Motor Electrical Signature Analysis

Motor Current Signature Analysis (MCSA)

             MCSA detect broken rotor bars using current signal only & it tells the user what is from the point of test towards the load.                    

Electrical Signal Analysis(ESA)

           ESA detects Motor fault using both Voltage and current & it tells the user about information from the point of test towards the supply

MCSA Vs ESA

 MCSA is primarily used by the vibration industry using special current probes which allow the vibration data collectors to take current input. This current is then converted from analog to digital, filtered and produced as an FFT (Fast Fourier Transform) spectra of amplitude versus frequency. ESA has been primarily used by the dedicated ESA instrument manufacturers and includes the voltage waveform as an input. The primary difference is that current tells the user what is from the point of test towards the load and voltage provides information from the point of test towards the supply. This allows the user to quickly determine where a particular signature exists.

ESA Advantages

1.    ESA provides the capability of detecting power supply issues, severe connection problems, airgap faults, rotor faults, electrical and mechanical faults in the motor and driven load, including some bearing faults

2.    It is important to note that the technology should not be considered a replacement for vibration analysis in mechanical analysis, but provides excellent data on motor condition from incoming power through to the rotor

3.    From the bearings to the mechanical load still remains in the realm of vibration, in most cases.

Fault Detection Using ESA

            One of the original concepts behind the development of ESA was to eliminate the loss of instrumentation to test MOV’s in the dangerous areas within nuclear power plants. The primary failure of these machines is the rotor which would overload and melt when limit switches failed. It was discovered that the rotor bar failure signature was unique enough that not only could the signature be quickly identified, but that condition values could be applied easily.

              When the Pole Pass Frequency sidebands (P1 and P2) of Figure 1 are compared to the values in Table 1, and the condition of the rotor bars can be determined. However, in this case, the motor is 4,160 Vac and the data was taken from the Motor Control Center (MCC) Current Transformers (CT). The result can be a dampening effect on those peaks resulting in the analyst needing to estimate the severity of the fault.

           Where SS is Synchronous Speed, RS is Running Speed, LF is Line Frequency and PPF is the Pole Pass Frequency

            Concerning most other faults detected with ESA, the number of rotor bars and stator slots in the design of the motor is necessary. Many of the ESA instrument manufacturers have built algorithms into their software which can assist the analyst in estimating either number.

            In figure 2, the motor is an 800 horsepower, 1785 RPM, 101 Amp, Louis Allis motor with 58 rotor bars and 72 stator slots. SM1 and SM2 are peaks related to the movement of the coil ends of the motor windings. As measured through the CT’s, the values are about -78 dB which would be more severe if the current was measured directly. With an RPM of 28.793 Hz (1727.6 RPM), the stator mechanical (coil movement) frequencies would be the number of stator slots times the running speed plus and minus the line frequency. In this case, 2013.1 Hz and 2133.1 Hz which relates to the fields passing through the coils ends and interacting with the rotor fields.

           Excessive coil movement will cause fractures in the coils as they leave the stator slot. In the case of the 800 horsepower motor, this movement coupled with oil contamination caused the winding to fail where the windings leave the slot.