Traction alternator rectifier maintenance and test procedure.

          Eleven cases of rectifier failure have been observed in in last eighteen months. Details are given below:

Loco No	Date of fail	Shed	Reason of failure
16007	3.3.2010	N	Thermal switch damaged due to flash over in rectifier panel
16281	24.8.2011	M	Rectifier cable damaged
16518	10.7.2011	M	Rectifier +ve bus bar insulator punctured
20074	31.3.2010	S	One fuse blown
12178	8.4.2010	S	Two nos. fuse blown and two nos. diodes short circuited.
12185	30-05-2010	S	Two nos. diodes short circuited.
20070	03-09-2010	S	One diode open circuited
20073	01-03-2011	S	Five fuse blown and three diodes short circuited
20070	10-08-2011	S	Six diodes open circuited and five nos. Fuse blown
12176	18-08-2011	S	Rectifier panel fuse blown.

Field visit and interaction with maintenance supervisors revealed gross lack of knowledge on maintenance and testing of rectifiers. All sheds are directed to implement the instructions issued below during maintenance of rectifiers.

1.   Cleaning:

Dirt causes poor heat dissipation of diodes and with increasing heat   diodes behaviour changes. Dirt and water vapour provides creepage path causing damage of rectifier.

Check rectifier panels for dust and dirt. If dirt presence is more check cyclonic filter condition and rectifier panel sealing and TA compartment partition wall sealing and take necessary corrective action.

1.1  Running schedule cleaning–

Remove the rectifier cover and blow clean compressed air at approximate 2 kg/cm² to remove dust, dirt and soot deposition. In case soot/oil deposit is found on glass epoxy items, heat sink and bus bars, it may be cleaned with the help of cloth soaked in cleaning solvent. Use long bottle brush cleaner for better accessibility.

1.2 Heavy  schedule cleaning For ALCO locomotives :

The complete rectifier panel shall be unloaded in yearly and above schedule. Clean each part properly with help of cloth soaked in recommended solvent. (Dober Chemical Corporation Cleaner 6006, or Turco Chemical Company Steamfas).

For EMD locomotives :

The complete rectifier panel shall be unloaded in yearly and above schedule. Clean each part properly with help of cloth soaked in recommended solvent. (Dober Chemical Corporation Cleaner 6006, or Turco Chemical Company Steamfas).

1.3 CAUTION-

The contact surfaces of the diodes and heat sink assemblies must not be cleaned with an abrasive material or wire brush. Such cleaning will destroy the finish and reduce heat rejection capability.

Water or cleaning solution allowed to contaminate the arc quenching sand inside the fuse body can cause the fuse to explode when it is required to isolate a shorted diode.

2.   Inspection for Blown FUSE –

For ALCO locos- Check fuses for any signs of cracks or burning. Flag of the micro switches on all the fuses should be in closed condition. Check functioning of micro switch and LED by manual operation.

For EMD Loco:- check fuses for any signs of cracks or burning. Check fuse indicating pin. If it protrudes 5mm from the normal position remove the fuse and check fuse continuity.  If the fuse is found open, check corresponding diodes of the phase.

3.   Inspection of Snubber circuit

3.1                Snubber resistors-

Visually check Snubber resistor for open circuit, overheating and loose terminals. Check resistance if any abnormality is noticed. Snubber resistors used in alternator-mounted rectifiers are of rating 20 ohm, 250 watts (for BHEL rectifiers) and 22 ohm, 50 watts (for HIND rectifiers). Replace the resistor if it's value is out of the above range or if there is physical damage. Snubber circuit has been removed in EMD make TA ,but it is still in service in CGL and DMW make TA 17 in EMD locos. Check resistance and capacitance value during running schedule.

3.2                Snubber capacitors-

Visually check Snubber capacitors for deformation due to short circuit, overheating and loose terminals. Check capacitance if any abnormality  is noticed. Snubber capacitors used in the alternator-mounted rectifiers are of rating 1.5 MFD 3000V (for BHEL rectifiers) and 0.47 MFD, 2000V (for Hind Rectifiers). The capacitors should be checked using an analogue multi meter. A healthy capacitor shall give a kick and show very high resistance. A capacitor showing short circuit or low resistance (less than 10 K ohm) should be rejected. The capacitance value should be measured using a portable capacitance meter. The capacitance value should be within +10% range. Record of measurement should be kept. Any capacitor showing a reduction in capacitance by 10% over the last measurement should be rejected.

4.   Diode inspection-

If any fuse in the rectifier panel is found open check corresponding diodes of the phase. To check the diode following test can be followed-

4.1                Diode resistance check -

Check forward resistance and reverse resistance of diode.

For Alco locos if reverse resistance is high (> 100 kΩ) and forward resistance is low (About 850 Ω) than diode is perfect but if the reverse resistance is low along with the forward resistance than diode should be tested for Reverse Voltage Test.

For EMD locos check the above and if reverse resistance is high (> 30 kΩ) and forward resistance is low (About 10-20 Ω) than diode is perfect but if the reverse resistance is low along with the forward resistance than diode should be tested for Reverse Voltage Test. The 30K-ohm value is for an individual diode isolated from the circuit. A diode not isolated from the circuit should register greater than 2K Ohms for EMD locos.

4.2                Reverse Voltage Test – It should be conducted by connecting diode in reverse direction and increasing voltage from 0 to 1000 V. Measure correspondingly reverse leakage current (Ir).

If the Ir is less than 30 mA accept the diode and if the Ir value is more than 30 mA, reject the diode.

It is better idea to have paired diodes i.e. diodes having same leakage current in one panel.

4.3                DC Megger Test- Use a hand-cranked or battery operated Megger to test the diode. Connect one lead to diode pigtail and other lead to base of diode. If using a hand crank meter, it is important to crank slowly at first, then gradually increase the cranking speed to medium. Depending on polarity, the reading should be zero (blocking) when connected one way, then by reversing test leads, there should be a definite megger reading of at least one Megohm or better.

4.4                DC Voltage Test-

Connect a 110-volt incandescent lamp in series with the suspected diode, and apply 64 vdc (from the locomotive batteries) to this series connected circuit, then, reverse diode leads and observe the lamp response.

• If the diode is good, a light turns ON when the diode is connected one way, and turns OFF when connected the other way.

• If the lamp turns ON both ways, the diode is shorted out.

• If the lamp remains OFF when the diode is connected both ways, the diode is open circuit.

4.5                AC Voltage Test-

Connect a 60 to 100-watt, 110-volt incandescent lamp in series with the suspect diode. Apply 110-115 volts, 60-hertz shop power to the circuit, then reverse diode leads and observe the lamp response.

• If the diode is good, the lamp turns ON at about half of its normal light level (half wave rectification), when the diode is connected in one   direction, and is OFF when connected the other way

• If the lamp is close to full light level, the diode is shorted.

• If the lamp does not light, the diode is open.

5.   Assembly-

Replace defective diodes with good diodes of identical polarity, and voltage class (or higher voltage class).

For EMD locos apply a thin coating of compound 8346481 to the base of the diode hex to cover the surface. Do not apply on threads. With special diode socket and 0 – 50 ft.-lbs. Torque wrench, torque diodes to 45 to 47 Nm (33 to 35 ft.-lbs.). Torque diode terminal lug bolts to between 15 to 18 Nm (11 to 13 ft.-lbs). Tightness as recommended by OEM should be ensured by using torque wrench.

For ALCO locos apply thin layer of heat sink compound such as Dow corning make DC340R or equivalent, smear it out with clean cotton then place diode on heat sink. Place the clamp, insert Allen screws (M8) then tighten it by torque wrench diagonally setting torque at 12.5 NM.

6.   Replacement-

All diodes should be replaced after 10 years of service due to thermal/mechanical degradation.