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Summary:
"The Air Gap in Rotating Machines: A
Necessary Evil" With
trilingual summary
Efficient operation of electromagnetic
devices requires that the magnetic circuit contain materials (such
as iron) offering low resistance to the passage of magnetic flux.
That minimizes the amount of electrical energy needed to create the
magnetic field.
However, gaps in the magnetic circuit
are normally unavoidable. They contain air. Because that offers high
resistance to magnetic flux, such gaps require undesirable increases
in magnetizing current and the associated electrical loss.
This is particularly true in motors and
generators. The air gap needed to separate the revolving rotor from
the stator should be as small as possible to reduce the magnetizing
power requirement, yet large enough to prevent contact between the
two despite manufacturing tolerances on their dimensions, or
movement resulting from mechanical deflection and looseness in
supporting bearings.
Few design guidelines exist for
selecting the air gap size best suited to any rotating machine. For
induction motors, in power ratings from 3/4 to 750 kilowatts,
practical values from 0.2 to 5 mm are typical; the higher the motor
speed, the larger the gap. A common empirical calculation involves
rotor peripheral speed, core stack length, and rotor diameter.
Although increasing the air gap will increase magnetizing current,
it will also tend to decrease stray load loss.
In any event, the gap must be large
enough to assure that eccentricity of the rotor with respect to the
stator will not cause the shaft stiffness to be overcome by
unbalanced magnetic pull, which could allow the rotor to strike the
stator.
Whatever gap is used, it must be
uniform. A non-uniform gap causes increased noise and vibration.
Variation is commonly limited to plus or minus 10 percent of the
average value.
In synchronous and d-c machines, two
separate fields interact in the air gap. The a-c field created by
the armature (stationary in the synchronous machine, rotating in the
d-c machine) distorts that supplied by the d-c field, reducing its
effectiveness and degrading machine performance. Increasing the air
gap lessens the effect of that "armature reaction." Hence, these
machines will have air gaps several times larger than those in
induction motors.
From "The Air
Gap in Rotating Machines: A Necessary Evil - published in Electrical Apparatus
September 2005
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