Lessons · Engineering · Turns ratio, and what it does to current
Turns ratio, and what it does to current
A transformer changes voltage in proportion to the ratio of its turns, and changes current in the opposite proportion. Power in equals power out apart from losses, so stepping voltage up by ten divides current by about ten.
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In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is why power is transmitted at high voltage. Cable loss goes with the square of current, so dividing current by ten divides the loss by a hundred, and the transformer is what buys that.
How to think about it
Write the ratio once and apply it in opposite directions to voltage and current. Then multiply volts by amps on both sides as a check -- they should match apart from losses.
Worked example
Voltage ratio = turns ratio.Step up or step down.
Current ratio is the inverse.Up in voltage means down in current.
Power in equals power out, minus losses.The check that catches errors.
Cable loss goes with current squared.Why transmission is at high voltage.
Your turn
A transformer steps 240 V down to 24 V. Write the turns ratio as a number (primary to secondary).
240 / 24 = to 1
Solve one, graded on the server
The trap
Applying the ratio the same way to current. Voltage and current move in opposite directions, or the transformer would be creating power.