Means for automatically compensating DC magnetization in a transformer

Claim 1

In an electronic watthour meter of the type having a transformer including an input winding for receiving alternating current from a power signal generating source, an output winding connected to meter circuitry for monitoring the alternating current flowing through the input winding, and a core which is potentially saturable through DC magnetization caused by an undersired DC component, of potentially differing values, which is deliberately superimposed on the AC component of the alternating current applied to the input winding of said transformer, apparatus for compensating for DC magnetization of said core comprising: (a) a compensating winding on the core of said transformer; (b) sensing means connected to sense the current flowing through the input winding of said transformer and generating an alternating output signal having a magnitude proportional to the sensed current and which includes the AC and DC components of sensed current; and (c) an averaging filter, including an amplifier, for filtering the AC component out of the alternating output signal over at least one cycle of the sensed current, to thereby provide, from said amplifier to said compensating winding of said transformer, a compensating current signal having a magnitude proportional to that value of the DC component of the sensed current, said compensating current signal effecting linear operation of said transformer by compensating for the DC magnetization of said core caused by the undesired DC component flowing in the input winding of said transformer to thereby effect maximum coupling of the AC component of the alternating current from the input winding to the output winding of said transformer to derive an input signal to the meter circuitry having a magnitude proportional to the magnitude of the alternating current flowing in the input winding of said transformer.