FIELD OF THE INVENTION
The present invention generally relates to an improved, safe and inexpensive
method of providing insulation to anti-friction bearings of large rating TEFC type
A.C. Induction motors driven from a variable frequency drive (VFD). More
particularly, the invention relates to an improved insulated End-Shield to protect
Deep Groove Ball Bearings mounted on the Drive-End and the Non-Drive end of
an A.C Induction motor from harmful rotor currents and damages due to
electrical pitting.
BACKGROUND OF THE INVENTION
Whenever an induction motor's electromagnetic circuit becomes unbalanced, a
stray flux path links the stator and rotor across an air gap. This flux linkage
induces a stray voltage (<40V peak) in the rotor circuit, which keeps on building
up until the insulating capability of the bearings lubricating oil film is exceeded.
At that moment, the voltage collapses and pulses of current passes through the
bearing. The extreme heat produced during this event blasts a minute pit in the
bearings and such repeated discharges permanently damage the bearings.

Motor operations also become very critical when there is an unbalance in the
supply voltage either form the utility or from a variable frequency drive ( VFD ).
The difference in 3- phases of supply voltage is known as the common mode
voltage (CMV). The CMV return to the source, which is the utility or the VFD from
the motor frame to the ground. When the CMV links the rotor, a charge gets
accumulated on the rotor. Since the unbalance source is external to the motor,
the rotor current flows through the bearings to ground and returns to the source,
taking the path of least resistance. To protect the bearings from an external
unbalance, both the bearings (DE and NDE) must be isolated from the motor
shaft and frame. To avoid shaft currents passing either to the inner or outer race
of bearings, they must be coated with ceramic, alternatively, a shaft ground
brush may be installed to bleed down the rotor charge and protect both the
bearings.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved insulated End-
Shield to protect Deep Groove Ball Bearings mounted on the Drive-End and the
Non-Drive end of an A.C Induction motor from harmful rotor currents and
damages due to electrical pitting.
Another object of the invention is to propose an improved insulated End-Shield
to protect Deep Groove Ball Bearings mounted on the Drive-End and the Non-
Drive end of an A.C Induction motor from harmful rotor currents and damages
due to electrical pitting which provides desired bearing insulation of a large
rating A.C. Induction motor driven from one of a utility and a variable frequency
drive (VFD).

SUMMARY OF THE INVENTION
According to the invention, there is provided an improved insulated End-Shield
to protect Deep Groove Ball Bearings mounted on the Drive-End and the Non-
Drive end of an A.C Induction motor from harmful rotor currents and damages
due to electrical pitting. The insulated end-Shield so produced provides desired
insulation between the rotor and stator of the motor by breaking the current loop
circulating within the motor.
The use of specially designed insulated End-shield replaces the use of insulated
Deep Groove Ball bearings at either side of motor, which are very costly and not
readily available in the market and its use to restrict the rotor currents is very
expensive affair.
Thus, the present invention proposes the use of insulated end-shield along with
insulation disc of Bakelite, insulating Washer and insulating Bolt. The invention
insulated End-shield is unique and its components such as Insulating ring,
Insulating disc and permits use of a non-insulated bearings which are easily
available in the market and less costlier than the insulated bearings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 schematically shows a non-insulated End-shield as per prior art.
Figure 2 - shows schematic of a Drive End and Non-Drive End bearing assembly
of TEFC Motor with non-insulated end-shield and insulated Deep Groove Ball
Bearing as per prior art.

Figure 3 - shows a machined non-insulated End-Shield for mounting of
Insulating Ring as per the invention.
Figure 4 - shows schematic of an Insulating Ring with diametric insulation as per
the invention.
Figure 5 - shows Schematic of an insulated end-shield as per the invention.
Figure 6 - shows schematic of a Drove-End and Non-Drive End bearing assembly
in a TEFC Motor with insulated End-shield and Non-insulated Deep Groove Ball
Bearing as per the invention.
Figure 7 - shows a cross-sectional layout of a TEFC type A.C. Induction motor
provided with an insulated End-shield of the invention mounted on either end of
the motor.
DETAILED DESCRIPTION OF THE INVENTION
Figure 2 shows typical schematic of Drive-End and Non-drive End bearing
assembly of a large rating A.C. induction motor (TEFC type). The rotor of a
motor rotates on Deep Groove Ball Bearings mounted of either end. In order to
provide insulation and prevent bearing damage from harmful shaft voltage
induced in the motor shaft, Insulated Deep Groove Ball Bearing (2G) is used.
Deep Groove Ball Bearings with outer race insulated is shown in Figure 2.

Figure 5 shows newly invented insulated End-shield (5A), which is manufactured
by combining two major parts (3A) and (4A) with the help of insulating disc (5B),
Insulating Bolt (5C) and Insulating washer 5D. Item 3A is obtained by additional
machining of Non-insulated End-shield (1A) made of cast iron, as shown in
Figure 3. Item (4A) is manufactured from normal fabricated steel by turning
operation in any center lathe and providing insulation of epoxy resin on outer
diameter as shown in figure 4. Insulated End-shield so produced as per new art,
suitably replaces non-insulated end shield as used in prior art.
Figure 6 shows a bearing assembly of Drive-End and Non-Drive End of A.C.
Induction motor fed from Variable frequency drive (VFD) as per new art. Newly
invented insulated End-shield (5A) is used along with non-insulated Deep Groove
Ball Bearing (6A) in place of Non-insulated End-shield (1A) and Insulated Deep
Groove Ball Bearing. Insulated Deep Groove Ball Bearing is at least 8 times
costlier than non-insulated deep Groove Ball Bearing. All other assembly
components like outer Bearing Cap (2A), Inner Bearing Cap (28), Grease flinger
(2C), shaft seal (2D), Felt seal (2E), circlip (2F) and hardwares are retained as
per prior art . Newly developed insulated End-shield is able to withstand high
voltage and offers insulation resistance in the range of lG-ohm to 1.2 G-ohm.
The resistance so obtained is quite able to protect the non-insulated Ball
bearings from harmful rotor currents.

WE CLAIM :
1. An improved and newly invented insulated end-shield of A.C Induction
motor with Deep Groove Ball Bearings to achieve a desired insulation to
restrict leakage of current, the improvement is characterized by :
- an End-shield (3A) additionally machined, and an insulating Ring (4A)
conjointly forming an Insulated end-shield (5A) which when mounted at
both driving end (DE) and non-driving end NDE providing desired
insulation to protect bearings from harmful rotor current;
- the additionally machined end-shield (3A) is made of cast iron and the
Insulating Ring (4A) is made of steel and provided with radial insulation of
epoxy resin; and wherein the insulating Ring (4A) is mounted over the
additional machined End-shield (3A) from inside of the motor with the
help of an insulating bolt (5C) and an insulating washer (5D).
2. The end shield as claimed in claim 1, wherein the top half (3A) of the end
shield is obtained by additional machining a cast iron non-insulated end-
shield, wherein the bottom half (4A) is made from fabricated steel by a
turning operation in any lathe machine.

ABSTRACT

The present invention relates to an improved and newly invented insulated end-
shield of A.C Induction motor with Deep Groove Ball Bearings to achieve a
desired insulation to restrict leakage of current, the improvement is characterized
by an End-shield (3A) additionally machined, and an insulating Ring (4A)
conjointly forming an Insulated end-shield (5A) which when mounted at both
driving end (DE) and non-driving end NDE providing desired insulation to protect
bearings from harmful rotor current; the additionally machined end-shield (3A) is
made of cast iron and the Insulating Ring (4A) is made of steel and provided
with radial insulation of epoxy resin; and wherein the insulating Ring (4A) is
mounted over the additional machined End-shield (3A) from inside of the motor
with the help of an insulating bolt (5C) and an insulating washer (5D).