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Rockler claims it won't work with soft start tools or capacitive start bench grinders. stay away from the 15Amp controllers as most don't vary the speed much. This one from Rockler is 20Amp and good for up to 3HP routers.
The Soft Start Reduced Voltage starter uses high speed switching devices called 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 (roughly average) voltage getting to the motor is reduced proportionately by the amount of time the switch is delayed. So if the 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 SCR is being gated at the Zero-cross 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 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. In addition, 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.
In short, the router speed control interferes with the SCR's ability to switch.