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Iron loss separation in high-torque BLDC Motors

Motors & generators with MotorSolve

High torque motors of the BLDC type are required to deliver high power, and are usually characterized by low operating speed and synchronous frequency, high number of poles and teeth, etc. Given the power requirement, loss minimization is usually an important design criterion for such machines. Depending on the operating conditions, iron losses may be a significant fraction of the total loss and qualitative and quantitative knowledge of their variation with respect to design parameters (electrical and geometrical) could be quite useful to the machine designer. Even fractional improvements in minimizing losses can be useful, and energy savings at this level can only be predicted by accurate FEA based analysis.

In this example, using MotorSolve BLDC, Infolytica’s template based motor design software, iron loss trends in a high-torque BLDC motor with respect to electrical loading, drive types (sinewave and six-step drive) and rotor back iron depth are computed. The need for FEA based analysis is demonstrated here, as well as a simple application of the results that helps the engineer set electrical loadings for this machine.

Iron loss separation in high-torque BLDC Motors

METHODS and RESULTS

HIGH TORQUE MOTOR WINDING CONFIGURATION

A BLDC motor, surface mounted with radial magnets with two magnets per pole and an exterior rotor is chosen for this example. The number of magnet poles and stator teeth are 66 and 72, respectively. The operating speed is 150 rpm. The rotor and stator material is M-19 29 Ga steel. A top view of this motor along with phase A (balanced) winding configuration generated using the automatic winding generator is shown in the figure to the left.

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FLUX DENSITY SATURATION

Consider the flux density of this motor at two different electric loadings; 5 A-turns (motor on left of figure) and at 500 A-turns (motor on right of figure). It is clear from the flux density field values (and the B-H characteristics of M 19- 29 Ga) that parts of the machine have been driven well into saturation. Consequently, FEA based analysis is absolutely essential for determining machine parameters and characteristics over such reasonable ranges for the electric loading (or for many other sets of parameters ranges). This also implies that accurate loss analysis also requires FEA based tools.

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IRON LOSS SEPARATION and FIELDS

MotorSolve is a hybrid FEA and lumped parameter based motor design software. In this section, some of MotorSolve's iron loss computation and analysis capabilities are presented.

The loss calculations are based on the Steinmetz equation. Using Epstein frame data, MotorSolve uses curve fitting to determine loss coefficients for the Steinmetz equation. Transient FEA solution, flux density variations in all parts of the model, and harmonic decomposition are used to compute and separate the hysteresis and eddy current losses. An example of the time-averaged hysteresis losses for the machine considered here is shown on the left. The loss fields are reported in W/m^3.

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EDDY CURRENT LOSS FIELD

The eddy current loss field is shown in this figure.

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IRON LOSSES for SINEWAVE & SIX-STEP DRIVE

Comparison chart between iron losses for sinewave (prototype design 1) and six-step (Prototype design 2) drive.

Some of these features have been applied to analyze the rotor iron loss properties of this motor. These results and an application are presented next.

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ROTOR CORE EDDY CURRENT LOSS vs ELECTRIC LOADING

In this section, the rotor hysteresis and eddy current losses as a function electric loadings, drive type and back iron depth are presented. First, the rotor eddy current loss data versus electric loading for sinewave and six-step drive types are shown.

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ROTOR HYSTERESIS LOSS vs ELECTRIC LOADING

Stator hysteresis loss vs. electric loading

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STATOR HYSTERESIS LOSS vs ELECTRIC LOADING

Stator hysteresis loss vs. electric loading

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STATOR EDDY CURRENT LOSS vs ELECTRIC LOADING

Stator Eddy Current Loss vs Electric Loading

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ROTOR OUTER DIAMETER vs IRON LOSSES

The rotor iron loss variations with respect to the rotor outer diameter, which also varies the back iron depth, keeping the rest of the parameters constant. The results are computed at 900 A-turns electric loading (corresponding to a saturated machine) and for a sinewave drive.

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ROTOR EDDY CURRENT LOSS DERIVATIVE vs ELECTRIC LOADING

A direct utility of the data is that the designer may compute the expected efficiency levels of the motor accurately as a function of electrical and geometrical parameters. Another simple, secondary application of such loss trends can be as follows. Consider the derivative of the loss curves as a function of the electric loadings (in the rotor). First, the derivative of the rotor eddy current loss is shown on the left.

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ROTOR HYSTERESIS LOSS DERIVATIVE vs ELECTRIC LOADING

Now, the derivative of the hysteresis loss is shown. These results allow the machine designer to select an appropriate electric loading parameter range so that the machine operates under conditions that steadies eddy current losses with respect to variations in the electric loading (which would be expected under realistic driver such as PWM drives). Considering the eddy current loss derivative data, a good range of electric loading parameters for this machine would be from approx. 700 - 1200 A-turns for both drive types.

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