Q6Electrical Machine Design
Question
4 marks
Q.6. Enumerate specifications of a Transformer that are required prior to designing a transformer.
Answer
Before designing a transformer, the required specifications include rated kVA, voltage ratio and connection (star/delta), frequency, number of phases, type of cooling, impedance/regulation limits, temperature rise limits, and applicable standards.
The specifications that must be established prior to beginning the detailed design of a transformer include:
- Rated output (kVA): the power rating the transformer must continuously deliver, which directly determines the required conductor cross-sections and core flux via the output equation.
- Voltage ratio and number of phases: the primary and secondary rated voltages (and hence the turns ratio), along with whether the transformer is single-phase or three-phase, and the winding connection (star, delta, or zig-zag) for each side.
- Frequency: the supply frequency (e.g., 50 Hz), which directly enters the volt-per-turn formula (Et=4.44fBmAi) and hence affects core sizing.
- Type of cooling: whether the transformer is oil-immersed (ONAN, ONAF, OFAF etc.) or dry-type (air-cooled), which determines the permissible current density and overall thermal design approach.
- Impedance / percentage regulation requirement: the desired percentage impedance (affecting fault current levels and voltage regulation under load), which constrains the winding geometry and leakage reactance design.
- Permissible temperature rise: based on the insulation class to be used and the applicable standard, setting the thermal design limits for windings, core, and oil (where applicable).
- Type of core construction: core-type or shell-type, and the specific core cross-section shape (square, cruciform, stepped), which affects both material utilization and cooling surface area.
- Applicable standards and special requirements: relevant national/international standards (IS 2026, IEC 60076, etc.) and any customer-specific requirements such as tap-changing facility, parallel operation compatibility, noise limits, or specific mounting/terminal arrangements.
Only once all of these specifications are firmly established can the detailed design calculations (core sizing, winding turns, conductor areas) meaningfully begin, since each of these upstream requirements directly constrains one or more of the downstream design choices.