Motor Star-Delta starting method
Star-Delta starting method is probably the most common method for
reduce the amount of current during starting an electric motor in
means of reducing the disturbances and interference on the
electrical supply. With this method the starting current is reduced to
1/3 of the direct starting current.
Electric Motor General Inspection
Visual
A visual inspection shall be made to assess the general condition of the outside
of the motor for cracks, broken welds and missing parts. Darkened paint in the
motor (indicating excessive heat)
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.
Bearings
The shaft shall be manually rotated to check for any obvious problems with
bearings or shaft.
Connection Terminals
Periodically retightening of terminal connections
Space Heater
Space Heaters shall be tested for insulation resistance for one minute at 500V,
a 10ΜΩ minimum resistance is acceptable.
No Load Run
If possible the motor shall be run on no load at nameplate voltage and checked
for balanced currents and vibrations.
Corrosion
Check for sings of corrosion. Corrosion may indicate internal deterioration or a
need for external repainting.
Fan Blades
The fan blades are behind metal guard on the back of the motor. Make sure it
is securely fastened to the frame and is not clogged with dirt and other debris.
The openings in the rear metal guard need to have full and free air movement;
otherwise, the motor will overheat and eventually fail.
Brushes(DC motors)
Observe the brushes while the motor is running. The brushes must ride on the
commutator smoothly with little or no sparking and no brush noise(chatter).
Commutator (DC motors)
Black spots on the collector rings must be removed by rubbing lightly with fine
sandpaper.
Megger Test
Megger Test to be done in intervals in order to measure the insulation health.
Electric Motor Dismantling
After the incoming inspection, the motor shall be dismantled to the
extent needed to either fully identify problem or to do overhaul.
• Disassembly of the motor should be conducted so as not to damage
the individual parts. Therefore, when disassembling is not allowed to
use excessive force, sharp blows.
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• Before starting to disassemble the motor mark the exact position of
the end shields on the motor frame. This will permit reassembling
the motor just as it was for true bearing alignment.
• Remove nuts and through bolts or cap screws holding the end
shields and remove the Rotor with its end shield. Care shall be
taken that the slings do not damage the bearing surfaces or the
rotor.
• The Stator windings do not be touched by any of the parts being
moved.
Electric Motor Assembly for Horizontal Mounted
• Make sure that the motor is securely mounted during alignment in
order to prevent damage to the bearing.
• In case of direct coupling, motor shaft’s must be aligned axially and
radially to the driven load.
• Incorrect alignment may lead to bearing, vibration and may cause
shaft breakage.
• Correct alignment can be ensured with the use of dial gauges.
• During a complete shaft rotation total misalignment should not exceed
0,05mm
Save & Proper Storage
Improper motor storage will result in seriously reduced reliability and
failure. An electric motor that does not experience regular usage while
being exposed to normally humid atmospheric conditions is likely to
develop rust in the bearings or rust participles from surrounding
surfaces may contaminate the bearings. The electrical insulation may
absorb excess moisture leading to motor winding failure.
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