SOP OF REFORMING OF DRIVES

BACKGROUND OF DRIVE REFORMING

• Variable Frequency Drives utilize Electrolytic Capacitors for storage of voltage in the inverter section of the drive. These electrolytic capacitors are made up of basically 3 parts. Two conductive plates (usually one being a metal substance similar to aluminum foil and the other being a porous material impregnated with an acidic substance similar to battery acid) that sandwich a layer of insulator material.

• The insulator material is generally made up of an oxide that is created when voltage is applied to the capacitor during manufacturing of the capacitor. Subsequent to this initial power up, every time the capacitor receives a charge it “rebuilds” this layer of oxide. As time elapses and there is no voltage applied to “reform” this layer the layer begins to degrade. This layer “thickness” is the determining factor for the voltage rating of the capacitor. If the layer degrades to a certain point one of two failures will occur.

• The two conducting materials will begin to conduct current at a high rate, and this will cause a boiling of the liquid inside the capacitor. Once this boiling begins the pressure will rise internally and the capacitor will rupture. This failure usually results in a complete destruction of the drive as these capacitors are saturated internally with an acidic mixture.

• The inrush current will arc across the oxide insulator and create a bridge between the plates. This bridge will short the plates and will cause a direct short in the power circuit that can result in other damage to the electronic components. 

REFORMING TIME

• The length of storage time after which reforming is necessary depends on the storage temperature, the manufacture and length of time voltage has been applied during the original forming in process

• To be safe capacitor should be reformed if the ambient storage temperature is 40degree celcius or higher or cell has not had full voltage applied for 12 month.

REFORMING METHOD 1

•Ensure that the converter module is disconnected from all possible power sources (all AC and DC inputs/outputs are disconnected).

•Ensure by measuring that the converter is dead (terminals (U1, V1, W1, UDC+, UDC-, U2, V2 and W2).

•Compose a reforming circuit (see the diagram below) and connect it to the input terminals of the converter module.

•Switch on the AC power supply of the reforming circuit for the time defined in section Reforming time.

•Three-phase or single-phase input, 50 or 60 Hz, without connecting a motor to the output. This voltage should be applied for a period of 1 hour (In six step, 17% of rated voltage in each step for ten minute duration).

•Output current: 0.5…1 A DC and Output voltage during reforming is 1.35…1.45·Ux. Ux denotes the nominal AC voltage of the converter.

•Switch off and disconnect the AC power supply of the reforming circuit.

•Wait for 5 minutes to let the converter DC capacitors discharge.

•Ensure by measuring that the DC terminals of the converter are dead.

•Disconnect the reforming circuit from the converter

•NOTE-: After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is  vital.

REFORMING METHOD 2A

•Ensure that the converter module is disconnected from all possible power sources (all AC and DC inputs/outputs are disconnected).

•Ensure by measuring that the converter is dead (terminals (U1, V1, W1, UDC+, UDC-, U2, V2 and W2).

•Compose a reforming circuit (see the diagram below) and connect it to the DC terminals of the converter module.

•Switch on the AC power supply of the reforming circuit for the time defined in section Reforming time.

•Output current: 0.5…1 A DC and Output voltage during reforming is 1.35…1.45·Ux. Ux denotes the nominal AC voltage of the converter.

•Switch off and disconnect the AC power supply of the reforming circuit.

•Wait for 5 minutes to let the converter DC capacitors discharge.

•Ensure by measuring that the DC terminals of the converter are dead.

•Disconnect the reforming circuit from the converter.

•NOTE-: After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is  vital.

REFORMING METHOD 2B

•Ensure that the converter module is disconnected from all possible power sources (all AC and DC inputs/outputs are disconnected).

•Ensure by measuring that the converter is dead (terminals U1, V1, W1, UDC+, UDC-, U2, V2 and W2).

•Compose a reforming circuit (see the diagram below) and connect it to the DC terminals of the converter module.

•Switch on the reforming circuit for the time defined in section Reforming time.

•Output current: 0.5…1 A DC and Output voltage during reforming is 1.35…1.45·Ux. Ux denotes the nominal AC voltage of the converter.

•Switch off the reforming circuit.

•Wait for 5 minutes to let the converter DC capacitors discharge.

•Ensure by measuring that the DC terminals of the converter are dead.

•Disconnect the reforming circuit from the converter.

•NOTE-: After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is  vital.

What are all the jobs to be executed in HT Motor Overhuling:

JOBS TO BE CARRIED OUT IN STATOR:

1. Complete dismantling of motor and removal of rotor from Stator.
2. Stator slot wedges to be removed & prepare new slot wedges for rewinding.
3. Stator winding connection to be opened  & winding data to be recorded
4. Stripping of the existing winding without any heat treatment or mechanical impact to protect the core     from unwanted Eddy Current loss and Hysteresis loss
5. The core  lamination  to  be  checked  & Flux loop  test  to  be carried  out to detect hot spot if any  & defective laminates to be replaced if required.
6. Prepare new coil with best insulating material and as per winding data recorded before stripping
7. Rewinding of stator with new copper coil.
8. The Coil should be braced properly in slot and should not be any loose fitting in slots.
9. New RTDs (PT 100) to be installed as per previous slot location.
10. Preheating, varnishing (Dr. Beck’s H-class) & curing the winding in oven.
11. Preheating & pouring Epoxy( Dr. Beck’s H-class) to the overhang portion of the winding to achieve rigidity.
12. Applying moisture protectant , finishing coat,  anti-tracking Red Gel of Dr. Beck on winding and curing the same
13. Final testing( IR,PI, Winding Resistance & Inductance, Surge Comparison, A.C.H.V, Tan- Delta as per IS  & VDE Norms)
14. Motor space heaters to be checked and replaced if defective
15. Both end cover / shield of bearing housing to be checked for concentricity of bearing housing.  If required,  remetalling  of  bearing  housing  in  end  cover/shield.
16. Checking of bearing:  if required replacement of new bearings

MECHANICAL JOBS OF ROTOR:

1. Inspection of rotor shaft after dismantling of the motor. .  If required remetalling.
2. Inspection of rotor stamping condition, replace/repair if required.
3. Dynamic Balancing of rotor.
4. Ensure to fix the Insulated bearing ring and washers in NDE bearing. Replace if damaged
5. Assembly / box up of motor.
6. Replace the cooling duct rubber sealing
7. Painting of motor.

SOP FOR EARTH RESISTANCE CHECKING

Theory of Operation:

1. The principle used in measuring the earth resistance is based on simple ohm’s law.

2. Four electrodes A,B,C & D are buried in the earth, the resistance of which is to be tested at a distance of 20 meter from each other as shown in fig 1.

3. AC Signal is applied to electrodes A and D and voltage developed across electrodes B and C due to flow of current through the earth is measured by ammeter.

4. If the current is constant, the voltage measured will be directly proportional to the earth resistance.

Steps to measure earth resistance as per Digital Earth Resistance Tester Manual:

1. Take work permit and Isolate the equipment. The Earth electrode C1, whose resistance to be measured must be disconnected from its connection to the site.

2. To obtain a correct measurement, the three electrodes must be in straight line and the well aligned.

 3. The Auxiliary current electrode C2 is placed at the distance D, it is approximately 30Mtrs from Earth Electrode C1.

 4. The potential probe (Reference Probe P2) is placed in the soil in a straight line-away from the earth electrode Normally, Spacing of 20Mtr.

 5. Now short the C1, P1 terminal of earth tester and connect that point to earth electrode, P2 to potential probe (Reference Probe) and the last point C2 is connected to Current Electrode or Auxiliary electrode. Connect the earth tester as shown in the Fig – 2. Press start and read out the RE (resistance) value.  

 6.  This is the actual value of the ground electrode under test.