When the DPDT switch is in the low position this circuit triac will control an induction motor from full to low RPM.
But at low RPM the motor internal air flow cooling is ineffective and will cause the motor to overheat in a short time.
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Do you have a link to the original project that explains this?
Long Goon... Do not mistake this as a true induction motor RPM control.
It is a lamp dimmer class power control.... Less power => more armature slip when a load is applied. Works best with a fan. At no load there will be very little rpm drop until a mechanical load is applied the a big drop in rpm will occur.
In the brush motor mode this will respond more linearly to load or no-load.