Hardware Reference
In-Depth Information
Figure 4.25: Connection of three symmetric coils
All the problems mentioned above can be avoided by using electronic com-
mutation system, where no brush or commutator is used. Following the fast
developments in power electronics and drive technology, the electronic com-
mutation technology has been progressing rapidly since 90's. The motor that
uses electronic system to detect the rotor position and to commutate armature
current is called brushless DC (BLDC) motor.
In DC and BLDC motor systems, a rotatingpartmoveswithrespecttothe
other part that remains stationary. The components producing the excitation
fi
eld should be installed on one side and armature winding that carries current
is placed on the other. In a DC motor, the excitation
fi
led producing compo-
nents are on the stator while armature winding is on the rotor. However, this
arrangement is reversed in the BLDC motor where the armature windings are
installed on the stator side and the components generating the excitation
fi
eld
are mounted on the rotor. In both DC motor and BLDC motor, the excitation
fi
eld can be produced by permanent magnets (PM). Such motors are called as
PM DC motor and PM BLDC motor, respectively.
When mechanical commutation system is used, detection of rotor position
and commutation of armature current are realized together using the same
device. However, in BLDC motor, the rotor position is detected by some
sensing mechanism, and the coil current is commutated using power electronic
system. Figure 4.26 shows the basic structure of a 3-phase BLDC motor,
and Figure 4.27 shows the electronic commutation circuit (H bridge). In DC
motor, commutation is determined by the armature coil position, whereas in
BLDC motor, commutation is determined by the position of the excitation
fi
eld produced by the permanent magnets on the rotor.
