AN2372

Theory of operation and control

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Theory of operation and control

In three-phase AC motors, currents flowing in the stator windings create a magnetic field

with a definite magnitude and orientation inside the motor. When a DC current passes

though these windings, it produces a static magnetic field. The permanent magnets in a free

spinning rotor interact with the stator flux and experience a force of attraction to fall in line

with the stator flux and lock with it. If now the stator flux orientation is changed by adjusting

the stator currents, the rotor that is already locked with the stator flux, also changes its

orientation to take the new position of the stator flux. If the stator is now excited with

sinusoidal varying currents, the stator flux inside the motor spins at the frequency of its

sinusoidal currents and pulls along the rotor at this frequency.

The ability of the rotor to stay locked with the stator flux depends on the strength of the

magnetic fields and the magnitude of load torque disturbances on the rotor. Once the rotor

is in motion, if at any time the it falls out of alignment with the stator flux, it cannot spin

anymore and comes to a halt. If the stator is still excited with sinusoidal currents, then the

rotor experiences pulsating torque in either direction at the frequency of stator currents.

Figure 1.

Self control of PMSM

However, this situation can be overcome if we force the sinusoidal angular values of stator

currents to correspond to the angular position of the rotor (plus an offset) as shown in

Figure 1. What this means is that even if the rotor tries to fall out of alignment for any reason,

since the stator current (which determines the stator flux magnitude and orientation)

depends only on the angular position of the rotor, it pushes/pulls the stator flux in the

direction of the rotor disturbance to maintain alignment, thereby giving improved stability

and control.

Under this condition, the PMSM motor acts like a DC motor where commutation is

performed by inverter switches and the speed is determined by the magnitude of applied

voltage. The frequency of applied sinusoidal voltage varies directly with speed and

automatically tracks itself to a value such that it matches with the V/f ratio for the motor. For

precise speed maneuvers, load angle tuning can be brought into play. For operations in field

weakening mode, applied voltage magnitude can remain at the maximum level and the load

angle should be increased appropriately.

ρ

δθ

++

Absolute Position Sensor

3 Phase

PWM

Generator

and Inverter

PMSM