Hardware Reference
In-Depth Information
opposite polarities, the hysteresis loop induces loss in the magnetic mater-
ial [77], [133]. Such loss is also related to the frequency and amplitude of the
magnetic fi eld.
In a spindle motor, the bonded NdFeB ring is surface mounted on the rotor
yoke and the magnets produce strong magnetic fi eld. So the motor fi eld is
determined almost by the rotor magnet. The local fi elds on the magnet and
rotor core can be considered invariable, and no iron loss takes place in the rotor
during the operation of spindle motor. When the motor speed is constant, both
the frequency and the maximum value of the fi eld passing through the stator
core are almost constant, and the iron loss can thus be considered independent
of motor load or drive current. In the rated operation, the iron loss of spindle
is normally much smaller than the copper loss.
EM power and EM torque are two important concepts used in the analysis
of electric machines. The former refers to the electrical power that is trans-
ferred from the stator, through the airgap, to the rotor with the help of the
magnetic fi eld. This power is equivalent to the product of the EM torque and
the speed of the motor. The equivalent circuit shown in Figure 4.61 can also
be used to describe the generation of the EM power where the current passing
through the back-EMF generates the electromagnetic power, and
P em = E · I = K e · ω · I
P em = T em ω = K t · I · ω.
During the operation of spindle motor, only part of input electrical power
is converted into electromagnetic power, or transferred to the rotor. Part of
the input electrical power is wasted as copper loss. Since the dominant fi eld is
generated by the permanent magnet on the rotor, the iron loss takes place only
when the rotor rotates. So the iron loss can be assumed to take place after
the electrical power has been transferred to the rotor. The equivalent circuit
shown in Figure 4.55 cannot describe any phenomenon that takes place after
the EM power has been created, e.g., the effects of iron losses.
There are also mechanical losses, e.g., friction loss in the bearings and
windage loss on inner and outer surfaces of rotor, when the motor is in oper-
ation. These losses depend on the mechanical structure of the motor, but are
independent of the drive current and load condition if the speed is fi xed. As
these losses are caused by mechanical factors, it is difficult to describe them
using electric and magnetic circuits. Therefore, the equivalent circuit of Fig-
ure 4.38 describes only the conversion process from input electrical power to
electromagnetic power.
According to the analyses presented in this section, we can get the power
fl ow in the spindle motor as shown in Figure 4.71. If no disk is mounted on
the rotor, the output power of the motor is zero. When the disks are mounted,
the input power must be increased to compensate for the mechanical losses
caused by the inclusion of disks. This power increment is the output power of
the motor which is mechanical power.
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