Measure transformer field strength or dummy run with secondary

[beergut]

G'day everyone, new project.

I need to rewind the secondary coil on a transformer to change the voltage and current output.
I dont know how many turns are on the primary.
Is the a quick and dirty way to measure magnetic field strength?
Or should I just put a known number of turns on the secondary, measure induced voltage and work backwards?
I have seen Arduino hall effect sensors cheap, anyone have experience with those?
Budget is extremely limited.

Thanks to anyone that reads this.

[Gyro]

The simplest way is to wind a known number of turns onto the transformer and measure the output (as you say). This will give you approximate turns per volt for the secondary. If you load the existing secondary to it's specified rating, The difference in measured voltage on your added secondary will give you an idea of the regulation contribution for the primary and core of the transformer (no load - full load %).

It is not normally necessary to know the number of turns on the primary, simple secondary measurements will get you close enough - mains voltage isn't precise and varies over a few volts anyway. There are tables online that will give you recommended diameter for a given current, and resistive voltage drop per unit length, which you need to factor in for load regulation contribution of the secondary.

Magnetic field strength measurements are unlikely to get you very far in practice.


P.S. The 'wind your own secondary' RS Components transformer kit instructions, attached to the first post of the following thread will give you some useful pointers...  https://www.eevblog.com/forum/reviews/r-s-transformer-kits/

[Kleinstein]

How much the transformer voltage drops on loading is mainly a function of the DC resistance of the wires (at least for small transformers of less than some 1 kVA - for large one stray inductance gets more relevant). So one can use the DC restance as a first and often good approximation unless the core has extra leakage (e.g. microwave oven transformer or welding transformers).

Using the coil voltage is a kind of way to measure the flux. 

[Terry Bites]

You've a long night ahead of you mate.

[MrAl]

G'day everyone, new project.

I need to rewind the secondary coil on a transformer to change the voltage and current output.
I dont know how many turns are on the primary.
Is the a quick and dirty way to measure magnetic field strength?
Or should I just put a known number of turns on the secondary, measure induced voltage and work backwards?
I have seen Arduino hall effect sensors cheap, anyone have experience with those?
Budget is extremely limited.

Thanks to anyone that reads this.

Hello there,

There are a couple of things to think about here.

First, when a transformer is wound the primary and secondary voltages and currents are known beforehand and the wire sizes are chosen to meet the power handling requirements of the construction.  What this means to start with is that there is an intimate relationship between the current and voltage of both the primary and secondary.
For example, and i'll use some round numbers here, if the input is 100 volts and the secondary is 10 volts at 10 amps, then since 10v times 10a is 100 watts, that means the primary will be drawing (approximately) 1 amp, and that means although the secondary wire size has to handle 10 amps the primary wire size only has to handle 1 amp.  That 1 amp primary current however limits what you can expect to get on the secondary.
If you were to change the secondary windings such that you can get 5 volts out instead of 10 volts, then you could expect to get twice the current because 5 volts times 20 amps is still 100 watts.  That would mean your secondary wire size would have to handle 20 amps if you want to get the most out of it.
If you instead were to change the secondary windings such that you want to get 20 volts out instead of 10 volts, then you can expect to get only 5 amps out because 20 volts times 5 amps is still 100 watts.

So you can see how the voltages and currents are related: the expected power drawn from the secondary must be the same or less.  If you try to get more the regulation could suffer, and worse the transformer could overheat with long run times.
The main life test is to run the transformer at full load for some 12 or more hours and see if it overheats.  You can sometimes get away with running for short times in the application, but only if that same application has control over the run time and limits it to the acceptable time limit.

If you can measure the diameter of the primary wire you can get an idea how much current that can handle in case you don't know the power rating of the transformer beforehand.  An acceptable wire size would be 600 circular mils per amp or less, but whatever you do testing has to be done to ensure the transformer guts do not melt.  It's not a pretty sight and could get dangerous.  Primary fusing is also required.

It's a good idea to wind some turns on the secondary and test the voltage and go from there.  There are some caveats though.
Transformers load because of primary resistance, secondary resistance, and leakage inductance.  Since you may not know the leakage inductance, you would have to test the voltage under load and that would normally be full load.
You would have to do this testing with the right wire size on the secondary.  That means if you only wind 10 turns on the secondary and it could really take 100 turns, then you'd have to load it about 10 times more than with the full amount of turns in order to see how much the voltage would drop under full load.