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 position feedback for adjusting frequency and amplitude of stator voltage reference properly 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.
Synchronous motor construction: Permanent magnets
are rigidly fixed to the rotating axis to create a constant rotor flux. This
rotor flux usually has a constant magnitude. The stator windings when energized
create a rotating electromagnetic field. To control the rotating magnetic field,
it is necessary to control the stator currents.
The actual structure of the rotor varies
depending on the power range and rated speed of the machine. Permanent magnets
are suitable for synchronous machines ranging up-to a few Kilowatts. For higher
power ratings the rotor usually consists of windings in which a DC current
circulates. The mechanical structure of the rotor is designed for number of
poles desired, and the desired flux gradients desired.
The interaction between the stator and rotor
fluxes produces a torque. Since the stator is firmly mounted to the frame, and
the rotor is free to rotate, the rotor will rotate, producing a useful
mechanical output as shown in Figure 2-5.
The angle between the rotor magnetic field and
stator field must be carefully controlled to produce maximum torque and achieve
high electromechanical conversion efficiency. For this purpose a fine tuning is
needed after closing the speed loop using sensorless algorithm to draw minimum
amount of current under the same speed and torque conditions.
The rotating stator field must rotate at the same
frequency as the rotor permanent magnetic field; otherwise the rotor will
experience rapidly alternating positive and negative torque. This will result in
less than optimal torque production, and excessive mechanical vibration, noise,
and mechanical stresses on the machine parts. In addition, if the rotor inertia
prevents the rotor from being able to respond to these oscillations, the rotor
will stop rotating at the synchronous frequency, and respond to the average
torque as seen by the stationary rotor: Zero. This means that the machine
experiences a phenomenon known as pull-out. This is also the reason why
the synchronous machine is not self starting.
The angle between the rotor field and the stator
field must be equal to 90ºC to obtain the highest mutual torque production. This
synchronization requires knowing the rotor position to generate the right stator
field.
The stator magnetic field can be made to have any
direction and magnitude by combining the contribution of different stator phases
to produce the resulting stator flux.
Figure 2-5 The Interaction Between the
Rotating Stator Flux, and the Rotor Flux Produces a Torque