FIELD OF THE INVENTION
The present invention relates to a technique for preventing damage to bearings
in rotating electrical machines. More particularly , the invention relates to an
improved arrangement for preventing circulating shaft current caused damages
to the end shield mounted bearings of explosion proof rotating electrical
machines.
BACKGROUND OF THE INVENTION
A rotor shaft of a rotating electrical machine is a mechanical component foj
transmitting torque and rotation. A bearing arrangement such as a rolling
bearing carries and supports the rotor through an end shield fastened to the
stator housing. The bearing arrangement consists of roller bearings, a bearing
housing , inner ring and outer bearing caps, flinger, dust seals etc. The lubricant
is provided to protect the bearing elements against corrosion and wear.
Explosion proofing of the electrical machine consists of enclosing the machine
in such a way that it will withstand, without injury, any explosion of an

inflammable gas that may occur within the machine and prevent the
transmission of flames which may ignite any inflammable gas if present in the
surrounding atmosphere. The explosion proofing in electrical apparatus and in
particular rotating machines is achieved by obtaining the joint gaps and length
of path through and across any joint surface or opening in the structure to a
certain predetermined length as per IEC 60079-1.
In the electric motors connected to a sine-wave electrical network, the
asymmetrical distribution of the magnetic flux in the motor induces a voltage in
the rotor shaft which leads to a low-frequency current flow through the bearings.
The bearing currents are caused by asymmetries of the electrical circuit,
manufacturing tolerances and material anisotropics. These currents circulate in a
closed circuit of shaft—bearing—bearing housing -end shield- stator housing.
(Fig.1).
In rotating electrical machines, the bearing currents and shaft voltages have
been recognized as a problem causing bearing current damage. A high shaft
voltage causes increased flow of current through the bearing that results in
mechanical damage and heating characterized by the appearance of either pits
or transverse flutes burnt into the bearing race liberating wearing metal particles
into the lubricant . Electrical pitting continues until the bearing loses its
coefficient of friction, further increasing the losses and breaking up bearing
surface. A low shaft voltage has lower current amplitudes but has been found to
still cause corrosive type of pitting due to grease decomposition.

Interruption of this current flow is achieved by isolating the bearing.
WO9701200A1 entitled " MOTOR SHAFT DISCHARGE DEVICE" shows
a grounding system for a motor comprising a conductive brush in resilient
contact with a contamination-free conductive member removably secured onto a
rotating part such as a motor shaft. The contacting surface is shielded from
contamination from motor bearings, for reliably shunting the shaft to the frame
of the machine to avoid high voltage transients across bearings that would
otherwise result in bearing damage. A brush holder positions the brush in
juxtaposition with the rotating conductive member and cooperates to
substantially enclose the region of contact between the brush and the conductive
member. The apparatus defines a cavity for extending the immediate enclosed
region of contact between the brush and the conductive member for the purpose
of receiving and holding residue produced by the brush .
This arrangement is however prone to generation of electric sparks/discharges
which may ignite any inflammable gas present in the surrounding atmosphere.
EP2696478A1 entitled 'A method for manufacturing an end plate arrangement of
a rotating electrical machine', teaches an end plate arrangement comprising an
inner ring , an outer ring and an insulation layer between these rings
manufactured using injection moulding. The end plate outer ring has a
protrusion and a matching groove for preventing rotation along with the shaft.
The end plate inner ring and the form of the insulation layer have axially locking
forms so that said forms prevent the axial movement of the inner ring and the
bearing inside it. (Fig. 2).

This arrangement may be usefully employed in small motors in non-hazardous
atmosphere; however the end shield may not be able to withstand the radial
forces transferred through it to the stator housing in case of medium and large
range motors. Also this arrangement does not comply with the requirements of
explosion proof enclosure for the motors operating in hazardous atmosphere.
Prior art further discloses the use of insulated bearings, wherein the outer ring of
the bearing is pre-coated with a thin layer of insulating material.
These bearings are very expensive and the insulating coating is prone to
scratches and damage during assembly and disassembly of the motor thus
reducing the effectiveness of isolation.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved arrangement
for preventing circulating shaft current caused damages to the end shield
mounted bearings of explosion proof rotating electrical machines.
Another object of the invention is to propose an improved arrangement
for preventing circulating shaft current caused damages to the end shield
mounted bearings of explosion proof rotating electrical machines which is reliable
and low cost.

SUMMARY OF THE INVENTION
The damage to the bearings is caused by circulating shaft currents . A reliable
and low cost method is provided for breaking the path of flow of circulating
shaft currents by isolating the non-drive end bearing housing.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1 - shows an illustration of source of shaft voltage and circulating shaft
currents
Fig. 2 - shows endplate arrangement with moulded insulation layer
Fig. 3 -Insulated bearing assembly
Fig. 4 - Insulated bearing housing
Fig. 5 - Insulation disc
Fig. 6 - Insulation washer and Insulation tube.
DETAILED DESCRIPTION OF THE INVENTION
A bearing housing (01) accommodates bearings (9) on a rotor shaft (15). A
grease flinger (04) mounted on the shaft abets against an inner race of the
bearing while a circlip (6) fixed in the groove on the shaft prevents axial

movement. An outer bearing cap (03) along with a shaft seal (05) mounted on
the shaft prevent collection of dust on the bearing. The outer bearing cap, held
onto the bearing housing by means of screws & washers (07 & 08), abets
against the outer race of the bearing and forms a path for on line greasing fitted
with a grease nipple (10) and a chamber provided for spent grease.
The inner ring assembly (11), the outer ring assembly (02) and the bearing
housing (01) form a flame path along the shaft such that flames caused due to
any explosion occurring within the motor enclosure are extinguished while
passing through the flame path and are not conveyed to the atmosphere
surrounding the motor. A spigot joint on a stator housing holds an end shield
(16) on the outer diameter (OD) while it is fixed to the bearing housing on inner
diameter(ID) through bolts. The joint gaps and length of path through and
across the joint surface between the end shield and the bearing housing and
the end shield and the stator housing are designed to a certain predetermined
length as per IEC 60079-1. An insulation disc (12), an insulation washer (13) and
an insulation tube (14) isolate the radial surface of the joint and the head &
shank of the screw respectively.
In the present invention, the jointing surface of the bearing housing is machined
and layers of insulation consisting of fire retardant material are applied, cured
and machined to specified size and tolerances complying to the standards for
explosion proofing of the electrical motor. The insulation provides the electrical
isolation at the joint between the bearing housing and the end shield. The
electrical as well as the explosion proof testing of the motor has been carried
out successfully.

WE CLAIM:
1. An improved arrangement for preventing circulating shaft current
caused damages to the end shield mounted bearings of explosion
proof rotating electrical machines, the improvement is characterized in
that:
the inner ring (11) including the outer ring (02), the rotor shaft (15)
and the bearing housing (01) forming a flame path along the shaft;
a spigot joint on the stator housing holding the end shield (16) on the
outer diameter of the bearing housing;
the joint gaps and length of the path through and across the joint
surface between the end shield and the bearing housing, and between
the end shield and the stator housing designed to a predetermined
length;
an insulation disc (12), an insulation washer (13) and an insulation
tube (14) configured to isolate the radial surface of the joint and head
and shank of the screw (7) holding the outer ring (02); and

the jointing surface of the bearing housing is machined and applied
with a plurality of insulation layers consisting of fired retardant
material which is cured and machined to specified size and tolerances.

ABSTRACT

The invention relates to an improved arrangement for preventing circulating
shaft current caused damages to the end shield mounted bearings of explosion
proof rotating electrical machines, the improvement is characterized in that the
inner ring (11) including the outer ring (02), the rotor shaft (15) and the bearing
housing (01) forming a flame path along the shaft; a spigot joint on the stator
housing holding the end shield (16) on the outer diameter of the bearing
housing; the joint gaps and length of the path through and across the joint
surface between the end shield and the bearing housing, and between the end
shield and the stator housing designed to a predetermined length; an insulation
disc (12), an insulation washer (13) and an insulation tube (14) configured to
isolate the radial surface of the joint and head and shank of the screw (7)
holding the outer ring (02); and the jointing surface of the bearing is machined
and applied with a plurality of insulation layers consisting of fired retardant
material which is cured and machined to specified site and tolerances.