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Simulating a claw-pole alternator in an automobile electrical system environment

Motors & generators with MagNet

Since a claw-pole alternator can be found under the hood of almost any car, it is one of the most optimized of magnetic machines, balancing manufacturing costs against efficiency. Simulating its dynamic electromagnetic characteristics is challenging, but careful analysis of the results can lead to further improvements. Here, using MagNet 7, Infolytica’s newest electromagnetic simulation software with an enhanced GUI, post-processing features and multi-core computational capabilities, Transient 3D with Motion analysis of this machine in the environment of an automobile electrical system, including a three-phase diode bridge and a resistive load is carried out. Presented below are some demonstrations of MagNet 7’s multi-core computational and analysis capabilities.

METHODS and RESULTS

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Multi-Core Computing Using MagNet 7

First, we present some results of computational speed up from MagNet 7 of the 3D transient simulation for this example. There results are summarized in this figure which clearly demonstrates the utility of multi-core computing.

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CIRCUIT DIAGRAM

The circuit side of the simulation is shown here. It includes a three-phase diode bridge and a resistive load (e.g. headlights or the rear window defroster). Since only one sixth of the claw-pole is modeled, the circuit quantities are scaled to model one-sixth of the electrical components as well. In the full model of the machine the 6 images of the coils in each phase are connected in a series-parallel configuration (two parallel paths each with three sets of coils in series). This means voltage sources (including the diode forward voltage drop) are scaled by a factor of a third, and resistors by a factor of two-thirds.

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MAGNETIC FIELD

The magnetic field at 4 ms is shown here. Only the rotating Claw pole has been shown here. Even in a motion solve MagNet can apply periodic constraints to both the stationary and moving sections of the model, reducing the size of the model and considerably speeding up the simulation.

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CURRENT DENSITY

The z component of the current density in the phase windings are shown here at 5.33 ms.

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claw pole flux density

Video -- TRANSIENT STARTUP BEHAVIOUR

This animation of the magnetic flux density shows the transient startup behavior. Although the field in the rotor appears nearly constant, the effect of the opposing stator flux is clearly visible when the video loops around and starts again.

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Video -- CURRENT INDUCED IN THE STATOR WINDINGS

The current induced in the stator windings is shown in this animation.

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claw pole phase currents

PHASE CURRENTS

The phase currents shown here begin at zero but quickly reach steady-state. The non-sinusoidal shape, showing a large third harmonic component, is due mostly to saturation effects.

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DIODE CURRENTS

This graph of the diode currents shows how they are shuttled between the different diodes such that there are never more than three diodes "on" simultaneously.

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PHASE VOLTAGES

Shown here are the phase voltages that change abruptly as the current switches from one diode to another. In operation, each node of the delta-connected windings will be at either the load voltage plus the diode forward voltage, or at ground minus the diode forward voltage. Therefore, since the sum of the phase voltages is zero, one phase voltage will always be positive, another negative, and the third zero.

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CALCULATING THE TORQUE

The torque required to spin the alternator can be calculated in two different ways. MagNet reports the torque on every body as calculated by the Maxwell Stress method. The torque can also be determined using power conservation: the sum of the losses in the electrical components and the power going into the magnetic field must equal the mechanical power input. Torque is one of the hardest quantities to calculate accurately, the time step chosen here is sufficient for the circuit quantities but if torque is of particular interest a finer time step and an increased discretization of the air gap would be advisable.

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