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 2-1.
- 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.