can be derived by simple considerations analogous to the dc case.
However, in ac, a phase uncertainty due to the not purely resistive characteristic of voltage circuit arises.
In sinusoidal conditions, if ew (in radians) is the phase of the real coil impedance, and cos is the load power factor, the relative uncertainty in active power measurements can be shown to be equal to –ewTg .
The phase uncertainty depends on the frequency.
By using more complex circuits, the frequency range of the dynamometer can be extended to a few tens of kilohertz.