Motor control reduced-voltage starting

Secondary Control of Wound-Rotor Motors Wound-rotor motors may be effectively reduced-voltage-started or have their speed controlled by using external secondaiy resistance. The addition of resistance into the secondaiy circuit of a wound-rotor motor reduces the starting current and affects the speed under load conditions. 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

Closed-circuit transition (as applied to reduced-voltage controllers) A method of motor starting in which power to the motor is not interrupted during the normal starting sequence. 

Operating overload The overcurrent to which electric apparatus is subjected in the course of normal operating conditions that it may encounter. Note Maximum operating overload is considered to be 6 times normal full-load current for ac industrial motors and control apparatus and 4 to 10 times normal full-load current, respectively, for dc industrial motors and control apparatus used for reduced- or fuU-voltage starting. It should be understood that these overloads are cuirents that may persist for a very short time only, usually a matter of seconds. 

The most significant cost to consider when purchasing a reduced-voltage soft-start motor control is the purchase price. But there are several other costs which must also be considered. In both new and existing installations, it is necessary to figure in the cost of instaUmg the device. Depending on the location selected, the cost to install and wire the device could exceed the cost of the device itself. 

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