Q9Electrical Machine Design

Voltage regulation of a transformer is defined as the percentage change in secondary terminal voltage from no-load to full-load (at a specified power factor), with the primary voltage held constant, expressed as:

where E2 is the no-load secondary voltage and V2 is the full-load secondary voltage; it quantifies how well the transformer maintains a stable output voltage as load current is drawn, with a smaller percentage regulation indicating better (more stable) voltage-holding performance.

Temperature rise in transformers refers to the increase in temperature of the windings, core, and (for oil-filled types) the oil, above the ambient temperature, caused by the copper loss (I²R, in the windings) and iron loss (hysteresis + eddy current, in the core) generated during operation. Since insulation degrades progressively faster at higher temperatures (following an approximately exponential relationship between temperature and insulation life, per the 'half-life every 8-10°C' Montsinger's rule commonly cited in practice), the permissible temperature rise is strictly limited according to the insulation class used (e.g., Class-A 60°C rise, Class-B 80°C rise, over a standard ambient reference typically taken as 40°C, giving corresponding maximum hot-spot temperatures per the insulation class limits discussed elsewhere in this paper), and verifying that a design's calculated temperature rise remains within this limit (via adequate cooling surface area, oil circulation, or forced cooling as needed) is an essential final check in the transformer design process.