Hello and welcome to this video about the
failure and analysis of asynchronous motors. Asynchronous motors are the most common type of
motors used. They are known for their advantages such as low purchase price, high efficiency, easy
regulation and simple but robust construction. Despite their high reliability, asynchronous
motors suffer from some malfunctions of machine parts. We can divide failures in
an asynchronous motor into failures of mechanical and electrical origin, as well
as stator, rotor and bearing failures. Let´s start with the bearings.
All parts of the bearing are subject to degradation. The cause of bearing failures can be
considered as mechanical stress during rotational movement and bearing currents. Mechanical
stress can be caused by poor installation, poor assembling, or by improper use,
overloading, and poor maintenance. The bearing currents can
be caused by shaft voltages (due to asymmetric electrical circuits or
power supplies) and capacitive currents (caused by the pulse frequency from the power
supply control of semiconductor converters). All mechanical (and some electrical)
motor faults have a unique signature in the vibration spectrum of the machine
and vibration analysis can recognize them. Failures such as misalignment, looseness,
unbalance and bearing faults are diagnosed according to the same rules applicable
for all other machinery parts. How do we analyze mechanical faults? Get the
answer for example from our video named How to become an expert in Vibration Analysis and our
other tutorial videos.
What is vibration analysis? Our series of Vibration analysis for beginners
videos will help you to understand this topic. Some electrical faults are recognizable
in the vibration spectrum too. You should measure the motor with and
without a power supply in order to find them. Some vibration signatures could disappear after
power off – those with an electrical origin. You can also focus on the exact
frequency of peaks in the spectrum. If you find a peak with your exact current
supply frequency (for example 50Hz and more often on its harmonic frequencies) it is probably
an electric issue because there is always some slip on a loaded electric motor and the motor
isn´t running on its exact rotation frequency. Electrical (and some mechanical)
motor faults have a unique signature in the frequency spectrum of the motor
current. The MCSA method can recognize them. MCSA stands for: Motor Current Signature Analysis. Excessive sidebands are created in electric
motors, which distort the frequency spectrum. Each fault then has its specific signature.
Individual defects can be distinguished from each other according to the amplitude
bands and the frequency or other signatures. The basis of this method is to measure the
course of the stator current of one or more phases in the time domain and
its subsequent spectral analysis.
Stator faults
Stator winding faults cause the majority problems in stators. Broken winding
insulation is the most common stator fault. MCSA can recognize broken
insulation between threads, which can lead to broken insulation between
phases and it is fatal for the motor. Thermal stress has the greatest impact
on the life and quality of insulation. Another undesirable effect is the
electrical stress of the transient voltage. In the case of more and more frequent
use of inverters for soft-start, rectangular voltage pulses are
modulated at the output of the inverter. Rotor faults
The rotor of an asynchronous motor consists of a shaft, insulated sheets pressed on
the shaft which form the rotor magnetic circuit and windings. Mostly the winding of
the rotor consists of a cage structure, which is formed by bars, which
are connected at the ends. Rotor eccentricity (meaning the unevenness of
the air gap between the rotor and the stator) and rotor bar interruption
are the most common faults. The cause of these faults can be
the use of poor-quality materials, overloading or heavy starts. In the case of rotor bars, the fault may increase
the resistance of the bar, or completely break the bar electrical circuit.
Rotor bar failures
result mainly in engine starting deterioration and generating parasitic moments. Also, the broken
bar causes additional faults in other bars because the current in them is greater due to the missing
bar current path (where one bar is broken). The ADASH VA5Pro vibration analyzer offers
the unique capability of analyzing vibration and current in one device. In addition, the MCSA
module expands the capabilities of the analyzer and allows you to do analysis of the current
signature from the spectrum – based on your knowledge and experience, or you can use the
automatic detection function. It is a similar feature to the ADASH automatic Fault Source
Identification Tool (FASIT) for vibration analysis. The device can automatically recognize
the main causes of failures such as unbalance, looseness, misalignment and bearing faults.
The MCSA module of the VA5Pro device is able to automatically identify rotor and stator faults,
eccentricity, broken rotor bars and power quality.
