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While advancements in technology really have made AC drives competitive, which is "better" depends on your application. The table below lists the advantages of both types.
Motor Maintenance and Troubleshooting
In general, motors are very reliable machines that require little maintenance. But while a typical electric motor might be a low-maintenance item, it still requires regular maintenance if it’s to achieve the longest possible service life.
SCHEDULE PERIODIC INSPECTIONS
The key to minimizing motor problems is scheduled routine inspection and service. Keep records of all maintenance schedules and procedures performed. The frequency and procedures of routine service vary widely between applications. Motors should be inspected periodically for things such as shaft alignment, motor base tightness, and belt condition and tension.
BRUSH and COMMUTATOR CARE
For DC motors, remove the covers and perform checks on brush wear, spring tension, and commutator wear or scoring. Replace the brushes if there is any chance they won’t last until the next inspection date. The commutator should be clean, smooth, and have a polished brown surface where the brushes ride. Observe the brushes while the motor is running. The brushes must ride on the commutator smoothly with little or no sparking and no brush chatter.
TESTING WINDING INSULATION
Twice yearly, test winding and winding-to-ground resistance to identify insulation problems. Motors that have been flooded or have low megger readings should be thoroughly cleaned and dried before being energized. The following are typical minimum motor insulation resistance values:
Rated Motor; Voltage; Minimum; Insulation; Resistance;
600 V and below 1.5 M-ohm 2,300 V 3.5 M-ohm
4000 V 5.0 M-ohm
KEEP YOUR MOTORS CLEAN
Wipe, brush, vacuum, or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling airflow. Heat reduces insulation life and eventually causes motor failure.
KEEP YOUR MOTORS DRY
Motors that are used continuously are not prone to moisture problems. It’s the intermittent use or standby motor that may have difficulties. Try to run the motor for at least a few hours each week to drive off moisture. Be careful that steam and water are not directed into open drip-proof motors.
Lubricate motors according to manufacturer specifications. Apply high-quality greases or oils carefully to pre vent contamination by dirt or water.
CHECK FOR EXCESSIVE HEAT, NOISE, and VIBRATION
Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise, or vibration.
EXCESSIVE STARTING IS A PRIME CAUSE OF MOTOR FAILURES
The high current flow during start-up contributes a great amount of heat to the motor. For motors 200 hp and below, the maximum acceleration time a motor connected to a high-inertia load can tolerate is about 20 seconds.
The motor should not exceed more than about 150 "start seconds" per day.
Ill. 73 Instruments used for motor troubleshooting. www.fluke.com. Megohmmeter Thermometer. Multimeter Clamp-on ammeter.
Electric motor failures can be due to mechanical component failure or electrical circuit failure. Any type of electrical testing involves risk, and complacency can lead to injury! When working on any type of motor, to reduce the risk of injury be certain to:
• Disconnect power to the motor and complete lock out and tag-out procedures before performing ser vice or maintenance.
• Discharge all capacitors before servicing the motor.
• Always keep hands and clothing away from moving parts.
• Be sure required safety guards are in place before starting equipment.
Electrical contact accounts for one-fifth of all construction deaths. Never work on energized equipment unless this is absolutely necessary for examination, adjustment, servicing, or maintenance. When you find you must work on energized equipment, always wear the appropriate personal protective equipment and use appropriate tools and equipment. Use the "buddy rule" and never work on energized equipment alone. Always have a partner working with you, in case of emergency.
Typical instruments used for troubleshooting motor operation problems include a multimeter, clamp-on ammeter, megohmmeter, and infrared thermometer. These instruments, are used to measure voltage, current, resistance, insulation resistance, and temperature.
The basic motor system consists of the power supply, controller, motor, and driven load. When a motor problem occurs, it’s first necessary to find which of the parts of the system is at fault. Power supplies and controllers can fail as well as the motor itself. Mechanical loads can increase because of an increased size of the load the motor is driving, or failure of bearings or coupling mechanisms.
Mechanical overloading is a prime cause of motor failure.
Once it has determined that the motor is at fault, you can proceed to locate the problem with the motor. A troubleshooting guide outlines a comprehensive variety of motor problems.
Generally the categories are arranged according to symptoms, offering brief suggestions concerning what to look for when investigating motor failures and often providing advice on how to correct the problem once it has been identified.
The following is an example of a troubleshooting guide that outlines fault symptoms common to most types of motors.
1. Symptom: The motor fails to start. Possible causes:
2. Symptom: The motor overheats. Possible causes:
3. Symptom: Excessive motor noise and vibration. Possible causes:
4. Symptom: Motor produces an electric shock when touched. Possible cause:
5. Symptom: Motor overload protector continually trips. Possible cause:
Troubleshooting Q&A may be used to quickly identify common problems and possible corrective courses of action. The following are examples that pertain to specific motor types.
Probable Cause and Course of Action:
Probable Cause and Course of Action:
Probable Cause and Course of Action:
TROUBLESHOOTING DECISION TREE
A troubleshooting ladder or tree may be used to guide you through the steps of the troubleshooting process. A trouble shooting ladder is sequential in nature, and its simplicity can often save time in arriving at the source of a motor problem. The following is a typical example of a trouble shooting ladder used to determine the cause of overheating of a three-phase squirrel-cage induction motor.
Step 1 Is ambient temperature too high? NO/YES Reduce ambient, increase ventilation or install larger motor.
Step 2 Is motor too small for present operating conditions? NO/YES Install larger motor.
Step 3 Is motor started too frequently? NO/YES Reduce starting cycle or use larger motor.
Step 4 Check external frame. Is it covered with dirt, which acts as insulation and prevents proper cooling? NO/YES Wipe, scrape, or vacuum accumulated dirt from frame.
Step 5 Feel output from air exhaust openings. Is flow light or inconsistent, indicating poor ventilation? NO YES Remove obstructions or dirt preventing free circulation of airflow. If needed, clean internal air passages.
Step 6 Check input current while motor drives load. Is it excessive, indicating an overload? YES NO Go to Step 11 Step 7 Is the driven equipment overloaded? NO YES Reduce load or install larger motor.
Step 8 Are misalignments, bad bearings, or damaged components causing excessive friction in driven machine or power transmission system? NO YES Repair or replace bad components.
Step 9 Are motor bearings dry? YES Lubricate. Does motor still draw excessive current? Step 10 Are damaged end bells, rubbing fan, bent shaft, or rubbing rotor causing excessive internal friction? NO YES Repair or replace motor.
Step 11 Are bad bearings causing excessive friction? NO YES Determine cause of bad bearings.
Step 12 Check phase voltage. Does it vary between phases? NO YES Restore equal voltage on all phases.
Step 13 Is voltage more than 10% above or 10% below nameplate? NO YES Restore proper voltage or install motor built for the voltage.
Step 14 Check stator. Are any coils grounded or short-circuited? YES Repair coils or replace motor.
1. From a safety perspective, what is the first step to be taken before performing any type of motor maintenance?
2. Outline five common motor maintenance tasks that should be performed as part of a motor preventive maintenance program.
3. Outline how to test for each of the following suspected motor problems.
a. Blown fuse or open circuit breaker.
b. Low voltage applied to the motor.
c. Defective motor windings.
4. List five possible causes of motor overheating.
5. The centrifugal switch of a spilt-phase motor fails and remains open at all times. How would this affect the operation of the motor?
6. The centrifugal switch of a capacitor start motor fails and remains closed at all times. How would this affect the operation of the motor?
7. List four possible causes of unbalanced voltages on the supply voltage of a three-phase motor circuit.
8. List five possible causes of excessive arcing at the brushes of a DC motor.
TROUBLESHOOTING SITUATIONS and SCENARIOS:
1 . Assume the tags used to identify the six motor leads of a compound-wound DC motor are suspected of being incorrectly marked or missing.
a . Outline how an ohmmeter would be used to identify the armature, shunt field, and series field leads.
b . What operating test could be made to ensure cumulative connection of the shunt and series field?
2. One of the three-phase line fuses to a squirrel-cage induction motor burns open while the motor is operating.
a . Will the motor continue to rotate? Why?
b . In what way might this operating condition damage the motor?
c. Should the motor be able to restart on its own? Why?
3. A defective motor start capacitor rated for 130 µF and 125 V AC is replaced with one rated for 64 µF and 125 V AC. What would happen?
4. The speed of a motor is to be reduced by one-half by using two different size pulleys. What must the relative diameters of the motor drive and load pulley be?
5. A motor feels hot to the touch. Does this always indicate it’s operating at too high a temperature? Explain.
1 . Explain how a squirrel-cage rotor produces a magnetic field.
2. List the different types of motor measurements that are used for troubleshooting motors.
3. Why does a single-phase motor have no starting torque if only a single winding is used?
4. How would you determine the running and starting winding of a single-phase motor from a visual inspection of the stator?
5. Arrange the following single-phase motors in the order of decreasing torque, with the highest torque first: split-phase, universal, shaded pole, capacitor.
6. How does slip affect motor speed?
7. Describe the major physical and electrical differences between the three major three-phase motor types.
8. Can a single-phase motor be operated from a three phase power supply? Explain.
9. Assume you have to purchase a motor and load laser alignment kit. Search the Internet for suppliers and prepare a report on the features and operation of the one you would consider purchasing.
10. An energy-efficient motor produces the same shaft output power (hp), but uses less input power (kW) than a standard-efficiency motor. Visit the website of a motor manufacturer and compare the price and features of a standard-efficiency motor with that of an equivalent energy-efficient motor.
11. Explain why motors are more efficient at full load.
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