Method for the protection of electric motors from critical operating states

Method for the protection of electric motors from critical operating states wherein, as the electric motor is operating, a motor current value and/or a motor voltage value is/are measured and compared with a permissible operating range wherein an output signal is generated when leaving the permissible operating range,

• wherein the permissible operating range is determined from the motor current value and/or the motor voltage value;
• wherein the motor current value and/or the motor voltage value is/are being measured while the electric motor is operating;
• wherein the determination of the permissible operating range includes calculating an upper and/or lower limit of the permissible operating range and continuously adjusting the upper and/or lower limit of the permissible operating range while the electric motor is operating;
• wherein the operation of the electric motor is characterised by an initial start-up operation followed by a normal operation, and the method includes determining the permissible operating range for the start-up operation and/or the normal operation;
• wherein the determination of the permissible operating range for the start-up operation comprises the determination of a start-up value and of a threshold value of the measured motor current value and/or of the measured motor voltage value, wherein the threshold value signifies the transition to normal operation;
• wherein the determination of the permissible operating range for the start-up operation also comprises recording a transition course of the measured motor current value and/or of the measured motor voltage value between start-up value and threshold value wherein a permissible operating range exists only if the amplitude of the derivative of the recorded course pattern does not exceed a predetermined maximum gradient.