decreasing exponentially and then started to flatten out at the longer motor lengths. Figure 1. Figure 1 shows the impact of the motor length increasing as the OD decreased in value while keeping the motor output power constant. Also shown is the effect of the mean air gap radius. Since we are keeping the lamination ID to OD ratio the same, the ID radius will be very close for both motor types and is not shown. The motor airgap radius will have a linear decrease in value and a slope very close to the outer radius slope. The outer rotation motor airgap will have a larger radius and is close to the outer radius of the motor 18 Magnetics Business & Technology * Spring 2016 Figure 3. Figure 3 shows the same curve for the outer rotation style motor. The magnet weight is the smallest component of the two motor styles. The lamination and copper are still the highest component weight but the rotor weight has caught up to the copper weight. www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2016

Editor's Choice
Permanent Magnets in a Changing World Market
Design Considerations Using Permanent Magnets
Insulated Iron Powders, SMC, Current State and Future Possibilities
Research & Development
The Design of a Permanent Magnet Motor for a Direct Drive Fan Application
Tailoring of Pole Shapes of Multipolar Injection Molded Magnets
Electromagnetics
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: Seventeen Mistakes People Make with Permanent Magnets

Magnetics Business & Technology - Spring 2016