# Calculating the value of the resistance for controlling the speed of AC fan

Discussion in 'General Electronics Discussion' started by sino007, Mar 27, 2017.

1. ### sino007

2
0
Mar 27, 2017
i have a single phase ac fan that i would like to control its speed i am a bit new in this field so please bear with me

i had the idea of fixing 3 speeds with 3 switches one for each speed: (25% 50% 90%), and 3 relays each switch activates a relay and each relay wired in series with a resistance of a certain value so we have 3 speeds for every resistance (see the attached image)

i dont know the type of the motor (synchronous , asynchronus ,wound rotor...)

the motor have the following informations: 220 v , 50 Hz , 33W

i didnt make any mesurements (voltage,current..) when the fan is on

my question is how to determin the value of the 3 resistances if i want to fix the following speeds (25% 50% 90%)

thank you

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2. ### Minder

3,029
641
Apr 24, 2015
Most of these fan motors are shaded pole motors and if so can be controlled by a simple Triac dimmer circuit, just 1/2 doz components.
Resistors are inefficient.
M.

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3. ### duke37

5,364
769
Jan 9, 2011
The motor is only 33W so three switches and three resistors should do it, What is the blue block for?

The motor torque will be much less at low speeds so it will be very difficult to calculate resistors.

4. ### sino007

2
0
Mar 27, 2017
thx for answering me
the blue black thing is an arduino its like a microcontroller, i m using it because i have tempeture and humidity mesurement plus some led bliking to do
can you tell me how to calculate the resistance or probabl the range of the résistance that i might use(min .. max)
thank u

5. ### duke37

5,364
769
Jan 9, 2011
The motor is equivalent to a resistor of about 3500Ω.
Try a 1kΩ and 2k2Ω resistor. You may need to dissipate more than 10W.

If you have an arduino then this could control a triac to get what speed you want. See #2. I do not know in detail how this could be done.

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6. ### Bluejets

4,417
937
Oct 5, 2014
Likely that it could be induction motor in which case the modern day control uses series capacitor.
Not much sense catering for 90% full speed as you would hardly notice the difference.
Usual approach is around 20% and 50% with the use of a dual value cap. in the one casing.
Appropriate voltage applies of course.
As to the values, not known for 30W as most ceiling fans are around 60w and use 1.8uF for low, 2.5uF for medium and no cap for full 100% speed.

In olden days (60's and 70's) a variable wound choke was used.
Thyristor control was attempted in the 80's and 90's but soon became unpopular as they created large amounts of switching noise which amplified massively within the body and blades of the fans.

7. ### Minder

3,029
641
Apr 24, 2015
If using a micro and wishing to use phase angle control with a Triac for a shaded pole motor.
There is Fairchild AN-3006 and Picmicro AN958 for detecting the zero cross point for phase angle control.
M.

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