Exciting current, when used in connection with transformers, is the current or amperes required for excitation. The exciting current on most lighting and power transformers varies from approximately 10% on small sizes of about 1 KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA.
The exciting current is made up of two components, one of which is a real component and is in the form of losses or referred to as no load watts; the other is in the form of reactive power and is referred to as KVAR.
Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondary winding. Transformers 600 volts and below are normally connected in additive polarity — that is, when tested the terminals of the high voltage and low voltage windings on the left hand side are connected together, refer to diagram below:
This leaves one high voltage and one low voltage terminal unconnected. When the transformer is excited, the resultant voltage appearing across a voltmeter will be the sum of the high and low voltage windings.
This is useful when connecting single phase transformers in parallel for three phase operations. Polarity is a term used only with single phase transformers.
BIL is an abbreviation for Basic Impulse Level. Impulse tests are dielectric tests that consist of the application of a high frequency steep wave front voltage between windings, and between windings and ground.
The Basic Impulse Level of a transformer is a method of expressing the voltage surge (lightning, switching surges, etc.) that a transformer will tolerate without breakdown.
All transformers manufactured by Acme, 600 volts and below, will withstand the NEMA standard BIL rating, which is 10 KV. This assures the user that he will not experience breakdowns when his system is properly protected with lightning arrestors or similar surge protection devices.
Impedance is the current limiting characteristic of a transformer and is expressed in percentage.
Insulation class was the original method used to distinguish insulating materials operating at different temperature levels. Letters were used for different designations.
Letter classifications have been replaced by insulation system temperatures in degrees Celsius. The system temperature is the maximum temperature at the hottest spot in the winding (coil).
Graphical representations of six insulation systems recognized by Underwriters’ Laboratories, Inc. (UL) are shown in Figure A (Below). These systems are used by Acme for a large part of the product line.
Temperature rise in a transformer is the temperature of the windings and insulation above the existing ambient or surrounding temperature.
Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. This is usuallyexpressed in terms of percentage.
For example: A transformer delivers 100 volts at no load and the voltage drops to 95 volts at full load, the regulation would be 5%.
Acme dry-type distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for whichthey are used.