18-04-2012, 12:45 PM
Implementation of a Speed Field Oriented Control of 3-phase PMSM Motor using TMS320F240
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Introduction
A brushless Permanent Magnet Synchronous motor (PMSM) has a wound stator, a
permanent magnet rotor assembly and internal or external devices to sense rotor
position. The sensing devices provide logic signals for electronically switching the stator
windings in the proper sequence to maintain rotation of the magnet assembly. The
combination of an inner permanent magnet rotor and outer windings offers the
advantages of low rotor inertia, efficient heat dissipation, and reduction of the motor size.
Moreover, the elimination of brushes reduces noise, EMI generation and suppresses the
need of brushes maintenance.
Two configurations of permanent magnet brushless motor are usually considered: the
trapezoidal type and the sinusoidal type. Depending on how the stator is wounded, the
back-electromagnetic force will have a different shape (the BEMF is induced in the stator
by the motion of the rotor). To obtain the maximum performance from each type of
PMSM, an appropriate control strategy has to be implemented. The trapezoidal BEMF
motor called DC brushless motor (BLDC) uses a "two phases on" strategy, whereas the
sinusoidal BEMF motor offers its best performances when driven by sinusoidal currents
(three phases on strategy).
This application report presents the implementation of a control for sinusoidal PMSM
motor.
The sinusoidal voltage waveform applied to this motor is created by using the Space
Vector modulation technique.
The Field Oriented Control algorithm will enable real-time control of torque and rotation
speed. As this control is accurate in every mode of operation (steady state and transient),
no oversize of the power transistors is necessary. The transient currents are constantly
controlled in amplitude. Moreover, no torque ripple appears when driving this sinusoidal
BEMF motor with sinusoidal currents.
PMSM Model
The operation of a brushless PM motor relies on the conversion of electrical energy to
magnetic energy and then from magnetic energy to mechanical energy. It is possible to
generate a magnetic rotating field by applying sinusoidal voltages to the 3 stator phases
of a 3 phase motor. A resulting sinusoidal current flows in the coils and generates the
rotating stator flux.
The rotation of the rotor shaft is then created by attraction of the permanent rotor flux with
the stator flux.
Speed and Position Definition
In electric motors, two measures of position and speed are usually defined: mechanical
and electrical. The mechanical position is related to the rotation of the rotor shaft. When
the rotor shaft has accomplished 360 mechanical degrees, the rotor is back in the same
position where it started.
FOC Control for PMSM
The goal of the Field Oriented Control [BPRA073] is to perform real-time control of torque
variations demand, to control the rotor mechanical speed and to regulate phase currents
in order to avoid current spikes during transient phases.
To perform these controls, the electrical equations are projected from a 3 phase nonrotating
frame into a two co-ordinate rotating frame.
This mathematical projection (Clarke & Park) greatly simplifies the expression of the
electrical equations and remove their time and position dependencies.
Expression of the Stator Current Vector
As phase current values are used in the general expression of the torque, the expression
of their values in the new rotating frame are needed afterwards.
The three sinusoidal currents created by the 120° (electrical) phase shifted voltages
applied to the stator are also 120° (electrical) phase shifted one from another.