Hardware Reference
In-Depth Information
also form a rotating
fi
eld. From equation 4.41, the speed of this harmonic
fi
eld
is
1
m
of the speed of the fundamental
fi
eld. Therefore, higher is the order of
the harmonic
fi
eld lower is the speed of the corresponding rotating
fi
eld. It can
also be shown that the summation of the triple MMF harmonics is zero for the
3-phase systems, and therefore, the effects of the triple MMF harmonics are
normally not considered in the discussions of energy conversion.
4.2.3 Force and Torque Generated by Magnetic Field
Magnetic
fi
elds contain energy, and that can also appear as force/torque act-
ing on the parts of the related electromagnetic system. The existence of the
EM force/torque is the necessary condition in realizing the electromechani-
cal energy conversion. The EM forces can be categorized into two types, the
Ampere's force and the reluctance force.
4.2.3.1 Ampere's Force and Ampere's Torque
Let us consider a straight conductor of length l conducting a current I in an
evenly distributed magnetic
fi
eld. In this case, the conductor is acted on by
the Ampere's force,
F
A
. The force can be expressed as,
½
F
A
= Il × B
F
A
= I · l · B · sin(θ)
(4.42)
where, θ is the angle between the conductor and the
fi
eld (see Figure 4.16).
The force is a vector and its direction is vertical to both the conductor and the
magnetic
fi
eld.
Figure 4.16: Ampere's force acting on a conductor
If the conductor is perpendicular to the magnetic
fi
eld B, then the directions
of the current, the magnetic
fi
eld and the Ampere's force are given by the left-
hand rule, as shown in Figure 4.17.
