Brushless DC Motor - Simple Analysis

This sample demonstrates how Infolytica's MagNet software can accurately calculate cogging torque in a brushless DC motor. The sample also shows the modelling simplifications that can be achieved in all of Infolytica's software using periodic boundary conditions. The motor's rotor and stator are made out of steel along with Neodymium Iron Boron magnets. Each pair of magnets are magnetized inward and outward radially.

The problem is set up to view the cogging torque on the rotor. Initially the rotor is given a speed of 1500 deg/s. A viscous friction is also applied to the rotor which causes it to slow down and eventually to oscillate between the teeth of the stator.

This model displays how Infolytica's software uses symmetry to its advantage in order to greatly reduce the analysis time. The motor is symmetrical every 120 degrees, so with the application of periodic boundary conditions only a third of the model is required. This speeds up the design cycle without compromising accuracy.

For more information on this brushless DC motor consult "A comparison of 2D and 3D analysis methods for the prediction of cogging torque in an electrical machine having skewed slots" in COMPEL Volume 20 Number 2 2001.

Results

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After MagNet's Transient 2D with Motion solver completes its processing, the user can view a number of results calculated by the software. To the right is the graph of the magnetic torque acting on the rotor vs time.

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On the right is the speed vs time graph of the rotor. Once the rotor's speed goes below a certain point the cogging torque prevents it from passing the next stator tooth and the oscillations will eventually decay to 0 due to the friction.