# How to calculate transformer rating?

Discussion in 'General Electronics Chat' started by woodyzai, Dec 3, 2009.

1. ### woodyzai Thread Starter New Member

Nov 14, 2009
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I got one question, my input is 24V DC, 21A, and I going to make a 240V, 500W inverter, I need to power a load about 300W.

I want to know how to calculate the size of the transformer needed for this project? What else precaution I should take since I am a newbie in electronics.

2. ### mik3 Senior Member

Feb 4, 2008
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Since the DC source cannot give you more than 504W then a 500VA transformer will be fine.

Is it going to be a true sinewave inverter?

What kind of load is it?

3. ### woodyzai Thread Starter New Member

Nov 14, 2009
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I was trying to make a modified sine wave inverter, the output is used to drive a small water pump which is 300W

4. ### GetDeviceInfo AAC Fanatic!

Jun 7, 2009
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you best check your data for an FLA, and it's starting method. Me thinks you'll be looking at around 1500VA, which has some implications on the input.

5. ### woodyzai Thread Starter New Member

Nov 14, 2009
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hmm.. what is FLA?

6. ### bountyhunter Well-Known Member

Sep 7, 2009
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You will need to know both efficiency and power factor.

Mar 8, 2009
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8. ### GetDeviceInfo AAC Fanatic!

Jun 7, 2009
1,619
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if your powering a resistive load (pf = unity) then wattage is your indicator. With a motor however, you have several other factors to consider. As indicated, you'll need to know your motors FLA (full load amps). This rating considers the motors power factor and efficiency. Then you need to make some assessments as to it's starting requirements, which depend on the starting method. Suppliers' data sheets typically contain this information.

Tack onto that the effeciencies of your converter/inverter.

9. ### Duane P Wetick Senior Member

Apr 23, 2009
419
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For a single phase transformer, figure about 90% efficiency, for 3 phase transformers, about 95% efficiency, for AC motors, about 86% efficiency. Don't overload your transformers as the voltage will sag when they go out of regulation; current goes up and they over-heat.

Cheers, DPW [ Spent years making heaters out of op-amps.]