Predictive (preventive) maintenance on Induction Motor.
1. What are the reasons for high current in motor?
a. High frequency – at 51 Hz current will be 105% of the normal current.
b. Low frequency – at 47.8 Hz current will be 102% of the normal current.
c. High voltage.
d. Under voltage.
e. Mechanical over loading.
f. Process requirement.
2. What are the reasons for unbalanced current in motor?
a. Loose power cable connection.
b. Voltage unbalance.
c. Short-circuited turns of coils of winding.
3. What are the reasons for vibration in the motor?
Vibration could be because of mechanical faults and electrical faults.
1. Mechanical faults.
a. Wrong alignment of the motor on foundation.
b. Wrong installation.
c. Improper fitting of bearing and cooling fans.
d. Periodic impulse loads such as reciprocating compressors.
e. Pulley of heavy weight which cause bending of motor shaft resulting in non
uniform air gap.
f. Damage of bearing or bad bearing.
g. Bad coupling.
h. If the axial alignment of the motor and the driven machine is incorrect and
rotor is allowed to contact its axial stops, high axial vibrations may occur,
together with high bearing temperature high and even bearing failure.
i. Machine base and foundation problem.
j. Under sized bearing.
2. Electrical faults.
a. Air gas dissymetry.
b. Broken rotor bars.
c. Slackened stator core.
d. Slackened rotor core.
e. Interturn short in the rotor winding in the two-pole machine.
f. Unbalance in rotor winding.
g. Unbalance power supply voltages.
If the vibration is because of electrical fault, de-energise the machine and
watch the vibration as it runs down.
The possible vibration frequencies observed are
a. Twice the power supply frequency – it indicates that the vibration is developed
by unbalanced power supply voltages, unbalanced air gap, unbalance in rotor
winding, slackened stator core etc.
b. Multiple of power frequency – the stator and rotor slots co-ordinate to develop
radial lines of force to deform and pulsate the cores.
c. Twice the slip frequency – magnetic unbalance due to unbalance air gaps,
slackened rotor core, interturn short in the rotor-winding etc. of two-pole
machine.
d. Beat (Humming) – in case of two-pole machine the beat is developed when
the vibration of twice as much as power frequency developed between the
stator and rotor is superimposed on the vibration of twice the slip frequency
developed due to irregular air gap.
For motors having class – B insulation the temperature should not be more than
110°C and for motors having class – F insulation the temperature should not be more
than 130°C. In case temperature is more, then the following could be the possible
reasons.
1. Electrical overloads.
a. Over and under voltage.
b. Over and under frequency.
c. Voltage unbalance. Voltage unbalance create unbalance of currents and
increase in temperature which will be 2*(% voltage unbalance)* (% voltage
unbalance)*.
(% Voltage unbalance) = 100 * maximum deviation from average voltage
average voltage.
For instance if voltages are 390V, 410V & 440V,
% Voltage variation = 100*(440-390+410+440) (440-390+410+440) = 6.45%.
3 3
Therefore increase in temperature rise = 2*(6.45)*(6.45) = 83°C (approximately).
d. Voltage transients and interruptions.
e. Loose connection at motor terminals.
f. Unbalance current.
g. Single phasing (if OLR protection is not working).
h. Long acceleration cycle.
i. Unusual system grounding conditions.
2. Mechanical overloads.
a. Locked rotor.
b. Heavy starting.
c. Bearing problem.
d. Overload in continuous duty and intermittent duty.
3. Environmental overloads.
a. Excessive temperature of cooling medium or ambient temperature.
b. Restricted flow of cooling.
c. Reduction in the density of cooling medium.
d. Heat transfer from machine parts connected to the motor.
4. Others.
a. Excessive number of switching operations.
5. What are the reasons for bearing temperature high?
Temperature of bearing should not be more than 90°C. In case temperature is higher
than the 90°C the following could be the possible reasons.
a. Inadequate lubricants inside the bearing.
b. Faulty bearing.
c. Bearing is jammed.
d. Over greasing.
e. Improper grade of lubricant.
6. What are the reasons for abnormal sound or noise?
Motors in general should run very quietly and no abnormal noise is desired.
However if noise is there, it could be because of following reasons.
a. Windage noise – the noise due to ventilating system, (whistling noise).
b. Bearing noise – the noise due to its rolling contact.
c. Unusual noise – some defects inside the motor (example – motor bar failure).
d. Deep heavy growling noises – some electrical fault.
For permissible limits of noise levels for rotating electrical machines IS: 12065:1987
is being reffered.
7. What are the reasons for harmonics in the motor?
Generally even harmonics are not expected to be present in three phase motors.
Triple-n harmonics like 3rd, 9th, 15th etc. are also not expected. The dominant odd
harmonics expected are 5th, 7th, 11th and 13th etc.
Presence of strong 2nd harmonics indicates unbalance voltage, unbalance winding
impedance, rotor defects, magnetic imbalance, faulty rotor skewing etc.
Very strong 3rd harmonics indicates magnetic saturation, ground leak currents,
overloads etc. Overloading causes overheating, resulting in non-linear magnetization
which gives high 3rd harmonic winding faults, short circuits. Hot spots in rotor or
stator also may indicate higher harmonics.
8. What are the possible reasons for not coming of rated speed during start?bable
causes
a. Starting load is too high.
b. Broken rotor bars (look for cracks near rings).
c. Open primary circuit.
d. Voltage is too low.
9. What are the possible reasons for motor to take long acceleration time?
Following may the possible reasons for motor to take long acceleration time.
a. Excess loading.
b. May be rewound motor with poor quality of winding conductor having high
resistance.
c. Defective squirrel cage rotor.
d. Applied voltage is too low.
10. What are the points contributes in insulation resistance of the motor?
If the measured insulation resistance of the motor is less than 1 MΩ / kV with a
minimum of 1MΩ, when the machine is cold it is to be dried out before full voltage
is applied to the terminals of the motors and the drying out is to be continued as long
as the insulation resistance rises or until a sufficiently high value that is not less than
1 MΩ / kV with minimum of I MΩ at 75°C is reached.
While proceeding for point as above said, following factors are to be kept in mind
which affect the insulation resistance measurement. They are,
a. Surface condition.
b. Moisture.
c. Temperature.
d. Magnitude of test voltage.
e. Duration of application of test voltage.
f. Residual charge in the winding.
g. Ageing of the insulation.
h. Mechanical stresses.
11. What are the minimum recommended PI values for AC and DC rotating machines?
Following minimum recommended PI values criteria is to be followed.
a. 1.5 for class – A insulation.
b. 2.0 for class – B insulation.
c. 2.5 for class – F insulation.
12. What is the minimum recommended absorption coefficient value for AC and DC
rotating machines?
Absorption coefficient = IR value for 60 seconds = 1.3 (minimum recommended value)
IR value for 15 seconds
Tips:
a. IR value decreases some what with an increase I applied voltage. However for
machines in good condition substantially the same IR is obtained for any test
voltage up to the peak value of the rated operating voltage.
b. If the IR value decreases significantly with an increase in applied voltage it is an
indication of imperfections or fractures of the insulation aggravated by the
presence of dirt or moisture or may be due to the effects of dirt or moisture alone,
or may result from numerous other phenomena not necessarily associated with
any defect or weakness.
c. IR value for good dry winding continue to increase for hours with constant test
voltage continuously applied, however a fairly steady value is usually reached in
10 to 15 minutes. If the winding is wet or dry or dirty the steady value is usually
reached in 1 or 2 minutes after the test voltage is applied.
d. The recommended minimum IR value for AC and DC machines is determined by
the following empirical relationship.
IR = kV + 1
Where IR = recommended minimum IR in mega ohms at 40°C of the entire
machine winding and kV = rated machine voltage in kilo volts.
Temperature correction is to be applied, if winding is not at a temperature of
40°C.
e. IR of the one phase of three phases winding with other two phases earthed, is
approximately twice that of the entire winding. Therefore when the three phases
are tested separately, the observed insulation resistance of each phase should be
divided by two to obtain a value which after correction for temperature, may be
compared with the recommended minimum value of IR.
13. What is use of Tan – Delta test? And what are the recommended values?
The very purpose of this test is to detect moisture content, voids, cracks and
deterioration in the insulation and same is to be conducted on HT motors.
Based on the guidelines given in the article ‘Diagmistic testing on the winding
insulation’ by J. S. Simon (IEE vol. 127 may 1980) the contamination level of motor
winding is to be assessed from the given Tan – Delta value.
Starting Tan – Delta values Degree of contamination
0 – 4%. Low void content.
4 – 6%. Clean.
6 – 10%. Some dirt.
10 – 14%. Dirt and moisture.
14 – 16%. Gross contamination.
Question and answers Electrical Maintenance Unit
- 193 -
16 – 20%. Heavy deposit of oil dirt.
Above 20%. Severe oil and carbon contamination.
14. What are important guidelines for conducting HV test?
Based on the recommendations given in IS: 4029:1977 decided DC test voltage
= (2E+1kV) 1.6 * M
Where E = rated voltage.
2.6 = AC to DC conversion factor.
M = derator factor which is a function to be decided on the basis of the age and
condition of equipment.
The DC voltage applied in steps and the leakage current recorded at each step. A plot
leakage current Vs test voltage is to be plotted as the test progress.
Some recommendations of IS : 4029 : 1977.
a. The HV test made on the windings on acceptance shall as far as possible not be
repeated. If however a second test to be made at 80% of the voltage given by the
empirical formula given above.
b. Test voltage for completely rewound motor = full test voltage for new motor.
c. Partially rewound or overhauled motor = 75% * full test voltage for a new motor.
d. Before the test for the old parts of the winding shall be carefully cleaned and
dried.
e. Before attempting of HV DC test a minimum PI value of motor should be
obtained.
15. What are the uses of high voltage surge test?
This test gives distinct wave forms giving indications of various defects such as,
a. Turn to turn short in same phases.
b. Coil to coil short in same phases.
c. Partial phase to phase short.
d. Complete phase to phase short.
e. Improper coil connections.
f. Reverse coil connections.
g. Open coil connections.
h. Short to ground partial.
i. Short to ground complete.
16. What is the thumb rule for motor current?
Thumb rule for NO LOAD current of motors.
Type of enclosure No. of poles % No Load current of rated current
TEFC 2 15 – 20
TEFC 4 30 – 35
SPDP 2 25 – 30
SPDP 4 35 – 40
SPDP 6 to 8 50 – 55
SPDP 10 80
Note: TEFC (Totally enclosed fan cooled) motors are low inductive having low
active material in comparison to SPDP(Screen protected drip proof) motors.
Thumb rule for calculating positive sequence and negative sequence current in
motors.
a. Positive sequence current: Average of all three phases currents.
b. Negative sequence current: Maximum deviation of any of the phase currents from
the average.
17. How you evaluate the insulation condition based on PI value?
Evaluation of insulation condition based on PI value
PI value Insulation condition Recommendation
1.0 – 1.5 Bad Drying is mandatory
1.5 – 2.0 Doubtful Drying is recommended
2.0 – 3.0 Adequate No drying is needed
3.0 – 4.0 Good No drying is needed
> 4.0 Excellent No drying is needed
18. What are the conditions monitoring for the motor bearings?
Bearing oil analysis is a useful tool in determining bearing performance and possible
deterioration. Periodic checks for oil colour, viscosity and acidity can aid in
preventing or anticipating bearing failure.
Oil analysis tests
Symptoms Possible cause Test Cost
Viscosity Water or high Water content Low
temperature ASTM 445 viscosity Low
ASTM 974 neutralization number Low
ASTM 664 neutralization number Moderate
Viscosity change
colour change
Oxidation
ASTM 2296 neutralization number Moderate
Spectroscopy Low
Particle count Moderate
Direct reading ferrography Moderate
Particles Bearing
deterioration or
foreign matter
Analytical ferrography High
ttps://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg https://www.youtube.com/channel/UC4_D50vMu1wbQrPaLFYo6Eg RRB JE, SSC AE/JE UPSSSC JE, SSC JE, CIVIL ENGINEERING MCQs, ELECTICAL ENGINEERING MCQs, preavious year quesion papers, dmrc, lmrc, drdo,rrb ntpc, ntpc, pgcil, dsssb, states board, GATE IES EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA https://t.me/pravendrarajpoot Daily news & current affairs in hindi & english fully updated Daily current affairs https://t.me/newsdailypkr Engineering Discussion group for your upcoming exams, you can ask your any query regarding your problem,👇👇👇 https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA PKR ELECTRICAL ENGINEERING I am sure this is the best place for you guys subscribe and get success IF YOU WANT TO JOIN ME ON TELEGRAM FOR PDF @newsdailypkr AE/JE EE, ESE, ECE, ME, CE, IT & CS EXAM MATERIALS & OLD PAPERS Electrical Engineering https://t.me/pravendrarajpoot facebook page:- Pravendra Kumar Rajpoot https://t.me/newsdailypkr https://chat.whatsapp.com/5AS7dNFTP4H4vVsiWsqHrT https://t.me/srk50 https://t.me/pravendrarajpoot https://t.me/joinchat/JObxeA7n6S4qvnegrGhTgA
No comments:
Post a Comment