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.