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The EXP3000/EXP3000R utilizes a multitude of tests to determine the power condition, health, load, and energy profile of electric motors.  The following test domains describe the functions of each test.

The EXP3000/EXP3000R performs four primary functions:

• Identifies possible energy savings.
• It recognizes all power circuit problems that degrade motor health.
• It examines the motor’s overall power condition.
• It monitors the load of the motor.
• It observes motor performance.

Power Quality - Voltage Level, Voltage Balance and Distortion tests

The Voltage Level Test:

• Identifies over and under voltage conditions.
• Compares the measured voltage levels with user defined thresholds.

The voltage level test informs of excessive over or under voltage conditions.

The Voltage Unbalance Test:

• Examines the single-phase voltage in the motor by calculating its percentage unbalance via NEMA derating.
• Assesses the voltage unbalance level compared with the stored threshold.

A nonbalanced voltage condition causes negative sequence currents within the stator, resulting in excessive heat.  The voltage balance test determines if a nonbalanced voltage condition exists in the motor. Thus, NEMA utilizes a derating curve that specifies a maximal load for each type of unbalance.

The Distortion Tests:

• Examines the Total Harmonic Distortion of the 3 single phases to neutral voltages.
• Assesses the level of Total Harmonic Distortion and compares it to the stored threshold.

The distortion test examines the conditions that cause excess heat in motors, i.e. strong voltage distortion.

Fig 2. Typical equivalent circuit of a three-phase induction motor

Rs = stator resistance
Ls = stator inductance
Lr = rotor inductance
Rr = rotor resistance
Lm = magnetizing inductance
R­load = equivalent resistance which’ dissipated power models the load

I2R losses occur when current runs through the Rs and Rr resistances.  The Rload resistance represents the power the motor delivers mechanically to the load.  Rload is a variable resistance and is sized to represent the output power of the motor.

In Fig. 2, Rload is not a component of the equivalent circuit for harmonic frequencies, since harmonics do not aid torque production.  The harmonic components of the stator voltages in Fig. 3 are faced with less equivalent impedance.  This translates into relatively large harmonic current components running through the stator and rotor resistances, which creates additional heat.
The voltage distortion test utilizes a derating factor in NEMA MG 1-30.01.1.

Power

The Power window pulls all the main information together which defines the power related qualities. It displays kW, kVA and kVAr as well as voltages, currents, power factor, total harmonic distortions for votlages and currents, plus the crest factor for voltages and currents. Additionally, the power details contain the NEMA derating percentages for the current and the voltage.

Harmonics

The Harmonics components investigate the effect of the non-sinusoidal components to currents and voltages of the system. It displays the most common measures, THD, and HVF [1,a-f]. Bar charts for all currents and voltages display the distribution of the harmonic content on the different frequency bands.

Machine Performance Tests - Effective Service Factor, Load, Operating Condition, Efficiency, Payback Period

• Divides the estimated percentage load into the NEMA derating factor.
• Identifies any thermal overloading in your motor.

The service factor test, overcurrent test, and efficiency test observes motors performance and helps establish money savings. It also identifies how close the motor is operating to its effective service factor. The ESF test predicts heat-based deterioration and provides an accurate thermal assessement of the motor.

Power conditions may place unnecessary stress on a motor. The instrument utilizes two NEMA derating factors: Harmonic content and Voltage unbalance. This NEMA derating number describes the maximum load at which a motor can run without a combination of load and sub-optimal power conditions. Not complying will cause excessive thermal stress.

Two elements effecting the ESF number: real operating voltage condition and steady state load conditions. The ESF number represents the thermal stress caused by these two conditions on the motor.

• Compares your motor's load level with the load level found on previous measurements.

For motors that operate at constant loads, the load history test warns if any changes occur in the operating load. It displays the current load level of the motor along with the loads measured on previous tests. This allows the operator to assess whether the load has changed from previous tests.

The Results of this test are useful in context with the plant operation. If the load has not changed due to any modifications in the process, than a variation in the load profile is significant.

Each induction motor has a torque-speed and current-speed characteristic operation curve. These curves will vary their signature if an induction motor's operation changes from healthy to a faulty condition. For example, increase operating temperture, fluctuating environmental conditions, varied power supply conditions, or broken rotor cages can alter a motor's operating condition.

Differences in the operating condition could indicate:

• A change in the operating process
• A condition that may influence the motor's operation

A resultant warning does not imply a defect in the motor, load, or power supply. However, it is important to monitor the motor's operating condition. Any identifiable changes could affect the future operation of the motor.

• Examines the efficiency of the motor under its particular operation conditions.
• Compares the performance of the motor with similar motor designs in a vast database.
• Identifies money savings.
• Aids in deciding whether the motor should be replaced.

Where:

Ploss = the power that dissipates, mainly heat
Pin = input power
Ploss can be seen as the energy in the power bill that is potentially wasted and degrades motor health.

The efficiency test displays the motor's operating efficiency and previous measured motor efficiencies.

Low results in the efficiency test suggest that motor retrofits may be advisable. A decline in efficiency may indicate an increase in the motor's operating temperature, causing faster motor degradation.

A manufacturer database with more than 20,000 different motor designs is scanned, refined, if possible motors of a similar design is able to perform at efficiencies to EPAct' 92. Such motors are then evaluated with respect to the currently operating load point, and the resulting efficiencies compared. If the efficiency of the motor under test is significantly lower than the target efficiency found on a EPAct motor, a warning or caution flag is issued.

• Searches for a list of a comparable motor in the database.
• Calculates possible payback period if replacing this mtoor with a higher efficiency design.

• Inspects the single-phase current by comparing it with the nameplate data of the stator current.

Too much current can overstress particular phases in the motor. The overcurrent test determines if the motor might be draawing more than its rated current on one or more phases, causing excessive heat in the motor and decreasing the insulation life.

The nameplate information for every motor includes current data. The overcurrent test compares the highest phase's current with the rated nameplate current and identifies any overheating for that particular winding.

A typical thermal assessment for a winding examines a particular phase's current. The current generates I2R losses, thus creating heat. Although not the entire cause of heat in the motor, I2R losses are usually the main contributor of excessive heat.

NEMA defines current unbalance according to the following equation:

Where IA, IB , and IC are the RMS values of the currents of the 3 phases, and IAvg._RMS is their average. Unbalanced currents are frequently caused by mildly unbalanced voltages. A common rule of thumb is that voltage unbalance can be the cause of up to eight times larger current unbalances. Machines will also show very large current unbalances under very light, or no load conditions – even when driven by a balanced voltage. These current unbalances are common in healthy machines. These current unbalances vanish rapidly when the machine is loaded.

Another way to analyze unbalances is via the negative to positive sequence ratio. This is the preferred method of analysis in the IEC standards.

Kirchhoff’s Current Law (KCL) asserts that:
 
The fundamentals of this test can be explained with the following figure:

KCL must hold in all cases, leading to:
 
Hence, theoretically, adding all 3 stator current together would deliver a ground-current monitor technique. 

This approach, however, is very susceptible to relatively mild error sources in connections, hardware and calibration related errors:

• Most errors in connecting the CTs (including phasing and “pinching wire” types of mistakes) will typically result in a very large Sum of Currentsresult, without meaning that there is any ground current flowing (false positives).
• CTs out of calibration, or any hardware malfunction during the data-gathering phase will also cause erroneous assessments (false positives).

Spectrum Domain - Rotor Bar, V/I Spectrum, Demodulated Spectrum, Harmonics

• Inspects the relative amplitude of the ‘rotor-bar sideband’.
• Assesses the rotor cage signature in relation to stored thresholds.

The rotor bar test, and operation condition test evaluate the overall condition of your motor. Broken rotor bars cause excess heat on the motor, decrease efficiency, shorten insulation life, and possibly cause core damage.

The Rotor Bar test identifies salient frequency components, which are generated by a degrading rotor cage in the current spectrum.  It also compares the salient frequencies to established thresholds. The particular frequency of interest, which the rotor bar test evaluates, is the following:

where frotor is the frequency of the broken rotor current, fnet is the frequency of the fundamental, and s is the slip, which is defined as

In this equation, speedsynch is the synchronous speed, while speedoperating is the operating speed of the motor.

Induction motors include rotor cages.  The entire motor can be viewed as a rotating transformer.  Equivalent turn ratios of this transformer cause large currents to flow through the rotor bars.
With induction motor startup, the rotor and rotor bars undergo extensive electrical stresses.  These strong rotor bar currents generate sizable oscillating forces when they interact with the magnetic field.

Startup forces can cause ruptures in the rotor bar, particularly if the rotor die casting process caused small imperfections in the rotor.  Rotor bar failure can also occur if a motor operates at extremely high loads.

Current distribution will deviate from optimal operating conditions if the rotor cage has a broken or severely cracked rotor bars or endring.  Adjacent bars to the damaged section will display larger current amplitudes than when the rotor cage was not damaged.

If the rotor has faults, the torque delivered to the load will pulsate.  The speed of the motor will slightly decrease while the operating temperature of the rotor and stator increase.  This will cause further deterioration of the motor.  This deterioration will increase the probability of motor vulnerability to further defects.

With a damaged rotor cage, the motor’s stator currents create several additional frequency components.  Rotor cage health can be gauged according to these components’ relative size.
Cases in which the EXP3000/EXP3000R does not assess the rotor bar test with pass/fail criteria are:

• Load level is too low.  If the load estimated is below 30%, rotor bar test will be blue.
• Not constant frequency during acquisition.  If the fundamental frequency changed during acquisition, then the rotor bar will remain blue.
• Low signal to noise ratio.  If the noise floor of the current versus frequency plot is too high, the EXP3000/EXP3000R will display a blue result on the rotor bar test.
  
  

The V/I Spectrum window enables to analyze the frequency spectra of the 3 line-neutral voltage waveforms and of the 3 line currents independently of each other. The current spectra have shown to contain information related to the vibration spectra of the machine. It is possible to identify roller-bearing faults by using the “Mark Frequencies” option using the right click menu. It is also feasible to find deteriorating alignment problems, load unbalances, looseness, eccentricity, and cavitation by analyzing these spectra, as has been shown in research work published in IEEE among others, by the following research teams:

T. Habetler, R. Harley          Georgia Tech University, USA
H. Tolyiat                            University of Texas A&M, USA
S. Nandi                              University of Victoria, Canada
A. Wallace, A. von Jouanne    Oregon State University

Note: It has been shown that is possible to identify the above mentioned faults using the current spectrum. However, the location in the spectrum that display the signatures of any particular fault differ noticeably from the locations where vibration experts suspect particular fault types. Experience has shown that it is significantly easier to diagnose mechanical faults using the torque spectrum.  It also needs to be noted that the torque spectrum’s noise floor is typically almost one decade lower when compared to the signatures, than where it is encountered in the current spectrum.

In the Demodulated Spectra, EXP3000/EXP3000R proceeds to calculate the 3D – demodulated spectrum of the torque signature. With the Channel control, the demodulated signal can be changed from torque to current or voltage of any one of the phases. The demodulated spectra tool analyzes the dynamic behavior profile of the motor load system.

The Harmonics components investigate the effect of the non-sinusoidal components to currents and voltages of the system. It displays the most common measures, THD and HVF [1, a-f]. Bar charts for all currents and voltages display the distribution of the harmonic content on the different frequency bands.

Torque Domain - Torque, Torque Ripple

• Warns you if the motor’s load requires oscillating, or varying torque.

The torque ripple test and load history tests examine your motor’s load profile.
The torque ripple test identifies the ripple that the load is requesting, resulting in a percent of a ripple to steady state.

The Torque Ripple Test shows the load stress imposed on the motor.  RMS quantities are an accurate measure of the average energy contained in the instantaneously changing current or voltage signal.  If only RMS quantities are monitored, it is impossible to find peak stresses in the motor’s load.

Instantaneous waveform analysis monitors the time-varying or instantaneous variations in the load.  Thus, the operator can localize peak pulses above the motor’s nameplate information, even if the average power delivered to the load does not exceed the nameplate data.
A significant portion of a motor’s performance can consist of transient load fluctuations.  Copper losses and their associated heating are proportional to the square of the currents, but not proportional to the currents.  Thus, the transient load fluctuations are just as significant as the average value of the operating torque. 

The torque ripple test examines the torque propelling the load by focusing on the interface between the motor and load.

Any changes in the load’s transient component should prompt further investigation.  For example, the influence of broken fan blades or worn roller bearings on a conveyor belt can be seen on the instantaneous torque display.

Torque ripple is the percentage of the transient torque to the steady state in percent:
%Torque ripple = 100*(Peak Torque- Steady State Torque) / Steady State Torque
The Torque Ripple test plots the transient behavior of the torque as a function of time.  It also calculates the % torque ripple of the acquired data.

Connections - Waveforms, Symmetrical Components, Phasors

This window shows the waveforms for all three currents and all three voltages for line operated mode.  If the EXP3000/EXP3000R is operated in VFD mode, it shows the voltage and current for the phase a.

The Symmetrical Components investigates the voltage, current and impedance unbalance regarding their effect from a positive sequence (accelerating) and negative sequence (retarding) effect to the shaft. Current, voltage and impedance information is handed in phasor format.

The details phasors window shows the a,b,c phasors for voltages and currents in line operated mode.  In VFD operated mode, it shows the instantaneous current phasor vs. the instantaneous voltage phasor.

The VFD Details window can only be activated, if the measurements were taken in VFD mode. This window displays the dynamic behavior of voltage level, torque, frequency and speed as a function of time.

Vibration Level Testing - Haystack, Operating Condition, Spectra, Motor Master+, Operating Point

This section only applies, if the Vibration option has been purchased. Many factors can cause excessive vibration, including soft foot, increasing eccentricities, and broken rotor bars. NEMA MG-1, 7.08.3 specifies permissible limits of vibration as a function of frequency.

• Displays the domain of vibration against frequency.
• Investigates whether any frequency component of the vibration spectrum trespasses the preset threshold.

The instrument calculates three major quantities for the mechanical behavior of the motor: displacement, velocity, and acceleration. Displacement measures low frequency vibration. Velocity resolves medium frequencies, and acceleration measures high frequencies.

The Vibration test uses NEMA graph MG-1 7.08.3 fiqure 1 to access the magnitude of certain vibration frequency components. This graph resembles a haystack. The bration test maintains the "haystack" shape of the thresholds while the user can specify its levels.

If any frequency component in the measured spectrum surpasses the specified thresholds for a particular frequency, then the test issues a caution or warning depending on the severity of the fault.

Each motor load application has a unique vibration signature, which depends on the mechanical setup of the system and the load currently operating. A change of the vibration signature could indicate one of two thing:

1. A change in the operating process;
2. A condition that may reflect a motor or load health related problem.

A resultant warning does not apply a defect in the motor, load, or power supply. However, it is important to monitor the motor's operating condition. Any identifiable changes could affect operation of the motor.

Spectra show a window that plots the frequency spectra of either the voltage, or current, for the phase A, B, or C, or the torque. It allows marking of frequencies, and analyze frequency contents of the obtained signatures.

• Allows the identification of inefficiently operating motors.
• Can calculate wasted energy and money savings.
• Assesses pay back periods for replacing motors with more energy efficient equipment.

• Compares the torque, speed, and average stator current values of motors against previously stored values.
• Alerts if any of the values deviated from previous operation conditions.

Stator current, torque,a nd operting speed typically describe the health of a motor. For example, maintenance personnel should be alerted if a motor must run at a lower speed in order to provide the same torque utilized onprevious conditions. This scenario could be caused by broken rotor bars, excessive heat, or different voltage conditions.