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I purchased a step-up/step-down voltage converter to use my 110-120V electronics in countries with 220-240V wall sockets. It's similar to the one pictured below.

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These transformers are sold in a variety of Wattage ratings. What is the power consumption of the transformer? Does it always consume its maximum wattage capacity regardless of what devices are attached to it? If I just turn on my 1000W transformer with no devices attached will it be drawing 1000W, or some lower amount? If I have devices attached that add up to 1000W, is it transformer drawing 1000W or a different amount?

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If you have no devices attached to the transformer, it will draw very little power depending on the transformer core design, a few watts at most. When you attach a load then the transformer, as its name implies, will transfer power from the input to the output. It will only transfer as much power as the attached device needs. If the device only needs 200 watts, then that is what the transformer will deliver. The 1000 watts is the maximum power the transformer is designed to deliver safely. You should not exceed that value, probably no more than 900 watts to play it safe.

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  • \$\begingroup\$ Important: The power rating of such a transformer is in VA, not W. So if you connect a device with a motor (e.g. a fridge) which says 250W, cos phi=0.5, its actually 500 VA, not 250. You don't have to pay the bill for the additional 250VA, but the transformer must be able to shift it back and forth between the motor and the grid. \$\endgroup\$ – Janka Jul 13 '17 at 22:59
  • \$\begingroup\$ @Janka: A nasty thing about reactive power is that a transformer actually has to couple it both from the supply to the load during part of the cycle and then back from the load to the supply during the other half, and incurs losses going in both directions. \$\endgroup\$ – supercat Jul 13 '17 at 23:19
  • \$\begingroup\$ @Janka, can you make a short explanation to why cos phi = 0.5? Is it common practice to assume that an inductive load such as a motor always give 60 degrees phase shift? \$\endgroup\$ – Harry Svensson Jul 13 '17 at 23:21
  • \$\begingroup\$ @supercat: Yes, losses go in both directions, but I wanted to focus on not forgetting reactive power, or you have to buy twice. \$\endgroup\$ – Janka Jul 13 '17 at 23:23
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    \$\begingroup\$ @Harry Svensson: it was for the sake of making the calculation simpler. But to give common figures, AC induction motors in the 250W range usually have a cos phi of 0.8 while running at the operating point but a lot less (~0.2) when starting. \$\endgroup\$ – Janka Jul 13 '17 at 23:27
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What power is delivered by a transformer to the load is also taken from the primary AC supply plus maybe 5% more power for the transformer inefficiencies.

With no load connected to the output there is still a residual current taken by the transformer primary and this is called called magnetisation current but, theoretically, this current doesn't constitute a power loss. However, the core will saturate a little bit in no-load conditions and this can cause some power losses.

Also, because the core is basically iron and, because iron conducts, there can be circulating eddy currents flowing in the core due to the induced voltages from the primary winding magnetism. This is why insulated laminations are used; they reduce eddy currents but there is always a small power loss in the iron.

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