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Location

This example model can be found in the software under Distributed Generation > VFD_PMSM.ecf

Description

Background

Variable frequency drive-based applications are gaining more popularity recently due to higher energy efficiency and increased control. This demo example demonstrates the two-quadrant operation (forward motoring and reverse motoring) of various applications like elevators, cranes etc. For instance, an elevator moving up with people inside represents the forward motoring mode wherein the load is lifted against the gravity, and coming down empty with gravity-assist as reverse motoring mode.

This example shows the implementation of Variable Frequency Drive (VFD) based Permanent Magnet Synchronous Machine (PMSM) example where the converters are implemented using: a) Switching Bridges and b) Switching Function. The converters used in this example are the 2-level Voltage Source Converters. The advantage of using Switching function based model for real time execution performance is demonstrated.

For more details about the converters model please refer to Switching Device and Switching Function

Model description

The model consists of a 380 V, 60 Hz voltage source (emulating a grid) feeding the PMSM through back to back voltage source converters as shown in Figure 1. The PMSM carries a load whose torque is proportional to the mechanical speed and is given by the equation: T_load=0.04 w_m.

The example consists of 2 separate models, one with Switching Device and the other with Switching Function. The control architecture and network parameters are the same for both these implementations. The only difference being the type of converter used and the respective controller. Switching function based converter uses the PWM average generator, which is a pulse generator averaged over the time step of the simulation. It uses an interpolation technique to calculate the transition moment within a sample time of the model. The switching bridge model uses the standard PWM generator using carrier-based pulse width modulation (PWM) converters. Both the converters are operating at the switching frequency of 4.2 kHz.

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Figure 1. Schematic diagram

Each converter of the model has a specific objective, as given below: 

Grid-side converter control

The grid side converter (GSC) is operated to control the DC link voltage amplitude and the reactive power . The outer voltage loop maintains the DC  ink voltage at 700 V, while the reactive power control loop is set to unity power factor. Below figures show the control architecture used.

Outer loop DC link voltage regulator

Inner loop Current regulator

Where,

Id : d-axis component  of source current

Iq : q- axis component of source current

Vd :  d-axis component  of source voltage 

Vq :  q-axis component  of source voltage

Machine-side converter control

The machine side converter (MSC) controls the generator speed. LC filters connected shunt to the network are used to eliminate the harmonics.

Outer loop Speed regulator

Inner loop torque and current regulator

Where,

Id : d-axis component of machine stator current

Iq : q-axis component of machine stator current

Φ : Magnetic flux of the machine

p : Number of poles

Model parameters

System parameters

Machine general parameters

Machine electrical parameters

Machine mechanical parameters

Converter parameters

Controller parameters

Scenarios

The following scenarios demonstrate the modes of operation of the PMSM based VFD example and the results are shown in the figure below. The sequence of the mode change is shown in the table below.To demonstrate the performance of the model few fault scenarios introduced during forward and reverse motoring modes.