Apparatus and method for detecting a rotating rectifier fault

A microcomputer implemented apparatus and method for detecting the occurrence of a shorted diode in the rotating rectifier of a brushless alternator. The alternator current, voltage and power output and the alternator temperature are measured. The anticipated exciter field current is determined for the alternator output and operating conditions and compared with the actual exciter field current. An actual exciter field current in excess of the anticipated current indicates a rectifier fault.

FIELD OF THE INVENTION 
This invention relates to an apparatus and method for detecting the 
occurrence of a fault in the rectifier of the rotating exciter of a 
brushless alternator. 
BACKGROUND OF THE INVENTION 
A typical brushless alternator has an exciter field winding which is a part 
of the stator structure of the machine. The rotor of the alternator 
includes an exciter armature in which an alternating output is developed. 
A rectifier which is a part of the rotor is connected with the exciter 
armature and provides direct current excitation for the main field 
winding. The alternator output is developed in the stator armature 
windings. The exciter field is connected through a regulator transistor 
with a DC source. A voltage regulator circuit provides a pulse width 
modulated drive signal to the regulator transistor in accordance with the 
output voltage of the alternator. 
A failed diode in the rotating rectifier causes malfunctioning of the 
regulator and must be detected to avoid damage to the system. Prior 
circuits have, for example, responded to the frequency and amplitude of 
the ripple current in the field winding, Calfee et al. U.S. Pat. Nos. 
3,210,603 and Hyvarinen 3,534,228, or compared the exciter field voltage 
with the vector sum of the alternator armature current and voltage in a 
differential protection system, South U.S. Pat. No. 3,705,331. 
SUMMARY OF THE INVENTION 
The present invention provides an apparatus and method for detecting a 
rotating rectifier fault with which the actual exciter field current is 
compared with the exciter field current which is anticipated for the 
alternator output and operating conditions. More particularly, in one 
embodiment of the invention, the anticipated field current is calculated 
in accordance with the alternator KVA output and power factor, and the 
alternator operating temperature. In another embodiment the anticipated 
exciter field current is calculated in accordance with either the high 
phase current or the average phase current of the alternator. 
In the preferred form of the invention, the alternator output and operating 
conditions are measured, connected by a multiplexer and analog-to-digital 
converter with a microcomputer which is programed to calculate the 
anticipated exciter field current and to compare the actual exciter field 
current with the calculated current. 
Further features of the invention will readily be apparent from the 
specification and from the drawings.

A brushless alternator system, elements of which are shown diagrammatically 
in FIG. 1, has an exciter with a stationary field winding 10. The rotor of 
the machine is driven by a prime mover (not shown) and includes an exciter 
armature winding 11 in which an alternating voltage, typically three 
phase, is generated. A rectifier 12 is connected with the armature and 
provides DC current to the main alternator field winding 13. Rectifier 12 
is typically a 3-phase, full-wave rectifier using six semiconductor 
diodes. The alternator output is developed in stator 14, providing 3-phase 
power to a load (not shown). Power for the exciter field is, in the system 
illustrated, developed in the stator winding 15 of a permanent magnet 
generator (PMG) which has a permanent magnet (not shown) mechanically a 
part of the main machine rotor. The 3-phase output of the PMG is connected 
with a 3-phase full-wave rectifier 16 which provides DC power to a pulse 
width modulated amplifier 17 that controls the current to exciter field 
winding 10. Voltage regulator 19 is responsive to the output voltage and 
current of the main machine providing an appropriate drive signal to pulse 
width modulated amplifier 17. 
Alternator 25, FIG. 2, has a 3-phase output A, B, C to which a load (not 
shown) is connected. Current transformers 26.sub.A, 26.sub.B, 26.sub.C 
measure the three phase currents I.sub.A, I.sub.B I.sub.C. The actual 
current I.sub.F to the exciter field winding 10 is measured by the voltage 
across resistor 27 in series with field current amplifier transistor 28. 
Low pass filter 29 removes the pulse frequency. 
An oil coolant flows through the alternator. The inlet and outlet 
temperatures are measured, as by thermal sensors 30, 31. The phase 
currents, phase voltages V.sub.A, V.sub.B, V.sub.C, exciter field winding 
current and coolant temperature signals are sampled periodically by an 
analog signal multiplexer 34. The signals are connected one at a time with 
an analog-to-digital signal converter 35 and the digital signals are 
connected with programed microcomputer 36. The microcomputer may, for 
example, be a part of a control unit (not shown) and detects a rectifier 
fault as one of a number of functions in the control and operation of the 
alternator. 
The microcomputer calculates the anticipated exciter field current in 
accordance with the alternator output and operating conditions. The 
anticipated field current is compared with the actual field current. If 
the actual current exceeds that which is anticipated by an excessive 
amount, an indication is given of the occurrence of a shorted diode in the 
rotating rectifier. The fault indication may be in the form of an alarm to 
an attendant or operator or it may cause the control unit to terminate 
operation of the alternator. 
The operation of the programed microcomputer 36 with respect to rotating 
rectifier fault detection will be described in connection with the 
functional diagram of FIG. 3. The three phase currents I.sub.A, I.sub.B, 
I.sub.C are multipled by the respective phase voltages V.sub.A, V.sub.B 
and V.sub.C at Multiply blocks 40, 41 and 42. The output of each Multiply 
block is the volt-amperes supplied to the load by one phase. The three 
volt-ampere signals are combined at Add block 43 to provide a signal KVA 
representing the total volt-ampere output of the alternator. This signal 
is connected with a Function block 44 which develops an output signal 
representing the exciter field current. The Transfer Function of block 44 
is shown in more detail in FIG. 4 where the exciter field current is 
plotted as a function of KVA load for a power factor of 1.0 and an 
operating temperature of 100.degree. F. 
The signal representing the KVA output of the alternator is also connected 
with a Divide block 45 at which the real power in kilowatts delivered by 
the alternator is divided by the KVA output to determine the load power 
factor PF. The anticipated field current signal from block 44 and the 
power factor from block 45 are combined in Function block 46 to provide an 
output representing the exciter field current compensated for power factor 
in accordance with the function illustrated in FIG. 5. With a load power 
factor less than 1.0 the anticipated exciter field current is greater than 
that developed by Function block 44. 
The signals representing the coolant temperature out and in are connected 
with Subtract block 48. The difference is divided by 2 at block 49 and the 
quotient added to the coolant In temperature at block 50, providing a 
signal representing the average temperature of the generator. The output 
of Power Factor Function block 46 and the generator temperature signal are 
connected with Function block 52 which develops a temperature compensated 
signal in accordance with the relationship illustrated in FIG. 6. The 
resulting calculated or anticipated field current signal is subtracted 
from the actual field current signal at summer 53. The difference and a 
reference or threshold signal are connected with comparator 54. If the 
difference exceeds the reference, a rotating rectifier fault is indicated. 
An alarm is given or the alternator shut down as desired. 
A simplified implementation of the invention is illustrated in FIG. 7. The 
signals representing the three phase currents I.sub.A, I.sub.B, I.sub.C 
are connected with a detector block 60 which may, for example, select the 
highest phase current or determine the average of the phase currents. The 
signal I.sub.ALT representing the alternator current is connected with 
Function block 61 which develops a signal representing the anticipated 
field current required for the alternator output. The anticipated field 
current signal is subtracted from the actual field current signal at 
Summer block 62. The difference and a threshold or reference signal are 
connected with comparator 63. Again, if the actual field current signal 
exceeds the anticipated field current signal by more than a selected 
amount, a rotating rectifier fault is indicated.