Side-by-side comparison
| Parameter | Auto Transformer | Two Winding Transformer |
|---|---|---|
| Winding Structure | Single winding — part of it serves as primary and secondary | Two separate electrically isolated windings |
| Galvanic Isolation | None — primary and secondary share a common conductor | Complete isolation between primary and secondary |
| kVA Rating for Same Output | Conducted kVA = (1 – 1/a) × output kVA; physically smaller | kVA rating equals output kVA; larger and heavier |
| Efficiency | Higher — less copper loss for same kVA | Lower than equivalent autotransformer |
| Cost | 30–50% less material for ratios close to 1:1 | Higher material cost |
| Voltage Ratio Limitation | Advantageous only when ratio < 2:1 (e.g., 415 V to 230 V) | Used for any ratio including high step-up/step-down |
| Safety | Hazardous if HV fault propagates to LV side | Safe — HV fault cannot reach LV side through conductor |
| Typical Application | Motor starting (65%, 80% taps), 11 kV/6.6 kV grid interconnection | Distribution transformers 11 kV/433 V, isolation in medical equipment |
Key differences
An autotransformer's copper saving comes from its conducted kVA: the winding handles only (1 – 1/a) of the output kVA, where a is the turns ratio. For a 415 V to 230 V step-down (a = 1.8), the autotransformer handles only 44% of the output kVA in copper — a massive material saving. But the shared conductor means a primary-side fault (say, 11 kV entering the 230 V winding) goes directly to the secondary with no isolation barrier. Two-winding transformers have full copper for both windings but provide complete galvanic isolation — mandatory in medical isolation transformers and in circuits where earth faults must be contained.
When to use Auto Transformer
Use an autotransformer when the voltage ratio is less than 2:1 and isolation is not required — for example, a three-phase autotransformer starter with 65% and 80% voltage taps to limit the starting current of a 45 kW induction motor.
When to use Two Winding Transformer
Use a two-winding transformer when galvanic isolation is required or voltage ratio exceeds 2:1 — for example, a 100 kVA, 11 kV/433 V distribution transformer feeding a residential area where the LV side must be fully isolated from the MV network.
Recommendation
For exam problems, choose autotransformer when the question mentions motor starting, variable voltage supply, or ratios between 1.1:1 and 2:1. Choose two-winding whenever the question mentions isolation, safety, or high voltage ratios. That single rule resolves most autotransformer exam questions correctly.
Exam tip: Examiners ask you to calculate the kVA rating of the common winding — use: kVA_common = output kVA × (1 – 1/a); for a 100 kVA, 2:1 autotransformer, the common winding handles only 50 kVA.
Interview tip: Interviewers at switchgear companies ask why autotransformers are not used for 11 kV/230 V step-down in houses — the correct answer is lack of isolation: a fault in the HV winding would put 11 kV on the household sockets, which is lethal.