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Old 11th Jun 2018, 10:13 am   #21
Dave Moll
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Default Re: Wiring a desk electric fan.

If brown is attached to one end of a capacitor, there will presumably be no DC path to whatever is connected to the other end. If the pair under test is first shorted (to ensure no charge in the capacitor), there may be a brief resistance reading as the capacitor draws a charging current.
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Old 11th Jun 2018, 10:30 am   #22
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Default Re: Wiring a desk electric fan.

The problem being, when I apply the prongs to the wires, wildly fluctuating readings flash on and off the screen of the meter, barely allowing me time to read them.
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Old 11th Jun 2018, 10:30 am   #23
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Default Re: Wiring a desk electric fan.

If black to green is 22R and green to grey is 22R, then I'd expect black to grey to be 44R. It would be worth rechecking these readings.

If blue to green is 30R and Green to black is 22R, then I'd expect blue to black to be 52R, so 55R is near enough.

When you say "no reading" I take it you mean open circuit or infinite resistance?
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Old 11th Jun 2018, 10:32 am   #24
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Default Re: Wiring a desk electric fan.

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The problem being, when I apply the prongs to the wires, wildly fluctuating readings flash on and off the screen of the meter, barely allowing me time to read them.
That might be down to capacitors charging. Use croc clips rather than probes and wait for the reading to settle.
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Old 11th Jun 2018, 10:44 am   #25
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Default Re: Wiring a desk electric fan.

When attempting to take the readings(which appears to be an impossible task) I have the arrow on the meter pointing directly below the off where it reads 200.. As on a compass N-S
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Old 11th Jun 2018, 10:46 am   #26
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Default Re: Wiring a desk electric fan.

''When you say "no reading" I take it you mean open circuit or infinite resistance?'

I mean nothing flashes on the meter screen. no reaction, zilch.
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Old 11th Jun 2018, 10:48 am   #27
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Default Re: Wiring a desk electric fan.

You need to start with your meter on the highest resistance range and then switch down the ranges until you get a sensible reading. This will allow for windings with a resistance in excess of 200R.

I take it your meter has a good battery fitted? A low battery can lead to the display flashing.
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Old 11th Jun 2018, 10:53 am   #28
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Default Re: Wiring a desk electric fan.

I think that can be taken as indicating an open circuit, or at least (as Graham says) higher than the maximum of the range used (at least as far as DC resistance is concerned).

Incidentally, for observing the resistance reading as a capacitor charges (as I described above) you really need an analogue meter, as you can see the pointer flick up and back. This is difficult to observe with a digital readout.
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Last edited by Dave Moll; 11th Jun 2018 at 10:55 am. Reason: rider about meter range
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Old 11th Jun 2018, 10:55 am   #29
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Default Re: Wiring a desk electric fan.

When I touch the wires with the prongs, a reading flashes up for a nano of a second. If I retain the prongs on the wires, nothing occurs until I make fresh contact when another reading flashes up, again for a nano of a second which bears little, or no relationship to the previous reading.

Taking these readings via the wires appears to be ''Mission impossible.''
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Old 11th Jun 2018, 11:00 am   #30
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Default Re: Wiring a desk electric fan.

A reading should not "flash up" on a DMM. The display should be visible all the time, even if the reading is varying.

What happens if you try to measure the resistance of say a 100W incandescent light bulb?
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Old 11th Jun 2018, 12:58 pm   #31
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Default Re: Wiring a desk electric fan.

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The problem being, when I apply the prongs to the wires, wildly fluctuating readings flash on and off the screen of the meter, barely allowing me time to read them.
It's quite likely that the meter is trying to set its resistance range automatically, but can't. That is not uncommon with transformers and inductors! The meter forces a constant current through the leads and samples the voltage to determine the resistance, but if there is a lot of inductance then the snapshot of voltage could well be before things have settled down, so could be unduly large. Then the meter 'thinks' the resistance is large, so switches the current down by a factor of 10, and measures the voltage again... but the act of changing the current gives an unduly large voltage surge so it gets confused again, and the display just changes chaotically.

The motor looks like an induction motor, with tapped windings - probably a shaded pole motor. Strictly these are nearly constant speed, but with a load (especially a set of fan blades as load, where the torque is roughly proportional to the square of the speed) they will often run well below synchronous speed, enabling crude, inefficient but simple and effective speed control to be achieved just by reducing voltage, adding series resistance, or adding extra windings (which is equivalent to reducing the turns per volt).

If it was my fan, I'd use an analogue meter like an Avometer to measure resistance, finding the pair with the highest resistance. I'd assume these were the slowest speed, and prove it by connecting to a Variac and winding up the voltage, feeling for the windings overheating as I did so and letting it spin. I'd investigate applying the Variac to other tappings, expecting the torque to be increased for the same voltage. I'd expect to spend a bit of time, trial and error, but as it is a rather nice fan, it should be well worthwhile!
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Old 11th Jun 2018, 1:13 pm   #32
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Default Re: Wiring a desk electric fan.

Does that apply to non-auto ranging meters, which is what the OP has? I can't say I've had any problems measuring the resistance of inductors and transformer windings using a DMM.
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Old 11th Jun 2018, 1:14 pm   #33
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Default Re: Wiring a desk electric fan.

That would be true of an auto-ranging meter, but the OP's meter is one of the usual basic kind.
I agree, though, that the inductance of the windings may result in confusing readings until they settle.

Edit: cross-posted!
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Old 11th Jun 2018, 2:07 pm   #34
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Default Re: Wiring a desk electric fan.

I was given a 3 speed fan that did not work at all.
It had 4 wires with one common and 3 wires connected to latching buttons with a fourth that reset the other buttons. Someone had pushed two buttons at once and burned it out.
It was just several coils in series with one wire connected to a tap according to the selected speed. There was nothing else in it.
I would just connect some AC with a lamp limiter between the highest resistance points and see if it runs.
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Old 11th Jun 2018, 2:07 pm   #35
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Default Re: Wiring a desk electric fan.

Firstly, I apologise for testing the patience of all contributors to this thread with my ignorance of electrical matters, reading meters etc. I can strip a Patek Philippe wristwatch, draft out legal appeals etc. but when it comes to the aforementioned, my knowledge is severely limited.

I do however have an old analogue volt meter(image enclosed( but I don't have a clue how to use it.
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Old 11th Jun 2018, 2:17 pm   #36
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Default Re: Wiring a desk electric fan.

That will work very well with some new batteries and it will not give false readings when measuring motor windings.
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Old 11th Jun 2018, 3:18 pm   #37
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Default Re: Wiring a desk electric fan.

That AVO meter would be ideal for the job. As far as the batteries go, it should require one "D" cell (1½V)* and one BLR121 (15V). The latter will probably need to be purchased from the internet (just search for BLR121), as it's not widely used outside AVO meters and not something you can pick up off the shelves of the likes of Wilko.

*If I recall correctly, some AVOs require an adaptor for the cell. Alternatively, wires can be soldered to the cell (using a fairly beefy soldering iron).
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Old 11th Jun 2018, 6:45 pm   #38
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Default Re: Wiring a desk electric fan.

Have we really discounted the possible use of DC for this particular device. To this end, the wires may have been snipped from a speed control rheostat that were common around that time; as were three wire DC supply networks.
No one has made any mention of a commutator which are used in universal motors but are series connected. If a squirrel cage rotor is used (no comm) then it will obviously be an AC supply but if DC we would need to lift the brushes and undertake further continuity and Ohmic measurements. If series connected then yes AC could be used but not so if shunt or compound wound.
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Old 11th Jun 2018, 9:13 pm   #39
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Default Re: Wiring a desk electric fan.

Quote:
Originally Posted by Lucifer View Post
Firstly, I apologise for testing the patience of all contributors to this thread with my ignorance of electrical matters...I can strip a Patek Philippe wristwatch...[but] my knowledge is severely limited.

I do however have an old analogue volt meter
That's very funny. It's not a rare thing or anything, but you have (one of) THE classic bits of test gear. Like someone apologising for ignorance of wrist watches and then pulling something out of a draw and saying 'it appears to be Swiss - Rulex? Ralex?'

Put some batteries in and measure again. The 15v cell can be made from a stack of 5 off CR2032 coin cells and a spacer (some 2p pieces if you've nothing else). You'll need to zero the ohms range on that Avo; if you can't work it out, ask on here.
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Old 12th Jun 2018, 12:02 am   #40
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Default Re: Wiring a desk electric fan.

I have found that a 15mm copper pipe stop end without the nut on it works as a spacer for the CR2032s. I have done two meters this way and they are both fine.
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