Motor Determination / Megohmmeter Lab

Name: ______Date: ______

In-class Activity

This lab is worth 10 points.

Use high voltage at a low current level to determine insulation conditions. Typically, used to test AC and DC motors, transformers, commercial wiring and cable testing. Most newer megohmmeters are digital, while older variants use an analog scale, which require the operator to wind up a high voltage charge.

Common Application Notes:

1. Moisture will influence the readings.

2. Due to high voltage be extremely cautious

3. Avoid working in an environment with flammable fumes

4. Verify all stored energy is discharged!!!!! CAPACITORS

5. Clean terminals to minimize tracking

Common Features of the Digital Megohmmeter Model 1026

  1. resistance is measured from 0.1Ω to 4000MΩ
  2. insulation resistance measured from 1kΩ to 4000MΩ
  3. 3 test voltage ranges from 250, 500 and 1000 volts.
  4. Safety Specifications include IEC 1010, Cat. III, 600v

Megohmmeter Functions:

ACV. Measures AC voltage by setting V setting. Press HOLD to take measurements.

DCV. Measures DC voltage by setting to V setting. Press HOLD to take measurements.

4000Ω Measures low resistance of a circuit, not to be used for the resistance of the motor windings. This feature is used for motor ground check.

MegOhm Insulation Resistance Test Function. This function applies a high voltage to a coil of wire, transformer or motor winding and determines the insulation of it. It does this by taking Voltage / Current to produce a Resistance value.

Do not activate the test before the leads are properly connected to the sample under the test.

Never take a measurement of a live sample, ie live motor.

Never take a megohmmeter reading of a VFD Drive or any Drive for that matter. Only a disconnected motor.

Manual Mode: Press and hold the TEST button to activate the test voltage source until you obtain a stable reading. A periodic beeping will warn that there is a high voltage output. Release the TEST button to stop test voltage output. The test voltage will continue to be applied until the TEST button is pressed again. Press the TEST button again to stop the test voltage output. A series of short beeps indicate that there is discharge occurring. When the beeping stops the discharge is complete. The test result will be displayed automatically.

Lock Mode: Press the LOCK button to enter the Power Lock operation mode. Press TEST button once to activate the test voltage source. A periodic beeping will warn that there is a high voltage output. Release the TEST button to stop test voltage output. The test voltage will continue to be applied until the TEST button is pressed again. Press the TEST button again to stop the test voltage output. A series of short beeps indicate that there is discharge occurring. When the beeping stops the discharge is complete. The test result will be displayed automatically.

The purpose of a Megohmmeter is to supply a high voltage potential across motor leads and measure the amount of current flow. If we apply 1000V and get a few microAmps that means the insulation has Megohms or resistance.

Motor windings, magnet wire, are insulated from each other by a thin film or varnish. Over time the heating and cooling will cause the varnish to break down. The megohmmeter will check the deterioration of the windings.

There is a very general rule of thumb that says the motor should have at least 1MΩ of resistance of insulation for each 1000V of equipment voltage rating. So a 120Vac motor needs at the very minimum 1MΩ.

Motor Winding Measurement Lab:

Determine the winding arrangement for the motor. Set the Megger to the 4000Ω range. You should read very low resistance.

Place the 3 phase motor in question on your test bench and using the Megger measure the resistance of T1 through T 2. Repeat for T 2 – T 3 and T 3 – T 1.

You will get a table similar to the following:

Terminals / Resistance
T1 – T2 / 20Ω
T2 – T3 / 18Ω
T3 – T1 / 20Ω

All motors should read about 10% of each other. If not then there is an issue. Either poor connection, or a shorted winding.

Say you do a measurement and get the following results:

Terminals / Resistance
T1 – T2 / 20Ω
T2 – T3 / O. L. Ω
T3 – T1 / O. L. Ω

This is an indication of an open leg. This problem is called single phasing. This means the motor is bad.

Some times the motor is not exactly like the diagram seen above. The below figure shows a 3 phase wye connected motor. There are usually 2 ways in which to wire this motor up, either a high voltage or a low voltage application.

The only way to truly know how that motor is wired up is to read the nameplate and then use an ohmmeter to measure the resistance of the coils in the motor. Below is a graph used to measure the coils. For the motor that you have measure all of the coils and determine how it is set up.

Fill in the table

Coil Termination # / T1 / T2 / T3 / T4 / T5 / T6 / T7 / T8 / T9
T1 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T2 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T3 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T4 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T5 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T6 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T7 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T8 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω
T9 / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω / Ω

Now draw a schematic of the motor’s windings and indicate where L1, L2 and L3 will be located.

Motor Insulation Measurement Lab:

1. Measure the insulation of the motor’s windings.

2. Set the Megger to read at the 500 V setting.

Connect the red (+) and black (-) leads together and press the test button.

The meter should read 0Ω. If the meter does not show that then it is defective. It

needs to be replaced.

3. Connect the red(+) test lead to a good ground or metal motor frame.

4. Connect the black (-) test lead to the winding leg under test.

Press and hold the Test Button.

As a side note many Meggers should measure the resistance by taking a few measurements. That is usually a series of tests are done and repeated within a 1 minute to 2 minutes. So be prepared to relax and wait for credible results.

6. Fill in the table

Terminals / Resistance
T1 – Ground / MΩ
T2 – Ground / MΩ
T3 – Ground / MΩ
T4 – Ground / MΩ
T5 – Ground / MΩ
T6 – Ground / MΩ
T7 – Ground / MΩ
T8 – Ground / MΩ
T9 - Ground / MΩ

7. Does the resistance reading confirm the windings are good or bad.

Further Questions:

8. Go on the internet and find out what I E C stands for.

9. There are 4 types of CAT designated for electrical meters what are the 4 types.

10. Why is it important to use the appropriate CAT meter? Besides safety.

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