Procedure for Cleaning, Washing and Drying of Insulation & Describe the Vacuum Impregnation Method of Varnishing
Q. Explain the procedure for cleaning, washing and drying of insulation and describe the vacun impregnation method of varnishing.
Methods of cleaning insulation :
The failure of insulation takes place due to absorption of moisture. Oil, dust, dirt, excessive heat, vibration , over voltage and aging are the main reasons for failure of insulation.
Removal of loose dust :
The loose dust accumulated over the surface of winding, in the ventilating duct can be removed by applying compressed air at a pressure about 2.1 to 2.8 kg / C2 from a portable vacuum cleaner. For this work the person should use mask, safety goggle etc.
Cleaning with water : Cleaning of electrical machinery with fresh water and detergent. It is the recommended procedure for cleaning H class insulation. The cleaning should be done as rapidly as possible. The machine should be rinsed with fresh water, excess moisture wiped off with clean dry cloth and the machine should be baked in dry oven.
Drying of insulation and revarnishing :
External heat : It is the method of drying insulation most frequently used. The various methods of applying external heat are :
- Baking in the oven : It is conveniently carried out by placing the machine, having winding. The winding of machine with varnish is dried in baking oven at constant temperature. The temperature is kept constant with the help of thermostatic control.
- By using boiler : The steam from the boiler heats up the top of machine. The heat raises the temperature of the windings and dries the windings . The temperature of the windings is measured by thermometers attached to the coils should not exceed 90° C.
Internal heat : The insulation of the winding can be dried with the help of heat produced in the winding, by I2R loss. The d.c. voltage can be obtained with the help of rectifier. Low d.c. current is passed through the field winding and armature is short circuited. For induction motor apply reduced voltage to stator and block the rotor such that rated current will pass through the stator. The copper losses are converted into heat , which is used to dry the insulation. Even for transformer secondary is short circuited reduced voltage is applied to primary. The copper losses (I2R) are used to heat the winding.
Vacuum impregnation : Vacuum impregnation is carried out in a vacuum impregnation plant; which consists of a double jacketed vacuum impregnating chamber. It is having a top removing cover. There is separate varnish Storing tank. A compressor cum vacuum-exhauster which can create a desired pressure or vacuum by using proper valves connected to the chamber.
- The wound armature, stator or rotor is placed in the chamber by operating the top cover.
- Then it is tightly closed.
- The chamber is then heated upto 100°C by circulating steam or hot oil through the chamber and the temperature is maintained for nearly four hours.
- At the same time, air from the chamber is pumped out by vacuum exhauster and maintained, which will help in vapourising the moisture present in the coils and team same completely.
- Varnish from the varnish tank is allowed to flow into the chamber till the varnish in the comes up to a level in which windings are completely immersed.
- The valve of varnish tank is closed. A pressure from compressor about 1.4 kg / cm² to 2.1 kg / cm² is applied above varnish will make the varnish to be forced in all the porous (air packets) etc. spaces in the interior coils. The pressure is maintained for an hour or half.
- The valve of the varnish tank is opened and varnish is made to flow back to the tale Ans pressure till excess varnish also gets drained out. Then the valve is closed.
- Then the armature / stator is baked in the chamber at 100° to 110° C for nearly 8 hour varnish is completely dried out or it becomes dry bone.
- The air pressure and circulating steam or oil in the jacket is then stopped and the stator is then removed by opening the top cover.