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Can you use a step down transformer as step up transformer by reversing the primary voltage as a secondary voltage?
If the transformer has two separate windings, then, YES a step-down can be used as a step-up, and vice-versa.
It is more correct to say you are reversing the high and low sides.
By definition Primary is the “IN” side and Secondary is the “OUT” side.
The Primary could be the high voltage side if it is a step-down,
or it could be the low voltage side, if it is a step-up. Design ConsiderationsCare must be taken when reversing the operation of a step-down transformer to insure that it does not exceed the ratings of the transformer. For example, let’s say we have a 12 VA step-down transformer that takes 120Vac in and is rated to provide 12V at 1 amp at the secondary. If we were to reverse it and apply 12Vac to the new primary (the old secondary), we would have 120Vac at the new secondary, but to keep within the original ratings it could only be loaded to 0.1A @ 120Vac.
As long as you treat the output rating of the step-down transformer as the input rating as a step-up transformer, and don’t try to draw current beyond what would normally have been applied to the high voltage primary, you should be fine. Potentially one could push up the voltage on such a transformer beyond what its stated application specified, while making sure not to exceed the power rating of the transformer and not exceeding the breakdown voltage of the transformer’s insulation, for example driving the above transformer at 24V to get 0.05A @ 240Vac. It is important to realize though that the rated input voltage for a step-down transformer will likely not be a safe input voltage if you use it as a step-up transformer – for example, applying 120V to the above transformer with the windings reversed would generate 1.2KV!
One important point to remember when reversing a three-phase transformer to a step-up is what kind of windings it has. Most three-phase transformers use a “delta” configuration on the primary side. This means that there are three “hot wires” and a ground, but not a neutral. (The white wire in a circuit). On the secondary side, transformers are usually wired up in a “Wye” (sometimes called a “star” configuration. Doing this allows the transformer to create a Neutral connection. If a step-down transformer is reversed to make it a step-up, the high (output) side will not have a Neutral connection. If one is not needed, say for a three-phase welding machine, this would not be a problem. However, if a Neutral is needed, say for lighting, reversing this transformer would not work. In this case, a listed step-up transformer will be needed. One final thing, if a step-down transformer is used as a step-up, a Neutral wire is not required to be connected to the input side.
Conclusion:
Standard step-down transformers may be reverse fed for step-up applications but there are several precautions that should be considered:
1. Higher inrush currents dictate that the input over current protection must be sized at the higher range allowed by NEC Article 450.
2. Transformers with compensated windings will have output voltage 3-4% below nominal at no-load and 6-8% below nominal at full load. The transformer’s taps can be used to correct for this under-voltage condition.
3. Taps can be used to adjust output voltage but cannot be used to correct for over excitation. Tap operation is reversed, so raising taps increases the output voltage.
4. The neutral on the input side of the transformer should not be tied to ground and should not be bonded to the transformer enclosure.
5. Never reverse feed a fan cooled transformer.
6. Always review applicable codes and standards and consult with the local authority having jurisdiction before reverse feeding transformers.
Thanks for Information
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