Shorted rotating diode detection and protection

An electric generator with a rotating diode fault detection device built in that operates by comparing a voltage buildup across the exciter DC supply with a preset threshold value and determining if a fault condition is present based on the comparison.

BACKGROUND OF THE INVENTION

The present application relates to an aircraft power generation system, and specifically to detecting faulty rotating diodes within a rotating DC rectifier.

It is common in the aerospace industry to use a 3 phase synchronous generator to generate electrical power in an aircraft. A synchronous generator requires a rotating magnetic field to generate 3 phase voltages at its terminals. The rotating magnetic field is provided by a field winding carrying direct current (DC) current. The source for the DC current is provided by a 3 phase bridge rectifier which in turn is powered by exciter alternator. The exciter alternator is another synchronous machine that is excited by a stationary field winding current. An exciter field driver is used to control the exciter field current. The exciter field driver is part of a generator control unit. One of the functions of the generator control unit is to regulate the output voltage of the aircraft generator by controlling the exciter field current by means of the exciter field driver.

In an aircraft generator, failure of a shorted diode in the rotating bridge rectifier leads to loss of capacity to regulate the generator output voltage. This failure needs to be detected quickly to prevent further damage to the synchronous machine and any potential damage to the generator control unit that regulates the output voltage of the generator.

There are several methods in the prior art that can detect a shorted diode failure. These methods range from detecting loss of generating capacity to detecting ripple content in the exciter field current. However, these methods are relatively slow and, in aircraft generators that are designed to provide motoring function during engine start, a failed diode can be damaging to the generator control unit if not detected quickly.

Additionally, it is known that if a short circuit were to occur somewhere else in the circuit (such as in the load) it could skew the results of some detection methods and cause either false alarms, or a failure to recognize a faulty diode.

SUMMARY OF THE INVENTION

A power generating system contains an exciter driver, exciter, a rotating bridge rectifier and a main generator. The exciter driver is powered by DC voltage supply generated by rectifying the output voltage of a 3 phase Permanent Magnet Generator (PMG). Also included is a voltage buildup detection unit which can detect at least one voltage buildup characteristic across the PMG DC rectifier. Additionally contained within the power generation system is a generator controller. The generator controller compares the voltage buildup characteristic from the DC rectifier to a threshold characteristic. In the case of a shorted rotating diode, there is significant voltage buildup. If the voltage exceeds the threshold, the exciter driver is turned off by the generator controller.

These and other features of the present invention can be best understood by one skilled in the art from the following specification and drawings. A brief description of the drawings that accompany the detailed description follows.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1illustrates a simplified synchronous generator commonly used in the art. A generator300such as the one illustrated contains a rotating rectifier10which is between the main rotor windings90and the rotating field windings50. Also shown inFIG. 1is a generator control unit500which is controlling the field60of the exciter generator. The controller has inputs200relating to each of the three phases and the neutral for the main stator windings30, and for the PMG windings100.

FIG. 2illustrates in more detail the circuit for a diode fault detection and protection circuit according to one embodiment. In this circuit the generator controller80has additional inputs accepting a PMG DC link voltage measured across a voltage clamp70. A faulty rotating diode can cause a voltage buildup across the DC link resulting in an increased PMG DC link voltage from the nominal. It is additionally anticipated that other measurements corresponding to the voltage buildup could be used in place of the voltage clamp70measurement and achieve the same result.

When the voltage buildup sensor400detects a voltage buildup, a signal is sent along wires210and211to the generator controller80indicating the level of voltage buildup detected. The generator controller80then checks the voltage buildup level against a preset threshold voltage buildup level and determines if a fault condition is present based on that comparison. Alternatively the generator controller80could check the rate of increase in buildup voltage to determine if a fault condition is present. It is also anticipated that both a voltage buildup rate and voltage buildup level could be checked to create a more robust system.

Voltage clamps, such as the voltage clamp70utilized in the illustrated embodiment ofFIG. 2, work by limiting the voltage across themselves to a maximum value. The voltage clamp70is connected in parallel with the device that the user desires to clamp. In this case the voltage clamp70is connected in parallel to the rectifier20. Limiting the voltage across the voltage clamp70has the effect of simultaneously limiting the voltage across anything that is connected to it in parallel. Some voltage clamps can only handle a certain amount of power before they start allowing an increase in voltage above their clamping value. This type of voltage clamp is called a limited authority voltage clamp. One embodiment of the current design utilizes the power limit in that it allows the voltage clamp70to remove voltage buildup that occurs as a result of fault conditions at or in the load, while simultaneously allowing voltage buildup as a result of a rotating diode fault. This use is possible because the voltage buildup across the voltage clamp in a rotating diode fault would exceed the voltage clamp's power limit.

After the generator controller80receives an input signal along wires210and211containing the voltage buildup measurements, the generator controller80checks those measurements against a preset threshold. In one embodiment the threshold used is a magnitude of voltage buildup. In this embodiment if the measured voltage buildup magnitude exceeds the preset threshold magnitude, then the generator controller80determines that there has been a rotating diode fault. In an alternative embodiment the generator controller80checks the rate of change of the measured voltage buildup to determine if the rate of change is above a preset threshold rate of change. In this embodiment if the rate of change is exceeded, then the generator controller80determines that there has been a rotating diode fault. In still a third embodiment the generator controller80checks both the measured voltage buildup magnitude and the rate of change of the voltage buildup. In this third embodiment, as in the other two embodiments, a value exceeding the threshold indicates that there is a diode fault in the rotating rectifier10component.

In addition to a voltage buildup sensor400measuring the voltage buildup, the measured value is transmitted to a generator controller80. The generator controller80checks for a diode fault. If a diode fault is detected, then the software sends a signal to the exciter field driver40tripping the exciter field driver40. When the exciter field driver40is tripped the exciter generator is turned off. This effectively de-energizes the circuit. De-energizing the circuit prevents the continued application of power to the faulty diode within the rotating rectifier10. Additional attempts to apply power to the exciter field driver40would fail as long as it remained tripped. This prevents the faulty diode from allowing any harm to the generator control unit500or to any other component as a result of the fault condition and allows a person to safely perform the appropriate maintenance to fix the problem.

Although the above describes an application in an aircraft starter generator, it is anticipated that various embodiments could operate in any system with a controller and where diode fault conditions are possible.

The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.