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Inplace
Inspection of DC Motors Electrical
Background You could be called upon to investigate an installed electric motor that has intermittent operational problems or is not operating at all. In most cases, you will be required not only to diagnose the problem but to actually accomplish onsite repairs or take action to have repairs accomplished as required. During this lesson, we will discuss some of the inspections and repair procedures that you must know to properly inspect, troubleshoot, and repair a dc motor. Determining the cause of electrical motor problems is usually more difficult than finding mechanical problems. But, as in any type of troubleshooting, your senses of smell, sight, and hearing are your best tools. In cases of electrical problems, it will be necessary to use generally approved troubleshooting procedures. Preparation for static inspection Contact tended unit personnel to obtain as much information as possible about the operating history of the motor, information about the failure, and other pertinent information. Examples of some questions to ask are: • Did the motor fail during operation?
Inspect the controller to determine if the overloads are tripped. Use a voltmeter to test incoming power leads in the controller to ensure that power has been secured. Remove inspection covers from the motor. Static inspection • Inspect the commutator for evidence of excessive arcing, blackening, burned spots, flat spots, threading, high or low bars, high mica, or a burned ring around the commutator.
• Inspect brushes for proper length, tension, freedom of movement in brush holders, contact surface, and frayed or broken shunts. • Inspect brush holders for condition of springs, evidence of arcing, and their distance from the commutator.
• Inspect accessible armature and field windings. Also, inspect insulation for evidence of overheating, burned spots, bare conductors, or mechanical damage. Pay particular attention to the condition of interconnections between field windings and their connection points. • Inspect windings for cleanliness and evidence of carbon dust, grease, oil, and salt deposits. • Inspect the power cable in the vicinity of the motor connection box for signs of burned or deteriorated insulation, discoloration, or breaks in the cable. Mechanical tests
Rotate the motor shaft by hand, if possible. Check for freedom of rotation, rubbing, or binding. Some brush drag can be expected. Check for excessive end or side play. Secure testing if the motor appears to be binding. Disassemble the motor if the cause of the binding cannot be determined. Refer to the manufacturer's technical manual and the lesson on inplace repair of dc motors if disassembly is necessary.
If the mechanical condition of the motor appears to be satisfactory, conduct a commutator runout test in accordance with procedures listed in the lesson on inplace inspection of dc motors (mechanical). Insulation resistance test of motor windings
Prepare for the test by consulting the manufacturer's technical manual to
determine internal and external armature and field connections. This ensures that
conducting an insulation resistance test on the commutator will read all motor windings. |
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test: Use a 500 volt-dc megohmmeter to test motor insulation resistance from the
commutator segments. Correct reading to 25°C. Record the results. Values less than those specified in the table of minimum allowable insulation resistance readings for motors and generators for that component will require isolating armature, controller, motor power cable, and field windings to determine the cause of low readings. General guidelines for isolating dc motor windings and associated circuitry for testing Refer to motor drawings in the manufacturer's technical manual during isolation procedures. The controller schematic drawing can be used if the manufacturer's technical manual is not available. The armature can be isolated by lifting or removing the brushes or by placing paper between the brushes and commutator. Readings will then be taken on all commutator segments. The controller can be isolated by disconnecting the motor leads in the controller and retaking readings from the commutator. The motor power cable can be isolated by disconnecting at the motor connection box after disconnecting at the controller and taking readings of individual conductors. Be sure to matchmark all leads before disconnecting them. Depending on the individual motor, the field windings can be isolated by disconnecting the external connections in the motor connection box. Motor disassembly and disconnection of internal connections will be necessary in cases of internal winding connections.
Take corrective action when the cause of the low reading is determined. Perform inplace cleaning of the motor as necessary. |
Click on image to
enlarge. Nomograph for Temperature Correction of Insulation Resistance
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Testing field windings  |
Ensure
that the power supply is tagged out by tended unit personnel.
A commutator in unsatisfactory condition will require inplace stoning or
armature removal to facilitate turning in a lathe. No action should be taken until all
electrical tests have been performed on the armature and field windings. Faulty armature
or field windings can directly affect commutator appearance and condition. Refer to NSTM
Chapter 300 (series), paragraph 5.4.3.6, for guidance o