# PT iron Flux Density?

Discussion in 'General Electronics' started by Sniper, Aug 3, 2004.

1. ### SniperGuest

I have the crazy idea of winding a PT from scratch.
I unwound a donor PT because it had a short in at least one winding and the
HV secondary put out 1200V.
The HV winding was under the primary so I've got to wind from former up.

Now, in looking through various texts the one item
I'm going to have to guess at is the
'lines of Flux' or Flux Density of the laminations from the PT.

I've seen values of 55000,60000,75000 and up to 120000 in books and online
texts
and the calculations for winding turns and turns-per-volt vary widely with
small changes
in the flux amount.

Is there any possible way to determine this information without knowing who
supplied the iron?

2. ### Fred StevensGuest

There is a way to determine the maximum flux density of a core. Here
is my suggestion:

1) Wind a few turns on the core (say N=100)
2) With a signal generator and a MOSFET (to handle the current),
impose a low duty cycle square wave on the coil.
3) Watch the slope of the charging current and increase the duty cycle
slowly until you see the slope start to increase. This is the point
where the core starts to saturate.
4) Note the value of the current at the onset of saturation ("Imax")
and the width of the pulse
5) Knowing V and di/dt, you can calculate L
6) The reluctance can then be calculated from R = (N*N)/L
7) The maximum flux before saturation will then be N*Imax/R
8) To calculate the maximum allowable flux density B=flux/Area, you
need to figure out the area that the flux is passing through.

I hope that helps.
Fred.  