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.
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
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.
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
Motor Winding Measurement Lab:
Determine the winding arrangement for the motor. Set the Megger to the 4000Ω range.
You should read very low resistance.
MEGGER
4000Ω
BLACK
RED
T2
T1
T3
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
T1 – T2
T2 – T3
T3 – T1
Resistance
20Ω
18Ω
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.
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
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.
T1
T2
T4
T5
T8
T7
T9
T6
T3
L1
Low Voltage Set up for 3 phase
Wye Configuration
L2
T1
T2
T4
T5
T8
T7
T9
T6
T3
L3
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
High Voltage Set up for 3 phase
Wye Configuration
L1
L2
T1
T2
T4
T5
T8
T7
T9
T6
T3
L3
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
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T1 T2
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
T3
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
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
Ω
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
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.
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Motor Determination / Megohmmeter Lab
Name: _________
Date: ___________
6. Fill in the table
Terminals
T1 – Ground
T2 – Ground
T3 – Ground
T4 – Ground
T5 – Ground
T6 – Ground
T7 – Ground
T8 – Ground
T9 - Ground
Resistance
MΩ
MΩ
MΩ
MΩ
MΩ
MΩ
MΩ
MΩ
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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