FIELD OF INVENTION
The present invention relates to a DC auxiliary generator having shunt field coil
arrangement. More particularly it relates to a DC auxiliary generator having shunt
field coil arrangement enabling battery charging at low idle speed of diesel
engine of locomotives.
BACKGROUND OF THE INVENTION
Traction machines are manufactured for various diesel electric locomotives. The
diesel engine is coupled with one traction alternator which works as mobile
power house in these diesel electric locomotives to provide power for traction
motors. In each alternator, two identical DC auxiliary generators are coupled to
alternator rotor with gear pinion arrangement. These DC auxiliary generators
have both series field winding and shunt field winding. Initially when diesel
engine is to be cranked or started, these two auxiliary generators work as series
field excited starter motors. Once the engine is cranked or started, these
auxiliary generators work as shunt field excited generators. One of the DC
auxiliary generator works as exciter to the alternator and feeds DC supply to
rotor field of the alternator. Shunt field coils of this auxitiary generator is
separately excited from a controller.
The second DC auxiliary generator gives supply for battery charging, other
auxiliary loads, lights and fans of driver's cabin etc. This DC auxiliary generator is
self excited type as the shunt field coils are self excited from residual magnetism
of the machine through a voltage regulator panel, which controls the field

current to maintain machine output voltage at 72 volts. Minimum 72V generation
is required from this auxiliary generator starting from low idle speed (350 RPM)
to 8th notch highest speed (1050 RPM) of the diesel engine, so that it can charge
battery in all condition irrespective of whether the locomotive is in running
condition or standstill condition. The low idle speed of the engine is 350 RPM
when locomotive is in standby condition.
So far this auxiliary generator is not being able to charge the battery at low idle
speed of the diesel engine because of its failure to generate the required voltage.
When the locomotive is kept in shade or station at standstill condition, due to the
fan, light and other loads of the locomotive, the battery gets drained. If now the
engine is stopped for some reason, a difficult situation is faced to crank or start
the engine again with drained battery since enormous power is needed (approx.
36 km) to start the engine.
In prior art this battery charging is possible at higher speed i.e. for diesel engine
RPM 450 to 1050 in running condition. But the problem is predominant when the
locomotive is in standstill condition and diesel engine is operating at low idle
speed of 350 to 450 RPM. At this low idle speed, the minimum field current
requirement is 5 to 5.5 Ampere to generate 72V armature output. In prior art
shunt field coil effective hot resistance is 16 ohms as shown in fig.1. For
generation of full output voltage i.e. 72 volts and even if VRP drop is considered
zero, the maximum field current possible is only 72/16 = 4.5 Ampere. Since this
field current is insufficient, it fails to generate the required voltage. This effect
goes in cascaded manner, hence at low idle speed the machine is not able to
generate 72V causing non-charging of battery and also allows drainage of
battery due to supply of power to connected loads like light, fan etc. in
locomotive.

Hence a necessity exists for designing an auxiliary generator that enables battery
charging of the diesel electric locomotive at low idle speed while keeping the
overall dimension of the machine same due to space limitation of the locomotive
and without changing the other characteristics of the locomotive and control
features to facilitate interchangeability of the present invention with older
machines already in service.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a DC auxiliary generator
having shunt field coil arrangement enabling battery charging at low idle speed
of diesel engine of locomotives which eliminates the disadvantages of prior art.
Another object of the invention is to propose a DC auxiliary generator having
shunt field coil arrangement which enables continuous battery charging at: all
speeds, particularly at low idle speed of diesel engine of locomotives.
A still another object of the invention is to propose a DC auxiliary generator
having shunt field coil arrangement which stops drainage of battery at low idle
speed of diesel engine of locomotives.
A further object of the invention is to propose a DC auxiliary generator having
shunt field coil arrangement which keeps the overall dimension of auxiliary
generator same to fit in the available space in the locomotive.
A still further object of the invention is to propose a DC auxiliary generator
having shunt field coil arrangement which is operable by the same control
system as existing in the locomotive.

BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1- shows a line diagram of prior art auxiliary generator
Fig. 2- shows a line diagram of the invented auxiliary generator
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
The present invented auxiliary generator (3) has novel feature of series-parallel
arrangement of shunt field coils (1) by connecting them in two different ways at
terminal block. In exciter mode all the six shunt field coils are connected in
series, whereas in the self generating auxiliary generator mode the shunt field
coils (1) are connected for example with three coils in series and two parallel
circuits (4) as shown in fig.2. This feature has enabled battery charging by
auxiliary generator in all speed range of the diesel engine, which is not possible
with the prior art, particularly at low idle speed of the diesel engine, when the
locomotive is in standstill condition.
It is necessary that for uniform battery charging, auxiliary generator (3) output
voltage should be maintained at 72 volts at all speeds of the diesel engine. The
working principle of this auxiliary generator is such that, whenever the engine
rotates, the residual magnetism of the field causes a small voltage generation,
which further increases the field current and it goes on in a cascading manner so
that its output finally reaches to desired 72 volts within few seconds. The field
current is controlled through an electronic voltage regulator panel (VRP)(2). The
function of voltage regulator panel (2) is to sense armature output voltage and
accordingly drop some field volts and feed only required field current at all
speeds to maintain armature output voltage of 72 volts.

In the present invention, the field circuit is arranged in such a manner that the
current requirement in individual coils (4) are kept same to facilitate
standardization and interchangeability aspects. As shown in fig.2 the shunt field
coils are arranged in series-parallel combination for example three coil in series
and two parallel circuits (4) with effective resistance R/4 ohms.
Hence, the overall circuit resistance is l/4th of the earlier resistance value i.e.
R/4= 16/4 =4 ohms. The current requirement in the circuit is double due to two
parallel paths, i.e. 10 to 11 Ampere instead of earlier 5 to 5.5 Ampere. The
required field supply voltage is now= (2X5.5)A X R/4 ohms =2X (5.5)A X 16/4
ohms = 11A X 4 ohms = 44 volts.
The output voltage being 72 volts, 72-44=28 volts is dropped by the voltage
regulator panel (2) and field is now able to render sufficient magnetization to
generate required output voltage so that uniform battery charging is ensured at
all speeds. At higher speeds the requirement of field current eases out further.
Hence the present invention totally eliminates the problem of battery charging
specifically at low idle speed of diesel engine.

WE CLAIM
1. A DC auxiliary generator (3) having a shunt field (1) consisting of a
plurality of shunt field coils (4) characterised in that,
the said shunt field coils are connected in series-parallel combination to
enable battery charging at low idle speed
2. A DC auxiliary generator (3) having a shunt field (1) as claimed in claim 1
wherein the overall dimension of the said generator is made to fit in the
available space in the locomotive.
3. A DC auxiliary generator (3) having a shunt field (1) as claimed in claim 1,
which is operated by the same control system as existing in the
locomotive.

A DC auxiliary generator (3) having a shunt field (1) consists of a plurality of shunt field coils (4) which are connected in series-parallel combination to enable battery charging at low idle speed of a diesel engine of a locomotive. The generator is made to fit in the available space in the locomotive and is operated by the same control system as existing in the locomotive.