CONDITION MONITORING SERVICES
CMT Technologies has entered into an agreement with PdMA Corporation for the supply and service of their equipment and for conducting an Electrical Motor Conditioning Monitoring service utilizing their sophisticated equipment.
CMT Technologies make use of the MCE-Max unit which provides us with the ability to do online (dynamic) testing as well as offline (static) testing.
We also provide Thermal Imaging Services. (Thermography)
Predictive Maintenance, Trending and monitoring of equipment.
MCE TEST DESCRIPTIONS (Offline testing)
The MCE Standard Test is a 2 to 3 minute test, which provides a comprehensive view of overall motor and circuit condition. High frequency AC and low voltage DC signals are transmitted through the circuit to collect the parameters utilized for diagnosis. Results are trended, compared to caution and alarm levels, and annotated in red or yellow to alert the user of potential problems.
Initially, lead checks are conducted to ensure accurate readings, preventing rust, dirt or other corrosive contact surfaces from adversely influencing test results. The parameters collected are discussed below:
Resistance to Ground parameters are obtained with either 500 VDC or 1000 VDC in a standard unit. Potentials as low as 100 VDC and as large as 5000 VDC are available with our new AIS (Advanced Insulation System). All readings are temperature corrected and standardized to ensure precise trending of insulation condition.
Capacitance to Ground this parameter capitalizes on the natural capacitance of the motor and circuit to trend dirt and moisture build-up on the insulation. It is utilized in combination of with the resistance to ground measurements to determine overall insulation condition.
Phase Resistance Extremely accurate phase-to-phase resistance values on the micro-ohm level are recorded to determine the quality of connections in the circuit and motor itself. An imbalance figure is then calculated for ease of interpretation and trending. Stator faults, both turn-to-turn and phase-to-phase, may be detected using these parameters.
Phase Inductance Once again, phase-to-phase inductance parameters are collected and an imbalance calculated to help determine rotor, stator and air gap health. Higher or escalating imbalances indicate the presence of an anomaly. Additionally, these parameters can be beneficial in identifying faulty capacitors within the circuit.
POLARIZATION INDEX (PI) AND DIELECTRIC ABSORPTION RATIO (DAR)
Utilizing detailed graphing and complete automation, the PI and DAR are used as advanced methods of evaluating insulation and can be highly effective at detecting the presence of contamination. Caution and alarm levels are set in accordance with IEEE specifications. The tests voltages selected are the same as those available for the Standard Test insulation to ground readings. The DAR applies a steady potential to the circuit for one minute, graphing results every 5 seconds and calculating a ratio of the reading at 60 seconds to that of 30 seconds. The Polarization Index (PI) is conducted in identical fashion, with the ratio calculated using the reading at 10 minutes compared to the one recorded at 1 minute.
ROTOR INFLUENCE CHECK (RIC)
The RIC is a test that examines the relationship between the rotor and stator fields. The rotor is incrementally rotated through one pole face, recording phase inductance at each position. The generated patterns are combined with standard test results to confirm the presence of rotor, stator and air gap problems. The collected parameters are sensitive enough to detect excessive porosity in cast rotors, cracked or broken rotor bars, or other defective conditions.
E MAX TEST DESCRIPTION
ONLINE TESTING (Dynamic)
Demands in today’s industry require motors to run as often as possible. Most proactive companies want to employ predictive maintenance practices on their motors. However, this valuable act can be a double-edged sword-important to do, but often requiring the motor to be shut down for access. Finally, there is monitoring equipment available that gives you the best of both worlds. Emax, from PdMA Corporation, is a portable dynamic tester that evaluates electrical motor condition without shutting down your process.
The power analysis portion of the equipment provides valuable insight into incoming power quality, power circuit condition, stator health and motor efficiency. The three-phase current and voltage signal is captured in less than a minute and stored for diagnostics and trending.
Utilizing a combination of unbalance parameters and harmonic analysis, Emax examines the power signal for indications of defects within the different fault zones. Power quality is diagnosed using total harmonic distortion (THD) and crest factor calculations. Distortion in the signal, often the result of nonlinear loads, produces additional heat within the motor that will reduce life if not acted upon.
The power circuit and stator are evaluated through the use of unbalance parameters and sequence data. Unlike the positive sequence currents that produce useful torque, negative sequence currents are caused by high resistive connections within the motor or circuit and generate heat-producing counter torque. Rising negative sequence currents coupled with a high impedance imbalance is indicative of developing stator faults.
Current analysis is performed through a combination of spectral graphs and software automation. Utilizing this portion of the equipment, the user can identify defects in the rotor and air gap fault zones and record motor start-up:
Current is collected in one of three ways: high resolution, low resolution or eccentricity. Each uses different sampling rates and resolutions to identify anomalies in a specific fault zone. Rotor bar defects and eccentricity, static or dynamic can be detected and trended.
Start-up, arguably the most stressful and informative time in motor operation, is recorded from start to finish. Graphing in-rush current and start-up time is extremely valuable when evaluating motor operation and condition. Changes in the start-up characteristics can be attributed to rotor or stator faults.
Trending data is a critical element in any condition monitoring process. But, spotting motor trends is most effective if the data collected can be organized and viewed as quickly and efficiently as it’s captured. Emax generates valuable reports immediately after testing, enabling you to assess the condition of your motors in the field or trend the data for later use.
In addition, when combined with the MCE, the Emax provides the most comprehensive condition analysis equipment available in a single package and with the professionals at PdMA Corporation behind this product, you can always rely on our expertise and technical support. Call on us for assistance in maximizing your electrical reliability efforts.
Rotor Influence Check (RIC)
What does it tell you?
The Rotor Influence Check (RIC) is a graphical representation of the rotor/stator relationship. By analysing variations in the magnetic flux while rotating the rotor, eccentricity and rotor defects are identified. The RIC can also be used to confirm stator faults.
A motor acts similar to an electromagnet. The rotor acts like the “core” and the stator acts like the windings of the electromagnet. A RIC shows how the rotor’s residual magnetism influences the stator inductance in different positions. As the magnetic field of the rotor interacts with more of the coils in each stator winding, the inductance of that winding changes. This influence causes repeatable patterns of change in the graph of the stator inductance, shown as sinusoidal waveforms.
Why Is This Important?
Broken rotor bars can cause extreme heat and vibration, which can result in winding failure, bearing failure, and loss of torque in a motor. Eccentricity, a non-uniformity of the air gap between the rotor and stator, can cause excessive vibration, which can result in winding and bearing failure.
What Does it Tell You?
The resistance-to-ground (RTG) measurement indicates the cleanliness and health of the insulation system. As the insulation ages, cracks and small holes develop. It also becomes brittle over time, as the wiring expands due to heating and contracts when it cools off. Aging and temperature variations also break down the molecular structure of the insulation.
These factors allow contaminants and moisture, which collect on the surface of the insulation, to penetrate to the conductor. Since current follows the path of least resistance, some of the motor current is diverted from the motor circuit to these alternate paths, and ultimately to ground. As the RTG value decreases over time, capacitance-to-ground often increases, indicating the presence of many current leakage paths to ground and the accumulation of contaminants.
Why is This Important?
A low RTG value indicates that the insulation needs to be cleaned. If the condition causing the low RTG is not corrected and the RTG value continues to drop, the insulation could completely fail and the motor windings could be damaged. This could requite a complete rewind of the stator. If the condition causing the low RTG is corrected, a less expensive motor cleaning or a clean, dip, and bake may suffice.
The capacitance-to-ground (CTG) measurement is indicative of the cleanliness of the windings and cables. As dirt and contaminants build up on windings and cables, CTG values increase. An increasing trend showing rising CTG values indicates that the motor needs to be cleaned.
Why is This Important?
Any two conducting materials called plates, separated from each other by a dielectric material form a capacitor. Dielectric material is anything that is “unable to conduct direct electric current.” A cable or motor winding surrounded by insulation provides one conductor and the dielectric material. The stator core and motor casing iron form the second plate.
Normally, when the outside of the insulation is clean and dry, it is not a good conductor. When dirt, moisture, and other contaminates begin to cover the stator windings inside the motor, they cause the outer insulation surface areas to become conductive. Since this surface is in contact with the ground, it allows an AC current path to ground. Cables in the power circuit are also subjected to the same affect, when moisture penetrates the outer casing.
With a build-up of material on them, dirty windings and cables produce higher capacitance values than clean ones do. Over time, CTG values steadily increasing indicate an accumulation of dirt and that cleaning is necessary. This can be correlated with decreasing resistance-to-ground (RTG) values.
Dirt and contamination also reduce a motor’s ability to dissipate the heat generated by its operation, resulting in premature aging. A general rule of thumb is that motor life decrease by 50% for every 10 °C (50 °F) increase in operating temperature above the design temperature of the insulation system. This holds true with the motor operating at or above a 75% load. Heat raises the resistance of conductor materials and breaks down the insulation, providing paths for unwanted current to flow to ground. If capacitance is higher than normal, a low RTG reading is an indication that such a path already exists.
What Does it Tell You?
Phase-to-phase resistance is the measured DC resistance between phases of the stator in an AC motor and between polarities of the armature and field coils in a DC motor.
In AC induction motors, use the phase-to-phase resistance values and resistive imbalances for trending, troubleshooting, and quality control. In DC motors, use trending and relative comparison to determine the condition of the phases in the motor and power circuits. This includes comparing readings taken from identical motors operating in similar conditions and comparing current readings against past readings for the same motor.
An increasing resistive imbalance or a changing resistance over time can indicate one or more of the following:
High resistance connections
Coil-to-coil, phase-to-phase, or turn-to-turn current leakage paths
Corroded terminals or connections
Loose cable terminals or bus bar connections
Poor crimps or bad soldier joints
Loose, dirty, or corroded fuse clips or manual disconnect switches
Mismatched components (incompatible materials, wrong sizes, etc.)
Undersized conductors (misassemble or improperly engineered)
Why is This Important?
The length, size, width, composition, condition, type and temperature of the conductors and connectors determine circuit resistance. When two different conductors are connected, dirt, corrosion or an improper connection increases the circuit resistance. Also, inadequate connections cause heating of the conductor, which increases resistance even more. This could be caused if only a few strands of a conductor or portions of a soldered joint are improperly connected to a terminal or if undersized connectors are used.
In a three-phase motor circuit, the resistance in the conductor paths should be as close to equal as possible. A “resistive imbalance” occurs when the phases have unequal resistance. This produces uneven current flow and excessive heat.
What Does it Tell you?
In AC motors, phase-to-phase inductance readings can:
Indicate the condition of the stator windings
Detect phase-to-phase and coil-to-coil current leakage paths
Reveal poor or incorrect rework
These readings can also be used to detect faults in power cables and main contacts in the power circuit. A rotor-influence-check (RIC) can be performed to further troubleshoot the motor to reveal faults such as:
Broken/cracked rotor bars or end rings
Porosity and lamination damage
In DC motors, inductance changes within the field or armature can indicate current leakage paths in the windings.
Inductance changes when leakage paths develop. These paths can be either within the winding coils, or directly to ground. Leakage paths result from mechanical, thermal, environmental, or electrical damage to the insulation system of the windings. Additionally phase-to-phase and turn-to-turn shorts can occur. In either case, current flow bypasses some coils, thereby reducing inductive reactance and increasing current in other phases of the stator. Temperature rises in the remaining conductors and in the surrounding insulation. This accelerates the deterioration, which can cause an avalanche effect, as heat produces more insulation failures, resulting in more leakage paths and more coils removed from the circuit, further increasing temperature.
As there are fewer winding turns in a given phase actively creating the magnetic field upon which the motor is functioning, the windings in the other phases compensate to meet the requirements of the load on the motor. These windings in turn draw more current than is normally supplied by a balanced motor.
Why is This Important?
A large inductive imbalance causes torque-induced vibration can be linked to mechanical degradation. Also, inductive imbalance can contribute to other problems, among which are:
Loosened rotor bars
Insulation failure at winding end turns or at exit of stator slots
POLARIZATION INDEX AND DIELECTRIC ABSORPTION TEST
What Do They Tell You?
The Polarization index (PI) and Dielectric Absorption (DA) ratios indicate the condition of the insulation system of the motor and power circuit. Both of these tests use ratios of measurements of insulation resistance taken at two different times. The PI is the ratio of the reading taken at 10 minutes and divided by the reading taken at 1 minute. The DA is the ratio of the reading taken at 60 seconds divided by the reading taken at 30 seconds.
There are three different currents that flow through an insulator when a voltage potential is applied. Since the RTG test measures the voltage and current to calculate insulation resistance, all of these currents must be taken into account.
First, the “charging current “ starts out high and drops to nearly zero after the insulation has been charged to full test voltage. This is normally negligible after the first few seconds of the test.
Second, the “absorption current” also starts out high and drops off. The majority of this current dissipated after one minute, but continues to decay for up to 5 to 10 minutes.
Finally The “conduction” or ”leakage current” is a small, mostly steady current which becomes a factor after the absorption current drops to a negligible value. This current should remain steady for the remainder of the test.
As the motor accumulates dirt and as the insulation ages and cracks, the PI and DA ratios decrease. Dirt accumulates based on the operation and environment of the motor. The insulation cracks as a function of heat and aging of the motor.
Because of the effects of each if these varying currents, the resistance to ground measured by any insulation tester varies with the amount of time the voltage is applied to the insulation. In order to trend or compare insulation RTG values, the charge time for all tests MUST be the same. If the charge time is not the same, the trend or comparison may not be valid.
Finally, the charging developed by these three different currents does not dissipate immediately when the voltage is removed at the end of the test. The insulation system must be allowed to discharge sufficiently between resistance to ground tests in order to obtain accurate results. A rule of thumb states that a motor takes four times the amount of charge time to discharge.
Why Is This Important?
Resistance-to-ground readings involve three different current components: charging, absorption, and leakage. The PI test allows the charging and absorption currents to decay so that only actual leakage current is measured. As a voltage is continuously applied, healthy insulation slowly polarizes and the absorption current diminishes. This causes a steady rise in resistance until the majority of the current is from the small amount leaking to ground. In poor insulation, leakage current is high enough to overshadow the lowering absorption current and provide little increase in the resistance over time.
DC COMMUTATOR BAR-TO-BAR TEST
What Does It Tell You?
Testing the resistance between commutator bars gives an indication of the comparative value of resistance that exists between all like electrical circuits in an armature.
Why Is This Important?
The commutator consists of insulated segments assembled into a cylinder and held together by insulated rings. Electric current is transferred to the armature winding by “brushes” made mainly of carbon and graphite. Brush wear creates carbon dust, a conductive contaminant, which penetrates into crevices, cracks and openings of the armature. Copper particles add to the contaminant accumulation when the wrong brushes are installed or the brushes are improperly installed, or when maintenance is inadequate. If the insulating material on the commutator bars or their risers has cracked, these contaminants can short entire windings.
Also, high resistance connections can develop at the risers causing open or high-resistance armature coils. Equalizing connections can break and cause an imbalance due to the loss of equalization.
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