FIELD OF THE INVENTION
The present invention relates to an improved method of balancing Rotors of A.C
Induction Motors by providing additional balancing plane on the rotor, and
eliminating one of the existing two balancing plane.
BACKGROUND OF THE INVENTION
During type testing of a high capacity for example, a 2400 kW A.C.Motor,
excessive overall heating of the motor is observed. Detailed investigation carried
out by the present inventors, revealed that the balancing ring mounted on the
rotor essentially contributes to temperature rise beyond a threshold limits.
It is also noted that the balancing ring obstructs the path of smooth air
circulation inside the machine which interalia partially or totally blocks the cooling
air flow. More over, the material of Balancing Ring is generally magnetic material
and consequently it gets over-heated due to the eddy current inevitably induced
in the Rotors due to Stator winding overhang of 2 pole Motors as the overhang
portion is substantially larger.
In a rotating machine of finite length, a minimum of two planes at strategic
locations along the length of the Rotor are required for dynamic balancing of the
rotor. In a single-ended ventilated motor, the Fan at the Drive end (DE) acts as a
first balancing plane. A second plane is created using a balancing disc at non-
driving end (N.D.E) of the motor.
Dynamic balancing of a Rotor in Induction Motors is normally carried-out to
correct the unbalance on various axis-symmetric components. It is known that
unbalance normally gets created as a result of manufacturing tolerances and
geometric irregularities on some rotating components due to manufacturing
limitation resulting into torsional and bending vibrations interalia the overall
motor vibrations.
Additionally, the Rotor of large rating 2 pole motors are generally Flexible Rotors,
i.e., they rotate with additional bending nodes between the Bearing supports. For
balancing of a Flexible rotor, two planes as per prior art were provided. This
method requires large weights and multiple trial runs on the dynamic balancing
machine for balancing the Rotor.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved method of
balancing of rotors of A.C. induction motors, which eliminates a plane causing
excessive heat generation by blocking the cooling air circuit and Eddy current
heating.
Another object of the invention to propose an improved method of balancing
rotors of A.C. induction motors, which creates an additional balancing plane and
allows reduction in the weights to be used in the dynamic balancing machine
during rotor-balancing process.
SUMMARY OF THE INVENTION
According to the invention, an improved method of balancing rotors in particular,
flexible Rotors in high capacity two-pole induction motors is improvised. The
invention proposes eliminating one of the two balancing planes of existing
motors for example, the balancing ring and S.C. Ring is machined with identical
groove to allow it to be used as two balancing planes.
These two additional balancing planes created with the S.C. Rings allows adding
balancing weights easily on the dynamic balancing machine, in order to achieve
a desired residual unbalance of the rotor. The balancing planes so provided act
as strategic locations required for balancing of Flexible Rotor of Motors.
With this new technique very-finer balancing of the Rotor can be achieved by
using smaller and lighter weights on the balancing planes.
SC rings are formed of copper, a non-magnetic material, thus eliminating the use
of prior art magnetic material for the balancing plane which inevitable gets
overheated in magnetic field of the Stator winding of the Motor.
According to the invention a technical solution to the problem of balancing and
temperature rise, is provided by disposing the SC Rings to act as balancing
planes thus obviating the need of using the prior art created plane using the
balancing disc. Such configuration of the motor results into an extra plane which
is advantageous for ease of balancing and also lesser weight requirement.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a sectional layout of a high-capacity 2-pole induction motors
indicating the balancing planes according to prior art.
Figure 2 - shows a cooling air circuit of the motor of Figure 1
Figure 3 - shows schematically a balancing method of prior art motors (the
balancing machine not shown)
Figure 4 - schematic of an improved method of balancing a higher capacity 2
pole induction motor according to the invention.
DETAIL DESCRIPTION OF THE INVENTION
Figure 1 shows a typical Cross Sectional Layout of a large capacity for example, a
2400kW H.T. Induction Motor in CACW (Closed Air Circuit Water Cooled)
construction. The water cooler is mounted at the top of the Motor. This motor
rotates on Sleeve Bearings. Rotor geometry including Core, and Shaft results in
flexible operation of the Rotor. The figure shows locations of the Balancing
planes (1,2) that is a Balancing Ring (2) at Non-Drive and a centrifugal Fan (1) at
D.E. side of Motor.
Figure 2 shows a typical Cross-sectional arrangement of prior Art with Cooling Air
Circuit. Internal cooling fan is mounted inside Motor Driving End side. This
cooling Fan (Centrifugal type) extracts the hot Air axially from the Motor thus
cooling the Stator Winding (3), Stator Core (4), and Rotor Core (5). This Hot Air
(6), in turn enters the Water Cooler (7) and gets cooled. The cold air (8) is then
recirculated back into the Motors from Non-Drive End side thus forming a closed
circuit.
In prior method, balancing of Rotors was performed on dynamic balancing
machines, by adding weights in two planes at different angles in order to achieve
a desired residual unbalance. In this technique of balancing, the Cooling Fan (1)
at DE acts as first plane and the Balancing Ring (2) at NDE acts as second plane.
As shown in figure 4, according to the inventive method of balancing Rotor, the
Balancing Ring (2) has been eliminated from Rotor which excessively heats up

and obstructs in the cooling air circuit which was evident during type testing of
the Motor. Two balancing plane were created on both side of the rotor core (5)
by machining grooves on both the S.C. Ring (9) as shown in figure 4.
With this modification, now three balancing planes (1,9,9) are available in the
Rotor, against two balancing planes in prior method.
WE CLAIM
1. An improved method of balancing rotors (5) in particular flexible rotors, of
A.C induction motors in a dynamic balancing machine to achieve a desired
residual unbalance in the rotors, the improvement is characterized by the
steps of:
- removing the balancing rings (2) from the rotor shafts (5) to
restrict heating-up of the motor beyond a threshold value;
- assigning a cooling fan (1) disposed at drive end (DE) to act as a
first balancing plane;
- machining grooves of identical size on the S.C. Rings (9) disposed
one each at the driving end (D.E) and non-driving end (N.D.E); and
- allocating the S.C. Rings (9) to act as second and third balancing
plane during testing of the rotor shaft (5) on a dynamic balancing
machine.
2. The method as claimed in claim 1, wherein the S.C. Rings (9) are formed
of copper which is a non-magnetic material and enabled to restrict
overheating of the stator winding (3) of the motor.

An improved method of balancing rotors (5) in particular flexible rotors, of A.C
induction motors in a dynamic balancing machine to achieve a desired residual
unbalance in the rotors, the improvement is characterized by the steps of
removing the balancing rings (2) from the rotor shafts (5) to restrict heating-up
of the motor beyond a threshold value; assigning a cooling fan (1) disposed at
drive end (DE) to act as a first balancing plane; machining grooves of identical
size on the S.C. Rings (9) disposed one each at the driving end (D.E) and non-
driving end (N.D.E); and allocating the S.C. Rings (9) to act as second and third
balancing plane during testing of the rotor shaft (5) on a dynamic balancing
machine.