You can calculate the fundamental component of the currents and voltages from instantaneous values.
If two instantaneous values I1 and I2 are taken 1/4 cycle apart, then:
The magnitude of the sinusoid is I = sqrt(I2? + I1?)
The rms value will be Irms = I/sqrt(2)
The angle ? = arctan(I2/I1)
? is relative to a cosine function that has a peak at the time of the second reading I2. It doesn't really matter as long as all values measured are on the same time basis, then the relative angles of all quantities will be correct.
If two instantaneous values I1 and I2 are taken 1/4 cycle apart, then:
The magnitude of the sinusoid is I = sqrt(I2? + I1?)
The rms value will be Irms = I/sqrt(2)
The angle ? = arctan(I2/I1)
? is relative to a cosine function that has a peak at the time of the second reading I2. It doesn't really matter as long as all values measured are on the same time basis, then the relative angles of all quantities will be correct.