Field current measurement device

A rotating machine including a generator having a rotor provided with a field winding, an exciter having an armature, a rotatable steel shaft connected to the generator rotor and the exciter armature, and current conductors enclosed by, and extending axially along, the shaft for conducting a dc generator excitation current from the exciter to the generator rotor field winding, which current creates a magnetic field with the shaft, the machine being provided with a member defining a stationary magnetic field path external to the shaft and magnetically coupled to the shaft for conducting a predetermined portion of the magnetic field created in the shaft, the path being disposed such that during rotation of the shaft, the intensity of the magnetic field in the path varies cyclically; and a magnetic field responsive component magnetically coupled to the path for producing an output signal respresentative of the rate of change of the magnetic field in the path.

BACKGROUND OF THE INVENTION 
The present invention is concerned with measuring the field current being 
conducted from an exciter to a generator via a pair of axial leads 
extending along the center of a shaft coupled to the generator and the 
exciter. 
It is known to associate a wound rotor generator with an exciter which 
supplies a dc field current to the generator rotor via a pair of axial 
leads extending along the center of a shaft which is common to the exciter 
and the generator. It is desirable to be able to measure this field 
current in connection with monitoring generator performance, testing of 
generator transient response, detection of field winding shorts and 
diagnosis of excitation system malfunctions. However, in the case of 
certain types of generators, such as those having a brushless excitation 
system, the generator rotor field winding current cannot be measured 
directly. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to measure such field current 
without requiring any structural modification of the exciter, the 
generator, or the shaft. 
Another object of the invention is effect such field current measurement in 
a simple and inexpensive manner. 
Yet another object of the invention is to effect such measurement by means 
of a structurally simple, passive device. 
The above and other objects are achieved, according to the present 
invention, in a rotating machine including a generator having a rotor 
provided with a field winding, an exciter having an armature, a rotatable 
steel shaft connected to the generator rotor and the exciter armature, and 
current conductors enclosed by, and extending axially along, the shaft for 
conducting a dc generator excitation current from the exciter to the 
generator rotor field winding, which current creates a magnetic field 
within the shaft, by the provision of: means defining a stationary 
magnetic field path external to the shaft and magnetically coupled to the 
shaft for conducting a predetermined portion of the magnetic field created 
in the shaft, the path being disposed such that during rotation of the 
shaft, the intensity of the magnetic field in the path varies cyclically; 
and magnetic field responsive means magnetically coupled to the path for 
producing an output signal representative of the rate of change of the 
magnetic field in the path.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a shaft 2 which extends between a wound rotor generator 
and a brushless exciter and which is secured to the generator rotor and 
the exciter armature. At the center of shaft 2, there are disposed, as is 
known, two axial conductors 4 and 6 which are insulated from each other 
and from shaft 2 by a body of insulation 8. 
When a dc current flows through conductors 4 and 6, there will be 
established a static magnetic field having the form represented by the 
closed flux lines 1 shown to be located totally within shaft 2. Since 
shaft 2 is made of steel having a much larger permeability than that of 
the surrounding air, most of this magnetic field will be confined within 
the shaft. The density of this magnetic field is relatively low, so that 
the permeability of the shaft steel will not vary with changes in the 
magnetic field. As a result, the intensity of this magnetic field is 
directly proportional to the magnitude of the field current flowing 
through conductors 4 and 6. Since conductors 4 and 6 rotate as a unit with 
shaft 2, the magnetic field will also rotate with the shaft, i.e. will be 
stationary relative to the shaft. 
If, now, a high permeability magnetic path is provided outside of the 
shaft, with the ends of such path being in proximity to the shaft surface, 
a portion of the magnetic field will be diverted to flow along that path. 
By way of example, as shown in the drawing, this path is provided by a 
C-shaped member 16 composed of a high permeability steel of the type 
customarily employed for transformer cores. The ends of member 16 face the 
periphery of shaft 2 and form a small air gap therewith. With member 16 in 
the position shown in the drawing, a portion 18 of the magnetic field in 
shaft 2 will be diverted to flow along the path defined by member 16. 
When member 16 is held stationary, and shaft 2 rotates, the magnetic field 
in member 16 will vary in magnitude at a frequency equal to the speed of 
rotation of shaft 2. The peak-to-peak variation of this magnetic field 
variation will be proportional to the intensity of the magnetic field in 
shaft 2 and thus to the magnitude of the field current flowing through 
conductors 4 and 6. 
In order to monitor the current flowing through conductors 4 and 6, a 
series-wound coil 22 is formed around member 16, as shown in FIG. 2. The 
time-varying magnetic field within member 16 will induce a voltage in coil 
2 and, for a given shaft speed, this voltage will be directly proportional 
to the intensity of the magnetic field in member 16 and, as a result, 
directly proportional to the current flowing through conductors 4 and 6. 
Therefore, the voltage induced in coil 22 can be calibrated to provide a 
direct measurement of the field current amplitude. 
Preferably, member 16 is formed so that its free ends, which are spaced a 
small distance from the periphery of shaft 2, are located adjacent 
diametrically opposite points on shaft 2. Member 16 can be made of any 
suitable high permeability material. For example, member 16 can be made of 
high permeability oriented steel laminations or a high permeability 
ferrite material. When member 16 is composed of laminations, their major 
surfaces will be perpendicular to the axis of shaft 2. The spacing between 
the free ends of member 16 and the periphery of shaft 2 is selected to 
provide a sufficient magnetic coupling between shaft and member 16, while 
assuring that the free ends of member 16 will remain out of contact with 
the shaft periphery. 
In general, and as is known, the magnitude of the voltage induced in coil 
22 for a given field current magnitude will be inversely proportional to 
the length of the air gaps between member 16 and the periphery of shaft 2 
and directly proportional to the number of winding turns in coil 22. 
One exemplary embodiment of the present invention could have the following 
dimensions, which have been determined by calculation, for the case of a 
small generator where shaft 2 has an outer diameter of the order of 15 cm 
and a bore diameter of the order of 7.5 cm, the bore enclosing insulation 
8 and conductors 4 and 6. For this example, it is assumed that the full 
load field current of the generator is 600 A. For this example, member 16 
could have the form shown in FIGS. 1 and 2, with a dimension parallel to 
the axis of shaft of the order of 2.5 cm and a dimension in the radial 
direction of shaft 2 of the order of 5 cm. The radial air gap between each 
free end of member 16 and the peripheral surface of shaft 2 can be 
selected to have a value of the order of 0.16 cm. If coil 22 is composed 
of 1000 turns, the peak voltage induced therein would be of the order of 
160 mV. Of course, these dimensions and the precise shape of member 16 can 
be varied without departing from the concept of the invention. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes, and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.