SPot Welder Transformer turns ratio - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: November 18, 2016, 07:00:15 am
content: I have a spot welder project for which i need to rewind the Secondary of a Microwave Oven Transformer. I intent to get about 400 to 600amps.Can anybody pelase suggest the number of turns on the secondary ? I read that 1 turn = 1 volts aproximately but how much amps ??Thanks in advance.

author: Ian.M
author_rank: Super Contributor
timestamp: November 18, 2016, 09:07:43 am
content: 'suck it and see' - also known as experimenting. Wind a temporary ten turn secondary with hookup wire and measure the voltage. That gets you the volts/turn. Then you need to calculate the total loop resistance of the proposed welding circuit, by calculating the resistance of each part from the specific resistivity of the metal used (presumably copper) and its dimensions. That will tell you how many turns are needed to drive 600A round the loop. Adding a turn adds more length and thus resistance so you'll have to recalculate a few times. There's also the primary resistance to consider so you get abit less current than the simple calculation suggests. Then you need to check the secondary VA doesn't grossly exceed the original transformer rating. A factor of two overload would be acceptable if the duty cycle is well under 50%. If the calculations don't come out within a reasonable multiple of the VA rating of the transformer, you'll need two identical transformers and wind the secondary through both of them so they are effectively in series with the primaries in parallel.

author: oldway
author_rank: Super Contributor
timestamp: November 18, 2016, 10:08:49 am
content: The resistance of the secondary circuit is very low and is not the most important factor limiting the secondary current.In this kind of transformer with primary and secondary wound byside, the short circuit impedance is high (more or less 20%) and the inductance X2 will be the most important impedance limiting the secondary current.In my opinion, the only way is to measure all the parameters of the transformer and eventually modify the number of turns to obtain the secondary current you want.First, you should know what is the nominal primary current of your transformer. There are 2 models of MOT, 800W and 1000W. How much power is your MOT ?Nominal primary current is :for 800W and 230V: 3.48Afor 1000W and 230V: 4.38AFor short time, you can overload 2 or 3 times the primary of the transformer.Then, you make the secundary with isolated big wire using all the space of the window.Short-circuit the secondary and feed the transformer with a variac starting from 0V.You increase the primary voltage, mesuring primary voltage, primary current and secundary current. (use a current clamp)Don't overload the transformer, stop when you reach the primary nominal current.Report the values here.

author: Cerebus
author_rank: Super Contributor
timestamp: November 19, 2016, 12:21:11 am
content: Quote from: anishkgt on November 18, 2016, 07:00:15 amThis isn't going to be what you want to hear. Nobody who would write "I read that 1 turn = 1 volts aproximately but how much amps" is safe to work with circuitry that produces 400 to 600 amps. Really. You need to learn a lot more, starting from basics, before you start trying to rewind high power mains transformers. If you persist without learning some basics you're going to electrocute someone or burn somewhere down with a flaming transformer. I'm sorry to sound so negative but that's the way it is.