How to meg a motor? Performing regular insulation resistance tests on your motor is critical for maintaining safe and reliable operation.
If you’re working with brand-new motors in your plant, then your electrical insulation should be in tip top shape. However, despite major manufacturing improvements to motors over the years, insulation is still susceptible to degradation from equipment use and environmental impact.
Variables like mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just natural humidity can cause insulation failure.
Over time tiny holes and cracks will form, allowing moisture or foreign particles to leak into the surface of the insulation – giving way to a low resistance path for leakage current. Over time you will observe a resistance drop, but typically it is gradual and regular electrical testing will identify this issue.
How to meg a motor?
First things first, you’re going to need an insulation tester, a megohmmeter, or an all-in-one rotating machine tester (if you’re tired of lugging multiple test instruments around your plant), which will give you a measurement in ohms or megohms. Keep in mind that this test is non-destructive, so you don’t have to worry about further damaging your motor’s insulation.
Your instrument will simply apply a voltage and measure the resulting current over the insulation’s surface – giving you the resistance value.
Also, it’s really important to remember that you should never, under any circumstance, ever connect a Megger insulation tester (or any IR tester for that matter) to energized equipment. Now that that’s covered, let’s talk about hooking up the test.
For AC motors and starting equipment, check out the below diagram from A Stitch in Time – our complete guide to insulation resistance testing.
Note that the starting equipment, connecting lines, and motor are in parallel, and the starter switch is set to “on”. It’s always better to disconnect component parts too and test them all separately, so you can know precisely where a weakness exists.
For DC generators and motors, you’ll need to raise the brushes, as shown in the figure below. You can also test the rigging and field coils separately, from the armature itself.
How do you interpret resistance readings?
Well, for motors, we always recommend you grab a copy of IEEE’s guide, “Recommended Practices for Testing Insulation Resistance of Rotating Machinery” as it is the most complete resource for dealing with the problem of interpreting insulation resistance measurements for motors.
But the biggest recommendation we can give you is as follows…
Periodic testing is key.
While there are guides and rules for minimum values of insulation resistance, your best indication of trouble in paradise is a consistent downward trend in IR measurements. And this can only be achieved if you’re testing periodically and keeping good records, of course.
The purpose of measurement of insulation resistance
Measuring the resistance of motor windings makes it possible to detect deterioration due to weather, corrosion, dirt, moisture, and excess vibration before the motor fails.
There are very clear limitations on the ability of insulation resistance testing, alone, to assess the condition of an electric motor for operation. One thing to be taken care that, there has to be a clear path between the insulation system and the machine cover.
How to test a motor using Megger?
Disconnect the motor from the power supply and connect a megger between the windings.
Establish a safe working zone to keep away unauthorized personnel from the megger as it will be supplying a high voltage. Incorrect use of megger can cause damage to the parts of the equipment and injuries to users.
Use a standard multimeter to test inter-phase resistance on all three phases.
- All readings should be approximately the same and will vary depending on the size and type of the motor.
- If a dead short (0 ohms) or overload (OL) is detected with the meter, the motor may be bad.
To measure a high resistance, apply high voltage (up to twice the operating voltage). On a 480 V motor, for example, apply 1,000 V.
Perform the readings in megohms.
For a motor with a nominal value of 240-480 V, it is worth mentioning that different companies have different minimum tolerances for the insulation resistance in the used equipment, ranging from 1 to 10 megohms. The insulation resistance in new equipment should be much higher, from 100 to 200 megohms.
Since the strength of the insulation varies with temperature and humidity, you may need to perform several measurements of the resistance over some time to get a consistent result.
- Place one megger lead on the ground terminal or wire and place the other lead on one of the phase wires or terminals.
- Press test on the megger.
- A very high resistance reading (greater than 10 megohms) will indicate the motor insulation is good.
- It is recommended to consult with the motor manufacturer’s user manual if required.
- Repeat the steps with the other two phases.
Can you damage a motor by megging it?
Keep in mind that this test is non-destructive, so you don’t have to worry about further damaging your motor’s insulation. Your instrument will simply apply a voltage and measure the resulting current over the insulation’s surface – giving you the resistance value.
What is the megger reading for a 480 volt motor?
IEEE recommends that for motor starting that the operational voltage, expressed as a meg-ohm value doubled plus 1000 is the minimum meg value for starting. For a 480V motor that would mean . 480Meg x 2= . 960 Meg + 1000, so a minimum ground resistance for starting a 480V is 1.960 Meg or round up to 2 meg-ohms.
How do you tell if a motor is bad with a Megger?
Place one meter lead on the ground terminal or wire and place the other meter lead on one of the phase wires or terminals. Press TEST on the meghometer. A very high resistance reading (generally greater than 10 megaohms) will indicate the motor insulation is good.
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