Dimmer AC motor speed control schematic  

An electronic speed control for an AC Motor can be designed with a Light Dimmer IC by limiting the minimum firing angle of the triac so that the motor won’t stall. This can be done effectively for high volume applications by making a minor change to the IC.

An alternative to modifying the IC is illustrated in Figure 2. Schematic shows how to control the Minimum Triac Firing Angle so that it is greater than that determined by the LS7231 Light Dimmer IC. The desired value of Minimum Firing Angle is set by an Auxiliary Resistor-Capacitor-Diac Circuit. When the IC produces trigger pulses at lower values of firing angle, the Auxiliary Circuit fires the Triac. When the IC produces trigger pulses at greater values of firing angle, the IC trigger pulse fires the Triac. When the IC is off, the Auxiliary Circuit is also off and the Triac does not fire.

The major components required to implement this function are the MPS8599 Transistor, MPSA13 Darlington, L201E5 Triac and the HT-32 Diac.