How many turns does my primany winding need?

[gary350]

I salvaged a bad transformer. I took it apart and made notes, number of turns on the primary and wire size. I saved the laminations to use another day, that was 20 years ago. I have lost my notes. Now I have a project for this transformer I need to calculate the number of turns on the primary and wire size for 120 VAC, 1800 watts.

Laminations measure, 6 1/4" x 5 1/4". The lamination stack is 2 7/8" thick. Lamination center is 2 1/8" wide.

Ep = 120

Bm = 1.0

f = frequency = 60

Ac = core area sq in = 6.109375

Np = [3.49 x Ep x 10 to 6 power] divide by [f x Ac x Bm]

Np = [3.49 x 120 x 10^6] / [60 x 6.109 x 1] = [418.8 x 10^6] / 366.54 = 418,800,000. / 366.54 = 1,142,576.5 This is WRONG

1.1 million turns is WRONG. I think the primary was closer to 200 turns.

 

[ericgibbs]

hi Gary.
A rule of thumb for Mains transformer is 6 turns for a 1inch square central core.

So my estimate would be [6/6.109]*120 = 118 turns [which is close too 114 , from your answer]

 

[colin55]

The primary will be between 3 and 6 turns per volt.
Select 4 turns per volt. Now work out the size of the wire to take half the winding area. It's as simple as that.

 

[Diver300]

Np = [3.49 x 120 x 10 x 6] / [60 x 6.109 x 1] = [418.8 x 10 x 6] / 366.54 = 418,800,000. / 366.54 = 1,142,576.5 This is WRONG

1.1 million turns is WRONG. I think the primary was closer to 200 turns.

You have typed something wrong into your calculator

418.8 x 10 x 6 = 25,128

You have used

418.8 x 10 ^ 6 = 418,800,000

Correcting that, your calculation comes out to 68 which is the same sort of order as Eric's number.

I think Colin's estimate of 3 to 6 turns per volt is too big for such a transformer. The larger the transformer, the fewer turns are needed per volt. I would have estimated 1 - 2 turns per volt. 3 - 6 turns per volt would be for a smaller transformer.

 

[ericgibbs]

hi,
For a 1800VA transformer, a rule of thumb is that CSA of the core in square cm must be equal to the square root of the VA. In this case 42cm sq.

As the transformer in the post has a 6 inch sq core , ie 38.7cm sq, it will have a VA limit of approx 1500VA.

For a 60Hz transformer, the turns per volt is 38/38.7 = 0.98 turns per volt.

So for a 120Vac primary thats 120 * 0.98 = 117 turns, I would wind on 120 turns.

Hi Diver, beat me too it.!

My original estimate.
So my estimate would be [6/6.109]*120 = 118 turns [which is close too 114 , from your answer

 

[gary350]

I noticed a microwave oven transformer has exposed windings it is easy to see there are 6 layers, 10 turns on each of the first 5 layers and 7 turns on the top layer making a total of 57 turns on the primary coil. The transformer core measures 1.375" x 2.5" = 3.437 Square Inches. Wire diameter is .059" diameter with my micrometer that is #15 enamel coated copper wire. The microwave oven is rated 1000 watts.

My transformers I want to build has 6.1 Square Inches of core almost double the microwave oven transformer and the power rating will be 1800 watts 15 amps at 120 VAC.

At 15 amps my wire size will need to be #14 enamel coated copper wire for 1800 watts, 120 VAC. This is 80% more power than the microwave transformer I wonder if I can do this, 57 turns x 1.8 = 102.6 turns on the primary. What do you think?????

 

[Diver300]

The microwave oven transformer won't be continuously rated. I would guess that the core is saturated. Check the primary current with no secondary load to see if it is.

With a smaller core, it should have more turns of primary, not less.

A larger transformer will normally have fewer primary turns, because it has more magnetic field due to a larger core area, so your calculation multiplying 57 by 1.8 is meaningless.

I suggest that you wind 200 turns of some fine, cheap and easy to handle wire as the primary.

Power it up and measure the primary current.

Take off 10 turns and measure again.

Repeat until the current starts to shoot up and the core starts to get warm. That is where you have saturated it.

 

[gary350]

OK..........I finally got it figured out.

Np = number of primary turns

f = frequency = 60

Ac = core area in sq inches = 6.1

Bm = gauss = 12,000. for 14 mil laminations.

Ep = primary voltage = 120


Np = [3.49 x Ep x 10^6] / [ f x Ac x Bm]

Np = [3.49 x 120 x 10^6] / [60 x 6.1 x 12000.]

Np = [418.8 x 10^6] / [43920000.]

Np = 95.3 turns