How to Test and Assess Stator Core Condition Using a Loop

How to Test and Assess Stator Core Condition Using a Loop Test

April 22, 2020

EASA © 2020 1

Carlos Ramirez - Technical Support SpecialistElectrical Apparatus Service Association, Inc.

+1 314 993 2220 | [email protected]


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April 22, 2020

EASA © 2020 2

Core Testing Introduction• Countries globally are working to increase

energy system efficiency• Electric motors are some of the largest

consumers of energy• Electric motors are used in industrial,

commercial, transportation and agricultural sectors

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Repair/Rewinding and Efficiency

• In 2003, EASA and AEMT conducted a study and published The Effect of Repair/Rewinding on Motor Efficency and Good Practice Guide• This study was produced in response to

the need of increased efficiency globally• EASA is working on an update of this

topic

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• Study is available in https://easa.com/

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EASA/AEMT study:• Shows that the efficiency of motors

repaired/rewound using good practices is not degraded, and in some cases can be improved

• Provides “Recommended Practices” and considerations for repairing motors excluding hazardous location and explosion-proofapplications

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https://easa.com/


April 22, 2020

EASA © 2020 4

Efficiency

• Efficiency of any machine can be expressed as Output power/Input power

• Motor output power is the power delivered in the shaft (T x RPM)

• Input power is expressed in watts or kW and always exceeds the output power

• In order to express the two powers in same units, a conversion factor is used: kW = hp*0.746 (NEMA Motors)

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Efficiency Cont.

• In service, simply dividing the rated output power by the input power is not exact in determining efficiency as motor loadingconstantly changes

• Precise and accurate measurement of input and output power with calibrated equipmentis required to measure motor efficiency

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Energy Losses in Induction Motors

• They are 5 types of losses in an induction motor: - Stator I2R loss- Rotor I2R loss- Stray loss- Core loss- Friction + windage loss

• Stator I2R loss , Rotor I2R loss and Stray loss aredirectly dependent and vary closely with the square of the load, or with the load

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Energy Losses in Induction Motors Cont.

• Table below shows estimated losses of typical medium 4- pole three-phase motors

Loss type % Total LossesStator I2R loss 35

Rotor I2R loss 25

Core loss 20

Stray loss 10

Friction + Windage 10

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Stator Core• Normally manufactured with large number of

laminations insulated from each other• Materials: Low-carbon, silicon-iron, or silicon-

aluminum iron

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Core Loss

• Depends on ferromagnetic properties of materials: – Lamination quality– Grade– Thickness– Processing

• Insulating coatings for electrical steels are specified in ASTM A976-13

• Core loss for different lamination thickness are specified in ASTM A677-12

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Interlaminar Insulation Coatings

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Electrical Steel Core Loss Properties

Thickness Inches (mm) Max Core Loss at 1.5 T and 60 Hz W/lb (W/kg)

0.0140 (0.36) 1.45 (3.20) to 2.05 (4.52)0.0185 (0.47) 1.65 (3.64) to 4.50 (9.92)0.0250 (0.64) 2.00 (4.41) to 5.50 (12.13)

Ref. ASTM A677-12

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Hysteresis Loop

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Eddy Currents

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Effect of Shorted Laminations

• Eddy-current losses vary as the square of the lamination thickness– Two shorted laminations = 4 times the heat in that area

• 5 shorted laminations = 25 times the heat

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Eddy-Current Losses

• Flux density, frequency and lamination thickness are key contributors to eddy current losses

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EASA © 2020 10

Loop Test Background

1952-AIEE paper: Interlaminar Test For Synchronous Machine Stator by H.R. Tomlinson

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Stator Core Loop Test - Why?

• Verify that the stator core is acceptable for continued use

• Verify that the repair process has not degraded the stator core plate condition

• Test can be performed using a commercialcore loss tester or manually using an AC source, cables and calibrated meters

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April 22, 2020

EASA © 2020 11

Stator Core Loop Test - EASA

• If a commercial core loss tester is not available, or the test cannot be carried out using such equipment, EASA Technical Manual provides a step-by-step procedure

• To avoid manual calculations, EASA also has included a Loop Test Calculator on its website

• It is advisable to verify the actual back-iron magnetic flux density using a search coil

• Most often the test is performed at 1.32 T(85,000 lines/in2)

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Loop Test Calculator - EASA

https://easa.com/resources/software/loop-test-calculator

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https://easa.com/signin%3Freturnurl=%252fresources%252fsoftware%252floop-test-calculator

https://easa.com/resources/software/loop-test-calculator

https://easa.com/signin%3Freturnurl=%252fresources%252fsoftware%252floop-test-calculator


April 22, 2020

EASA © 2020 12

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Stator Core Loop Test - Dimension and Calculation

Measure and record (inches – mm):- L = Length less air ducts- D1 = Inside diameter- S = Slot Depth- B = Back iron depth- Calculate mean diameter of core back iron D = D1 + (2 x S) + B- Establish test voltage and frequency - Calculate loop turns and round off as necessary; if you need

to round off the turns more than 3/10, adjust the voltageinstead

- Estimate the amperes and cable ampacity- Calculate back iron weight and watts/pound or watts/kg - Calculate the induced voltage in search coil

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Stator Core Loop Test

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Stator Core Loop Test - Procedure

• Select single-phase power supply if posible, with variable voltage and same frequency as the motor rating (Ampacity higher than estimated amperes)

• Also required: Ammeter, voltmeter, wattmeter, thermometers, pyrometer, infrared thermal scanner or similar

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April 22, 2020

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Stator Core Loop Test – Cont. • Wind the loop and

search coil compactlythrough stator frame; use a larger cable ifpossible

• Connect the wattmeteras close to the loopturns to avoid influenceof cable´s watt loss and use an oversized cable to reduce cable losses

• Record ambienttemperature and applypower to the loop coil

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Stator Core Loop Test – Cont. • Record meter readings

at the begining of thetest

• If current is other thanestimated, do notadjust voltage or turns; variation is expecteddue to magnetizingcharacteristics or coredamage

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• If measured current is higher than cable ampacity, replace the cable with a larger cable

• If actual induced voltage varies more than+/- 5% of calculated value, remeasure the dimensions, especially backiron size and verify applied voltage oradjust the voltage and current– Low: Increase voltage or remove turns– High: Reduce voltage or add turns

• Calculate watts/pound or watts/kg (represent back-iron loss with error dependant of test cable losses)

• The larger the cable, the greater the portion of calculated watt losses that can be attributed to thecore

Stator Core Loop test – Cont.

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Temperature Reading• Record ambient temperature at start of the

test• Core will start to warm up and if laminations

are in good condition, the heating will be uniform and the temperature will rise no more than 5-10°C during 30 minutes

• Heating rate will vary with the size of backiron• Core areas with damage will be detected as

hot spots

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Temperature Reading - Hot SpotsHot Spot: Any area with a temperature 15°C above the ambient temperature.

Damaged spots heat up much faster than healthy areas

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Temperature Reading Cont.

• Hot spots closer to the surface can be detected after a few minutes, but in large 2-pole machines this can take 20 minutes or more

• During the test, frequent measurement of temperatureas necessary for a quick detection of hot spots

• Once the hot spots have been detected, end the test toavoid further damage and mark the damaged areas

• Record the core’s temperature at the start and the end of the test

• Calculate and record the core temperature rise(ending temperature minus starting temperature)

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Assessment Criteria - Core in Good ConditionCore will likely be in good condition if the test results are as follows: • Core temperature rise limit is ≤ 15°C• Localized heat is not prevalent• Watts/pound or Kg are favorable when

compared to published data or measurements taken in similar cores

• In a good core, the losses will vary from 1 to 5 watts/lb (2-11 watts/kg) depending on lamination material, grade, thickness and processing

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Assessment Criteria - Core Damaged Repair or replace the stator core if you detect any of these conditions:• The temperature of the core is high or increases

rapidly• Localized heating (hot spots) is found• Watts/pound (watts/kg) is high when compared

with reference dataNote: Watts/lb (watts/kg) represents only the back iron losses

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Loop Test in Stators That Are To Be Rewound• Test before and after burning out the stator• Windings should be burned out and stripped

properly• Both tests should be done under the same

conditions using the same voltage, frequencyand turns using same cable and samedimensions

• If W2 is below the maximum acceptable level and W2/W1 ratio does not exceed 1.2, the core is in good condition; a ratio > 1.2 means the core should be repaired or replaced

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Guidelines for Obtaining Reliable Results

• Use RMS values and calibrated meters for the test

• Connect the wattmeter as close to the loopturns as possible

• Use the same voltage for the before and after calculations

• Start the test with the cable and stator core at ambient temperature and record volts, amps, induced voltage and temperature

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Guidelines for Obtaining Reliable Results• If the measured current deviates from the

calculated current, do not adjust the voltage or turns to match it

• Make sure both the before and after tests are done under same conditions; use the same voltage, turns and same dimensions

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Safety Rules • Do not touch the loop coil or core surface while

the circuit is energized• Do not use shielded cable as voltage will be

induced into the cable shield• During the test, do not leave on place any

metallic objects in the stator bore

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April 22, 2020

EASA © 2020 20

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April 22, 2020

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Survey / Recording

• Please take time to fill out the brief survey that will appear in your browser window at the conclusion of this presentation

• A link to a recording of this presentation will be emailed to you tomorrow. This recording will be available for 30 days from today

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Thank [email protected]

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Documents

How to Test and Assess Stator Core Condition Using a Loop