Induction motor DIY speed control with electronics? - Page 1
Forum section: electronics / projects

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 15, 2022, 01:41:46 pm
content: This year the heat wave was just too much, bought a 70cm 140W fan.But the b*** is noisy as hell even at lowest speed, like you had a FBI chopper looking for criminals in your neighbourhood.Impossible to sleep with this noise, nor this heat...I might return it, but I like the power, can put it in one room and create a wind tunnel across the house...Well, as we know, induction motors speed depend on voltage, frequency and number of poles.The typical 30-100W home fan use this type of motor, and several series inductances to select the speed (Thus, lowering the voltage).Triac drivers are not good for this motor type, they draw a lot of power below its target speed.So I was wondering if there's any kind of clever circuit allowing fine speed control not implying AC inverters or bulky inductors.

author: Circlotron
author_rank: Super Contributor
timestamp: July 15, 2022, 01:59:34 pm
content: I've slowed down a fan by putting some AC rated caps in series with the supply. Several power factor caps as used in fluorescent lamps would be quite suitable. You would need to experiment with different sizes to find the right value for rpm, air delivery and noise limits.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 15, 2022, 02:23:35 pm
content: Fan motors are usually designed to be "dimmed", or slowed down with merely voltage reduction - can be phase angle chopping or actual voltage reduction. This means these motors can run with high slip without torque breaking down too badly. As a result, efficiency sucks at any speed.You can try any triac dimmer. But I don't like the buzzing sound triac dimming causes - so just use a transformer (or autotransformer). Job done. Series capacitor indeed is another possibility.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 15, 2022, 02:31:06 pm
content: Hah, it started to make a very annoying buzzing noise today... speed and power is the same, I guess it's some part vibrating, but I'm not breaking the warranty looking for the culprit.Yeah I know caps and inductors will do the job...I might try switching full-waves, hopefully will remove the noise...

author: mag_therm
author_rank: Frequent Contributor
timestamp: July 15, 2022, 02:59:32 pm
content: Take care when advising to put "..caps in series.." with small induction motor.Because if -jXc gets near jXL, then :High voltage across motor,High current through motorThese motors are usually "impedance protected" and jXL > Rone small motor I measured:jXL = 106 OhmR = 4.56 Ohmfor about 1.2 Amp run current on 120VSo if putting a cap on that motor to reduce its current, it has to be at least -jXC = 300 Ohm or so.Better not just try a random capacitor from the junk box.

author: Gyro
author_rank: Super Contributor
timestamp: July 15, 2022, 03:52:12 pm
content: I've seen speed control done on these fan motors using cycle skipping. You can't go very far before the low frequency drone on the skipping becomes just as objectionable though.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 24, 2023, 06:36:00 pm
content: Let me update this.I got a new fan which (Of course) was still noise as hell at lowest speed.We're having a serious heat wave in Spain so I had to use it to not melt down while sleeping.Again the noise is not terribly but I can't sleep properly, I wake up 100 times during the night.So today I stealed few different caps at workand tried them now.4.7uF did the trick for it for 45W 240V (50Hz).Speed 3 (max) does 1280rpmSpeed 2 (mid) does 1230rpm (Really can you call this "mid"?Speed 1 - Didn't measure it before modifying, but it was barely slower, probably in the 1100-1150rpm range.With series 4.7uF: 820rpm. Waaaay more tolerable, only a light breeze is heard.