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Trouble shoot and replace motors.pdf

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Trouble shooting and Replacement of
LT motors
J Niranjan
J Niranjan
Motor Malfunctions
With a few simple troubleshooting techniques
motor problems can usually be identified
quickly because there is little complex circuitry
and virtually no control devices within the
motor
J Niranjan
Recognize Faulty Operations
The table below shows some of the serious problems
associated with the motor and their causes
Condition Probable Cause
Motor does not run
does not hum
• Blown fuse
• Open circuit breaker
• Loss of supply
• Open lead to motor
• Motor is burned out
• Motor overload device tripped
J Niranjan
Recognize Faulty Operations
Motor fails to start does
not run but hums possibly
trips overload heater
Open phase
Low supply voltage
Mechanically overloaded
Motor overheats while
running under load
 Overloaded
 Low voltage
 High voltage
 Low frequency
 Clogged ventilating ducts
 Shorted stator coils
 Worn bearings
 Rotor rubbing on stator
Motor vibrates excessively  Load/coupling misaligned
 Shaft bent
 Worn bearings
 Broken rotor bar
J Niranjan
Recognize Faulty Operations
Motor runs slow  Overloaded
 Low voltage
 Low frequency
 Shorted stator coils
 Broken rotor bars
Unbalanced line
current on polyphase
motors during normal
operation
 Line voltage
 Shorted stator coils
 Stator coils incorrectly connected
 Poor connection of feeder for that phase
Motor trips ground
fault relay
 Grounded connection in motor junction
box
 Grounded feeder conductor
 Stator coil shorted to stator core or
frame
J Niranjan
Trouble shooting Procedures
Condition Probable Cause
Motor does not run
does not hum
• Blown fuse
• Open circuit breaker
• Loss of supply
• Open lead to motor
• Motor is burned out
• Motor overload device tripped
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Trouble shoot and replace motors.pdf

  • 1. Trouble shooting and Replacement of LT motors J Niranjan
  • 2. J Niranjan Motor Malfunctions With a few simple troubleshooting techniques motor problems can usually be identified quickly because there is little complex circuitry and virtually no control devices within the motor
  • 3. J Niranjan Recognize Faulty Operations The table below shows some of the serious problems associated with the motor and their causes Condition Probable Cause Motor does not run does not hum • Blown fuse • Open circuit breaker • Loss of supply • Open lead to motor • Motor is burned out • Motor overload device tripped
  • 4. J Niranjan Recognize Faulty Operations Motor fails to start does not run but hums possibly trips overload heater Open phase Low supply voltage Mechanically overloaded Motor overheats while running under load Overloaded Low voltage High voltage Low frequency Clogged ventilating ducts Shorted stator coils Worn bearings Rotor rubbing on stator Motor vibrates excessively Load/coupling misaligned Shaft bent Worn bearings Broken rotor bar
  • 5. J Niranjan Recognize Faulty Operations Motor runs slow Overloaded Low voltage Low frequency Shorted stator coils Broken rotor bars Unbalanced line current on polyphase motors during normal operation Line voltage Shorted stator coils Stator coils incorrectly connected Poor connection of feeder for that phase Motor trips ground fault relay Grounded connection in motor junction box Grounded feeder conductor Stator coil shorted to stator core or frame
  • 6. J Niranjan Trouble shooting Procedures Condition Probable Cause Motor does not run does not hum • Blown fuse • Open circuit breaker • Loss of supply • Open lead to motor • Motor is burned out • Motor overload device tripped
  • 7. J Niranjan Trouble shooting Procedures Blown Fuse Check to see that all line fuses are good. Fuses may be associated with a line circuit breaker or fused disconnect switch. Fuse checking must be done in accordance with the Praxair fuse test procedure, which includes selecting the proper PPE, de-energizing the circuit, verifying the circuit dead, lockout, tagout of the disconnect. If the fuse is discovered to be open, then the cause of the blown fuse should be investigated. Also, the motor should be rotated by hand to see that it is not mechanically locked.
  • 8. J Niranjan Trouble shooting Procedures Open Circuit Breaker Check all circuit breakers supplying the motor circuit. If the circuit breaker is found off, take action to determine why the breaker was turned off. If the breaker is found in the tripped position, check for conditions that would cause an overload or fault as above. Loss of Supply Verify the source voltage is present at the motor starter, or if the starter is operational, check voltage leaving the starter. This is diagnostic work and must be conducted under control of an HWP with appropriate PPE. If proper voltage is present on the leads leaving the motor starter the motor or motor circuit is open. Proceed with the next steps
  • 9. J Niranjan Trouble shooting Procedures Open Lead to Motor This is done with the motor starter or disconnect switch open, locked and tagged out. Use the motor checker. (Operation of motor checker explained in future slides) If a lead shows open (high resistance) open the motor junction box and look for a burned open or loose connection.
  • 10. J Niranjan Trouble shooting Procedures Motor Is Burned Out This is usually evident by the color of the enclosure. A burned out motor will have blistered or heavily discolored paint If the motor is in fact burned out, check for the reason. Occasionally, a motor will simply burn up due to defective construction, or deterioration of insulation, but the overload device should have tripped off. In any case, investigate the cause of the burn out.
  • 11. J Niranjan Trouble shooting Procedures Motor Overload Device Tripped This occurs as a result of three conditions: (1) the motor is overloaded; (2) the overload relays (thermal elements) are undersized; (3) the motor is partially shorted. The first step is to lockout the motor and try, then check the motor nameplate for rating, then be sure the shaft turns free. Check the resistance and inductance using Motor Checker. Next, re-set the overload and restart the motor, then check the current on all three legs. (Difference shall not be more than 5%). Also look for evidence of a loose connection in the motor starter.
  • 12. J Niranjan Trouble shooting Procedures Motor fails to start does not run but hums possibly trips overload heater Open phase Low supply voltage Mechanically overloaded
  • 13. J Niranjan Trouble shooting Procedures Open Phase Verify all 3 phases are at the proper voltage leaving the motor starter or disconnect switch. Loss of one phase can result from a blown fuse, defective circuit breaker, contactor or overload device. Also suspect would be bad connections, particularly in a wet and damp environment. Low Supply Voltage When supply voltage is low, a motor may not start under load. Voltages lower than 20% of the nominal 440 volts may not be sufficient to start the motor, consequently the overload relay usually trips. Causes of low supply voltage range from utility problems, overloaded circuits or panels, or undersized conductors to the motor.
  • 14. J Niranjan Trouble shooting Procedures Mechanically Overloaded A good example of this condition is a compressor that has stopped with high head pressure. Often, a compressor motor cannot start against head pressure, so it appears that the motor is locked up. Investigate mechanical loads before attempting to restart the motor
  • 15. J Niranjan Trouble shooting Procedures Motor overheats while running under load Overloaded Low voltage High voltage Low frequency Clogged ventilating ducts Shorted stator coils Worn bearings Rotor rubbing on stator
  • 16. J Niranjan Trouble shooting Procedures Overloaded The overloaded motor will draw excessive current. Check the current under load and compare with the nameplate data. If the motor is undersized, replace the motor or reduce the load. Low Voltage Low voltage, as discussed earlier will cause the motor to draw excessive current for the load, thus causing overheating. After the current is checked (above) then check the voltage to the motor. High Voltage High voltage, in excess of 20% can create an overheating problem due to higher magnetic losses. Look for an over- voltage condition when checking voltage above.
  • 17. J Niranjan Trouble shooting Procedures Low Frequency Low frequency, below 45 Hz will cause overheating due to loss of inductive reactance. The motor will also run slower. Low frequency usually occurs when the system is running on a generator that is not adjusted for the proper speed. Check the supply frequency and take action accordingly. Clogged Ventilating Ducts Most industrial motors run fairly hot under load. But they run hotter with the ventilation ducts clogged with dust or other material. Every l0 degC over the rated temperature can reduce insulation life by 50%! Carefully inspect and clean the vent ducts if necessary.
  • 18. J Niranjan Trouble shooting Procedures Shorted Stator Coils Stator coils can short in two ways: (l) the insulation between turns can break down, thus decreasing the inductance of that winding, (2) turns can make contact with the laminated core also due to insulation breakdown. Shorted turns appear as higher current on one phase with balanced voltage, or as uneven heating of the motor. Turns shorted to the stator core are more serious because the live conductor places voltage on the core. The ground fault relay will trip in this case Shorted turns may be detected by the use of a low resistance ohmmeter or Motor checker, stator to winding shorts may be detected with a conventional ohmmeter or a megger.
  • 19. J Niranjan Trouble shooting Procedures Worn Bearings Bearing wear is detected by noise, vibration, heating and complete seizure of the motor in extreme cases. Bearing wear is caused by several factors: • Misalignment • Lack of lubrication • Moisture and dirt • Excessive temperatures Eventually, an excessively worn bearing will fail to the extent that the rotor will begin rubbing the stator, the shaft will break or the motor will seize up.
  • 20. J Niranjan Trouble shooting Procedures Rotor Rubbing On Stator On most motors, the air gap between the rotor and stator is only 0.06 ” on small motors up to 0.20” on larger motors. Although not common, the motor end shields may be come distorted, the shaft bent, or the bearings so worn that the rotor actually rubs the stator of the motor. Rotor-stator rub is detected by noise, heating, sparks flying out of the vents, and loss of power.
  • 21. J Niranjan Trouble shooting Procedures Motor runs slow Overloaded Low voltage Low frequency Shorted stator coils Broken rotor bars
  • 22. J Niranjan Trouble shooting Procedures Overloaded The Overloaded motor tends to slow down proportionate to the torque vs. Rpm curve, until the level of stall torque is reached, then the motor speed will drop dramatically. If the overload device does not activate, the motor will burn out. Check the voltage and current supplying the motor and compare to the nameplate rated current. Also check the rating of the overload relays if the motor seems too hot.
  • 23. J Niranjan Trouble shooting Procedures Low Voltage and Low Frequency Both will cause a motor to run slowly. Under heavy loads, however, the motor will begin to overheat. Low voltage can result from utility conditions or undersized feeders, while low frequency usually only comes from a generator. Check voltage, then check frequency with the multi-meter. Shorted Stator Coils As discussed earlier cause loss of power consequently the motor will slow down under load, but again current drain and heating will increase
  • 24. J Niranjan Trouble shooting Procedures Broken Rotor Bars Though rare, occasionally a shorting ring will work loose from the ends of the rotor bars. When this happens, a portion of the secondary of the transformer is opened, thus reducing the rotor current flow and eliminating rotary torque. Mis-wiring Typically does not occur in the field. On some occasions, a motor may come from the factory or back from the motor shop with a particular coil in a winding connected in reverse. The reversed coil opposes the magnetic flux generated for that pole and acts as a brake for the rotor. A reversed coil within a winding may or may not create excessive current draw or heating, depending on the characteristics of that particular motor.
  • 25. J Niranjan Trouble shooting Procedures Unbalanced line current on poly-phase motors during normal operation Line voltage Shorted stator coils Stator coils incorrectly connected Poor connection of feeder for that phase
  • 26. J Niranjan Trouble shooting Procedures Unbalanced line current on a motor is more a symptom of another condition as opposed to being a malfunction of its own. Some current imbalance may be inherent in a motor due to anomalies in the construction, but this imbalance should not exceed 5% between phases. Shorted Stator Coils or Mis-wired Coils should also be considered if no other conditions can be detected. The motor may appear to run properly with this condition, as long as there is minimal load. A Poor Connection in any point in the feeder conductor can create resistance sufficient to cause a drop in current for that phase. However under load, the poor connection will manifest itself by creating heat at the poor connection.
  • 27. J Niranjan Trouble shooting Procedures Motor trips on ground fault relay Grounded connection in motor junction box Grounded feeder conductor Stator coil shorted to stator core or frame
  • 28. J Niranjan Trouble shooting Procedures A Grounded Connection in the motor junction box is the most common cause of shorts to ground in a motor. This is due to the tight proximity of wires in the box and the sometimes questionable methods of taping connectors. This is the first area that should be checked. A Grounded Feeder Conductor anywhere in the motor circuit will create a ground fault. Water in an underground conduit can also create a ground fault. As discussed previously, a Stator Coil Shorted to the Core will result in ground fault relay operation
  • 29. J Niranjan CAUSES OF PROBLEMS AND FAILURES Heat—High temperature causes degradation of insulation. Whether caused by high ambient temperature, overloading, or poor air circulation this condition accelerates aging. Moisture and corrosive substances can damage the motor, even though it may be rated for hazardous duty. Dirt, dust, abrasive and corrosive materials can enter the motor and attack windings and switch contacts in single- phase motors. Under-voltage and under-frequency create excessive current drain and subsequent heating. Voltage imbalance on larger motors (400 — 500 HP)
  • 30. J Niranjan CAUSES OF PROBLEMS AND FAILURES Misalignment between the motor and its load can create excessive stress on bearings and distort the shaft Improper application, for example an open motor is installed in a hostile environment, or a motor is undersized for the load that creates an overheating condition. Lack of compatibility with inverter power where a VFD (variable frequency drive) is installed as a replacement for a conventional starter. Spikes on voltage waveform stress insulation, resulting in failure. Lack of Maintenance—Dry bearings, air passages clogged with sawdust, dirt or fibers cause excessive heating
  • 31. J Niranjan Replacement Procedures •MEGGER THE REPLACEMENT MOTOR •LOCATE THE POWER SOURCE AND REMOVE POWER Be sure the motor is stopped before opening the main disconnect—this will prolong the contact life of the disconnect switch contacts. If the motor is still running, stop it by the normal method. Note the direction the motor is turning as it comes to a stop. If the driven load has no directional arrow, this step can save time later. Be sure that any other sources of energy cannot turn the motor or the driven shaft when the motor coupling is disconnected. Examples of such energy are: Air, gas or fluid pressure against a pump, Spring or hydraulic pressure, Wind blowing through a set of large fan blades, a winch or cable winder with tension on the rope.
  • 32. J Niranjan Replacement Procedures • UTILIZE LOCKOUT/TAGOUT PROCEDURES • BREAK THE ELECTRICAL AND MECHANICAL CONNECTIONS Remove the shaft coupling, drive shaft or belts. Before removing belts, be sure to release the tension. Open the motor junction box and carefully unpack the wires. They may be literally jammed into the junction box and brittle from heat. Note the connection configuration. If no metal tags or wire numbers are around the stator leads, then tag or label the leads
  • 33. J Niranjan Replacement Procedures •UTILIZE LOCKOUT/TAGOUT PROCEDURES •BREAK THE ELECTRICAL AND MECHANICAL CONNECTIONS Remove the shaft coupling, drive shaft or belts. Before removing belts, be sure to release the tension. Open the motor junction box and carefully unpack the wires. They may be literally jammed into the junction box and brittle from heat. Note the connection configuration. If no metal tags or wire numbers are around the stator leads, then tag or label the leads Disconnect and label heater leads, RTD leads where applicable. Disconnect the conduit/cable gland from the motor junction box and move it and the wires out of the way.
  • 34. J Niranjan Replacement Procedures • REMOVE THE MOTOR Loosen the motor on the mounts Remove the mounting bolts. If the coupling is still in place, be careful to slide the motor away from the coupling. Do not allow stress to be put on the coupling or the driven shaft, as this may create alignment problems later. Lower the motor to the ground and put it in a safe place. Before removing, any accessories take a measurement of the distance from the end of the shaft so that you can put the pulley back on for proper alignment. Remove any accessories such as pulleys, couplings or other hardware and install them on the new motor • INSTALL THE NEW MOTOR Place the new motor into position using the same sling or lifting arrangement as with the removal. Never allow hands, feet or other parts of the body to be under a suspended load. Once the motor is in place lightly tighten mounting bolts.
  • 35. J Niranjan Replacement Procedures • TAPE CONNECTIONS ACCORDING TO PRAXAIR SPECIFICATIONS • MEGGER THE FEEDER With all covers and guards installed, the feeder to the motor should be meggered again. Because all motor connections are made up, the most practical place to megger the feeder from is the output of the motor starter (the load side of the feeder). This will test the motor feeders and windings all at the same time This step is critically important in case a motor junction box screw has pierced a connection, or a feeder has become shorted with the movement of the conduit/due to the glanding.
  • 36. J Niranjan Replacement Procedures • RECONNECT THE WIRES First, review the connection diagram on the motor nameplate for the applied voltage (it is possible that the motor being replaced was a single- voltage motor with only three leads). Be sure the leads are properly connected for the voltage or starter configuration. Remember to reconnect any auxiliary leads, such as heaters or thermocouples. Make sure the motor frame is properly grounded • VERIFY CONNECTIONS ARE CORRECT FOR VOLTAGE AND ROTATION Make a final check for voltage and rotation connections. If you reconnected the new motor exactly the same way as the old, then there should be no problem. If the wires were not marked on the old motor as to rotation, then you have only a 50% chance of getting it right.
  • 37. J Niranjan TEST OPERATION AND RESTORE TO SERVICE • CLEAR THE AREA AND REMOVE THE LOCKOUT • RESTORE POWER • TEST ROTATION One person should go to the motor controller and prepare to operate the motor momentarily while another person watches the motor shaft. The person at the controller then “bumps” the motor . The observer then notes whether or not the motor is running in the right direction. The observer also notes any irregular sounds when the motor starts. The motor should coast down normally. If the motor starts in the wrong direction, then reversing any two wires from the starter will reverse the motor
  • 38. J Niranjan TEST OPERATION AND RESTORE TO SERVICE • CONDUCT FINAL ALIGNMENT • TEST OPERATION Again, any affected personnel should be notified that the starter is about to be tested and that the motor will be starting. The motor should be brought on in the normal mode by remote controls. The motor should be checked for excessive vibration by feel. If targets are installed, a baseline set of vibration readings should be taken.
  • 39. J Niranjan TEST OPERATION AND RESTORE TO SERVICE RESTORE SERVICE For the final time, notify affected personnel that the equipment will be restored to normal service. When the motor has been started in the normal mode, it should be again observed proper operation. The motor should be checked for heating by feel. If the motor feels hotter than normal, then the current should be checked at the MCC by a clamp-on ammeter and compared to the nameplate rated current. It is also a good practice to check the motor more frequently within the first 24 hours after the replacement has been completed, since problems may surface when the motor has come up to operating temperature, or vibration has set in.
  • 40. J Niranjan TEST OPERATION AND RESTORE TO SERVICE DOCUMENT THE REPAIR, CORRECT DRAWINGS No job is complete until the documentation is done. The replacement should be documented as a “Corrective Maintenance” in EAM 7i. Any changes made in wiring to accommodate a new type of motor should be noted and documented. In cases of a sudden or unexpected motor failure, a RCA (root cause analysis) should be conducted immediately.
  • 41. J Niranjan Using the ‘Motor Checker’ – EMC 22- Introduction
  • 42. J Niranjan Using the ‘Motor Checker’ - INTRODUCTION
  • 43. J Niranjan Using the ‘Motor Checker’ - INTRODUCTION
  • 44. J Niranjan Using the ‘Motor Checker’ - INTRODUCTION Fault Analysis A difference in the resistance and/or inductance of individual windings which is greater than 10% will indicate a fault in any motor. The resistance and inductance readings on the motors connected in star or delta will show approximately half the effect of a fault in a single winding. Thus a spread of more than 5% measured on a connected motor corresponds to a deviation by more than 10% in an individual winding
  • 45. J Niranjan Using the ‘Motor Checker’ - INTRODUCTION
  • 46. J Niranjan Using the ‘Motor Checker’ Instrument
  • 47. J Niranjan Using the ‘Motor Checker’ Instrument
  • 48. J Niranjan Using the ‘Motor Checker’ Instrument
  • 49. J Niranjan Using the ‘Motor Checker’ Instrument – Connections between various phases
  • 50. J Niranjan Using the ‘Motor Checker’ Instrument – Resistance measurement
  • 51. J Niranjan Using the ‘Motor Checker’ Instrument
  • 52. J Niranjan Using the ‘Motor Checker’ Instrument
  • 53. J Niranjan Using the ‘Motor Checker’ Instrument Inductance Measurement
  • 54. J Niranjan Using the ‘Motor Checker’ Instrument Using the Range Chart For Obtaining the approximate resistance and Inductance, values, note the reading obtained on the percentage scale and the range selected. From the Range chart calculate the value