Beginners' guide to Motor Soft Starter? (part 3)
The SSRV starter uses high speed switching devices (SCRs or Silicon Controlled Rectifiers) to switch on for only a portion of each half of the sine-wave line power. By doing so, the RMS (roughly average) voltage getting to the motor is reduced proportionately by the amount of time the switch is delayed. So if the Silicon Controlled Rectifier (SCR) is not allowed to begin conducting (know as being "gated") until the sine-wave is already 1/2 over with, the output RMS voltage 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 Silicon Controlled Rectifier (SCR) is being gated at the Zero-cross point and the motor is getting full line voltage.
The speed at which the Silicon Controlled Rectifier (SCR) gating is backed up is called the Ramp Time. Typically it can be anywhere from a fraction of a second to 60 seconds. Although longer times are possible, most AC motor 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.
And the ramp time can be overridden by a Current Limit setting, which determines the motor current through feedback sensors and stops the gate advancement in order to maintain a 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 Silicon Controlled Rectifier (SCR) firing becomes unnecessary and it is often beneficial to use a Bypass Contactr to shunt power around the SCRs. SCRs are not perfect conductors, and will reject about 1.5 watts of heat per running load amp per phase. So on a 3 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.
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Know more here: Motor soft starters, softstarters
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