Arc free phase control alternatives for AC motor starters

An arc-free phase control circuit for an AC motor, comprising:

• a first relay for connecting a terminal of an AC power supply source to an AC motor;
• a TRIAC for connecting another terminal of the AC power supply source to the AC motor according to a first output terminal signal;
• a second relay connected in parallel to the TRIAC and for connecting another terminal of the AC power supply source to the AC motor according to a second output terminal signal;
• a voltage sensing unit configured to sense a TRIAC trigger power supply voltage rectified from an output of an isolation transformer with a primary input of a power supply source and output a TRIAC trigger power supply voltage signal to a first input terminal;
• a current sensing unit configured to sense a current applied by the AC power supply source and output an applied current signal to a second input terminal;
• a zero-cross point detection unit configured to detect a zero-cross point of an AC power supply source and output the zero-cross point to a third input terminal;
• and
• a controller configured to receive the TRIAC trigger power supply voltage, the applied current, the zero-cross point, and the induced voltage from the first to third input terminals, and to phase-control the TRIAC at every half cycle of a commercial frequency,
• wherein, when the AC motor is started or stopped according to a stability of the TRIAC trigger power supply voltage of the first input terminal, in order to protect a peripheral circuit from a short circuit accident only at a first time a TRIAC trigger current is allowed, the controller memorizes a zero-crossing time point of an AC power supply voltage and then outputs an ON signal to a first output terminal after a lapse of a certain phase delay time greater than 135° and less than 180° to drive a TRIAC gate so as to apply the current to the TRIAC in a voltage-dropped state, senses the current of the TRIAC which is in a half-wave state through the second input terminal to calculate a circuit impedance and an allowable peak current, and immediately turns off, if a short circuit current is anticipated at a rated AC power supply voltage, so as to protect the peripheral circuit from the short circuit accident within a half cycle, and
• wherein, while in a TRIAC mode, the controller receives a coil induced voltage through the third input terminal for a TRIAC-off time and calculates a speed of the AC motor and if the calculated speed is low, then adjusts phase-control a cycle to one third or one fifth of the commercial frequency according to the calculated speed of the AC motor.