Step Down Transformer - Output Switch 110 & 240V

[Connoiseur]

The sticker on my 240V:110V step down transformer says it is 600W but JakeHuston says it is rated for 300W?  (I'd previously posted to JakeHuston asking the question, but no reply).

600 or 300 is a rather big difference so I'd like to know the answer and not plug in a 110V device that is too big for this transformer. 

I have read somewhere you should add 50% to a device's rated capacity as a safety margin i.e. if the 110V device is 500W then best to use a transformer with >750W.  (I know that's an oversimplification as there are other factors at play e.g. high energy start-up items like hair dryers etc but I'm just looking for a general rule-of-thumb).

Any comments or enlightenment will be appreciated.

 

Actually, the rating specified is misleading. Transformers are not rated in watts but in VOLT AMPERES (VA). Which is simply the product of voltage applied across the transformer and the current drawn by the transformer with the load connected.

The peak value of current and voltage occurs at different time instants as determined by the POWER FACTOR of the load; which again can be LAGGING or LEADING. In short if current peak lags the voltage peak, it is lagging power factor (motors, unloaded transformers, old fluorescent tube ballast have lagging power factor) and vice versa.

A transformer should never be loaded beyond its VA rating for anything more than a few seconds. Now Wattage is related to volt amperes as
Watts = VA * Power factor
Note: power factor value lies between 0 and 1.

So assuming a typo, the transformer is rated 600VA which means you can draw 600/110 = 5.45A - losses in transformer; so you should be left with about 5A to use.

Remember that not all loads have the same power factor, motors and cheap chinese SMPS/adaptors have substantially lower power factor than incandescent bulbs or high end IT equipments, which means you can extract far less "Watts" from your transformer. Higher the power factor higher is the wattage available (at PF of 1 wattage = 600).

Excuse me if it made no sense; English is my third language.

[terryt]

The sticker on my 240V:110V step down transformer says it is 600W but JakeHuston says it is rated for 300W?  (I'd previously posted to JakeHuston asking the question, but no reply).

600 or 300 is a rather big difference so I'd like to know the answer and not plug in a 110V device that is too big for this transformer. 

I have read somewhere you should add 50% to a device's rated capacity as a safety margin i.e. if the 110V device is 500W then best to use a transformer with >750W.  (I know that's an oversimplification as there are other factors at play e.g. high energy start-up items like hair dryers etc but I'm just looking for a general rule-of-thumb).

Any comments or enlightenment will be appreciated.

Not that JakeHuston has only three posts. Note also that the post in question has no context and does not fit into the narrative of the thread.

In short, the post has nothing to indicate you should pay any attention to it. Just ignore it.

IanB:  Thank you.  I shall ignore JakeHuston's post.

[terryt]

....Excuse me if it made no sense; English is my third language.

Connoiseur, thank you for your comments. 

Anything I didn't understand in your post wasn't because of your English but more a case of my very limited understanding of things electrical in general, and transformers in particular.

[Gyro]

Excuse me if it made no sense; English is my third language.

Show off! Your English is excellent. A lot of us around here struggle with our first language. 

[terryt]

I'm sorry, I need to recalculate. For some reason I thought that there 300W

Thanks for getting back to me, no harm done.

[radar_macgyver]

Only manual was a tiny strip written totally in Chinese, with a singlr line of characters on it, and no images at all.

Oh cool, they sent you a fortune without the cookie!

[Kilrah]

Not that JakeHuston has only three posts. Note also that the post in question has no context and does not fit into the narrative of the thread.

And given the commercial link in the signature the only purpose of this account is obviously spamming/advertisement. The game is to post things that while they make no real sense in the context they appear just enough on topic not to be removed.