Apparatus for controlling electric motor

In a control apparatus for controlling a driving main circuit consisting of a diode converter for converting AC power to DC power and a PWM power converting device connected to a DC output side of said diode converter for driving an AC electric motor, said apparatus for controlling an electric motor comprising;

• a current detector for detecting a DC current converted by said diode converter to be supplied to said electric motor;
• a high-pass filter for filtering therethrough an AC component of said DC current detected by said current detector;
• a control circuit in response to an output of said high-pass filter for controlling said PWM power converting device in a pulse-width-modulating manner so that said converting device may convert said DC power into the power of a predetermined value for driving said electric motor;
• said control circuit including:
• an A/D converting circuit for analog-to-digital converting the output of said high-pass filter;
• means providing frequency-to-voltage conversion of a frequency instructing signal to provide a voltage instructing signal;
• means for determining the deviation of said voltage instructing signal from the output of said A/D converting circuit and amplifying said deviation to provide a deviation signals;
• a PWM counter and a comparator for comparing the output of said PWM counter with said deviation signal to provide a voltage instructing PWM signal;
• and
• an adder for adding together a frequency instructing PWM signal provided in a frequency signal converting circuit by conversion of said frequency instructing signal and said voltage instructing PWM signal as the output of said comparator and for outputting the sum to said PWM power converting device.