Testing a 400 Hz transformer on a 50 Hz bench
A 400 Hz aircraft transformer is marked 115 V / 26 V, 100 VA. It arrives in a workshop that has only a 50 Hz mains supply and a variable autotransformer. The technician plans to apply the full 115 V at 50 Hz to the primary to 'check the secondary voltage and confirm the turns ratio' before returning the unit to service. Winding resistances are normal, insulation resistance is good, and the transformer will be tested unloaded. The bench supply is protected by a 2 A fuse. Nameplate ratio is 115:26, i.e. 4.42:1, and the rated secondary current is 100 VA / 26 V = 3.85 A.
1. What would an ideal-transformer calculation predict, and why is that prediction wrong in practice?
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Ideal prediction: V_s = 115 × (26/115) = 26 V, roughly independent of frequency - the turns ratio alone fixes it.
Reality: for a real core, V ≈ 4.44·f·N·Φ_max, so the peak flux is set by V/f. Rated flux occurs at 115 V, 400 Hz. Applying 115 V at 50 Hz gives 400/50 = 8 times rated flux, which drives the core deep into saturation.
Consequences:
- Magnetising current rises by more than the flux ratio (the core permeability collapses), limited mainly by air-path reluctance - tens of amps are possible from a 100 VA core.
- The 2 A bench fuse would very likely blow, or the autotransformer would be overloaded.
- If it survived, the secondary would not be a clean 26 V; the peaked magnetising current distorts the flux and hence the secondary waveform.
- Sustained saturation overheats and can damage the winding insulation.
So the ideal-ratio answer is arithmetically right and operationally wrong: the transformer is a flux-limited device, not a pure ratio device.
2. Describe a safe 50 Hz test that would still verify the turns ratio.
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Keep the flux at its rated value, i.e. scale the voltage with frequency:
- Maximum safe primary voltage at 50 Hz = 115 × (50/400) = 14.4 V
- Expected secondary = 26 × (50/400) = 3.25 V (equivalently 14.4/4.42)
Procedure: set the autotransformer to zero, connect the primary, wind up slowly while watching primary current (it should stay at a small magnetising value, typically well under 0.2 A for a 100 VA core), and record primary and secondary voltages simultaneously. A ratio within a few percent of 4.42 confirms the winding and its polarity (check phase with a two-channel comparison or a DC pulse test if polarity marking is in doubt).
Limitations to state: this proves ratio, continuity and polarity only. It does not prove insulation at rated voltage, full-load regulation, temperature rise, or the 100 VA rating. Full verification needs a 400 Hz source (static inverter or motor-generator set) at rated voltage and load.
3. The technician instead back-feeds the 26 V winding from the 50 Hz mains at 26 V and measures the primary. Is that safe, and what should he expect to read?
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Not safe. The 26 V winding is rated for 26 V at 400 Hz. At 26 V and 50 Hz the flux in the core is again 400/50 = 8× rated, exactly the same saturation problem, just fed from the other side. The primary would see an enormous magnetising current reflected through the ratio and the fuse would blow.
Correct back-feed test, scaled for frequency:
- Maximum voltage on the 26 V winding at 50 Hz = 26 × (50/400) = 3.25 V
- Expected primary voltage = 3.25 × 4.42 = 14.4 V
Judgement point: the V/f rule applies whichever winding is energised, and to any inductive component - the frequency on the nameplate is part of the rating, not just an operating condition. A technician who remembers only 'the ratio is fixed' will destroy the unit, and possibly the bench supply, on the first application of full voltage.