Last reviewed 16 Sept 2026 · 9 min read
Transformers
A transformer is a static device that transfers electrical energy between circuits by mutual induction, changing voltage and current levels at the same frequency. It works only on AC (a steady DC flux induces no EMF).
Construction
| Part | Details |
|---|---|
| Core | Laminated silicon steel (thin insulated sheets) to reduce eddy current and hysteresis losses; CRGO steel in power transformers |
| Windings | Primary (input) and secondary (output) — copper/aluminium |
| Core type | Windings surround the core limbs — suits high voltage, easier insulation and cooling |
| Shell type | Core surrounds the windings — better mechanical support, lower leakage — suits low voltage, high current |
| Accessories | Tank, transformer oil (insulation and cooling), conservator, breather (silica gel), Buchholz relay, bushings, tap changer, radiators |
EMF equation and ratios
Turns ratio (transformation ratio) Ideal transformer: ;
- → step-up; → step-down
- Impedance referred across: (to primary)
Losses
| Loss | Cause | Dependence |
|---|---|---|
| Core (iron) losses | Hysteresis + eddy currents in the core | Depend on voltage and frequency — constant at all loads |
| Copper losses | in windings | Vary as square of load — |
Transformers have no mechanical losses (no moving parts) → very high efficiency (commonly above 95%, large units above 99%).
Efficiency
= fraction of full load; = rated kVA.
Maximum efficiency occurs when copper loss = iron loss:
All-day (energy) efficiency — important for distribution transformers (energised all day, lightly loaded much of the time) — designed with low iron losses.
Voltage regulation
(Some texts divide by the full-load voltage.) Approximate formula: (+ for lagging, − for leading pf). Leading pf can give negative regulation (voltage rises on load).
Tests
| Test | Performed on | Measures |
|---|---|---|
| Open-circuit (no-load) test | Usually LV side energised at rated voltage, HV open | Iron (core) losses; no-load current; magnetising parameters |
| Short-circuit test | Usually HV side at reduced voltage, LV shorted, rated current | Full-load copper losses; equivalent resistance and reactance |
Autotransformer
- Single winding with a tapping — part of the winding common to primary and secondary.
- Saves copper (more so when ratio is close to 1); smaller and more efficient; but no electrical isolation.
- Uses: starting induction motors, variacs, interconnecting grids of close voltages.
Other types and cooling
- Power transformers (generating stations/substations, high efficiency at full load), distribution transformers (e.g. 11 kV/433 V), instrument transformers (CT, PT), isolation transformers.
- Cooling: ONAN (oil natural air natural), ONAF (oil natural air forced), OFAF, OFWF, and dry-type (AN) for indoor.
DC machines
A DC machine converts mechanical energy to DC electrical energy (generator) or vice versa (motor); the same machine can work as either.
Construction
| Part | Function |
|---|---|
| Yoke | Outer frame; mechanical support and path for flux |
| Field poles and field winding | Produce the main magnetic flux |
| Armature core (laminated) | Carries armature conductors; rotates |
| Armature winding | Lap ( parallel paths — high current, low voltage) or wave ( — high voltage, low current) |
| Commutator | Mechanical rectifier — converts alternating EMF in armature to DC at terminals (generator) / reverses current in conductors (motor) |
| Brushes (carbon) | Collect current from the commutator |
| Interpoles | Improve commutation |
EMF equation (generator)
= poles; Φ = flux per pole (Wb); = speed (rpm); = total armature conductors; = parallel paths ( lap, wave). Generator: (+ brush drop)
Types by excitation
- Separately excited — field from an external source.
- Self-excited: shunt (field in parallel with armature), series (field in series), compound (both; cumulative or differential; short or long shunt).
- Self-excited generators need residual magnetism and field resistance below the critical resistance to build up voltage.