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AC Motor Speed Controller

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Advanced Member level 2
Mar 21, 2015
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Hi All,
i am designing a AC Motor Speed controller using 8051 Controller. (Circuit diagram attached).
The Motor i am using is 220 V AC 180W motor with Forward Reverse and feedback.
To control the speed of the AC motor i am using Zero detection Circuit at MCU interrupt Pin. as the motor has a feedback terminals which provides me the analog voltage as per the speed of the motor which is further connected to MCU ADC pin as shown in the circuit diagram, which helps me the control the speed of the motor. In software i am scanning the ADC regularly and as per the ADC voltage received, i trigger the Triac.

For Eg.
I have set the treshhold of 0.70 Volt for speed 10%. so when the ADC Voltage is higher then 0.70V, which indicates that the motor is spining faster, i trigger the triac approx 8ms after the zero crossing to slow down the motor and as soon as the voltage decrease down less then 0.70V, which indicates the motor is getting slower, i trigger the triac a bit early step by step after zero crossing is detected. so this is somewhat a logic where i am controlling the Speed of ac motor.

Now what my problem is, when the motor is operated at 10% Speed. the motor rotates very smoothly without any load. but as soon as the load is connected to the motor the motor stops spinning. and sometimes vibrates a lot. Have also attached the waveform of all the reading taken. as soon as the motor stops spinning and the voltage drops below 0.70 V, the Triac gets triggered after 2.5ms after ZCD. but then too the motor does not rotates. even i have checked the motor waveform, i am getting a complete sine wave at motor output.
But at the same time when i drive the motor completely ON, say triac is always ON, then even after the motor is overloaded, the motor does not stop.

Can anybody please help me out for what the problem is.


  • Triac (12.08.22).png
    Triac (12.08.22).png
    74.5 KB · Views: 123
  • Test Report.jpg
    Test Report.jpg
    4.8 MB · Views: 132

I even tried implementing the new circuit as per the attachment, but still the results are same.


  • Triac Driver.png
    Triac Driver.png
    25 KB · Views: 109

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