Clawpole in motion

Since a clawpole 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, MagNet's Transient 3d with Motion solver simulates this machine in the environment of an automobile electrical system, including a three-phase diode bridge and a resistive load.

Results

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The circuit side of the simulation is shown at right. 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 clawpole 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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The magnetic field at 5.5 ms is shown here. The relatively uniform flux distribution shows good rotor iron utilization. Note that although half of the rotor is shown here, it was generated by replicating the solution on the one-sixth model. 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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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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The current induced in the stator windings is shown in this animation. To make the direction of current flow more obvious, the field shown is the z-component of the current density. As a result, the end-windings stay green (zero) since the current flow there is confined to the x-y plane.

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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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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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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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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 airgap would be advisable.