Electrical Motor Diagnostics for Generators Part 1 - The Basics
Electrical Motor Diagnostics (EMD) is a term for test methods and instruments designed for rotating and coil-wound machinery electrical and mechanical analysis. These instruments are used for all motor system related analysis from the generator and prime mover, through the transmission and distribution system, to the electric motor and driven load. These technologies, for the purpose of this paper, will include Motor Circuit Analysis (MCA), a de-energized test method, and Electrical Signature Analysis (ESA), a more advanced method of Motor Current Signature Analysis (MCSA).
In this paper, we will discuss the concepts behind the testing and analysis of both salient and turbine generators to detect some common generator faults. These faults include bearings, winding shorts in the rotor and stator, insulation to ground faults in the rotor and stator, exciter faults, misalignment and rotating field eccentricity.
Motor Circuit Analysis
MCA is a low voltage method for testing electric machinery cables, connections, windings and rotor for developing faults. The technique involves individual readings of DC Resistance (R), Impedance (Z), Inductance (L), Phase Angle (Fi), Current/Frequency Response (I/F) and insulation to ground (MegOhm) testing. Resistance is used for detecting loose connections and broken conductors, insulation to ground is used for detecting ground faults, Z and L are matched to evaluate the insulation condition for winding contamination, and, Fi and I/F are used to detect winding shorts. One of the key aspects of MCA is the ability to detect early winding defects that can be trended over time and a time to failure can be estimated.
As a vast majority of the rotating machinery, that MCA is used to evaluate, requires balanced phases, pass fail criteria for individual readings can be developed for both assembled and disassembled machines (Reference Tables 1 and 2). These values indicate a guideline and values outside of these guides normally identify component failures that have occurred, or are developing. These developing issues can be compared against Attachment 1 of this paper.
In addition to the power of detecting a motor system defect, the values are trend able without the requirement of temperature adjustments for a majority of faults. This allows for the ability to evaluate condition and provide estimates for time to failure by monitoring changes to the phase to phase unbalances over time.
Table 1: Pass/Fail Considerations for Assembled Machines
When a motor does not have a rotor in place, such as in a motor repair shop with a stator only, the tolerances change:
Table 2: Pass/Fail Criteria for Disassembled Machines
For trending and analysis purposes, MCA is a comparative tool using percent unbalance and difference between tests methods. In the percent unbalance method, the difference between like coils (i.e.: between phases in a three phase motor) is trended over time. This method is best for resistance, impedance and inductance. While resistance values are impacted by temperature, for instance, the relative difference between phases is not. By using the percent unbalance method, the user or software do not have to rely upon performing temperature correction calculations. Impedance and inductance are not significantly impacted by temperature. Therefore, the unbalance method is the most convenient way of detecting faults over time. The difference between tests method is used for phase angle and I/F in which the lowest value for each is subtracted from the highest value for each.
Table 3: Reading Change Table for AC Rotating Equipment
Electrical Signature Analysis
Motor Current Signature Analysis (MCSA) refers to the evaluation of current waveforms only, including the demodulation of the current waveform and FFT analysis. Electrical Signature Analysis (ESA) is the term used for the evaluation of the voltage and current waveforms. This provides an increased advantage to diagnostics as power-related, motor-related and load-related signals can be quickly compared. A key consideration when using ESA is that voltage signatures relate to the upstream of the circuit being tested (towards power generation) and current signatures relate to the downstream of the circuit being tested (towards the motor and load).
ESA uses the machine being tested as a transducer, allowing the user to evaluate the electrical and mechanical condition from the control or switchgear. For accurate analysis, ESA systems rely upon FFT analysis, much the same as vibration analysis.
Table 4: Rotor Analysis
Table 5: Signature Multipliers
Where RS = Running Speed
The pass/fail values of the signatures identified in Table 5 are presently based upon the experience of the user. In the case of motor (downstream) analysis, these values relate to current and for generator (upstream) analysis, these values relate to voltage.
The Fast Fourier Transform (FFT) of both current and voltage signatures are normally calculated in dB instead of linear scale. Analysis of the differences in peaks is determined by comparing the dB value measured down from either the peak current or peak voltage value.
The Combined Use of MCA and ESA
MCA requires that the equipment is de-energized while ESA requires that the equipment is energized. These differences offer the user specific analysis strengths for each technology that support each other.
In the case of MCA, it has specific strengths in the areas of:
Control and other connections
Cable insulation system health to ground and between phases
Stator winding health to ground and between phases and conductors
Air gap issues between the stator and rotating assembly
Rotor winding health: wound, induction or synchronous
This includes the ability to provide early failure detection of insulation degradation.
ESA has specific strengths in the areas of:
Severe insulation breakdown
Loose or open coils or stator
Loose or open rotor or rotor coils
Air gap problems, including static and dynamic eccentricity
Bearings and mechanical condition, including alignment
Attached mechanical systems
When used in combination, the technologies provide some overlapping capabilities, but specifically they provide a complete overview of the system being evaluated, with a high degree of accuracy.
There are two basic types of generator systems. These include turbo-synchronous machines and salient-pole synchronous machines. There are a large variety and variation of each type, so we will cover the basic assembly of both in this paper.
The turbo-synchronous machine is most commonly used in high-speed generators (two and four pole) used for high voltage power generation. The general assembly resembles a three phase induction motor with the following specifics:
The stator (armature) resembles a three phase motor winding. The DC fields of the turbine rotor cut through the conductors and generate power which is supplied to the distribution system from this component.
The turbine rotor (fields) resembles the squirrel-cage rotor of an induction machine. This component carries the DC power from the exciter and is driven by a prime mover such as a jet engine or steam turbine. It tends to be long and narrow for horizontal machines.
The exciter can be separate from the machine, in which brushes supply power to the rotor, or brushless, in which a small DC generator is mounted directly to the turbine rotor shaft. The exciter provides DC power to the turbine rotor.
The salient-pole machine is one of the more common smaller, low voltage, low-speed (1800 RPM or less, 4-pole) generation systems. The distinction is that the rotor contains a series of individual wound-coils which also contain an amortissieur winding, in most cases.
The stator (armature) resembles a three phase motor winding. The DC fields of the salient-pole rotor cut through the conductors and generate power which is supplied to the distribution system from this component.
The salient rotor (fields) incorporate an even number of pole pieces that radiate out from the rotor shaft. These poles carry the DC current that generates the rotating DC fields.
The exciter can be separate from the machine, in which brushes supply power to the rotor, or brushless, in which a small DC generator is mounted directly to the rotor shaft. The exciter provides DC power to the rotor.
Both machines can be evaluated in the same way that you would evaluate electric motors of similar design. In the case of ESA, you would evaluate the signatures using voltage spectra versus current spectra.
Energized testing of generators requires the ability to view the voltage signature of the generator and this requires equipment that can perform Electrical Signature Analysis, not just Motor Current Signature Analysis. The purpose of this first paper has been to provide a review of MCA and ESA, as well as a discussion of the construction of turbine and salient generators. The general analysis of generators is performed in a similar manner of any other AC machine.
Penrose, Howard W. Ph.D., "Electric Motor Diagnostics," MARTS 2004 Proceedings, May, 2004
Penrose, Howard W. Ph.D., "Practical Motor Current Signature Analysis: Taking the Mystery Out of MCSA," ReliabilityWeb.com, December, 2003
Penrose, Howard W. Ph.D., "Estimating Electric Motor Life Using Motor Circuit Analysis," 2003 IEEE Electrical Insulation Conference Proceedings, 2003
Penrose, Howard W. Ph.D., Motor Circuit Analysis: Theory, Application and Energy Analysis, SUCCESS by DESIGN Publishing, 2001
Sarma, Mulukutla S., Electric Machines: Steady-State Theory and Dynamic Performance, PWS Publishing Company, 1996.
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