Question.

A cage rotor motor has been flooded with sea water, the insulation resistance can drop down to zero MΩ. describe the procedure for putting the motor back into service

Answer.

If a cage rotor motor has been flooded with sea water, the insulation resistance can drop down to zero MΩ. The main problem is to restore the insulation value of the stator winding to a high value. This is achieved in three stages:

(i) Cleaning

(ii) Drying

(iii) Re-varnishing

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 stater 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 well 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.

Starter and other motor control gear should be regularly inspected to check and maintain the following items:

a)Enclosure - 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 foc'sle.

b)Contactors and relays - Check for any signs of overheating and loose connections.
Remove any dust and grease from insulating components to prevent voltage breakdown by surface tracking.

c)Contacts - Examine for excessive pitting and roughness due to burning. Copper contacts may be smoothed using a fine file and copper oxide, which acts as a high resistance, can be removed using glass-paper. DO NOT file silver alloy contacts or remove sliver 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 forequal pressure. Check power and control fuse contacts for signs of overheating - lubricate moving contacts on fuse-holder.

(d) Arc chutes - Ensure that the magnet armature of contactors moves freely. Remove any dirt or rust from magnet faces which may prevent correct closing.

(e) Connections - Examine all power and control connections for tightness and signs of overheating. Check flexible leads for fraying and brittleness.

(f) Overcurrent relays - Check for proper size (relate to motor full-load current). Inspect for dirt, grease and corrosion and for freedom of movement. A thorough OCR per- formance test can only be carried out by calibrated current injection.

(g) Control operation — Watch the sequence of operation during a normal start-up, control and shut-down of the motor. Particularly look for excessive contact sparking. Remember to check operation of emergency stop and auto restart functions.