transformer wattage

paulmars

a step down transformer lets say 2:1 requires 1/2 the wattage, correct? For every watt it uses it provides 2 watts, excluding of course whats lost in the windings. I have a 220 to 110 transformer. The output says "110v", that's all. The input side says "220v "VDE" step down transformer max 300 watts". Now if the max it can safely use (220 side) is 300, then the 110v side could pull 600 watts safely, right?"

Bondo

Ayuh,... I believe that's the Max Output....

Sliver

A 300VA (watts) transformer max capacity is 300VA on both the primary (input) and secondary (output). the 2:1 ratio refers to voltage primary to secondary, turns of wire primary to secondary, current secondary to primary.

so a 300VA transformer at 240v primary with a 2:1 ratio has a secondary of 120v

300va/240v=1.25A is the current @ full load on the primary

300VA/120v=2.5A is the current @ full load on the secondary

darren

You are mistaken on your theroy of transformers. The power on the high side will be the same on the low side.

The only thing the transformer change is the voltage and since this is changed the current will change as well.

Yoyizit

And if you look at the nameplates on these little wall 'formers, only about 70% of the power in makes it back out. For huge PoCo 'formers it might be 99%.

Knucklez

don't confuse 300VA with watts. "VA" is a different unit of measure, represents total power which is a mixture of both watts and vars (real & reactive power).

the only way a 300VA rated transformer can supply 300W is if the load is purely resistive. all power delivered to a resistive load is given in watts because:

a) the voltage is in phase with the current

and

b) there is no reflected or shuttled current

so all power is "real" and produces "work" - which in a resistor, means heat.

--
with motor loads or inductive loads like lighting which is both inductive + resistive, not all of the power delivered from transformer to the load is "real". some of it is "immaginary" or "reactive" which means that the voltage is NOT in phase with the current, or that some of the current has been used "establish the field" or some other strangeness.

so 300VA rated transformer - when applied to a realistic load which in most residential cases is inductive or inductive + resistive .. most of the transformer rating is used by the load to produce work (real power in watts) but some of it is used as reactive power.

confused yet?

Knucklez

bobelectric

Watts is Watts............

Sliver

Actually not really... I'm just used to using VA with transformers, I just got lazy and didn't change all the units to watts.

Besides since a transformer has to supply both VARS and Watts shouldn't they only be rated in VA?

What if i did a PF correction to 1.0?

Knucklez

yes, transformers should always be rated in VA.

question about power factor = 1.0. but now you are adding additional components, not just a transformer anymore.

anyway, that's a good point. i suppose PF = 1 is delivery of active power only and does not necessarily have to be with a resistive load. though usually power factor correction is done to shape the source power, not the load power. i had it in my mind we were talking about transformers and their loads, not transformers and their source power.

Knucklez