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Figurę 2.3. Ideał waveforms for BLDC (brushless d.c. motor), a) PM air gap flux density, b) PM flux per phase a, c) emf in phase a, d) ideał currents for motoring, e) ideał currents for generating.

For zero advance angle = 0 - the phase currents are in phase with the emf (i_a, E_a) for motoring (figurę 2.3.d).

At any instant, due to assumed instantaneous current commutation, only two phases are in conduction. Conseąuently only two thirds of the PMs are utilized.

For 120° wide magnets delta connection is used while 180° wide magnets reąuire star connection. To reduce the fringing (leakage) flux between neighboring magnets their span is 150° - 160°.

Returning to the linear phase flux linkage variation with rotor position (Fig.2.3b) and assuming idealizing air gap flux distribution over a?M = n (180°) we have:

(2.i)

The maximum flux linkage (for N tums and B„ air gap flux density, pole pitch r and stack length L) per phase i/ą_m is:

(2.2)

The phase emf, E_a, is:

E_a

dy/_aPM(fl)d# d0 dt

2

n

(2.3)

where co, is the electrical angular speed.

As two phases conduct at any time the ideał torque T_e is constant:

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