Apparatus for monitoring two electric quantities by combining three consecutive samples of each quantity

Apparatus for monitoring a variable derived from a first substantially sinusoidal electric quantity having a first period and a second substantially sinusoidal electric quantity having a second period nominally substantially equal to said first period, said variable being independent of time, comprising:

• a source of sampling pulses of a predetermined sampling period;
• two samplers responsive to said sampling pulses and respectively responsive to said first and second electric quantities for respectively deriving first and second discrete signals from said first and second electric quantities at individual sampling points defined by said sampling pulses, said first and second discrete signals being representative of samples of said first and second electric quantities, respectively;
• at least one multiplier unit coupled to said samplers for calculating at least one product of samples which are representative of at least one pair of said discrete signals to produce at least one product signal representative of said product, one and the other of said at least one pair being derived from said first and second electric quantities;
• respectively:
• arithmetic means coupled to said multiplier unit for carrying out at least an algebraic addition of a signal representative of at least one of the product signals produced by said at least one multiplier unit with another signal in response to the discrete signals derived at three consecutive ones of said sampling points to produce a first result signal representative of said variable, said variable being further independent of phase relations between said sampling pulses and respective ones of said electric quantities;
• and
• a discriminator circuit coupled to said arithmetic means and responsive to said first result signal for producing a second result signal when said variable exceeds a given reference value.