SKF AWAIV-12, Руководство пользователя

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Note the phase shift between the voltage and current. The ratio of the voltage and current
amplitudes along with this phase shift are used to determine the coil’s impedance.
The voltage and current are related by a “complex” impedance Z defined as:
The impedance Z will have a component in phase with the voltage (called the real part) and
an out-of-phase component (called the reactive part).
The real part of the impedance not only represents the component of current in-phase with
the applied voltage, it represents the part of the coil’s impedance that absorbs power. The
reactive part of the impedance represents the ability of a coil to make a magnetic field. So,
the motivation for measuring a coil’s impedance is clear: the ability of a coil to make a
magnetic field, which is so important to the operation of a motor, is represented by the
reactive component of the impedance of a coil.
Specifically, the measurement of inductance, which is proportional to the reactive impedance,
is most often used when measuring a coil’s inductive or magnetic properties. The reactive
impedance (X) and inductance of a coil (L) are related as follows:
where f is the frequency of the source. By measuring the changes in the inductance L,
changes in the coil’s ability to make a magnetic field are determined. From a physical
standpoint, the number of turns in a coil, the material properties surrounding the coil (that is,
the motor core), and the shape of the coil all combine to determine the coil’s inductance. The
following equation shows how these parameters combine to determine a coil’s inductance:
where the constant A describes the physical shape of the coil, the constant B describes the
material properties of the coil’s core, and N describes the number of turns in the coil. For
example, a solenoid’s inductance is found to be:
where µ
0
is the magnetic permeability of air, µ
r
is the relative permeability of the coil’s core
(approximately 1000 for electrical steels), N is the number of turns, A is the solenoid area,
and l is the solenoid length.
There are other formulas for a coil’s inductance, but the key thing to take away from these
formulas is the contribution to the inductance value from the physical shape of the coil, the
contribution to the inductance from materials properties, and the contribution to inductance
by the number of turns (squared).
A motor’s designer carefully chooses the shape and turn count of the coil along with the core
material to generate the magnetic field required to produce the desired motor shaft torque.
SKF Static Motor Analyzer—Baker AWA-IV User Manual 113
Motor testing theory and reference