Manufacturers of insulation resistance testers go to a lot of trouble to produce test lead sets that will help make their instruments safe and convenient to use. The sharp fall at b indicates there was an insulation fault. Understanding insulation resistance testing pdf 0206 rev. Always use a lead set thats designed for the instrument, appropriate for the test voltage youre planning to use and suitable for the test object youre. Insulation resistance testing is a nondestructive test procedure. Threephase dualvoltage motor insulation resistance testing. How to use a megger to measure and test motor insulation. These motors run conveyor belts, pumps, fans, and many other applications. During the measurement and immediately afterwards, some of the terminals. It is commonly used in the industry for acceptance testing prior to energizing the cable and for maintenance testing programs. Wb82 i work in a power plant of a large utility company. At a, the insulation resistance decreases due to aging and dust accumulation.
Insulation resistance varies very little with rating. Regardless of the configuration, either star or delta, the measurements are normally made phase to phase and comparisons are made to determine if the readings are comparable. Surge comparison test and pd test of the 300 horsepower motor figure 3. Motors are susceptible to the gradual failure of their insulation resistance. The test method is determined by the type of equipment being tested. If a fraction of current flows, it can be measured by these highly sensitive meters. We are testing the motors insulation, more specifically between the motor windings and the motor casing. Before conducting a transformer winding resistance test, it is important to observe all safety warnings and take proper precautions.
Jul 06, 2016 the insulation resistance tester or megohmmeter, generally known by its trade name megger, can provide critical information regarding the condition of motor insulation. If someone works with ac 3 phase motors, then it is important to know how to connect these motors in star and delta connection, and how to detect an electrical problem. Insulation resistance values are taken between the windings collectively as because all the windings on the hv side are internally connected together to form either star or delta. With one probe still connected to ground, connect the other test probe to a motor terminal. Threephase dualvoltage motor insulation resistance testing threephase dualvoltage motors are used for most daytoday operations in todays industrial plants. The three tests discussed here are used primarily to test motor, generator and transformer insulation. Therefore, the final conclusion was to plan an overhaul within the next three to six months. Troubleshooting with insulation resistance test instruments. Megger mit1020 10kv insulation resistance testers are all. If there is a 4 pole isolator,the neutral can be isolated,so no problem but most isolators are 3 pole. Motors of all hp, voltage, and phase rating have similar values of insulation resistance. There are two schools of thought regarding the voltage to test insulation at.
The insulation resistance tester or megohmmeter, generally known. Electrical testing usually consists of insulation resistance meg ohm m. Well learn how to identify windings, check windings resistance and insulation resistance. To effectively test the motor, you must disconnect the field coils where possible. The test method is determined by the type of equipment being tested and the reason for testing.
If there is a 4 pole isolator,the neutral can be i. It is used to convert electrical energy into mechanical energy. Insulation resistance test insulation resistance failure of an electric motor is one of the first signs that the motor is about to fail. Standard for acceptance testing specifications for. An insulation spot test is an insulation resistance test that can be used to verify the condition of the insulation over the life of the motor.
If someone works with ac 3phase motors, then it is important to know how to connect these motors in star and delta connection, and how to detect an electrical problem. The complete guide to electrical insulation testing. The graph below shows an example of the insulation resistance readings on an electric motor. How to interpret resistance readings as previously mentioned, insulation resistance readings should be.
Loop testing firstly, you must establish which type of supply you are testing. Test inter phase resistance by disconnecting the leads from each terminal with a flathead screwdriver, then touch any two terminals with the two probes. Most, if not all our motors only have 3 leads coming out of the connection box. How to test three phase ac motors learning electrical. It differs from an ohmmeter in that, unlike the ohmmeter, it is able to measure resistances in ohm values of more than one million. Introduction the induction motor is one of the most important motors used in industrial applications. If there are no checks, a motor with poor insulation, for example, may not only be dangerous to touch when voltage is applied, but also be subject to burn out. When measured through the drop cable, the resistance of the drop cable must be subtracted from the ohmmeter readings to get the winding resistance of the motor. The megger test is checking the condition of the ground wall insulation of the motor.
At c, the fault has been repaired rewinding of the motor so the insulation resistance has returned to. What was good insulation has become a partial conductor. In an industrial facility, the recommended procedure is to perform periodic tests and record the results so damaging trends can be detected and corrected to prevent an outage. For a three phase motor, insulation resistance is usually measured between each motor winding or phase and between each motor phase and motor frame earth using an insulation tester or megger. Megging a motor is a term used to test a 3 phase motor to check the resistance and. The test should also be performed after a motor is.
In order to measure the insulation resistance in electrical wiring and electrical motors, electricians use the megohmmeter, also known as a megger. Insulation resistance testing on a 3 phase motor youtube. How to check the motor winding resistance it still runs. How insulation resistance is measured you have seen that good insulation has high resistance. Aug 16, 2014 it is unnecessary to perform the insulation resistance test of transformer per phase wise in a three phase transformer. Guide to low voltage motor testing test equipment depot. Electric motors are made up of a multitude of components that when. This motor can be wired in either delta or wye but only one voltage. In order to test the condition of winding insulation, a surge test has to be performed. What equipment is necessary for conducting insulation. Defects in turntoturn, coiltocoil, and phase to phase that cannot be detected by other methods will be revealed. Induction motor, no load test, blocked rotor test, vibration test, temperature test, phase sequence test, insulation resistance test. Threephase dualvoltage motors are used for most daytoday operations in todays industrial plants.
The measured resistance indicates the condition of the insulation between two conductive parts. Figure 4typical record of insulation resistance of a mill motor. Mar 28, 2018 in this video you will find out how to test 3 phase induction motor. Test interphase resistance by disconnecting the leads from each terminal with a flathead screwdriver, then touch any two terminals with the two probes. How to megger a motor also known as an insulation test. The above variables should be controlled as much as possible to promote accurate insulation resistance measurement data. To accomplish this, a test procedure should be established and used consistently over time. Tests using multivoltage megger insulation testers 42. An electrical induction motor contains windings, or wire wrapped around a cylindrical shaft. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. To determine whether a 3 phase motor is still good or has gone bad, a simple ohmmeter test across the windings of the motor will reveal its true state of health.
The insulation resistance tester or megohmmeter, generally known by its trade name megger, can provide critical information regarding the condition of motor insulation. Irp with small discharges starting at about seven minutes, indicating an aging insulation system the irp was performed at 2500 vdc and resulted in only a few small capacitive discharges. Megging a motor presented by katie nyberg for galco tv. Measurement of winding resistance and insulation resistance of 3 phase 11 kv 433 v, 16kva oil cooled type transformer conference paper pdf available august 20 with 5,618 reads how we. On a 3 phase motor, what resistance would you like to see. Figure 3typical scale on the megger insulation tester. Ir value will be usually high several hundred megaohms for a dry insulation system. The insulation resistance tester, generally known by its trade name megger, is capable of providing critical information regarding the condition of motor insulation. The test measures the insulation resistance between the phases andor between phase and ground. In an industrial facility, the recommended procedure is to perform periodic tests and record the results so that damaging trends can be detected and corrected to prevent an outage. How to use a insulation resistance tester on a three phase. To operate correctly, the coiled wire should have a predetermined. Several different types of insulation resistance tests are performed when troubleshooting electric motors.
The measurement of insulation resistance is carried out by means of a megohmmeter high resistance range ohmmeter. This magnetic field causes a metal bar inside the coil to rotate and power the motor. At c, the insulation resistance value after the motor has been rewound. How to test a 3 phase motor windings with an ohmmeter. This is a basic explanation about insulation resistance and what it means and how to apply it to every day electrical work.
Three phase dualvoltage motor insulation resistance testing three phase dualvoltage motors are used for most daytoday operations in todays industrial plants. If the motor is not put into operation immediately upon arrival, it is. Even though this test was developed 80 years ago, surge testing still forms a part of testing a 480v motor to find whether it is good or bad. When an electric current flows though the wire, a magnetic field is induced inside the coil of wire. Insulation resistance or ir test is carried out by maintenance engineers to ensure the healthiness of the overall insulation system of electrical equipment it reflects the presence or absence of harmful contamination, dirt, moisture, and gross degradation. Curve b shows same values corrected to 20c see page 22, giving a definite downward. A typical 3 phase squirrelcage motor has six connection leads in the electrical connection box for the three coils. A 1954 camera is ideal for all applications in the building, electrical maintenance and. If the insulation resistance of a new, cleaned or repaired motor.
The following sections of the ansineta standard for acceptance testing specifications for electrical power equipment and systems must be incorporated by reference as part of any subsection. This test is used to find the insulation resistance of motor for 1 min or 10 min through high voltage insulation tester. Insulation testing on 3 phase board electrical inspection. Our two videos listed below will properly demonstrate how to conduct these tests correctly. Motor guide february 2014 low voltage motors motor guide.
Insulation testing as directed by bs7671, when performing an insulation test on a 415 v circuit, you should select the 500 v range on your insulation tester. Insulation resistance testing test equipment depot. Understanding insulation resistance testing jm test systems. Insulation resistance should indicate 1 megohm or more. In order to successfully test and tag a 3 phase appliance, there are three tests you can perform. How to meg a motor with a megohmeter checking motor condition. Figure 2 absorption curve of test conducted on 350 hp motor. Keep in mind that the purpose of the test is not to. A spot test should be performed at regular intervals every six months, for example to track the changes in insulation integrity.
Dc voltage of 500 or v is applied between the windings and the ground of the motor. Megger guide to electrical insulation testing pdf instrumart. Pdf measurement of winding resistance and insulation. Insulation resistance 19 spot or timed insulation resistance test 20 polarisation index pi 21 dielectric absorption ratio dar 22 temperature correction 24 guard terminal 25 three phase test 28 voltmeter28 phase rotation 28 continuity28 diode test 29.
How to test a threephase motor with a megger hunker. I was wondering how others do insulation testing on a 3 phase board,in particular,neutral to earth and phase and neutral combined to earth. In this video you will find out how to test 3phase induction motor. The basic steps in ascertaining the health of a threephase ac motor are given below. The measurement of insulation resistance is carried out by means of a. Qualifications of testing organization and personnel 3. As an electrician you may be required to use a 500 v or v insulation resistance tester sometimes. Feb 26, 2014 this is a basic explanation about insulation resistance and what it means and how to apply it to every day electrical work. As shown below, the indicated terminal matrix blue lines shows the way the windings of a 3 phase motor should be tested with an ohmmeter.
How to check the windings of a 3phase ac motor with an. Curve d indicates a good insulation with an excellent polarization index of 5. As an electrician you may be required to use a 500 v or v. Insulation resistance testing introduction insulation resistance testers can be used to determine the integrity of windings or cables in motors, transformers, switchgear, and electrical installations.
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