Hardware Reference
In-Depth Information
excitation
fi
eld and the armature current
fi
eld can be considered separately, and
the total airgap
fi
eld can be obtained by summing up the two
fi
elds according
to the principle of superposition. However, if the stator core or the rotor core or
both operates in the saturation state, the total airgap
fi
eld will be smaller than
the sum of the two individual
fi
elds. The difference increases with increasing
magnitude of armature current. From the point view of motor control, this
effect is equivalent to a reduction in the excitation
fi
eldwithincreaseindrive
current. This phenomenon is known as armature reaction.
Figure 4.41: Airgap
fi
eld formed by exciting
fi
eld and armature winding
fi
eld.
The conditions for generating armature reaction are,
1. the saturation of stator core, or rotor core, or both is serious, and
2. the
fi
eld produced by the armature winding current can affect the airgap
fi
eld obviously.
It was mentioned in sections 4.2.3 and 4.2.3 that the EM torque and back-
EMF of a DC or a BLDC motor can be described as
T
em
= K
t
I,
(4.63)
E = K
e
Ω.
If the armature reaction of the motor is severe, the torque constant K
t
and
back-EMF constant K
e
cannot be taken as constants. They are reduced when
the input current is increased. The nonlinearity of the torque constant and the
back-EMF constant makes the precision control of the motor complicated.
