FIELD OF THE INVENTION
The present invention relates to an improved hybrid insulation system for stator
winding of induction motors.
BACKGROUND OF THE INVENTION
Insulation system for stator winding is the heart of any induction motor which
greatly influences the useful operating life of the motor. Also the insulation
system has major impact on the initial cost of the motor due to expensive
insulating materials and requirement of skilled labor. The failure related to
insulation system has high techno- economic repercussions, as huge time and
expensive materials are involved in the repairs. Also, expertise is required for
repairs related to insulation system which may not be readily available.
Therefore, any improvement in the insulation system of a motor can reduce
initial cost and can greatly increase reliability of operation.
The two major insulation systems known for manufacturing of stator capsules for
large size induction motors are
1) Resin rich roll impregnation, and
2) Resin poor vacuum pressure impregnation.

Resin rich system is characterized by application of polyester film tape along with
mica paper tapes for inter turn and ground insulation. After coil housing and end
connections, the complete stator capsule is roll impregnated in low viscosity resin
and then appropriately cured to complete the insulation process.
Resin poor system on the other hand is characterized by epoxy bonded glass
fabric backed mica paper tape or polyester film backed mica paper tape for inter
turn and use of mica tapes with glass cloth carrier and polyester film carrier
alternately for ground insulation. The complete stator capsules is put under
vacuum and then pressure impregnated with epoxy resin.
Both these systems are extensively used for insulating windings of large range
high voltage induction machines. However, a combination of resin rich tapes for
inter turn and resin poor tapes for ground insulation system along with magnetic
wedge is not used by any manufacturer over the world.
Apart from this peculiar shaped stator slots are used for resin poor system to
accommodate non-magnetic wedges which result into poor electrical
performance. In the new insulation system developed by BHEL, slots generally
best suited for resin rich system have been used which result into much
improved electrical and magnetic performance of the motor.

OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved hybrid
insulation system for stator winding of induction motors, which eliminates the
disadvantages of prior art.
Another object of the invention is to propose an improved hybrid insulation
system for stator winding of induction motors, which is formed of a combination
of resin-rich and resin-poor tapes.
A still another object of the invention is to propose an improved hybrid insulation
system for stator winding of induction motors, in which the stator slots are
modified to reduce cost and improved performance of the motor.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved hybrid insulation system for stator
winding of induction motors manufactured under vacuum pressure impregnation
technique, the improvement is characterized in that tip height of the stator slots
and typical shape of the wedges is selected at lower values compared to those
of corresponding capacity of known induction motors such that magnetizing
current is reduced and resultant power factor improved; non-magnetic wedges

Replaced wiith magnetic wedges to reduce magnetic noise; each copper
conductors insulated with resin-rich tapes with thickness decided on voltage level
of the motor; consolidated stack prepared based on insulated conductors further
insulated using resin poor tapes; stator coils after housing resin rich material
disposed below the magnetic wedges with magnetic putty provided to fill the
wedge space; and separators disposed between the top and bottom stator coils.
In the new insulation system, slots suited for resin rich system have been used
which resulted improved electrical and magnetic performance of the motor.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows the slot shape and magnetic wedges used in the improved hybrid
insulation system according to the invention.
Figure 2 shows the prior art slot shape used in insulation system.
DETAILED DESCRIPTION OF THE INVENTION
As shown in Figure 1, the stator slots in the insulation system are configured to
have reduced tip height and typical wedge shape, which interalia reduces
magnetizing current and hence improves the power factor of the motor. Figure 2

shows the slot profile used in the invention, which is identical to the slot profile
used in resin-poor insulation system.
According to the invention, magnetic wedge is used in place of non-magnetic
wedges which in turn reduces magnetic noise.
The copper conductor of the system is insulated using resin rich tapes that is a
combination of polyester film and mica paper tapes. These tapes are
comparatively cheap compared to resin poor tapes. The thickness of the tape
covering, is decided based on voltage level of the motor.
A consolidated stack is then insulated using resin poor tapes which is alternately
disposed layers of glass backed mica paper tap and polyester backed mica paper
tape.
The thickness of the insulation is decided depending on the voltage level of
motor.
After completion of the ground insulation, stator coils are compressed before the
stator housing, to maintain slot clearances as per known resin poor insulation
system.

After coil housing, 1mm packing of resin rich material is used below the magnetic
wedge. A magnetic putty is used to fill the wedge space along with appropriately
placed pieces of hard epoxy wedges. A plurality of 3 mm thick HM693 separator
is used between the top and bottom coil. After the end connections are made,
the complete capsule undergoes Vacuum Pressure impregnation and Curing
process to complete the insulation process.
ADVANTAGES OF THE INVENTION
1) Use of low cost resin rich tapes reduces overall cost of motor
2) Use of magnetic wedges reduces magnetic noise of motor
3) Lesser magnetizing current due to improved stator slot shaped
increases power factor of the motor.

WE CLAIM :
1. An improved hybrid insulation system for stator winding of induction
motors manufactured under vacuum pressure impregnation technique,
with improvement is characterized in that:
- trip tip height of the stator slots and width of the wedges is selected at
lower values compared to those of corresponding capacity of known
induction motors such that magnetizing current is reduced and resultant
power factor improved;
- non-magnetic wedges replaced with magnetic wedges to reduce magnetic
noise;
- each copper conductors insulated with resin-rich tapes with thickness
decided on voltage level of the motor;
- consolidated stack prepared based on insulated conductors further
insulated using resin poor tapes;
- stator coils after housing resin rich material disposed below the magnetic
wedges with magnetic putty provided to fill the wedge space; and
- separators disposed between the top and bottom stator coils.

2. An improved hybrid insulation system for stator winding of induction
motors substantially as herein described and illustrated with reference to
the accompanying drawings.

ABSTRACT

The invention relates to an improved hybrid insulation system for stator winding
of induction motors manufactured under vacuum pressure impregnation
technique, with improvement is characterized in that tip height of the stator slots
and typical shape of the wedges is selected at lower values compared to those of
corresponding capacity of known induction motors such that magnetizing current
is reduced and resultant power factor improved; non-magnetic wedges replaced
with magnetic wedges to reduce magnetic noise, each copper conductors
insulated with resin-rich tapes with thickness decided on voltage level of the
motor; consolidated stack prepared based on insulated conductors further
insulated using resin poor tapes; stator coils after housing resin rich material
disposed below the magnetic wedges with magnetic putty provided to fill the
wedge space; and separators disposed between the top and bottom stator coils.