Experiment Name: Perform insulation resistance test of any one motor/transformer.

 

Bundwan Polytechnic

Department of Electrical Engineering

Subject: Electrical Testing and commissioning 

By- Palash Das

Lecturer in Electrical Engineering

Experiment To find out the insulation resistance (IR) of any motor or transformer.

Experiment Number:

Aim: To find out the insulation resistance between conductors .

To check the health of winding insulation

To identify moisture, dirt, or deterioration

To prevent electrical faults and hazards

Experiment Date:

Experiment Submission Date:

Theory:

Insulation resistance can be measured by Megger.

 Megger consists of a built in mega ohm meter and a d.c. generator. The generator is driven by hand or by a motor.

Meggers (Mega ohm meters) are available for d c. voltages of 500 V and 1000V. Generally in practice 500V Meggers are used.

Measurement of Insulation Resistance motors or transformer ::

Insulation resistance of an electrical equipment refers to the resistance between conducting part and earth expressed in mega homes . This  insulation resistance can also be measured between two circuits separated by insulation.

Circuit/ Image of the instrument:

Safety Precautions:

Disconnect motor from power supply.

Discharge all stored energy (e.g., in capacitors).

Ensure motor is de-energized and locked out.

Procedure:

1. Disconnect the motor from the power source.

2. Clean terminals to remove dirt or moisture.

3. Identify motor terminals (U1, V1, W1 for 3-phase motors).

4. Connect Megger leads: One lead to the winding terminal.

5. The other lead to the motor body (earth/ground).

6. Apply test voltage for 1 minute:

7. Repeat for all three phases (U1-E, V1-E, W1-E).

8. Optionally, test phase-to-phase insulation (U1-V1, V1-W1, W1-U1).

IR Value Interpretation

>100 MΩ Excellent

10–100 MΩ Good

1–10 MΩ Marginal — Monitor closely

<1 MΩ Poor — Needs maintenance

Specification:

Voltage grade- 500 Volt. 

Table/ data value:

Conclusion:

The tested motor condition is healthy. Here OL means enough insulation resistance between conductor and metalic parts.

Viva question:

1. What is megger?

2. What are the voltage grade of generally used megger available in market?Ans:500Volt and 1000 volt

3. What is the difference between insulation resistance and earth resistance?

Ans: Différence between insulation resistance and earth resistance :

Insulation resistance is measured between conductor and earth. It is of the order of several tens/thousands Mega Ohms or infinity for healthy insulation. It is measured by means of megger (mega-ohm meter), with built-in DC generator.

Earth resistance is measured for a earthed metallic body or neutral point. It is measured by means of earth resistance tester. For good effective earthing, the earth resistance should be of the order of 0.5 Ohm.

4. What should be the minimum value of insulation resistance?Ans- 0.5MOhm

5. What is the value of earthing resistance of a large sub-station? Ans-0.5Ohm

6. What is the ratio of salt and charcoal mixed around the earth electrode? Ans- 1:5

Source:Book-EC&MEE (EIMT) by Tarlok Sing. EIMT by J.B. Gupta.