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Closed Loop Fluxgate (v0)

A project log for DC Current Transformer

Investigating fluxgate current sensors (DC capable) with detours into analog electronics and switched mode power supplies.

jbbjbb 03/11/2014 at 12:250 Comments

(Edited by jbb 21/3/14 for typos)

As we saw in the previous two posts, we can use a fluxgate to measure an unknown primary current.  The use of two fluxgates provides (partial) cancellation of induced ripple Electro Motive Forces (EMFs) in the primary circuit.

However, we also saw that the current range of the fluxgate sensor is limited to around half of Isat (see project log 1), i.e. about 3A.  We want a measurement range of around ±25A, so we have a problem - the sensor is only good for 10% of the desired range!

Additionally, we want the induced voltage Ep to be as low as possible, which happens when Ip = 0 (see project log 2).

The solution to both problems is closed loop control. We will add a third feedback winding with Nf turns.  This gives us a pair of 3 winding transformers as shown:

We now have an extra degree of freedom to play with: the current through the feedback windings If. With a bit of closed loop control, we can adjust If such that:

With a little manipulation:

This will yield 0V output from the fluxgate current sensor, and 0V of induced voltage Ep.  The model is shown below:

A few points:

So, let's have a look at the waveforms:

We see that:

Fantastic, we have a working DC current sensor!  Now what about 50Hz AC?

Well, after a few ms of transient behaviour at startup, we do get a sinusoidal current out.  However, we have quite a bit of phase shift (power measurements using this would be useless!).  In a future post we will talk about bandwidth extension with a third ferrite core functioning as a current transformer.

Next post: practical excitation circuits for the fluxgate.

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