Check the insulation resistance with a mergger. Its values should be less than 1 megohm per rated kilovolt of the machine plus 1 megohm, subject to a minimum of 2 megohm, when cold. If this is not the case, presence of moisture or dust in the windings is indicated, and the machine needs to be cleaned and dried out. As the windings are impregnated with the highest grade of thermosetting varnish, such cases will be very rare and perhaps will indicated that the machine had been stored in a very damp ambient for too long.

1 Drying out

The drying out operation, when necessary should be entrusted to qualified and skilled personnel. One method of drying out the windings at site is to lock the shaft against rotation, short circuit the rotor windings in the case of slipring motors, and apply on the stator terminals a single- phase ac voltage having a mangnitude of about 10% of the rated voltage of the motor. Heat will be generated not only in the stator windings but also in the rotor windings due to induced currents. This way both the windings may be dried out simultaneousl

For low voltage machines and ac welding set may be used as the source of low voltage power. A dc welding set may be used, but in this case no current will be induced in the rotor windings, and it will fail to get heated; in such cases the stator and the rotor windings will have to be dried out separately. Before connecting the low voltage source, see that the external delta or star point connections, if any are made. The low voltage source should then be connected to the terminals of any two phases receive equal attention. At the end of each of these intervals the temperature, the end windings and the insulation resistance should be recorded and plotted on a graph paper.

In case of absence of ventilation, care should be taken that the temperature of the windings does not became dangerously high. The temperature should be controlled by adjusting the applied voltage or otherwise. The temperature of the windings should not be allowed to exceed 100oc during the drying out process. It will be found that the insulation resistance will decrease at first and then remain fairly constant for a long period. The heating should not be stopped at this stage, but continued until the resistance begins to exhibit a steady rise and achieves a maximum value. The machine is then completely dry and is ready for service. Depending upon the size of the machine and the degree of dampness, it may take from a day to several days to achieve this condition. Close supervision is essential during the process of drying out by this method. After drying out the winding, it is advisable to spray an electrical grade paint on the overhangs, or if possible to give varnish treatment to the windings.

Drying out operation should be carried out keeping the drain plug provided open which should be closed after drying out operation.

2 Condition of lubricant and lubrication.

Remove the shaft locking clamp, if fitted and if the machine has been held in storage for a longer period, check the condition of the grease by opening the bearing covers at both the ends of the machine. If the grease appears deteriorated, it must be removed and replaced with a fresh charge. Insert the correct amount of recommended grease only

2.1 For 2 Pole motors : Lithium based grease GRADE 2 IS : 7623 or Equivalent.

2.2 FOR 4 Pole and above motors : Lithium based grease GRADE 3 IS : 7623 or Equivalent.

2.3 Site Conditions

Attention to site conditions is essential for the satisfactory operation of all apparatus. Splashing liquids and abrasive and conducting dust are potential agents of failure and should be excluded from the site where the apparatus is intended to be installed. If this is not possible, it should be checked that the motor enclosure provides adequate safeguard against them. It is presumed that such a study was made at the time of ordering the machine itself but it pays to exercise a second check at the time of installation and commissioning also.

Unless otherwise mentioned, the enclosure of motors are designed to provided a degree of protection against ingress of solids and liquids that is not less than IP55 for totally enclosed types as defined in IS:4691. it need hardly be emphasized that whatever be the type of machine, there must be an unrestricted and undisturbed supply of the cooling medium to the motor, the temperature of which at any time during operation shall not exceed that specified at the time of placing the order.