Fault Diagnosis of Rotating Rectifier Based on Harmonic Features

Theoretical analysis of the rotating rectifier commutation process was described and the influence by the commutation reactance during commutation process was detailed. The harmonic content of the exciter current was quantitative analyzed when the rotating rectifier in the normal operating mode, the open circuit fault mode and the short circuit fault mode. Moreover, this feature is used as the basis for fault diagnosis of the rotating rectifier. The results which were verified by simulation and experiments show that the harmonic content of the exciter current can be used as a basis of the fault detection of the rotating rectifier.


Introduction
Rotating rectifier is an important component part of the brushless motor. The detection and diagnosis about the condition of the rotating rectifier are significant to ensure the safe operation of the brushless motor. When the brushless motor is running, the armature current of the exciter induces the harmonic current in the exciting current of the exciter through magnetic coupling. The analysis of the electromagnetic coupling process is the foundation of the rotating rectifier fault diagnosis [1][2]. The existing literature on the harmonic content of the exciter current, when rotating rectifier on fault state, is more on direct verification by simulation or experiments and few on quantitative analysis. This paper makes fast and accurate judgments on the fault diagnosis of the rotating rectifier, through the quantitative analysis on the harmonic content of the exciter current when the rotating rectifier in the normal operating mode, the open circuit fault mode and the short circuit fault mode

Analysis on commutation reactance influence
The excitation system of brushless motor is equivalent to the simplified circuit in Figure 1 The field winding of the main generator is equivalent to a high-inductive load on the DC side of the rotating rectifier. The idealized assumptions on Figure 1 are as follows [3][4][5][6][7]:  The AC side of the rotating rectifier could induct three-phase symmetrical AC voltage, which is expressed as:  The output current of the rotating rectifier is DC current.  The armature winding resistance and the forward voltage drop of the diodes are ignored. On the General operating condition, the rotating rectifier can be divided into three operating modes according to the number of the conductive diodes [8][9][10].
 Conduction mode: There are two conductive diodes of the rectifier bridge.  Commutation Mode: There are three conductive diodes of the rectifier bridge.  Short-circuit mode: There are more than three conductive diodes of the rectifier bridge and AC-side is in three-phase short-circuit state. The commutating reactance of the rotating rectifier is expressed as:

Analysis on commutation reactance threshold
When the commutation of the rotating rectifier overlap angle: , the three-phase armature winding current and voltage analysis are as follows: For phase A current, when 0 t   , diode D1, diode D4, diode D6 are in conductive state, as shown in Figure 2 (a), the current commutation process is from phase B to phase C. When

 
, phase B and phase C commutation process ends, as shown in Figure 2

. Illustration of commutation process
It is calculated: Ignoring the diode voltage drop, the average load DC voltage is calculated as follows: After simplification:  degrees respectively leading and lagging.

Analysis on the harmonic content of the excitation current in general condition
It is assumed that there is a brushless excitation generator with rated frequency of 50 Hz. It can be calculated that the commutation reactance of the rotating rectifier 0.34 l x  by the exciter parameters. The main generator field winding impedance can be equivalent to a large inductance connecting the resistance of 0.77Ω in series. The effective value of the exciter output voltage According to the generator excitation voltage mathematical equation: Where: It can obtain that when the rotating rectifier operating in general conditions, the amplitude of the 6th degree harmonic is the largest in the harmonic current of the excitation current.

Analysis on the harmonic content of the exciting current in open circuit fault
Therefore, when the rotating rectifier in a single diode open case, the harmonic content of exciter excitation current is characterized that the amplitude of the first harmonic is the largest In the harmonic current of the excitation current, and the amplitude of Second harmonic, the amplitude of third harmonic amplitude, the amplitude of fourth harmonic corresponding to the ratio of the amplitude of the first harmonic as follows:i fd2 =0.77; i fd3 =0.39; i fd4 =0.03.

Analysis on the harmonic content of the excitation current in short circuit fault
When the No. 1 diode of rotating rectifier is short-circuit, , the effect of the armature resistance can be ignored because the commutation reactance is much larger than the armature resistance. To highlight the main contradiction, when one diode short-circuit, it approximate that the waveform of armature winding current is the three-phase sine wave. On the basis of neglecting the armature resistance and the DC side current of the rotating rectifier. It Consider that phase A winding has only negative current flowing, and phase B, phase C winding have only forward current. The simplified circuit is shown in Figure 3. Obtained: And the DC component of the three-phase current are obtained: The obtained three-phase current expression is substituted into the mathematical equation of motor, and it can get that the excitation current of the exciter is mainly composed of the DC component It can considere that the excitation current of the exciter mainly contains the DC component and the first harmonic content When the rotating rectifier in a short-circuit fault.

Verification by simulation and experiment
The simulation model brushless excitation generator is built in the Matlab shown in Figure 4. The rotating rectifier under general operating condition, open circuit condition and short circuit condition is researched by simulation and experiment, and the corresponding excitation current is acquired. The harmonic analysis and contrast on the excitation current waveform of Simulation and experiment are shown in Figure 5.
Waveforms of excitation current harmonic content of excitation current  The results of simulation and experiment on the harmonic content of excitation current Under different conditions have little difference with the theoretical analysis, which can prove that the quantitative analysis on the harmonic content of the excitation current of the exciter was correct, and the feasibility of using harmonic content of the excitation current as a basis for fault diagnosis of rotating rectifier is also verified.

Conclusion
In this paper, the commutation process of rotating rectifier under general operating condition, open circuit condition and short circuit condition are analysed, and the different harmonic content of the exciting current of the exciter under these three conditions are obtained quantitatively by theoretical analysis, simulation and experiment. It can get that the harmonic content of the exciting current have corresponding characteristics when the rotating rectifier in different conditions. Therefore, the harmonic content of excitation current of the exciter can be used as the fault diagnosis basis, which can improve the rapidity and accuracy of the fault diagnosis of the rotating rectifier.