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bionicmerlin 22nd Aug 2019 11:03 pm

Limit desk fan
3 Attachment(s)
I got this fan a couple weeks ago at a radio rally . Sold as non working.The seller had said itís not safe to use as it had all sticky goo coming out the motor. Infact all it is the gromet turning to goo .
The fan works fine and Iím going to put a nice new 3 core cotton flex on it.
Now the problem it doesnít work on the low speed. It has a coil to drop the voltage and itís open circuit.
It runs nicely at 150 volts on my variac so Iím looking for some help on options to lower the voltage.there must be a more modern way of reducing the voltage.
I could put a dimmer switch in line but Iím sure some of you good people will come up with a better option. The fan is 90w load.

McMurdo 23rd Aug 2019 8:50 am

Re: Limit desk fan
I'd have thought a little onboard dimmer module (motor rated of course!) would've been the ideal way, if you can get it to fit.
Some lab beaker stirrers we used to repair had dimmer-style speed controls (universal motor) and ISTR they were fussy about the triac contained, DV/DT triggering and all that..but a purpose dimmer, somtimes sold as a 'drill speed controller' will be suitably designed. PS..some dimmer controls might make the motor buzz annoyingly, though a small motor should be ok IMHO

Brigham 23rd Aug 2019 11:03 am

Re: Limit desk fan
I'm not familiar with the coil arrangement. Does it only have two wires?

Herald1360 23rd Aug 2019 11:15 am

Re: Limit desk fan
I'd be inclined to pull the coil (series choke) apart and attempt to rewind it. Once you've found out roughly what gauge wire was used, a simple fill the space winding should work OK. If it's just in series with the mains to the motor and the winding can take the current, there's little chance of any problem if it's not quite right. If the fan's too slow, just remove turns......

You might get away with a suitable capacitor but be very wary of any resonance at 50Hz if you try it.

usradcoll1 23rd Aug 2019 3:26 pm

Re: Limit desk fan

Originally Posted by Brigham (Post 1170574)
I'm not familiar with the coil arrangement. Does it only have two wires?

It's a reactor type speed control. It creates a phase shift that alters the speed of the motor. Most fans today use AC capacitors. Ceiling fans use two or three caps for speed control.
Dave, US radcoll1, a little rusty on my AC math. :shrug:

McMurdo 23rd Aug 2019 5:11 pm

Re: Limit desk fan
1 Attachment(s)
The Xpelair Taurus desk fans (80's age) we used to fix by the hundred used in the sewing rooms of the UK's garment industry in the early 1990's had either switched windings with buttons on the base for the basic ones, or the luxury ones were continuously variable speed with a triac controller. The chinese generic ones that came in later years used either switched windings or switched capacitors, but you need a decent sized base to accomodate all that, those 18in fans had big bases :D

Hartley118 23rd Aug 2019 7:02 pm

Re: Limit desk fan
At Vent Axia, we abandoned triac speed control because of the ‘buzzy’ acoustic noise problem. Speed control was by inductors or auto transformer, but I find series capacitors just as effective. I’d start by trying 1or 2 uF.


bionicmerlin 23rd Aug 2019 10:28 pm

Re: Limit desk fan
Thanks everyone for your suggestions. I think I will try all 3 options and see whatís most likely to work. Andy

broadgage 27th Aug 2019 10:40 pm

Re: Limit desk fan
I have used a fluorescent lamp choke to reduce the speed of a desk fan.
IIRC it was an Expelair Taurus as pictured in a previous post, with a failed semiconductor speed control.

I simply used the choke and a three position switch to give OFF/LOW/FULL speed control.

bionicmerlin 31st Aug 2019 10:38 pm

Re: Limit desk fan
Well the wire was so fine on the transformer so I didn’t go that route so in the end I went for the capacitor option . 1.5 MF did the trick.
For some reason the switch operated in the neutral,that’s not ideal so now it’s altered to switch the live. I replaced the horrible black rubber flex for some nice 3 core cotton covered
Thanks everyone for your help and suggestions Andy

McMurdo 1st Sep 2019 5:26 pm

Re: Limit desk fan
dont forget to add a discharge resistor! ;D

bionicmerlin 1st Sep 2019 7:12 pm

Re: Limit desk fan
I hadn’t thought about that . Thanks for reminding me .Can you recommend a value Andy

julie_m 1st Sep 2019 8:07 pm

Re: Limit desk fan
Anything of a few hundred kΩ will be fine; just watch the power dissipation (which is just V*I, and I=V/R; you can do it in one step as P=V**2/R). A 470kΩ resistor would be dissipating about 0.11W, if the motor went short and left the full 230V across the R||C network.

bionicmerlin 2nd Sep 2019 12:37 am

Re: Limit desk fan
Thanks Julie . I will get on with that in the next day or so Andy

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