FIELD OF THE INVENTION
The present invention generally relates to the field of commutators for DC machines. More particularly, the invention relates to a multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly.
BACKGROUND OF THE INVENTION
It is known that the prior art uses mica V ring for traction machine commutator assembly.
Problem of mica V ring are the frequent crushing and low Insulation Resistance of the commutators at elevated temperature encountered during commutator manufacturing. Crushing of mica, many a time resulted in High Voltage (HV) failure of the commutator/armature during stage testing. On the other hand, commutator machines having Insulation Resistance values especially in humid conditions below the specified limits are not accepted by the user. Hence, it is necessary to develop a better insulation by modifying the existing mica V ring.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly, which exhibits better insulation properties even at extreme humid conditions.
Another object of the invention is to propose a multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly, which provides better mechanical strength while maintaining the dielectric strength during manufacture of the commutator.

Yet another object of the invention is to propose a multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly, which minimizes not only stage failure during High voltage testing but also minimize failure in later stages.
A further object of the invention is to propose a multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly, which increases reliability of in-service commutator and machines.
SUMMARY OF THE INVENTION
Two problems are most predominant in commutator assembly of DC machines Firstly, failure of the commutator in high voltage test, secondly low Insulation resistance (IR) during manufacture of the machine. According to the present invention, a mica V ring is made up of mica, and Polyamide Polyimide is sandwiched between the layers. The prior art mica V rings exhibits crushing of mica (after dismantling) in the commutators during high voltage test. Secondly, Insulation Resistance of the prior art commutators is found lower than the specified limit. Conditions get worsened in the humid weather. Substantial drop in Insulation Resistance may lead to failure of the machine during manufacturing and service.
To overcome above mentioned problems, all aspects of failure were studied and analysis was carried out based on which prior art mica V ring was modified. To improve Insulation Resistance during extreme humid weather and to prevent crushing of mica at the time of manufacturing and service, layers of Polyamide Polyimide were introduced between the layers of mica.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- Cross sectional view and isometric view of a mica V ring of prior art.
Fig. 2- Cross sectional view and isometric view of a mica V ring according to invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The invention shall include following elements for manufacturing:
A Mica V ring developed according to the invention is shown in Fig. 2, which is a multilayer Polyamide Polyimide sandwiched mica V ring complying the technical requirement as per IEC 371-3-1. Mica V ring manufacturing shall include of mica layers with suitable bonding agent, layers of Polyamide Polyimide and again mica layers with suitable bonding agent. The process is followed by baking/consolidation process with metallic molds kept under pressure at rated temperature .
The constructional elements of invention are as follows:


1. RESIN BONDED MICA SHEET (Fig. 1 and 2. Item 1)
This mica V ring is made from flexible molding micanite sheet prepared from muscovite type mica splitting with bonding agent consolidated under heat and pressure. Sheet used here js smooth and free from inclusion of any foreign particle.
2. POLYAMIDE PAPER (Fig. 2. Item 2):
Polyamide paper used in mica V ring is made from specially formulated polyamide fibers with a high degree of thermal stability and high electric strength even at extreme humid conditions. The material shall be free from surface or edge defects, inclusions, voids, pin-holes and cracks.
3. POLYMIDE SHEET (Fig. 2, Item 3):
Polymide film used here is made by drawing or extrusion process. Polyimide polymer make up is the result of a polycondensation reaction between pyromellitic diahydride and diaminodiphenylether. The sheet has a very good mechanical and electrical (insulating) properties, low water absorption, high resistance to dilute acids, alkalies, oils, greases, alcohols and hydrocarbons. This material can easily withstand temperature up to 220°C.
4. BONDING AGENT:
It is an isopthalic resin having good bonding properties with mica splittings/layers with higher dielectric strength (i.e. low dielectric loss factor). Thermal stability of resin is to be of temperature index 155°C or higher.

ADVANTAGE OF THF IMPROVED PRODUCT OVER THE PRIOR ART
Mica V ring (fig. 2) developed here is made up of Polyamide Polyimide sandwiched between the mica layers.
Present art is made up of Polyamide Polyimide used between the layers which increases insulation level more than double with respect to prior art. Polyamide Polyimide used between the layers also having better mechanical properties, hence, prevent breakage and crushing of mica during manufacture. By using present art stage failure during High Voltage test have tremendously minimized and Insulation Resistance of machines found far above the specified limit. Thus results in better performance of machine in service.

WE CLAIM :
1. A multilayer, resin bonded, Polyamide Polyimide sandwiched mica V ring for traction machine commutator assembly, comprising:
- a resin bounded mica V-ring made from flexible molding micanite sheet
- a polyamide paper formed of polyamide fibers with a high degree of stability including high electric strength even at extreme humid condition disposed in the form of a layer on the mica v-ring; and
-a polyimide sheet in the form of a polyimide film laid over the polyamide paper,
Wherein the micanite sheet is prepared from muscovite type of mica splitting
with a bounding agent consolidated under heat and pressure, wherein the bonding agent is an isopathalic resin, wherein the polyamide polyimide is
sandwiched between the mica layers of the mica V-ring.
2. The mica V-ring as claimed in claim 1, wherein the polyimide film is made by drawing or extrusion and is enabled to withstand temperature up to 220°C.
3. The mica V-ring as claimed in claim 1, wherein the bonding agent has a thermal stability equivalent to temperature index 155°C or higher.