Magnetization Current Standard

There is no standard.

The formula is

B=(3.491*E*10^6)/(f*N*Ac*SF)

B=Maximum Gauss

E= Volts RMS (Must be sine wave)

f=frequency in Hertz (was cycles per second until a few decades ago)

N=Number of turns

Ac=Core area in inches^2

SF=Stacking Factor, Usually = 0.95 to 0.98 (Depends on burr on core and core thickness)

Hertz (was cycles per second until a few decades ago)

I seem to remember some sort of almost politically correct nonsense about this but I've forgotten what it was all about. I use the terms interchangeably. Am I destined for a whipping?

As I remember, there were quite a few that thought it would have been more appropriate to honor Charles Proteus Steinmetz rather thaan Hertz, for obvious reasons.

Inrush current depends also on the point of wave at which the transformer is connected.

You may find useful info on :

http://www.electrical-installation.org/enwiki/Protection_of_LV/LV_transformers

I only design transformers for hobby purpose. Professional have better techniques.

I see a transformer as a large primary inductance that keeps the magnetizing flux well below saturation to reduce losses to which you add a corresponding number of turns for the secondary. Wire, magnetic core sizes and insulations have to be evaluated before beginning. Once a first design is completed, we often have to re-adjust if an optimum result is desired. A trick for hobbist or small quantity: Use a bigger core than expected. It saves many frustrating iterations when you cannot fit all the turns and insulation needed...

I first evaluate the magnetizing inductance by considering only the primary turns and the magnetic core.

Then divide the voltage by the L + R of the primary at the operating frequency. I then use this value to check for the core saturation and re-adjust the inductance. This give good results after (hopefully) a couple of iterations.

The number of turn for the secondary is then easy to calcuate.

The leakage inductance is harder to evaluate. I usually do a tight design and measure it afterward. Iterations and compromises are necessary.

Have fun!

Magnetizing current will be dependent on the type or core material you planned to use in your transformer! The lowest value I had used in power transformer computations is 65,000 gauss, since that matches the magnetic characteristics of the soft silicon laminated steel core material that was made available for me to use! Refer to the B-H curve and corresponding Ampere-turn per unit area of whatever core material you plan to use.

Substitute that value as one of your constants in the transformer formula that was previously or as posted earlier.

Good luck!

GA

Thanks for the input on what magnetizing current is and how it contributes to losses. What I had seen in an older manual was for a 138kV transformer was 6.5 A per phase or 3.1% of the full load current.

But I'm not concerned with the efficiencies of the transformer (yet); what I am looking at are disconnect switches. I've seen it noted before that to determine whether quick whips or other interrupters were required (for the switch on the transformer) you need the magnetization current. If the circuit breakers and tline switches are open at the substation then the current being drawn across that switch should just be the magnetization current.

I know what some switches are rated for with and without the quick whips, but I was hoping that there was on official standard for "guesstimating" the magnetization current for the transformer. From the sounds of it my best bet would be getting in contact with a company that makes transformers and get some comparative values.

My feeling would be to consult the disconnecting switch maker's about their breaking capacity for magnetising current and compare it with 5% of rated transfo full load current before worrying about actual magnetising currents. Often the available disconnect switch rating is a lot higher than the circuit rating. In any case, you should note that if the system is lightly loaded, the transfo voltage might be 10% high - which will increase the magnetising current significantly.