The type of insulation used in dry-type–transformer design and construction has a definite bearing on the size and operating temperature of the unit. Currently four classes of insulation, each having a separate NEMA specification and temperature limit, are being used.
A look at these will facilitate selection of the proper unit to meet prescribed installation and operating conditions.
1. Class 130 insulation-system transformers. When properly applied and loaded in an ambient not over 40 C, these transformers will operate at not more than a 60 C temperature rise on the winding.
These units can be used as control-type transformers when higher temperatures might affect other temperature-sensitive devices in the enclosure or as distribution transformers in locations (textile mills, sawmills, etc.) where combustible flyings might be present in the surrounding atmosphere.
2. Class 150 insulation-system transformers. These units have a higher-temperature insulating system and are physically smaller and about half the weight of Class A units of corresponding rated capacities.
When properly loaded to rated kilovolt-amperes and installed in an ambient not over 40 C, Class 150 units will operate at a maximum 80 C rise on the winding. For years dry-type distribution transformers have been of the Class 150 type.
3. Class 200 insulation-system transformers. These units also have a high-temperature insulating system and, when properly loaded and applied in an ambient not over 40 C, will operate at no more than 130 C rise on the winding.
The units are smaller in size than similarly rated Class 150 units and currently are available from a number of manufacturers in ratings of 25 kVA and lower, both single- and three-phase design. One manufacturer designs in-wall, flush-mounted dry-type transformers as Class 200 units.
4. Class 220 insulation-system transformers. These units are insulated with a high temperature system of glass, silicone, and asbestos components and are probably the most compact ones available. When properly loaded and applied in an ambient not over 40 C, Class 220 transformers will operate at a maximum 150 C rise on the winding.
This class of insulation is used primarily in designs in which the core and coil are completely enclosed in a ventilated housing. Generally, this stipulation covers units with ratings of 30 kVA and larger. Some experts recommend that the hottest spot on the metal enclosure be limited to a maximum rise of 40 C above a 40 C ambient.
It should be noted that Class 150 insulation is being replaced with Class 200 or 220 insulation in transformers of recent design.
Another significant factor which concerns all dry-type transformers is that they should never be overloaded. The way to avoid this is to size the primary or secondary overcurrent device as close as possible to the full load primary or secondary current for other than motor loads.
If close overcurrent protection has not been provided, loads should be checked periodically. Overloading a transformer causes excessive temperature, which, in turn, produces overheating. This results in rapid deterioration of the insulation and will cause complete failure of the transformer coils.
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