Download Causes of Motor Failures in Hot Cells

C1615-10 Standard Guide for
Mechanical Drive Systems for
Remote Operation in Hot Cell
Facilities
Dale Wahlquist/Steve Shaw
Idaho National Laboratory/Merrick
June 18, 2009
C1615-10 Guide Topics
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Materials of Construction
Motors
Motor Mountings
Cause of Motor Failures
Bearings and Bushings
Torque Transfer Components
Torque limiters
Lubricants
Linear Positioning Systems
Position Indicators
Switches
Materials of Construction
• Avoid or minimize using plastics, elastomers, resins,
solid state devices, standard wire insulation, paints,
coatings, polytetrafluoroethylene (PTFE), or other
materials susceptible to radiation damage.
• Substitute more radiation tolerant materials for
components subjected to significant radiation.
• Evaluate the ease of replacement of failed
components.
• Polyetheretherketone is a plastic material that has
been used successfully for seals, valve seats, and
other applications because of its resistance to beta and
gamma radiation damage.
Motors
• Motor longevity in a hot cell environment depends on the quality of the
motor, the amount of moisture in the atmosphere, and the materials of
construction
• Motors with sealed-for-life bearings are preferred
• Capacitor start, single-phase, AC motors have been used successfully in
hot cells, and are generally less expensive and smaller that equivalent
horsepower DC motors
• Lower voltage motors are generally preferable to high voltage motors
when used in argon gas hot cell atmospheres
• Motors with high temperature insulation, Type H, are more radiation
resistant
• Motor brushes should be replaced with high-altitude brushes
• Avoid DC motors with built-in electronics circuits
Motors Selection Table
Gearmotors, Brakemotors, Stepper, Linear
and Induction
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Gearmotor lubricant must be radiation resistant
Separate brakemotor electronics and place them in a non-radiation area
Stepper motors can’t support a load if power is lost
Stepper motors become thermally hot regardless of whether they are
turning or not
• Electrical noise may be an issue with controlling stepper motors
• Three phase induction motors are simple, dependable, and work well in
hot cells
• Linear motors have been used successfully in hot cells, but consideration
must be given to duty cycle, position sensors, and convection cooling
requirements
Causes of Motor Failures in Hot Cells
• Motors with brushes fail because the brushes fail or electrical
cabling connectors fail
• Motor windings are rarely the cause of failure
• Wire insulation embrittles over time and the wires short
circuit
• Use silicone rubber coated fiber-reinforced sleeving over wire
insulation
• Very dry, inert atmospheres, causes some motors to overheat
– use high altitude brushes (with silver-loaded, selflubricating carbon)
Bearings and Bushings
• Lubrication selection is critical to
increased life of bearings
• Bearings should be sealed and
lubricated for life
• An alternative to the standard
lubricated bearing is one that has
been modified with high-altitude
graphite blocks that provide dry
lubrication
• Bushings should typically be
metallic and self-lubricating
Torque Transfer Components
• Gears should be metallic
• Precision alignment of gears is crucial to proper operation
• Consider using the lowest diametrical pitch possible to
minimize the need for precision fit between mating gears
• Use radiation resistant lubricants
• Use 3:1 safety factor when designing gearing systems
• Composite belts, chains, and sprockets are typically not used
in hot cells because they are difficult to replace or may be
susceptible to radiation damage
Lubricants
• Hot cell environments and radiation can be damaging to lubricants
• Consider supplying vendors with acceptable, radiation resistant lubricants
• Perfluoroalkylpolyether lubricant leaves no visible residue, has good high
temperature capability, and good radiation resistance
• Alkylaromatic-type base oils provide the best radiation resistance
• Components requiring lubricants should be sealed-for-life where feasible
• Liquid lubricants will generally endure more radiation than semi-solid
lubricants
• Oils and greases are considered “moderators”
• Oils and greases may require disposal as mixed waste once they are
radioactively contaminated
• NLGI Grade 0 oil containing molybdenum disulfate or NLGI Grade 1.5
grease with sodium aluminate thickener has been used successfully
Radiation Resistance of Lubricants
Position Indicators
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Optical Encoders are not suitable for high radiation environments
because the optical disk turns brown from prolonged radiation
exposure
Magnetic encoders are preferable to optical encoders
Consideration should be given to having redundant position
indicators
Brushless resolvers have been used successfully in radiation
environments
Linear Variable Differential Transformers (LVDT) have been
used successfully in hot cells to measure pressure, weight, and
liquid level
Switches should be mounted for ease of replacement
Photoelectric light sensors should be avoided due to radiation
damage
Inductive proximity sensors have no moving parts and are
suitable for radiation environments
Hazards
• Potential shock or electrocution if master-slave manipulator
comes in contact with exposed wires or power cables
• Typical hot mechanical drive systems are not guarded and
caution may be required when in operation
• Protect against failure of high speed moving parts to prevent
damage from projectiles to windows or sensitive equipment
Questions?
[email protected], 208 533-8875
• [email protected] (303) 353-3818