As you will have gathered, maintenance of cage-rotor induction motors tends to mainly involve the stator windings and bearings. Cage-rotors require little or no special care in normal service. Inspect for signs of damage and overheating in the cage winding and its laminated steel core.
Make sure that all core ventilating ducts are clean and clear.
If an internal fan is fitted it must be in good condition if it is to provide adequate cooling.
If cage-rotor induction motor has been flooded with seawater and its insulation resistance is down to zero MO.
Wee need to restore the insulation resistance of the stator winding to a high value. This is achieved in three stages:
Salt contamination can be removed by washing with clean, fresh water. Any grease or oil on the windings has to be removed using a degreasant liquid such as Armaclean.
Dry the stator windings with low power electric heaters or lamps with plenty of ventilation to allow the dampness to escape.
Alternatively, the windings can be heated by current injection from a welding set or from a special injection transformer. Be sure to keep the injected current level weII below the motor’s full load rating.
With the windings clean and dry, and if the IR test remains high over a few hours, apply a couple of coats of good quality air-drying insulating varnish.
The motor starter and other control equipment should be regularly inspected to check and maintain the following items:
Enclosure of ship rotor:
Check for accumulations of dirt and rust. Any corroded parts must be cleaned and repainted. Examine the starter fixing bolts and its earth bonding connection – particularly where high vibration is present, e.g. in the steering flat and the forecastle.
Maintain of contactors and relays on ship
Check for any signs of overheating and loose connections. Remove any dust and grease from insulating components to prevent voltage breakdown by surface tracking. Ensure that the magnet armature of contactors moves freely. Remove any dirt or rust from magnet faces which may prevent correct closing.
Maintain of contacts on ship motor
Examine for excessive pitting and roughness due to burning. Copper contacts may be smoothed using a fine file. Copper oxide, which acts as a high resistance, can be removed using glass-paper. Do not file silver alloy contacts or remove silver oxide as it acts as a good conductor. A thin smear of electrical contact lubrication helps to prolong the life of all contacts. When contacts have to be replaced, always replace both fixed and moving contacts in pairs. Check contact spring pressure and compare adjacent contact sets for equal pressure. Examine power and control fuse contacts for signs of overheatig – lubricate the contact blades on fuse-holders.
Connections of ship motors
Examine all power and control connections for tightness and signs of overheating. Check flexible leads for fraying and brittleness.
Maintain Overcurrent relays on ship system
Check for proper settings (relate to motor FLC).
Inspect for dirt, grease and corrosion and for freedom of movement (not possible with an electronic type of OCR).
A thorough OCR performance test can only be carried out by calibrated current injection.
Control operation of ship el motors
Observe the sequence of operation during a normal start-up, control and shut-down of the motor. Particularly look for excessive contact sparking (only possible with open-type contactors). Remember to check the operation of emergency stop and auto-restart functions.
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