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Flux Lines and current density

Co-simulating current vector control of an IPM motor

Motors and generators with MagNet

This is an example of how the whole can be greater than the sum of its parts. The current vector control of an IPM (interior permanent magnet) motor is simulated here using the combined power of the PSIM circuit and systems simulator from PowerSim and MagNet.

This is a co-simulation in which both PSIM and MagNet run their transient solvers simultaneously, with a constant data exchange between the two to keep the shared quantities (voltages and currents) synchronized.

PSIM is designed specifically for power electronics and motor control, which makes it an ideal partner with MagNet, which specializes in the analysis of the dynamic performance of motors and other electromagnetic machines.

Co-simulating the vector control of an IPM motor

METHODS and RESULTS

PSIM CIRCUIT of CLOSED LOOP VECTOR CONTROL

PSIM circuit which implements a closed loop vector control of the motor speed. The link circuit element which couples to MagNet is located in the lower right quadrant.

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MESH of the IPM MOTOR in MAGNET

The properties of the link circuit element in PSIM is general enough to allow MagNet to take advantage of symmetry. In this case, the four windings of each phase are connected in a series-parallel configuration, which means both the current and the voltage are scaled by a factor of two in the PSIM link. Shown here is the mesh used by MagNet for its part of the simulation. At each time step, as the rotor changes position, the air gap is remeshed, which allows motional effects to be modeled quickly and accurately.

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VECTOR CONTROL SPEED vs ACTUAL SPEED

Using vector control, a feedback loop monitors the motor speed and varies the motor torque to maintain a desired speed. The graph here shows the commanded speed (1800 rpm, shown in blue) and the actual speed (shown in red). The transient simulation starts in PSIM, which invokes MagNet transparently to jointly run its electromagnetic simulation. While both simulations are running they are constantly passing data back and forth to synchronize the voltages and currents.

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FEEDBACK CIRCUIT VECTOR VOLTAGES

This graph shows the vector voltages commanded by the feedback circuit (red and green waveforms). The system blocks in the lower left quadrant of the circuit implement an approximate inverse model of the motor, which means that the speed feedback need only generate small corrections to these voltages (blue and violet waveforms).

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CONTROLLER PHASE VOLTAGES

PSIM has many components in its library which simplifies modeling motor drive circuitry. This graph shows the phase voltages commanded by the controller, which are obtained from the vector voltages by a single DQ to ABC transformation element.

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PULSE TRAINS for each PHASE COIL

The PWM circuitry amplifies the commanded phase voltages by generating a 10 kHz variable width pulse train for each phase coil. Shown here is a brief snapshot (1 ms window) of the three pulse trains. These are fed to the link circuit element which manages the data transfer between PSIM and MagNet.

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PHASE CURRENTS RETURNED to PSIM

MagNet also takes the mechanical loads on the motion component into account, and this with the velocity effects such as back emf are used to calculate the resulting phase currents by performing a finite element analysis. These currents are returned to PSIM, and are shown here.

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COMPARING the CURRENT MAGNITUDE

This graph shows both the commanded current magnitude (violet waveform) and the actual vector currents (magnitude is green, vector currents are red and blue). A limiter component in the feedback loop keeps the commanded current magnitude under 10 Amps. The actual vector currents are obtained from the phase currents using a ABC to DQ transformation element from PSIM's component library.

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PLOTTING the MAGNETIC TORQUE

The previous graphs were all generated using the SIMVIEW circuit post-processor, which allows various circuit quantities and algebraic functions of circuit quantities to be graphed quickly and easily. Once the simulation is complete, MagNet's post-processor can be invoked to plot the magnetic quantities, such as flux linkage and torque, as well as fields, such as flux density and current density. The magnetic torque on the rotor is plotted here. The video in the Information Center at the top right of this web page shows the time variation of both the current density and the flux lines.

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DATA EXCHANGE BETWEEN PSIM and MAGNET

The link which orchestrates the data exchange between PSIM and MagNet is designed to allow different time steps in each application while both are running a transient simulation. In this example the PSIM simulator is running with a time step of 1 us, which is needed to accurately simulate the circuit operating at the PWM switching frequency of 10 kHz. MagNet is running with a time step of only 0.2 ms, which translates to only one MagNet time step for every 200 PSIM time steps. A summary of the simulation data follows.