resistors > Starting resistors

Technical explanation

Are nowadays used only for starting large three-phase asynchronous motors with slip-ring rotor. The starting resistor is switched into the rotor circuit and switched off in phases. It restricts the starting current of the motor and increases its operating component. The resistor and its phased composition determine the starting ratio of the drive mechanism. This phased resistance is effected either by allowing an equal starting peak current flow ( I 2 ) between each separate step, or (and this would be ideal) by presetting a specific starting ratio. The I 1 / I 2 quotient (I 1 = current value for switching to the next step) indicates to what extent the next step is smaller than the previous one and determines the number of start-up phases. In the past, the switch mechanism of the phased start-up process consisted of hand- or motor-operated switch drums or cam-type control switches activated on the basis of a fixed switching program. Today, the industry mainly uses safety switches, controlled on the basis of a time unit, current value or rotor frequency. Smaller drive mechanisms are generally provided with a resistor with fixed gradation, whereas larger motors have resistors with dual gradation in order to limit the number of rotor safety switches. With 5 or 6 rotor safety switches you get 2 5 – 1 or 26 -1 steps respectively, which are software-controlled and selected according to the desired starting behaviourd surface.