Transformer for single phase motor - Page 1
Forum section: electronics / beginners

author: TacticalCode
author_rank: Contributor
timestamp: May 22, 2016, 05:59:41 pm
content: Dear forums,I am too stupid to find a transformer.I'm looking for a multi-tap (or multi-winding) transformer to transform 230V mains down, ideally in 25% steps, in order to control a fan's speed/noise. 25, 50, 75 and 100% output voltage for 4 different settings is what I'd like to get, doesn't have to be exact, just 3 (highest lower than ~80%) or 4 taps for different speeds/noise levels/voltages.The motor I want to control is a 60W universal single phase motor for 230VAC. So I'd need a >60VA/1A Transformer, right? (Triacs cause humming=noise?)Am I looking in the wrong category? Don't they (farnell/digikey) stock such transformers? Are there even such transformers? The only multi-tap transformers I could find were low-voltage (6-32V), or single tap 1:1, 1:2 etc.Could one of you kind people please explain to me how to find such a transformer on one of the usual distributor sites?Many Thanks!

author: Ian.M
author_rank: Super Contributor
timestamp: May 22, 2016, 06:45:23 pm
content: Universal motors are brushed and run on AC or DC. Single phase usually implies an induction motor, which is AC only and the speed is set by the mains frequency. Many shaded pole motors driving a fan or other load that doesn't require significant starting torque can be speed controlled to some extent by reducing the effective input voltage, increasing the slip. If you try that with other types of induction motor they tend to draw excess current and overheat.So the first thing you need to know is what type of motor do you actually have.*IF* its a universal or a shaded pole motor, you can then try it on a Variac to determine how much voltage your reduced speeds need. You can bet that equal steps wont do what you want. If you are running it slow, you'll need to monitor the winding temperature as there is a real risk of overheating.Transformers are expensive, so its common to switch fan speeds for shaded pole motors by inserting carefully chosen motor run caps in series to drop the voltage. They can also be controlled by TRIAC dimmer style circuits. Series capacitors with universal motors can be problematic due to the commutation exciting the series resonance of the winding inductance and the capacitor. If the audible noise from a dimmer style controller is objectionable, its preferable to rectify (but not smooth) the AC and then chop it at between 20-50KHz for PWM speed control.IF a transformer is actually your best option, one approach would be a 28V-0-28V transformer with two 115V primaries. Wire the primaries in series for 230V operation then use it as an autotransformer to get 115V for half voltage. The secondary delivers 56V, near enough 1/4 voltage. If connected to buck the incoming mains, it will give you 174V, 2/3 voltage.

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2016, 07:25:09 pm
content: Look for a ceiling fan speed control, which will do the job. Low cost, and a common thing on small industrial ceiling fans, and works for a fan up to around 100VA.http://www.ebay.com/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=ceiling+fan+speed+control&_sacat=0Look for one which is available to you, and which has the right mounting arrangement. Even have remote control versions for a low price.

author: TacticalCode
author_rank: Contributor
timestamp: May 22, 2016, 07:32:11 pm
content: Thanks for your answers. The motor is 1-Phase AC... So I don't know if "universal" also applies, not too sure on the terminology... Anyway, I've read online that a transformer is the best way to control these Fan/Vent motors, since phase chopping causes hum and potentially other problems.How do you think PWM compares to switching taps via relais in terms of power dissipation? The controller will sit in a weatherproof box (no airflow). I'd assume rectifying then switching 230V*sqrt2 DC would be dissipating more power than magnetic losses + contact resistance. But I could very well be wrong (read below quote)Looking at off-the-shelf solutions (ebay), there are some cheaper fan controls, but those are likely phase controllers, and I'd still have to hack in a PID for temp control. (The fan will be part of a homemade AC unit, but I don't want it to be too loud). So with a transformer, some relais, µC and temp probe, I should still be cheaper off than buying fan control with temperature feedback.Quote from: TacticalCode on May 22, 2016, 05:59:41 pmYes, Dummy! Turns out there are transformers for (but nor limited to) this very application, called autotransformers. (Spartransformator, for anyone speaking German having the same problem...)The very first google result for "Spartranformator Stufen" (Autotransformer taps in English) is a datasheet for these things stating these are specifically for controlling fans/vents.Quote from: TacticalCode on May 22, 2016, 05:59:41 pmI found some, most being over-spec (read: too expensive) for the job. Ebay has some for around 40€. Not as cheap as I imagined, but still cheap enough...