Current and Flux density

I have always used about 700 CM/A which, if I do the conversion correctly, is about 2.8 A/mm². This is essentially a continuous duty rating, which is what you need for a power transformer. By the way CM = circular mil.

I might be able to convince myself to go as low as about 550 CM/A with high temperature insulation and very good cooling. That would equal about 3.5 A/mm², but I would not go beyond that ever.

As to the silicon steel core, there is no way anyone can double the maximum flux density since the typical design value will already be perhaps 85% of the maximum. There are higher value steels available. Perhaps the commercial unit is using this?

Can you give a little more specific information?

I don't know which country you are in, but I find that auto transformers sold in many countries in the Middle East, Africa, India etc. are much too small for their nominal rating and consequently run dangerously hot. I am not surprised they are half the size of the text book transformer. A final twist is to replace copper windings with aluminium without increasing the size.

Hi,

I am from India. That is what I am worried about. The commercially available auto transformers are very much small in size than what we get from books. But, ths is also a fact that not al these small sized xmers heat too much on load. I designed a 0.5kva xmer with the current density of 10A/mm2 (getting this max. value from the book "Electrical machine design" by A K Sahni), and the xmer worked ok. Too much confusing!!!!!!!

You're essentially saying you ran 10 A through an 18 AWG wire, not in free air, and it didn't get hot. That's pretty good. Do you have any special copper you use?

Thanks for your comment. As I am a beginner in designing I don't mind about any strange comment.

Can you please check what is wrong in the following design? Any suggestions are most welcome.

7 step Auto-transformer

Power = 300 VA

Primary Voltage (Vp) = 90V

Secondary Voltage (Vs) = 230V

Core selected 3No., 2inch EI CRGO Stampings, Tongue Width (TW) = 3.175 cm , Window Area (WA) = 7.562 cm

Gross Core Area (Ag) = 3.175 * 2 * 2.54 cm2 = 16.129 cm2

Core Area (Ai) = 0.9 * 16.129 = 14.5161 cm2

Turns per Volt (Te) = 10^4 / 4.44 * f * B * Ai

= 10^4 / 4.44 * 50 * 1 * 14.5161

= 3.103 = 3 (appox.)

Primary turns (Tp) = 90 * 3 = 270

Secondary turns (Ts) = 140 * 3 * 1.05 = 441 (adding 5% for voltage drop)

Pimary current (Ip) = 300 / 90*0.95 = 3.508

Secondary current (Is) = 300 / 230 = 1.304

Effective current in Primary (Iep) = Ip – Is = 3.508 – 1.304 = 2.204

Taking Current Density = 5 A/mm2

Area of primary conductor (Ap) = 2.204 / 5 = 0.4408 mm2 (i.e. 22 SWG Cu Wire)

Area of secondary conductor (As) = 1.304 / 5 = 0.2608 mm2 (i.e. 23 SWG Cu Wire)

Space taken by primary winding = 270 / 176 = 1.534 cm2

Space taken by secondary winding = 441 / 242 = 1.8223 cm2

Total space taken = 1.2 (1.534 + 1.8223) / 0.6 = 6.713 cm2

Remarks: 1. Little bit heat up.

2. Little noisy after 200V.

What do you mean by "strange comment"?

In your earlier post, you said you were using 10 A/mm². In this detailed calculation, you're using 5 A/mm². Now I'm confused about your question.