Background

IMA electricians must be able to repair motor generator sets inplace. Large motor generator sets are highly susceptible to accidental damage when moved. Except for rewinding and machining, almost all repairs can be done inplace. Also, it is much easier to move only the parts that require work outside of the installed space.

You will learn about component disassembly, tests and inspections to be conducted, repair procedures, reassembly, and testing of motor generator sets. The motor generator set used as an example in this lesson consists of an ac wound rotor induction motor driving a synchronous ac generator.

Refer to NAVSEA 0900-LP-060-2010, Volume 1, Chapters 2, 4, 6, and 8 for additional information regarding the subject matter included in this lesson.

Preparation for inplace tests and inspections

Ensure that the power supply, including the alternate source, is secured and tagged out by tended unit personnel. Use a voltmeter to confirm that all sources of power have been secured.

Internal resistance tests of windings

Disconnect the motor and generator stator leads and conduct phase resistance balance tests. Record the results.

Lift or remove motor slip ring brushes and conduct a phase resistance balance test of the motor rotor windings. Record the results.

Conduct a motor rotor phase resistance balance test by reading from slip ring to slip ring.

Lift or remove generator rotor (field) slip ring brushes and conduct an internal resistance test. Record the results and compare them to those listed in the manufacturer's technical manual.

Analyzing internal resistance tests

The resistance of each phase must be within 5 percent of the others (the difference between the high and low readings must not be greater than 5 percent), as per NAVSEA 0900-LP-060-2010. Results of phase resistance balance tests of motor and/or generator stator and motor rotor windings that are not within these limits will require delivery of components to the shop for  rewinding.

Results of an internal resistance test of generator rotor windings that are not within limits stated in the manufacturer's technical manual will also require delivery of the rotor to the shop for rewinding.

Insulation resistance tests

Disconnect all input and output leads from the item under test.

Failure to lift brushes could result in damage to control system solid state devices.

Lift the brushes from the slip rings on the motor and generator ends.

Inplace cleaning

Motor generator sets will be cleaned inplace to raise their insulation resistance values.

Conduct an insulation resistance test using a 500-volt dc megohmmeter. Correct the readings to 25°C. Record the results. Readings less than those specified in the nomograph (above right)  will require delivery of the component to the shop for inshop cleaning and further repairs.

Inspection of brushes and brush holders

Visually inspect the brushes and brush holders.

Ensure that replacement brushes are of the same designation.

Replace any brushes found to be chipped, cracked, or worn below the manufacturer's recommendations or the requirements listed on the equipment PMS card.

Replace brush holders that show evidence of metal loss due to arcing. Install a replacement brush. Adjust the brush holder to allow 1/16 to 1/8 inch clearance between brush holders and slip rings.

Excessive brush wear is a symptom of problems that can be caused by:

• Not changing brush polarity at regular intervals
• Installation of brushes of incorrect designation
• Incorrect brush tension
• Faulty brushholders or incorrect clearance from slip rings
• Slip ring contamination caused by atmospheric contaminants such as silicon

Silicon contamination can result from various silicon-based materials including silicon tape, silicon varnish, shrinkable tubing, silicon insulation on wiring, additives in some lubricating greases, and room temperature vulcanizing rubber compounds such as RTV.

Bearing grease MIL-G-24139 contains silicon additives that can cause damage to slip ring surfaces.

Vapors from materials containing silicon condense on slip rings and form an insulating film on their surfaces. This film appears to be dark brown or black in color when the slip rings are at their normal operating temperature. After the machine is secured and begins to cool down, the film will absorb moisture from the environment and become grayish in color.

To correct the effects of silicon contamination, remove the source of contamination and polish the slip rings with a flexible abrasive cleaning device.

Brush seating

Ensure that all brush holders are in satisfactory condition. Adjust the clearance between the slip rings and brush holder to 1/16 to 1/8 inch.

Install brushes and set brush springs to the proper tension. Rotate the slip ring about six revolutions in the direction of rotation.

Lift the brushes and inspect the brush faces for proper seating. If inspection reveals that the brushes have not been properly seated, rotate them a few more revolutions. Check for buildup (clogging of the sandpaper).

Do NOT blow out the windings with low-pressure air.

Vacuum all carbon dust from the machine.

Inspection of slip ring film and condition

Film formation on a bright slip ring occurs within seconds after the machine is started. The film is a critical factor in determining brush performance and collector surface wear. The film color will vary with the type and grade of brush or surrounding atmospheric and load conditions. Copper and copper alloy slip rings should be brown colored under satisfactory operating conditions.

Electrolytic action is the greatest single cause of slip ring wear. Metal transfer occurs when the slip ring polarity is positive and the brushes are negative. The opposite polarity is most desirable and causes the brown film buildup. Periodic brush polarity changes will minimize slip ring wear.

Slip ring patterning is caused by cantilever vibration of the brush in the brush holder and can result from the following conditions: brush tension too low or too high, worn bearings, or rotor out of balance.

Grooved slip rings are normally caused by worn brushes or hard spots in a brush.

Cleaning slip rings

Clean slip ring surfaces with an untreated canvas wiper. Polish with a flexible abrasive cleaning stone. This method works well for minor cleaning applications. Four hundred-grit sandpaper can be used to clean slip rings, but it is not recommended and should be considered an emergency cleaning method only.

For slip ring conditions that cannot be corrected by cleaning, resurfacing is recommended.

Inplace slip ring resurfacing criteria

Slip rings will be resurfaced when slip ring eccentricity exceeds 0.002 inch (measured with a dial indicator).

Resurfacing is also required when slip rings contain deviations caused by excessive brush wear due to incorrect brush tension, worn bearings, or failure to change polarity of the brushes.

Preparation for resurfacing slip rings with a fixed stone device

Ensure that the power supply is secured and tagged out by tended unit personnel.

Test all incoming power sources with a voltmeter before commencing work.

Mount the stone traversing rig in a manner that allows the travel axis of the stone to be parallel to the rotating axis of the machine.

Inspect the ends of the motor shaft to determine whether the shaft has been modified to accommodate the fitting of an air motor and adapter. Modify the shaft as necessary.

Use an air motor and shaft adapter combination to drive the rotor during resurfacing operations. NEVER operate the unit electrically.

Calculate and record the rotor speed (revolutions per minute, or r/m) during resurfacing operations.
r/m = 2865 ÷ Slip Ring Diameter in inches

Resurfacing slip rings with a fixed stone device

Slip ring resurfacing is normally done by personnel from the outside or inside machine shop. Electrical personnel will normally assist in this evolution and should be familiar with all procedures involved.

Obtain the proper grade of resurfacing stone and install it in the stoning device.

Apply the air to the air motor and bring the unit up to the calculated resurfacing speed.

Apply enough feed pressure to assure good cutting. Do not use excessive pressure because this will cause the stone to vibrate. Periodically stop the machine and inspect the slip rings.

Secure the stoning when all slip ring imperfections have been removed. Measure slip ring runout with a dial indicator. Ensure that the total indicated runout of any slip ring does not exceed 0.002 inch.

Measure the outside diameter of the slip rings. All slip rings must have the same outside diameter when stoning is completed.