FIELD OF THE INVENTION:
The present invention relates to an Alternating Current (AC) machine, specially
designed for 3-phase drive IGBT based 25 KV AC Electric Multiple Unit (EMU). In
particular, the present invention relates to a traction transformer suitable for
IGBT based 3-phase drive ACEMU with provision of separate hotel load windings
for coach air-conditioning and method of manufacturing the same.
BACKGROUND OF THE INVENTION:
Electric Multiple Unit (EMU) is a multiple unit train consisting of self-propelled
coaches. An EMU requires no separate locomotive to haul the train. An EMU has
inbuilt traction motors in one or more of its coaches to generate the traction
power. EMUs are popular owing to their faster accelerations and pollution free
operation.
A traction transformer is the heart of the ACEMU traction system and complete
traction power as well as auxiliary power flows to different circuits through the
traction transformer. A 25 kV-overhead supply is stepped down through the
traction transformer to the desired levels and is fed to various systems through
different controllers.

Conventional EMU systems are based on DC drives. The DC drive system
includes traction transformer, tap changers, rectifier etc to feed controlled DC
supply to the traction motors. However, owing to the high maintenance
requirements, DC drives are losing their popularity. Today state of the art 3-
phase AC drive systems are gaining popularity due to their low maintenance
requirements and increased energy efficiency. The 3-phase AC drive system
includes traction transformer and electronic converters to convert single phase
AC supply to controlled 3-phase supply - which is fed to 3-phase induction
motors.
The 3-phase drive systems require transformer with high winding impedances to
limit the system fault currents to safeguard the electronic converters. The 3-
phase drive systems also require transformer with very high degree of magnetic
de-coupling between different secondary windings to avoid problems of
circulating currents between the different converters. Higher overload withstand
capability is required to take care of intermittent overloads during train starting
and train braking due to regenerative electrical braking method.
The traction transformers deployed at present are with traction and auxiliary
windings only for powering the ACEMU trains. Provision of separate Hotel load
winding to feed the coach air conditioning load is not there in conventional
traction transformers.

OBJECTS OF THE INVENTION:
It is therefore an object of the invention to propose a transformer for 3-phase
EMU drive systems having provision of separate Traction & Hotel load winding to
cater to the train traction load & coach air conditioning load respectively.
Another object of the invention is to propose a transformer utilizing core type
construction and concentric windings to achieve high winding impedances.
Another object of the invention is to propose a transformer to control the tank
(stray) losses by utilizing core type construction and concentric windings.
A further object of the invention is to propose a transformer which achieves high
magnetic de-coupling between different secondary windings.
A still further object of the invention is to propose a transformer with improved
overload withstand capability of the transformer.
A still another object of the invention is to propose a transformer with improved
sealing against moisture utilizing Vacuum Pressure impregnation (VPI) technique
for transformer windings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1 shows prior art winding arrangement.

Fig. 2 shows winding arrangement of the present invention.
Fig. 3 shows schematic diagram of prior art transformer.
Fig. 4 shows schematic diagram of the present invention with cooling
arrangement.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
OF THE INVENTION:
Fig. 1 shows the traction transformer winding arrangement of prevailing art
where traction and auxiliary windings are arranged to power ALEMU vehicles. In
the above winding schematic, 1 Secondary (10) and 2 Auxiliary windings (11,
110 are configured to empower ACEMUs. The said windings offer impedances as
low as about 10%, 3%, 6% for secondary and 2 auxiliary windings (11 & 110
respectively. However the present invention winding schematic as shown is Fig. 2
shows a separate provision of hotel load winding (HL) offering impedance of
about 16.9%. The 2 secondary windings (TR-1 & TR-2) as schematized in the
transformer offer impedance as high as about 38.7%. Such high winding
impedances facilitates to limit the system fault currents to safeguard the
electronic converters.
The constructional features as shown in Fig. 3 of prior art traction transformers,
in accordance to which ACEMU collects the power from 25 kV-AC overhead
electric lines (OHE). 25 kV is then stepped down by utilizing the traction
transformer to lower levels. The traction windings (10) feed the traction power to

traction motors. Auxiliary windings (20) are used to feed power to train auxiliary
circuits such as compressors, blowers, battery chargers, lights etc.
Fig. 4 is the schematic diagram of the present invention wherein provision of
separate hotel load winding is incorporated to cater the coach air conditioning
load has been made by adding a new winding - with improved insulation
schemes.
The proposed transformer of the present invention is manufactured with flame
resistant fibre such as Nomex and Glass-fibre insulations with fully enclosed
windings inside fibre glass cylinders with oil directed air forced cooling
arrangement (30, 31, 32) with cold oil directly impinged on to transformer
windings for effective cooling of the transformer and thereby with improving the
overload withstand capability of the transformer.
The proposed transformer of the present invention has a very high degree of
magnetic de-coupling between different secondary windings to avoid problems of
circulating currents between the different converters.
The core type construction and concentric windings as used in the present
invention are utilized to achieve - (1) high winding impedances
(2) to control the tank (stray) losses
(3) high magnetic decoupling between
different windings

While the best mode of carrying out the invention have been described in detail,
those familiar with the art to which the invention relates will recognize various
alternative designs and embodiments for practicing the invention within the
scope of the appended claims.

WE CLAIM:
1. An Alternating Current (AC) machine for IGBT based 3-phase drive 25 KV
AC Electric Multiple Unit (EMU) to cater to the traction load and coach air
conditioning load of a traction vehicle, wherein is provisioned a separate
winding arrangement comprising:
traction windings
Hotel load winding
with high winding impedances, the said windings being enclosed inside a
sheath configured by a cooling circuit which operates in conformity with a
cooling means, for effective cooling of the transformer.
2. The transformer as claimed in claim 1, wherein the said secondary
windings are constructed with very high degree of magnetic de-coupling
between each other.
3. The transformer as claimed in claim 1, wherein the transformer is
protected with a flame-resistant fibre and glass fibre materials with
windings being fully enclosed inside a glass fibre sheath.
4. The transformer as claimed in claim 3, wherein the glass fibre sheath is
equipped with oil directed air forced cooling arrangement.

5. The transformer as claimed in claim 1, wherein all the embodiments are
composed as in a single unit.
6. The transformer as claimed in claim 1, wherein the use of said winding
arrangement improve the overload withstand capacity and achieve high
winding impedances with reduced tank losses.
7. A method of manufacturing an AC machine for EMU to cater to the
traction load and coach air conditioning load of a traction vehicle as
substantially described and illustrated herein with reference to the
accompanying drawings.

ABSTRACT

The present invention relates to an Alternating Current (AC) machine for IGBT
based 3-phase drive 25 KV AC Electric Multiple Unit (EMU) to cater to the traction
load and coach air conditioning load of a traction vehicle, wherein is provisioned
a separate winding arrangement comprising traction windings, Hotel load
winding with high winding impedances, high degree of magnetic decoupling,
high overload withstand capability, the said windings being enclosed inside a
sheath configured by a cooling circuit which operates in conformity with a
cooling means, for effective cooling of the transformer.