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Are Triac based fan speed regulators harmfult to ceiling fans ? One shop guy told me that it's being replaced with capacitor based regulators now a days. But then how can the fan speed be controlled via microcontrollers without using triacs ?

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    \$\begingroup\$ I'm voting to close this question as off-topic because we're not going to be discussing second-hand opinions of "some shop guy". If the question is "how to control fan speed with a microcontroller", then ask it, and research and make clear why you can't use a triac :) \$\endgroup\$ Commented Jun 23, 2017 at 8:55

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Are Triac based fan speed regulators harmfult to ceiling fans ?

Triac based fan speed controllers are not harmful to ceiling fans that are designed for speed control. Even fans that are designed for another type of speed control are not at much risk since the main source of potential difficulty is difficulty in dissipating motor losses at lower speed leading to higher operating temperature. If the motor is designed for sufficient cooling at reduced speed with increased slip. it doesn't matter too much how the increased slip is accomplished.

Triacs can cause a buzzing or humming noise that can be annoying. They can also cause electrical noise on the power lines and even radio frequency interference.

A capacitor in series with an auxiliary motor winding is a common method of giving a single-phase motor starting torque and a defined operating direction. Reducing the capacitor value is a common method of speed control. See the question: "varying run capacitor for speed control of single phase motor." To use that method with a micro controller, the micro controller could be used to control switching devices to switch in different capacitor values. The switching elements would carry only the auxiliary circuit current not the entire motor current. As described, that would provide several discrete alternative speeds like the common three-speed switch. However, a modulated switching method could be used in conjunction with capacitors to provide the efect of a continuously variable capacitance.

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