FIELD OF INVENTION
The invention is related to the field of synchronous machines in general and to
detection of rotor winding earth fault in synchronous machines in particular.
BACK GROUND OF INVENTION
The present invention relates to the detection of a ground fault in the rotor
winding of a rotating machine in order to produce a warning in time to prevent
damage to the machine.
In large rotating machines, the rotor winding, which is isolated from ground, is
supplied with a direct current from an exciter. The rotor winding is carried by a
rotor body which is itself grounded and the winding is enclosed by insulation
which electrically isolates it from the rotor body. Eventually, the resistance of the
winding insulation to ground will decrease to extent which produces a low
resistance path between the rotor winding and ground.

Since the rotor winding is electrically floating, the occurrence of a ground
condition at one point of the rotor winding will not, by itself, result in damage
since there is no return path for current flowing to ground. However, if a second
such defect should appear, current will be shunted from a portion of the rotor
winding to the rotor body and if this current is of sufficient magnitude, it can
result in arcing and localized heating which can severely damage the rotor
structure.
For the above reasons, large rotating machines are typically equipped with
ground leakage detection equipment for monitoring such rotor winding insulation
faults. This equipment can take a variety of forms. One known system of this
type includes brushes which are periodically brought into contact with the rotor
shaft. The existence of a single ground fault is detected by imposing a low
voltage signal on the winding and the rotor. The existence of a ground fault
creates a closed current loop which can be detected.
In another type of system for this purpose, a voltage monitoring unit is coupled
to the ends of the rotor winding via slip rings and is operated to provide
indications of the voltages between ground and each end of the winding.

In various other known systems, test voltages are applied periodically to a
machine winding.
In all of the above-described systems, the monitoring system is connected to the
rotor winding by brushes through the slip rings. These slip rings are mounted on
the rotor shaft and needs additional shaft length and space thus making the
machine longer and expensive.
OBJECT OF THE INVENTION
In prior art access to the rotating winding for brushless excitation of synchronous
AC generators condensers, motors etc is provided by mounting separate slip
rings on the shaft or mounting bracket with slip rings at the non drive end of the
shaft thus increasing the machine length.
It is primary object of the present invention to monitor rotor winding ground
faults in a manner which avoids the above described shortcomings of the prior
art.

DESCRIPTION OF INVENTION
In the new method, access of brushes to the winding is provided without
mounting separate slip rings.
Brushless exciter of such machines usually have one positive and one negative
ring, called DC rings, to connect rotor field winding terminals with the (+)ive and
(-)ive terminals of the rectified brushless exciter terminals. The DC rings
therefore replace the slip rings of the prior art which were used for collecting
ground fault signals. This in turn reduces the length of the rotor in accordance
with the invention.
In the present invention these DC rings of exciter are suitably modified to
function as slip rings also and provide access for the non continuous contact type
brushes. These type of brushes are available in market and contact of the
brushes on slip rings can be made as and when required by remote operation.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with help of accompanying drawings which
depict an exemplary embodiments of the invention. However, there can be other
embodiments, all of which are deemed covered by the description.
Figure 1: Schematic diagram showing the conventional arrangement for Rotor
earth fault with slip rings mounted outside the machine.
Figure 2: Schematic diagram of new arrangement for rotor earth fault protection
using modified DC rings of exciter as slip rings.
Figure 3: DC ring of Exciter.
Figure 4: DC ring mounted on rectifier wheel.

Fig. 1 shows the cross sectional view of a rotating machine which includes a
rotor (1) carrying a rotor winding (2), winding (2) being normally isolated from
the rotor body and from round by suitable insulation. The ends of rotor winding
(2) are connected to an exciter circuit, through the Rectifier wheel (4) and DC
rings (5), which supplies a DC voltage across winding.
As shown in the Fig.l (prior art), there is no space available inside the machine
for mounting earth fault slip rings (6). These slip rings can be mounted inside the
machine, for which frame length has to be increased or they can be mounted
outside the machines, as shown. In both the cases machines length is increasing.
In the new arrangement as shown in Fig.2, the existing DC rings (5) of exciter
are suitably modified to act as slip rings also for providing access to the brushes
of earth fault protection.
Mounting of DC rings on rectifier wheel is shown in part view Fig.4. DC rings (5)
are mounted on one side of the rectifier wheel (4) by 6 nos. suitable insulated
fasteners(9) inserted through the holes (7) in the DC rings (Fig.3 ). Thickness of
the DC rings have been suitably increased to accommodate the brush

dimensions. Gap between the two DC rings and rectifier wheel is maintained with
the help of suitable insulated spacers (8) . After mounting DC rings (5) on the
rectifier wheel (4), outer diameter of the DC rings is turned to make it concentric
with the inner diameter of rectifier wheel, Outer surface of the DC rings is given
suitable surface finish to minimize the brush wear.
To collect the rotor winding ground fault signal, a magnetically actuated brush
gear device (10) is provided, as shown in Fig.2. Brush gear device (10) is
supported on a suitably designed support bracket (11), which can be mounted
on any suitable nearby stationary component in the machine. In present case it
is mounted on the stator frame of exciter(3). This commercially available Brush
gear device (10) when actuated at intervals by DC current, makes the contact of
brush on the outer surface of modified DC rings to collect the ground fault signal.

WE CLAIM
1. An improved rotor for detecting earth fault of rotor winding through DC
rings of brushless exciter in synchronous machines, comprising DC rings
(5), rectifier wheel (4), non-continuous contact type brush gear (10),
Characterized in that DC rings are mounted on one side of the rectifier
wheel (4) and modified to function as slip rings provided with access for
non-continuous contact type brush on brush gear device (10) wherein the
said brush gear device is actuated at intervals to collect ground fault
signals.
2. A rotor as claimed in claim 1, wherein DC rings (5) were modified to work
as slip rings also, for collecting ground fault signals thereby reducing the
length of the rotor.
3. A rotor as claimed in claim 1, wherein the DC rings (5) mounted on the
rotor shaft connect rotor field winding with the rectifier brushless exciter
which supplies DC voltage across the winding.

4. A rotor as claimed in claim 1 , wherein the DC rings (5) are mounted on
one side of the rectifier wheel by insulated fasteners (9) inserted through
holes (7) in the DC rings, and the DC rings are turned to make them
concentric with the rectifier wheel and are provided increased thickness
and suitable surface finish to minimize brushwear.
5. A rotor as claimed in claim 1, wherein the brush gear (10) is supported
on a support bracket (11) which can be mounted on a nearby stationery
component.

An improved rotor for detecting earth fault of rotor winding through DC rings of
brushless exciter in synchronous machines, comprising DC rings (5), rectifier
wheel (4), non-continuous contact type brush gear (10),
Characterized in that DC rings are mounted on one side of the rectifier wheel (4)
and modified to function as slip rings provided with access for non-continuous
contact type brush on brush gear device (10) wherein the said brush gear device
is actuated at intervals to collect ground fault signals.