To measure the resistance in a three-phase motor, you will need a megohmmeter, otherwise known as a megger. Unlike an ohmmeter, a megger can measure high-voltage resistance values, which are generally the kinds of values you will encounter with three-phase motors.
Can a megger damage a motor?
Any megger with a higher voltage output than 500 volts DC should be used by an experienced technician. A high voltage for too long of a time may further weaken or fail motor windings and the winding insulation could be damaged by the testing procedure.
How do you test a motor for insulation?
This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor.
A general rule-of-thumb is 10 Megohm or more.
|Insulation resistance value||Insulation level|
|50-100 Megohm||Very good|
What should be the megger value of motor?
The rule may be stated: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms.
How do you megger a 3 phase motor?
Connect one probe of the megger to the earth connection in the motor. So insulation resistance to earth can be checked then connect the second probe to the first winding and check the value, then do it for the second and third, and if all the values are almost the same then its ok.
What should you not Megger?
Megger Test Safety Precautions
Use megger test on high-resistance measurements only, such as insulation measurements or to check two separate conductors on a cable. Never touch the test leads while the handle is being cranked.
Is a Megger test destructive?
Non-destructive testing evaluates the properties of the DUT without causing or having the potential to cause damage. Tests that fall into this cat- egory include Insulation Resistance-to-Ground (RTG) commonly called a Megger® test, Polarization Index, Insulation Resis- tance Profile, and Capacitance-to-Ground (CTG).
How do you check motor insulation with a Megger?
How to Test a Three-Phase Motor With a Megger
- Turn off the power supply to the motor, usually accomplished by switching the circuit breaker. …
- Place one megger probe to the any mounting bolt on the breaker box to test for grounding continuity, then touch the other probe to a motor terminal.
How do you test a motor with a multimeter?
To test your motor for short to ground, you’ll need to set the multimeter to ohms and disconnect the motor from its power source. Then inspect each wire and look for infinite readings. Alternatively, if you get a reading of 0, you might have a cable issue.
How do you increase the insulation resistance of a motor?
Cleaning and drying the winding will usually improve the IR. If the winding temperature is at or below the dew point, moisture may accumulate on the surface, causing low IR values. When the winding is otherwise in good condition, drying will resolve this issue.
How do you test a 3 phase motor insulation resistance?
Phase Resistance: Take the insulation tester and set it to 500V. Take each end and place it on different permutations of L1, L2 and L3 and record each reading. Phase to Earth Resistance: Take the insulation tester, using the same setting, and check each lead from phase to the frame of the motor.
How many ohms should a 3 phase motor have?
The reading should be between 0.3 to 2 ohms. If it is 0, there is a short. If it is over 2 ohms or infinite, there is an open. You can also dry the connector and retest to possibly get more accurate results.
Should motor windings have continuity?
The windings (all three in a three-phase motor) should read low but not zero ohms. … It will usually be low enough (under 30 Ω) for the audible continuity indicator to sound. For proper motor operation, all windings must have megohm readings to ground, i.e. to the motor enclosure.
How do motor windings fail?
Electrical failures disrupt the power supply to the motor. This includes single-phased winding failures (wye- or delta-connected) caused by an opening from a blown fuse, open contactor, broken power line or bad connection. Insulation failures are typically caused by contaminants, abrasion, vibration, or voltage surge.