Testing of Rotating AC

machines - Part II

Compilation By: Prof. S. N. Jani

1

Induction Motor Testing

• All induction motors are tested before

shipment from the factory.

• This testing can be subdivided in two

groups:

1. Routine tests

2. Complete or prototype tests

• IEEE Std 112–1996 applies to induction

motor testing.

2

1. Routine tests

• The primary purpose of the routine test is to

insure freedom from electrical and

mechanical defects, and to demonstrate by

means of key tests the similarity of the motor

to a “standard” motor of the same design.

• The “standard” motor is an imaginary motor

whose performance characteristics would

agree exactly with the expected performance

predictions.

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• Depending on the size of the motor, some or allof the following tests could constitute routinetests:

4

1.Winding resistancemeasurement

2. No-load runningcurrent and power

3. High-potential test4. Locked-rotor test5. Air-gap measurement6. Direction of rotation

and phase sequence7. Current balance

8. Insulation resistance measurement

9. Bearing temperature rise

10. Magnetic center at no-load

11. Shaft voltages12. Noise13. Vibration

2. Prototype tests• The purpose of a prototype test is to

evaluate all the performancecharacteristics of the motor.

• This test consists of the following tests inaddition to the routine tests:

1. No-load saturation characteristic2. Locked rotor saturation characteristic3. Locked rotor torque and current4. Loss measurement including stray load loss5. Determination or measurement of

efficiency6. Temperature rise determination7. Surge withstand test

5

Insulation Resistance test

• During general maintenance work and before

the machine is started up for the first time or

after a long period of standstill, the insulation

resistance of stator and rotor windings must

be measured.

• The insulation resistance measurement

provides information about the humidity and

dirtiness of the insulation.

• Based upon this information, correct cleaning

and drying actions can be determined.

• For new machines with dry windings, the

insulation resistance is very high.

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• The resistance can, however, be extremely

low if the machine has been subjected to

incorrect transportation, storage

conditions and humidity, or if the

machine is operated incorrectly.

• NOTE: Windings should be earthed briefly

immediately after measurement in order

to avoid risk of electric shock.

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The Basics of Insulation Resistance Testing

• How significant is insulation resistance testing?

• Since 80% of electrical maintenance and testing involves

evaluating insulation integrity, the answer is "very

important."

• Electrical insulation starts to age as soon as it's made.

• And, aging deteriorates its performance.

• Harsh installation environments, especially those with

temperature extremes and/or chemical contamination,

cause further deterioration.

• As a result, personnel safety and power reliability can suffer.

• Obviously, it's important to identify this deterioration as

quickly as possible so you can take the necessary corrective

measures.

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• Correlation between the insulation

resistance and the temperature:

• R = Insulation resistance value at a

specific temperature

• R40 = Equivalent insulation resistance at

40°C

• R40 = k x R

• Example:

• R = 30 M Ω measured at 20°C

• k = 0.25

• R40 = 0.25 x 30 MΩ = 7.5 M Ω

9

Minimum values for insulation resistance

• Criteria for windings in a normal condition:

• Generally, the insulation resistance values for dry

windings should exceed the minimum values

significantly.

• Definite values are impossible to give, because

resistance varies depending on the machine type

and local conditions.

• In addition, the insulation resistance is affected

by the age and usage of the machine.

• Therefore, the following values can only be

considered as guidelines.

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• The insulation resistance limits, which are givenbelow, are valid at 40 °C, and when the testvoltage has been applied for 1 minute orlonger.

Rotor• For induction machines with wound rotors:

R (1-10 min at 40 °C) > 5 MΩ• NOTE: Carbon dust on slip rings and uncovered

copper surfaces lower the insulation resistancevalues of the rotor.

Stator• For new stators:• R(1-10 min at 40 °C) > 1000 M Ω. If the

measuring conditions are extremely warm andhumid, R(1-10 min at 40 °C) values above 100M Ω can be accepted

• For used stators:• R(1-10 min at 40 °C) > 100 M Ω

11

Stator winding insulation resistance

measurement• The insulation resistance is measured using an

insulation resistance meter.• The test voltage is 1000 V DC (Usually).• The test time is 1 minute, after which the

insulation resistance value is recorded.• Before the insulation resistance test is

conducted, the following actions must betaken:

1. Check that the secondary connections of thecurrent transformers (CT's), including sparecores are not open.

2. Verify that all power supply cables aredisconnected.

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3. Verify that the frame of the machine and

the stator windings not being tested are

earthed.

4. The winding temperature is measured.

5. All resistance temperature detectors are

earthed, Possible earthing of voltage

transformers (not common) must be

removed.

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14

Figure 1: Connections of the stator windings forinsulation resistance measurementsa) Insulation resistance measurement for starconnected windingb) Insulation resistance measurement for deltaconnected windingc) Insulation resistance measurement for onephase of the winding. The 'MΩ' represents theinsulation resistance meter.

Rotor winding insulation resistance

measurement

• The insulation resistance of the rotor windingis measured with an insulation resistancemeter.

• The test voltage of the rotor windings shouldbe 1000 V DC.

• Required notes and measures:

1. Verify that all power supply cables aredisconnected from the main supply.

2. Verify the slip ring unit connection cables aredisconnected from their supply.

3. Verify that the frame of the machine and thestator windings are earthed.

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4. The shaft is earthed

5. The rotor winding phases not been tested

are earthed. The rotor winding can be

internally connected in a delta or star

connection. If this is the case, it is not

possible to measure the phases individually.

6. The carbon brush connections are checked

to be in good order.

7. The measurement device is checked.

8. The stator winding temperatures are

measured, and considered as a reference

value for the rotor winding temperature.

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• The insulation resistance meter is connectedbetween the whole rotor winding and the shaftof the machine, see Figure 2 Insulationresistance measurement of the rotor winding.

• After performed rotor winding measurements,the rotor winding phases must be brieflyearthed in order to discharge the windings.

17

Figure 2: Insulation resistance measurement of the rotor windingIn the figure above the rotor is star-connected.

Stray load loss• The stray load loss is that part of the total loss

that does not lend itself to easy calculation. It

consists of two parts, viz., losses occurring at

fundamental frequency, and losses

occurring at high frequency.

• The stray load loss can be determined by the

indirect method or by the direct method.

• By the indirect method, the stray load loss is

obtained by measuring the total losses using

the input-output method and subtracting from

them the sum of stator and rotor I2R losses,

the core lose and the friction and windage loss.

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• The method thus entails subtracting two

relatively large quantities from each other

and is, therefore, not very accurate.

• For greater accuracy, and for the

determination of efficiency by the loss

segregation method, the direct

measurement techniques must be used.

• In this, the fundamental frequency and

high frequency components are measured

separately and require two tests:

• The rotor removed test

• The reverse rotation test

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• The fundamental frequency losses can be

measured by the rotor removed test, in

which consists of measuring the power input

with the rotor removed from the motor.

• The high frequency component is

measured by the reverse rotation test,

which entails (requires) measuring the power

input to the motor, with the rotor being

driven in the reverse direction to the stator

revolving field, and at synchronous speed.

• For details of this test, see IEEE 112–1996.

20

𝑷𝑺𝑳 = 𝑷𝑺𝑳𝒔 + 𝑷𝑺𝑳𝒓

Stray load Loss at Fundamental Frequency

𝑷𝑺𝑳𝒔 = 𝑷𝒔 − 𝑺𝒕𝒂𝒕𝒐𝒓 𝑰𝟐𝑹 𝒍𝒐𝒔𝒔; with rotor being

removed

Stray Load Loss at High Frequency

𝑷𝑺𝑳𝒓 = 𝑷𝒓 − 𝑷𝒎 − (𝑷𝒓𝒓 − 𝑷𝑺𝑳𝒔 − 𝑺𝒕𝒂𝒕𝒐𝒓 𝑰𝟐𝑹 𝒍𝒐𝒔𝒔) ;

with rotor being rotated at synchronous speed

in the reverse direction of stator field.

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𝑷𝒓 = 𝑴𝒆𝒄𝒉𝒂𝒏𝒊𝒄𝒂𝒍 𝑷𝒐𝒘𝒆𝒓 𝒓𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝒕𝒐 𝒅𝒓𝒊𝒗𝒆 𝒓𝒐𝒕𝒐𝒓

𝒘𝒊𝒕𝒉 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝒃𝒆𝒊𝒏𝒈 𝒂𝒑𝒑𝒍𝒊𝒆𝒅 𝒊𝒏 𝒕𝒉𝒆 𝒔𝒕𝒂𝒕𝒐𝒓

𝑷𝒎 = 𝑴𝒆𝒄𝒉𝒂𝒏𝒊𝒄𝒂𝒍 𝑷𝒐𝒘𝒆𝒓 𝒓𝒆𝒒𝒖𝒊𝒓𝒆𝒅 𝒕𝒐 𝒅𝒓𝒊𝒗𝒆 𝒓𝒐𝒕𝒐𝒓

𝒘𝒊𝒕𝒉𝒐𝒖𝒕 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝒃𝒆𝒊𝒏𝒈 𝒂𝒑𝒑𝒍𝒊𝒆𝒅 𝒊𝒏 𝒕𝒉𝒆 𝒔𝒕𝒂𝒕𝒐𝒓

𝑷𝒓𝒓 = 𝑬𝒍𝒆𝒄𝒕𝒓𝒊𝒄𝒂𝒍 𝑰𝒏𝒑𝒖𝒕 𝑷𝒐𝒘𝒆𝒓 𝒕𝒐 𝒔𝒕𝒂𝒕𝒐𝒓 𝒅𝒖𝒓𝒊𝒏𝒈

𝒓𝒆𝒗𝒆𝒓𝒔𝒆 𝒓𝒐𝒕𝒂𝒕𝒊𝒐𝒏 𝒕𝒆𝒔𝒕

𝑷𝒔 = 𝑬𝒍𝒆𝒄𝒕𝒓𝒊𝒄𝒂𝒍 𝑰𝒏𝒑𝒖𝒕 𝑷𝒐𝒘𝒆𝒓 𝒕𝒐 𝒔𝒕𝒂𝒕𝒐𝒓 𝒘𝒊𝒕𝒉 𝒓𝒐𝒕𝒐𝒓

𝒓𝒆𝒎𝒐𝒗𝒆𝒅

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23

Machine Rating in kW Stray Load Loss percent of rated load

1 – 90 1.8 %

91 – 375 1.5 %

376 – 1850 1.2 %

1851 and greater 0.9 %

Efficiency tests

• Efficiency is the ratio of the motor output

power and the motor input power.

• Efficiency =

• =

•

• =

• It can thus be calculated by a knowledge of

power input and power output, or of power

output and losses, or power input and losses.

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Input

Output

LossesOutput

Output

Input

LossesInput

• The losses in the induction motor consist of

the following:

• Stator I2R loss

• Rotor I2R loss

• Core loss

• Friction and windage loss

• Stray load loss

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• IEEE Std 112 gives 10 different methods for

the measurement of efficiency.

• Only three of these methods will be

described here, one each for fractional-

horsepower, medium and larger

induction motors.

• For a more complete description, see IEEE

Std 112–1996.

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Method A— Input-Output method.• This method is suitable for fractional-

Horsepower motors.

• In this method, the motor is loaded by meansof a brake or a dynamometer.

• Readings of electrical power input, voltage,current, frequency, slip, torque, ambienttemperature and stator winding resistance areobtained at four load points, more-or-lessequally spaced between 25% and 100% load,and two loads above the 100% point.

• Motor efficiency is then computed using theprocedures laid out in Form A in IEEE Std 112.

27

Method B—input-output with loss

segregation.• This method is the only method suitable for

testing motors designated energy efficientthrough 250 horsepower size range.

• The method consists of several steps whichneed to be performed in a set order.

• By this method, the total loss (input minusoutput) is segregated into its variouscomponents with stray-load loss defined asthe difference between the total loss and thesum of the conventional losses (stator androtor I2R losses, core loss, and friction andwindage loss).

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• Once the value of the stray load loss is

determined, it is plotted against torque

squared, and a linear regression is used to

reduce the effect of random errors in the test

measurements.

• The smoothed stray load loss data are used to

calculate the final value of the total loss and

the efficiency.

• The tests required to be performed to develop

the loss information are described below.

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1. Stator I2R loss is calculated from a knowledge

of the rated stator current and the resistance

of the stator winding corrected to the

operating temperature.

2. Rotor I2R loss is calculated from a knowledge

of the input power at rated load, the stator I2R

loss, the core loss and the per unit slip.

3. Rotor I2R loss=(measured input power—stator

I2R loss—core loss)×per unit slip.

4. The core loss and friction and windage losses

are determined from the no-load running

current and power test.

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5. The motor is run with no load at rated

voltage and frequency.

• The friction and windage loss is obtained by

plotting the input power minus the stator I2R

loss vs. voltage, and extending this curve to

zero voltage.

• The intercept with zero voltage axis is the

friction and windage loss.

6. The core loss is obtained by subtracting the

sum of stator I2R loss at no-load current and

rated voltage, and the friction and windage

loss from the no load power input at rated.

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Method F (and variations)—equivalent-

circuit method.

• This test is usually used for a motor whose size

is greater than 250 hp, and its size is such

that it is beyond the capabilities of the test

equipment.

• This method uses the equivalent circuit of the

induction motor to determine the performance

from circuit parameters established from test

measurements.

• The test provides acceptable accuracy for

starting and running performance.

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• It also yields the most accurate determination ofthe losses and hence the efficiency.

• This method uses two locked rotor tests: one atline frequency, and the other at reducedfrequency (a maximum of 25% of ratedfrequency).

• These tests, in conjunction with the runningsaturation test, delineate (define) the classicalequivalent circuit parameters of the motor.

• From the no load saturation test, themagnetizing reactance, the stator leakagereactance and the magnetizing conductancecan be determined.

• The rated-frequency locked-rotor test measuresthe stator and rotor reactance and the rotorresistance under initial starting conditions.

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• The low frequency locked-rotor test measuresthe stator and rotor leakage reactance androtor resistance at close to the runningfrequency.

• The stator and rotor leakage reactances forequivalent circuit are separated using the ratio ofthese parameters provided by design.

• Also calculated value of full-load slip, and eithertested value of stray load loss, or loss assumedaccording to Table 4.6 are used.

• The machine performance is then calculatedusing the parameters established from the test.

• Losses as determined from no-load tests areintroduced at appropriate places in thecalculation to obtain overall performance.

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Air gap

• An important factor in electrical rotating

machines.

• Failure mechanism associated with it

and ITS EFFECTS.

• There should be an airgap between rotor

and stator of the rotating machine so that

rotor can move without any friction except

air friction.

• If this air gap is not evenly distributed

around the 360° of the motor, uneven

magnetic fields can be produced.

• Magnetic imbalances.

• Cause movement of the stator windings,

resulting in winding failure.

• Electrically induced vibration, resulting in

bearing failure.

• The air gap fault zone describes themeasurable distance between the rotor andstator.

• Air gap eccentricity is a condition thatoccurs when there is non uniformity inthe air gap.

• When there is an eccentricity in the airgap,

1. Varying magnetic flux within the airgap.

2. Imbalances in the current flow, whichcan be identified in the currentspectrum.

• This unevenness in the space between the

rotor and stator will affect the alignment of

the RIC test results. (The RIC* is a graphical

representation of the magnetic coupling between

the rotor and the stator.)

1. Static eccentricity occurs when the

centerline of the shaft is at a constant offset

from the centerline of the stator. For

example misaligned* end bell.

2. Dynamic eccentricity occurs when the

centerline of the shaft is at a variable offset

from the centerline of the stator, such as a

wiped bearing.

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What is RIC?

• The Inductance measurements taken fromeach phase of the stator windings andcompares them at different rotorpositions to further define the condition ofthe rotor.

• This test is known as the Rotor InfluenceCheck (RIC).

• Figure shows us the results of a RIC testperformed on a healthy AC induction motor.

40

41

Note that each of the three inductance patterns are 120o apart and travelthrough two complete cycles over 360°. This occurs as a result of themotor under test being a 4-pole motor. Each pole consists of 90o.

Failure Mechanisms

• By definition, air gap eccentricity is a mechanical

fault with the motor.

• There are several possible causes for the

presence of variances in the distance between a

rotor and a stator.

• The five basic types of air gap eccentricitiesthat can occur are:

1. Rotor Outside Diameter is eccentric to the axisof rotation,

2. Stator bore* is eccentric,

3. Rotor and stator are round, but do not havethe same axis of rotation,

4. Rotor and shaft are round, but do not have thesame axis of rotation,

5. Any combination of the above.

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• The following are only a few of the

possible causes of an air gap eccentricity:

1. Incorrect mounting of the motor to

its bedplate can lead to an air gap

distortion.

• A loose or missing bolt allows shifting of

the motor’s mounting foot during thermal

expansion of the frame.

• This shifting over time could lead to a

distortion of the frame.

• The common term for a motor incorrectly

mounted is soft foot.

2. During construction of the motor, out-of-

roundness of either the rotor or stator will

lead to an air gap eccentricity.

• Industry standards: for total indicated

roundness should be performed at

different locations along the length of each

of these components.

• Couple measurements are at the

circumferences of each component,

depending on the speed and size of the

motor, there are recommended tolerances

from 5% to 20% variation in the air gap.

3. Eccentricity can develop due to incorrect

tensioning of drive belts coupled to a motor.

• Incorrect alignment could also lead to a

situation similar to this with both leading to a

bowing of the rotor during operation.

4. Distorted end bells, cocked bearings, or a

bent shaft will all cause an Air gap

eccentricity,

• During the manufacturing of the rotor, uneven

mechanical stresses could be introduced into

the cage and lamination stack.

• That will lead to bowing (bend) of the

completed rotor.

EFFECTS of AIR gap eccentricity

• Increased levels of vibration due to the

uneven magnetic pull it creates between the

circumference of the rotor and stator bore.

• These elevated levels of vibration can result in

excessive movement of the stator winding,

which could lead to increased friction and

eventually a turn-to-turn, coil-to-coil, or

ground fault.

• Additionally, this vibration can accelerate

bearing failure, which could seize the shaft

and overheat the windings or allow

additional movement of the shaft leading to a

rotor/stator rub (stroke).

EFFECTS of AIR gap eccentricity

• The uneven magnetic stresses applied to

the rotor coupled with the increased

vibration will also contribute to

mechanical looseness developing in the

rotor.

• Any of these occurrences could lead to a

catastrophic failure (terrible) of the

motor, which could require a complete

rewind and possible restacking of the

iron.

Predictive Maintenance Guide on

Motors and Variable Frequency Drives

• Electrical maintenance personnel have foryears been limited to troubleshootingmotors with no more than a multimeterand an insulation resistance tester(megohmeter).

• The IR tester unfortunately does notprovide enough information whether anelectrical problem exists or not.

• The troubleshooting of motors which arecoupled to VFDs has become moredifficult.

• VFD->Harmonics->Pollute power supply tomotor.

• There has always been an on-going

struggle to utilize technology to identify

problems in motors.

• Recently technologies, such as vibration

analysis has been developed to aid in the

identification of problems in motors.

• When vibration analysis shows a two

times line frequency (2*FL) spike, it is

assumed that it must mean an

electrical problem.

• However, it must be kept in perspective that

there are many other variables that may

be responsible for producing a 2*FL spike;

• therefore, removing a motor from service

for an electrical repair due to only a high

2*FL could be a mistake, possibly an

expensive one.

• Just measuring the insulation resistance of

motor windings may not be enough to say

that the motor is fine for continued

service or it can be put back in service

after it has tripped off-line.

• The fact is numerous (many) reasons can

exist which causes a motor to trip that will

not be seen by an insulation test, such

as a turn-to-turn short.

• Breakdown in the insulation between

individual turns of a winding can occur

inside a stator slot or at the end turn and

be completely isolated from ground.

• Phase-to-phase shorts can occur the

same way.

• If these faults are left un-attended, they

can result in rapid deterioration of the

winding, potentially ending in a complete

motor replacement.

• Restarting of a motor that has tripped

should be considered only after these faults

have been ruled out.

• Troubleshooting an electric motor that is

suspected to have an electrical problem

requires checking the insulation system as

well other components in the motor.

• To confidently assess the electrical

condition of a motor and ensure that it will

run reliably, there are six electrical

areas in a motor analysis that must be

looked at during the troubleshooting effort.

• Missing any of these areas could result in:

1. Missing the problem

2. Not having enough information to make

a correct decision.

The six areas are illustrated in following

figure:

• Predictive technologies and tools are

available today to troubleshoot and test

these areas of interest.

• PdMA Corporation is just one of the

several entities that have developed

technologies and tools for diagnosing motor

problems.

• PdMA offers two instruments that go beyond

the conventional insulation resistance

(megohmmeter) tester and multimeter

for predictive maintenance and

troubleshooting.

• These instruments are EMAX and MCE.