Technically a soft starter is any device that will reduce the torque delivered to the power train. Mechanically, this can be a clutch, fluid drive, magnetic coupling, short coupling or any of the variety of devices that allow the motor to start-up across the line while slowly applying the shaft torque to the load to avoid “torque shock”. Electrically it can be any system that reduces the torque by virtue of reduced voltage or a change in motor connection.
Changing the motor terminal voltage reduces the torque because the motor output torque varies by the square of applied voltage. So if 50 percent voltage is applied to a motor, it will produce 25 percent of its available torque at that point.
Reduced Voltage starting can be accomplished in several ways as well, A common method is to use an auto transformer that drops the motor voltage during starting, then is switched out so that the motor gets full voltage when running. This method is called reduced voltage auto transformer starting. Similar to this are reactor and primary resistor starters which drop the voltage through those devices as well. All of the above technologies can be and often are referred to as “Soft Start” devices, but more recently this terminology has come to usually mean one specific type, the solid state reduced voltage starter.
The solid state reduced voltage starter(SSRV) uses high speed switching devices call SCRs(silicon controlled rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS voltage getting to the motor is reduced proportionally by the amount of time the switch is delayed. So if the SCR is not allowed to be conducting until the sine wave is already 1/2 over with, the output RMS will be 1/2 of the line voltage. By moving the gate point further back in the sine wave, the RMS voltage is increased until the SCR is being gated at zero crossing point and the motor is getting full line voltage. The speed at which the SCR gating is backed up is called the Ramp Time, and can typically be anywhere from a fraction of a second to 60 seconds. Although longer times are technically possible, most AC motors applications will not allow this because the increased current caused by the reduced voltage will begin to exceed the thermal safety limits of the motor itself, particularly the rotor. In addition the ramp time can be over ridden by a current limit setting, which determines the motor current through feedback sensors and stops the gate advancements in order to maintain the particular current setting. This feature is useful when the power system has limited delivery capabilities, such as weak utility lines or portable generators.
Finally once the motor is at full voltage the SCR firing becomes unnecessary and it is often beneficial to use a bypass contactor to shut power around SCRs. SCRs are not perfect conductors, and will reject approx 1.5 Watt of heat per running load ampere per phase. So on a phase 100A motor, the SCRs will be rejecting 450W of heat into the enclosure continuously. A bypass contactor is a good way of avoiding that heat buildup without introducing dust, moisture or other contaminants into the enclosure.
