AlexM

Hi,

I don't hold out much hope of getting an answer, but you never know!.

I have one of those awful garish fibre optic christmas tree things, with a 25w 12v halogen bulb in the base and a rotating coloured disk driven by a 12v ac synchronous motor.

My kids have blown up the 12v ac adaptor (12v ac 3000ma) and I cannot find a direct replacement. Can anyone with suitable electrical engineering experience tell me:

a) a good source for a regulated, safe 12v ac 3a toroidal transformer which doesn't cost lots. Apparently voltage regulation needs to be good.

b) I have a 12v ac 'electronic' type psu designed for low voltage lighting. If I connect this, the light works, the dc cooling fan works (there is a little bridge rectifier in the unit), but the syncronous motor doesn't. Why? is the output of the electronic transformer not 12v @ 50/60hz?

As far as I know these transformers use a much higher frequency internally than 50/60hz to increase the efficiency of the voltage conversion.. maybe the output is not at mains frequency.


P.S. the lead lengths are comparable to those allowed for a lighting circuit, so capacitence should be ok.

Suggestions?.

Cheers,

Alex (obviously out of my depth ).

[Edited by AlexM - 12/13/2001 1:34:27 PM]

ChrisB

A couple of people to try (Don't know if they have the right item)...

http://uk1.farnell.com/Welcome/welco...gistered.jhtml

http://rswww.com

Any good?

[Edited by ChrisB - 12/13/2001 1:35:46 PM]

AlexM

Chris

Thanks for the links.. I did try the RS site, but I'm too ignorant to determine compatibility in this application . I THINK I need a 12vac 50va toroidal transformer, but who knows?

Cheers,

Alex

KF

I would just question how you know the kids blew up the adaptor.

That is to say, is it not possible that the motor went first, and took the transformer out, and that now you have replaced the transformer you find the motor still doesn't work.

50va should be fine, as you are drawing 3A at 12V - but given that you are comfortable talking about bridge rectifiers, I think you probably knew that

KF.

AlexM

KF,

I'm definately in the 'a little knowledge is likely to get you electrocuted' category. Well, maybe not at 12v, but you get the idea.

The original power supply blew up after 30minutes use and Homebase couldn't even swap it for one from their demonstrators as three out of five had died too. I think that there is a problem with the batch, or the quality isn't up to snuff. The original PSU gets very,very hot when left on for more than an hour.

The tree is fine, but I want to get another PSU working that will last a little longer.

Cheers,

Alex

P.S. It isn't actually a bridge rectifier as it doesn't appear to have any diodes in the circuit - just a couple of small caps.

P.P.S. Sorry to the 60 or so of you who have read this thread so far... I know.... it's unbelievably boring. At least I can acknowledge the fact


[Edited by AlexM - 12/13/2001 3:28:29 PM]

matt.bowey

How can you tell its an ac synchronos motor?? How many wires does it have feeding it?? & are there any other capacitors or inductors in the circuit.

As far as changing the frequency, I would be suprised if the adaptor has the ability to rectifty the 50Hz ac then invert it back to some higher frequency. Higher frequency means less iron (ie size/weight) but I would expect efficiency to reduce due to higher iron loss.

AC motors can be difficult to start, you sound intelligent enough to have turnt the power supply current limit up to something sensible. Motors often have a large inrush current to get them going.

With more detail, maybe we can get it going.

Matt

AlexM

Matt,

Luckily for me, it is labeled '12v synchronous motor 3rpm'. It has two wires, and the motor appears to have some reduction gear inside the casing.

No capacitors or other components are in circuit between the power supply and the motor. The motor is tiny 0.2w or somthing, so I can't imagine that it would be that difficult to start?.

I don't know what the output frequency of a halogen light transformer is, but the RS FAQ mentions that it switches at 35khz, presumably to improve power capacity while reducing the core size as you say. The power supply is rated 12v 50va, and presumably needs to be run at something like it's rated load to work properly.

Do you think the motor might not turn if the output waveform is more like a square wave than a sinusoidal wave?.

Thanks for sticking with me so far!.

Cheers,

Alex

KF

You know for sure that the 50va is giving AC. I only ask because the other two devices (fan and light) would probably work even with a DC output. Forgive the silly questions
KF.

Norman D. Landing

I am amazed at the knowledge resources here on Scoobynet.

I too have a problem.

I have a toebone that I need to connect to a shoulderbone via a series of other bones.

Can anybody source me a .......hipbone.


Edited to say I'm only joking not trying to sound obnoxious,.....I'll get me coat

[Edited by Norman D. Landing - 12/13/2001 5:08:46 PM]

KF

Mr Landing,
I think you have to speak to Pete's Cronies for that one
KF.

EDIT: Hang on, I'll get me coat too...

[Edited by KF - 12/13/2001 5:34:41 PM]

AlexM

KF,

Definately yes..

Norman D. - I know, I know... I'm already embarassed at how nerdy this thread is, but Scoobynet has a lot of erm... technically orientated readers so I might just get lucky



[Edited by AlexM - 12/13/2001 6:26:31 PM]

Dave T-S

Try Maplin - can't remember the website address but you'll find it by searching under "Maplin"

AndyC_772

iTrader: (2)

Alex: I don't think your thread is boring or nerdy - but then again I'm a professional electronic engineer, so maybe I have a good excuse

It sounds to me as though you're along the right track. Do you have a multimeter or voltmeter? That would help a lot.

If your lighting PSU switches at 35kHz, then it's a so-called 'switched-mode' power supply, which is a common type that does indeed improve power capacity whilst reducing the size of the transformer required. They're normally used when small size and high efficiency are required, which is most of the time! Your PC uses this type of power supply

They do also tend to produce a DC output, not the 50Hz AC that your motor requires. That explains why the cooling fan works but the motor doesn't. It's also possible that it's producing 35kHz AC, given that a light bulb wouldn't care either way; that would probably also drive the DC fan OK (given the presence of the bridge rectifier) but not turn the motor.

If you have a voltmeter available, you can do some further investigation on the 12v output from your working PSU. Measure its output voltage with the meter set to DC, then repeat the measurement with it set to AC. If the power supply has a DC output than it will register zero (or something close to it) when the meter is set to AC and vice-versa. If you seen an AC voltage but no DC, then measure its frequency if you can - some meters have a frequency counter built-in.

Of course, none of this really helps, because either way you're stuck with a power supply that doesn't work, but it would be good to nail down why it doesn't work. I'm sure you can get a suitable replacement from Farnell, but I don't have their catalogue to hand so I can't recommend anything until I get back into the office tomorrow.

Let me know how you get on,

Andy.

KF

OK. Having forgotten almost all of my Electronics degree I fire this parting shot and run.
It was my understanding that a synchronous motor couldn't be started. You start them as induction motors. How? I forget. Why? I also forget. Perhaps the startup sequence is part of the defunct PSU's operation.
Dunno. Good luck.
KF.

matt.bowey

You start synchronous motors by putting some inductance of capacitance in series, this gives some phase shift, blah, blah....
Once up to speed this extra L or C is removed.

I doubt that the motor can operate with 35kHz, even with a high number of poles, its still going to be running mighty fast! And what would be the point? as its output is only only running at 3rpm.

From this distance, I cant see whey your PSU doesnt do the job. To repeat what our friend said above, maybe its the motor that is knackered? does it still turn freely? what is the resistance of the armature winding?

[Edited by matt.bowey - 12/14/2001 8:05:48 AM]

Norman D. Landing

Perhaps you need to funnel the Dilithium crystals through a trans-warp conduit thereby alowing a phase shift in the coil inducers ?






Edit : I never meant it was nerdy, I'm just jealous as I dropped out of my HND Electrical Engineering after 1 year

[Edited by Norman D. Landing - 12/14/2001 10:29:11 AM]

AndyC_772

iTrader: (2)

No, Norman, that wouldn't work - the resulting antimatter explosion would destroy the motor, the tree and the planet they were sitting on

I doubt that the motor in a Christmas decoration is heavy enough to need any special starting procedure. If that turns out to be the problem, I'll drink a beer as forfeit.

Much more likely is that you just need a transformer. I've found something that looks promising in the Farnell catalogue - p/n 552-549 - but that's another lighting transformer, so the one you already have may be very similar. This one definitely has an AC output, so you really need to determine whether the one you already have is AC or DC. It costs £27.54 + vat and has screw terminal connectors, so you'll need cables and a willingness to make mains connections.

If you want something cheaper then there are plenty of 'bare' transformers, but you'd need to find a way of mounting them in boxes for safety.

Andy.

The_Gza

Err - not trying to be cheeky (honest ) but why not just buy a new tree instead?

Given that the transformer suggested above is nearly £30, I'm sure you can buy a new tree for about that price? I've got something similarly tacky and it wasn't expensive at all, plus it's still going

KF

Or a new motor

AlexM

A new tree is out of the question as I can't be arsed to take off the decorations and put them on a new tree. In fact, I think after christmas the tree can live in the garage and they can remain in situ for next year...

I have another 12vac psu, which I'll use to diagnose the motor.

Anybody got a spare 3pm syncronous ac motor then?

Happy christmas everybody!

Cheers,

Alex