Elmo HARSFEN0602, HARmonica software manual 113, Winding shapes

ElmoHARSFEN0602HARmonicaSoftwareManual PRELIMINARYDRAFT

With continuous commutation all the three motor coils are powered simultaneously to yield magnetic field exactly at the direction of the rotor. This brings ε near zero continuously,

with minimal torque losses and ripple torques.θ

The continuous commutation mode is native for the Harmonica and is used most of the time. The continuous commutation is much more complex to implement then six-steps commutation. In fact it requires two independent current controllers to control both the amplitude and the direction of the windings magnetic field.

Continuous commutation reduces the dynamic demands from the current controller, since current demands are almost never switched abruptly.

9.6Winding shapes

The implementation of continuous commutation implies that we know how to direct the magnetic field of the windings.

For a general motor, we have

T= K ⋅ (Ia h(θ) + Ib h(θ −120 ) + Ib h(θ − 240 ))

With

TTorque

KConstant

hWindings shape function.

Ia ,Ib ,Ic The A,B, and C phase currents respectively.

For optimal efficiency, it is easy to show that the phase currents must be
Ia = I0 h(θ),Ib = I0 h(θ −120 ),Ic = I0h(θ − 240 ) for some value I0 .	(1)

In another words, the phase currents must be proportional to the corresponding commutation function values. If (1) is satisfied, the magnetic field produced by the winding currents is perpendicular to the rotor magnet.

Most motor are wound to sinusoidal or Trapeze winding forms, but no motor can be exactly sinusoidal or trapeze. Trapeze motors are normally chosen for six-step commutation,

	0.4
	0.3
	0.2
	0.1
h(theta)	0
	-0.1
	-0.2
	-0.3
	-0.4
	0	50	100	150	200	250	300	350	400

Theta (degrees)

whereas sinusoidal motors are normally chosen for continuous commutation.

Figure 8: Winding shape function for a Trapeze motor

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