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Simulating Pulse Width Modulation with a Brushless DC Motor

Motors and generators with MotorSolve

MotorSolve BLDC contains an extensive list of post-processing options that allow the user to analyze models using finite element, analytical and hybrid approaches. One of MotorSolve BLDC's salient features is its dynamical simulation capabilities. These include simulation options using an ideal driver (transient analysis) and that using PWM drive circuits. In this gallery, some examples of simulations using the PWM drive are presented. The figure shown here is an example of a wye-connected 3 phase drive circuit used in such simulations. PWM capabilities of MotorSolve BLDC include delta connected circuits and sinusoidal and six-step drive types. Also, switching losses are taken into account in the simulations.

Simulating pulse width modulation with a brushless dc motor

METHODS and RESULTS

4-POLE 12-SLOT MOTOR with PHASE A WINDING

Consider a 4 pole 12 slot motor with phase A winding, as shown on the figure to the left. The examples presented below are for this model.

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LINE CURRENTS for the THREE PHASES

PWM simulations are performed easily in MotorSolve BLDC. The user simply inputs the operating parameters, elects to perform a PWM Analysis, selects the solution entities of interest and MotorSolve BLDC automatically generates the results with-a-click. Consider the following settings applied to the model shown above: PWM 3-phase bridge simulation, six-step drive type, wye connected windings operating at 1000 rpm. The line currents for the three phases with these settings are shown on the figure to the left. The 'spikes' seen in the various phases represent current switchings.

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INSTANTANEOUS BACK EMF on PHASE A at VARIOUS ROTOR SPEEDS

A number of interesting results are available to the designer from the PWM simulations including torque, back emf, line and winding voltages, power input and output, flux linkage, etc. The user may generate instantaneous, time-averaged as well as harmonic contents of these entities. For example, the instantaneous back emf on phase A at various rotor speeds are shown on this figure. As expected, the back emf is seen to scale appropriately with rotor speeds.

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HARMONIC COMPONENTS of the BACK EMF

Harmonic components are also available. The harmonic components of the back emf at 1000 rpm for one of the phases is shown here.

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TIME-AVERAGED TORQUE VS SPEED

The motor designer may be interested in how outputs vary as a function of rotor speeds, advance angles or for various prototypes. MotorSolve BLDC's PWM capabilities allow the user to make such comparisons readily. Consider for example, the time-averaged torque versus speed variation for the motor above. As the rotor speed and consequently the back emf increases, the torque generated decreases as the rail voltage becomes comparable to the back emf. This is captured clearly in the results shown here.

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TIME-AVERAGED TORQUE VS ADVANCE ANGLE at VARIOUS ROTOR SPEEDS

Consider now the time-averaged torque versus advance angle at various rotor speeds. As the angle between the winding currents and the q-axis increases, the torque generated is seen to decrease at low rotor speeds and increase initially for higher rotor speed values (due to field weakening) before following the same trend as that for low rotor speeds. Hence, to generate the same level of torque over a wide range of rotor speeds, increasing the advance angle is seen to help.

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SUMMARY

These are some basic examples of the type of analysis that may be done in MotorSolve BLDC using PWM simulations. MotorSolve BLDC is capable of generating other many interesting PWM simulation results that complement its extensive post-processing capabilities.