The selection of induction motors designed to match the load requirements of any particular application need to ensure that the temperature inside the motor windings does not rise to a level that exceeds the critical temperature. A major reason for winding failure is insulation deterioration. The use of paper wrapping on strap conductor is slowly being replaced by synthetic polymer coatings or wrapping with synthetic cloth. The most common insulation today for highvoltage windings in transformer is an enamel coating on the wire, with kraft paper used between layers. The electrical insulation system for materials used in generators, electric motors, transformers, and other electrical components is divided into different classes by. Pdf this special issue of transactions on dielectrics and electrical insulation is dedicated to transformer insulation. Table 1 typical basic insulation levels basic insulation level, kv standard 1. Here we can clearly see that there are two separate windings separatingisolating transformer. Instrument transformer basic technical information and.
Troubleshooting with insulation resistance test instruments. Depending on the thickness of the material, a distinction is made between pucaro presspaper in rolls and board material. Industrial control transformers schneider electric. The insulation of ct depends on the rated voltage of the ct. Transformer winding failures have statistically been attributed as the most frequent cause for transformer faults ansiieee, 1985, followed by load tap changers, and bushings. Type t transformers class 9070 type t transformers the type t units are designed for the global market and are the best choice when size and cost are of concern. Ensure that no leakage paths between phases or transformer body.
Temperature tolerance class b is the most common insulation class used on most 60 cycle us motors. Transformer insulation is based on basic impulse insulation level together with voltage rating. Insulation iec 60085 thermal class 3 iec 60085 thermal class 3 nema class 4 nemaul letter class um ot erature ed thermal endurance c3 materials 90 y 90c 105 er, 5 105 a 105 a 105c 120 er, 6 120 e 120c and erature b b c 155 estos lifetime erature 155 f 155 f 155c 180 erature erature 180 h 180 h 180c 200. This has changed to actual temperature rating class a class 05c, class b class 150c, class f class 180c, and class h class 220c. Insulation systems and thermal ratings as defined by underwriters laboratory ul standard 1446, an insulation system is. At b, the sharp drop indicates an insulation failure. A transformer used to limit the transfer of electrical noise. This is our most popular and complete offering of industrial control transformers. These solutions include both minor and major insulation for ground, layer, interwinding.
Insulation class standards established by the national electrical manufacturers association nema to meet motor temperature requirements found in different operating environments. The electrical insulation system for wires used in generators, electric motors, transformers, and other wirewound electrical components is divided into different classes by temperature and temperature rise. The drawing shows a transformer with a twochamber coil form. Potential transformers pts or voltage transformers vts pts or vts are the most common device used. In this paper, the transformer life and the remaining lifetime prediction is focused mainly on thermal degradation of its paper insulation. Depending on whether it is a small transformer or power transformer. Iec 600341 insulation class and temperature rise avsld. Cellulose based insulation insulation materials transform erboard, paper and components of modern power and distribution transformers are made of 100 % cellulosic fibers. Aug 22, 2018 the classes of insulation systems at least on nema are based on temperature. A spot test should be performed at regular intervals every six months, for example to track the changes in insulation integrity. A transformer sold for isolation is often built with special insulation between primary and secondary, and is specified to withstand a high voltage between windings. The material properties allow classification in insulation class a 105 c and are manufactured in accordance with the iec 60085 standard. Pdf transformer electrical insulation editorial researchgate.
The insulation class indicates the magnitude of voltage, which an instrument transformer can safely withstand between its primary and secondary winding and between its primary or secondary winding and ground core, case or tank without a breakdown in the insulation. Drytype resinencapsulated transformers are confined in cells that. Drytype transformer insulation insulation components. Isolation transformers provide galvanic isolation and are used to protect against electric shock, to suppress electrical noise in sensitive devices, or to. Reliable insulation for more reliable transformers.
The megger insulation tester is a small, portable instrument that gives you a direct reading of insulation resistance in ohms or megohms. The combination of an ambient temperature of 40c and allowed temperature rise equals the maximum winding temperature of a motor. Compliance options for ul 1446 once an eis is identified as suitable for the application, ul will work with the customer to initiate the system adoption process and establish a ul report under the electrical insulation systems objy2 category. Insulation class transformers by schneider electric. These devices use standard transformer theory with 23 windings one primary with one or two secondary windings with a magnetic connection via an iron core. Insulation resistance test of transformer is performed on the transformer to. Class f is usually used in large motors 115 deg c system total. As well, a class 180 insulation system transformer can be designed with 80 degree celsius rise.
What is the difference between a transformers temperature rise. The transformer had undergone oil purification on ten occasions since 1995 in an attempt to remove moisture and. Current transformers windings and rated insulation levels. Drytype transformer insulation abb offers a wide range of flexible insulating laminates for drytype transformers, both vacuum cast resin and open wound. Most transformers manufactured in europe before 1990 use kraft paper as winding insulation, which was later replaced. The paper insulation clearly was at end of life criteria. A transformer with a class 220 insulation system can be designed for a maximum temperature rise that is lower than the standard 150 degree celsius.
The name iso is from the greek word isos which means equal. Besides life expectancy, insulation systems ratings also help to determine the overload capability of a transformer. Due to increasing demands for enhanced transformer performance, insulation materials are subject to evergreater electrical and thermal loads. To withstand operational conditions, windings and leads must be properly insulated, supported and cooled. The transformer had undergone oil purification on ten occasions since 1995 in an attempt to remove moisture and improve the dielectric.
For practical insulation circuits, where theta is larger than 85 o, the power factor and the dissipation factor are numerically the same. In principle the singlephase transformer consists of a closed laminated iron core with two main parts. Isolation transformers block transmission of the dc component in signals from one circuit to the other, but allow ac components in signals to pass. What is the difference between a transformers temperature. This does not mean that the entire transformer is expected to reach these temperatures, rather, that the hottest point on the transformer may reach this temperature without damage.
The insulation class is the maximum temperature which the transformer can be operated before breakdown of insulation and loss of transformer life can occur. Insulation that is wrapped around bare conductor in place of enamel coated wire. Also, a class 180 insulation transformer can be designed with 80 c rise. Permissible temperature rise hotspot temperature margin. Drytype transformer insulation insulation components and. Industry standards have established insulation classes. Note that a transformer with a class 220 insulation system can be designed for a maximum temperature rise that is lower than the standard 150 c. Why are transformer insulation system ratings important. Dry type transformer is usually a power or distribution transformer without oil.
The transformer was removed to a works facility and inspected. Temperature classes of electrical insulators threebond europe. Pucaro presspaper transformer insulation paper insulation. Maximum ambient temperature minimum ambient temperature. Disconnect the electrical current from the transformer and grounding it. Insulation and temp jefferson electric transformers. Jul 16, 2018 transformer insulation classes the insulation rating is the maximum allowable winding hot spot temperature of a transformer operating at an ambient temperature of 40c.
The grounded specimen test gst is referred to as the measurement of an insulation sample that has one of its terminals grounded. Besides life expectancy, insulation systems ratings also help to. The classes of insulation systems at least on nema are based on temperature. Mechanical strength and elongation to support high speed winding resist damage during installation to prevent turn to turn failure temperature class for ul systems, must meet eis minimum thickness requirement window insulation ground insulation. Previously, insulation rating was denoted by letter system. A standard transformer with 220c insulation and a 150c temperature rise, will be rated to run full load in an average 30c ambient environment over 24 hours with a maximum 40c ambient temperature. Compliance options for ul 1446 once an eis is identified as suitable for the. Temperature tolerance class f is the most common for international and 50 cycle motors. For good insulation, the resistance usually reads in the megohm range. Insulation iec 60085 thermal class3 iec 60085 thermal class3 nema class4 nemaul letter class um ot erature ed thermal endurance c3 materials 90 y 90c 105 er, 5 105 a 105 a 105c 120 er, 6 120 e 120c and erature b b c 155 estos lifetime erature 155 f 155 f 155c 180. Lowvoltage strip can be bare with paper insulation between layers.
At a, the effect of aging and dust accumulation is shown by decreasing values. Verify insulation integrity due to the moisture and impurity contents of insulation. At c, the insulation resistance value after the motor has been rewound. There are two aspects of transformer design related to temperature. An isolation transformer is a transformer used to transfer electrical power from a source of alternating current ac power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. What is a dry type transformer and what is its insulation. It is important to understand both aspects when choosing a transformer for a specific installation. Class e insulation consists of materials or combinations of material which experience or test can be shown to be capable of operation at the class e temperature. For liquid filled units the ul listed insulation class is 120 degrees c for mineral oil and 140 degrees for less flammable seed oil. Rated insulation level means the power frequency and impulse voltage withstand test voltages specified for. Insulation may be grouped into the following recognised classes.
The different classes are defined by nema, underwriters laboratories ul, and iec. Electrical test equipment application solutions from megger. Insulation spot test 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. The windings can be made of foil, instead of wire coils, to keep thermal losses low. The insulation must be just the opposite from a conductor. Transformer insulation classes the insulation rating is the maximum allowable winding hot spot temperature of a transformer operating at an ambient temperature of 40 c. There are three basic styles used to transformer system voltages to control voltages. The insulation class of a power transformer is decided on the test level of which insulation is capable of withstanding. There is also insulation on the outside of those parts of the winding which are enclosed by the iron core. Each class designates the maximum temperature, in celsius, that the insulation can withstand without damage class 220 can withstand 220c, etc. The complete guide to electrical insulation testing.
Insulation systems are classified by the temperature rating. The insulation system of a transformer is rated in degrees celsius at its maximum temperature rating the class number the degree c of the insulation system. This has changed to actual temperature rating class a class 05c, class b. Other magnetics may have lower or higher ambient ratings depending on the design and application. Transformer oil is used for insulation and cooling. Insulation class standards established by the national. The electrical insulation system is sometimes referred to as insulation class or thermal classification. It can be designed for either 115 or 80 degree celsius rise. Class 155 and 105 transformers are not typically designed for other than their standard temperature rise. Class a 105 oc insulation consists of materials such as cotton, silk, and paper when suitably impregnation or coated or when immersed in dielectric liquid as oil. Class a is not worth talking about because it is so low.
One or more electrical conductors of insulated copper or aluminium are wound around each limb n times. This is often use for reinforced insulation in class ii. In the late 1940s, when silicone resin became a popular insulator material, the hclass insulator was added to the. A system of these materials immersed in a mineral oil is an industry proven standard transformer insulation system recognized to. Electrical electrical units, amps and electrical wiring, wire gauge and awg, electrical formulas and motors. Must be specified if it is a threephase transformer. The megger insulation tester is essentially a highrange resistance meter. Basics, maintenance, and diagnostics vii contents continued page 4.