An apparatus for calculating an effective voltage is provided, which includes a voltage reducer, an analogue to digital converter, a first detector, a second detector and a calculator. The voltage reducer steps down instantaneous output voltages of a generator. The analogue to digital converter converts each analogue signal of the instantaneous output voltages into a digital signal. The first detector detects a first signal of digitized instantaneous output voltage. The second detector detects a second signal, which differs in terms of phase by 120 deg. from the first signal. The calculator calculates an effective voltage according to the first and second signals, by executing a calculation with an expression: E={2(V_a²+V_aV_b+V_b²)/3}^1/2, where E represents the effective voltage, V_a the first signal of instantaneous output voltage and V_b the second signal, respectively.