I have a small electrical appliance with fuses.  The fuses keep
blowing when I turn it on.  How do I repair this?  I bought more fuses
and took the thing apart and everything seems to be connected cleanly,
so what should I do now?

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Clarification of Question by andythekid-ga on 03 Dec 2004 06:39 PST

The appliance I am working on is a plate rocker, which is basically a
machine that has a tray attached to a bar that on the other side is
attached to a motor.  Because the bar is offset when attached to the
motor, it pushes the tray up and down as it spins.  I use slow burn
fuses, which have not helped them from not blowing.  They are 250V 5A
fuses.

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Clarification of Question by andythekid-ga on 07 Dec 2004 15:38 PST

Thanks for all your help!  I am not really well versed in the lingo,
but I will describe what is going on inside:  From the plug into the
wall, there arae 3 wires.  1 is a ground and the other two go to the
on/off switch.  From there, the wires go into this switch that lets
you choose between 220 and 100V.  From there, the wires go into this
little box that says "Finepcr model 110x110x11, freq:50/60Hz,
Capacity: 40VA."  I cant get into that little box because there are
these paper-like thing wrapping the whole thing up that go into the
box.  It all seems very technical.  Anyways, from there, the wires go
into the knob that switches the speed and then into this small black
box that has 2 inputs and 2 outputs.  From there, they go to the
motor.  I hope this helps!  The guy in my lab who assigned me the job
has been giving me dirty looks all week.
Thanks,
andy

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Request for Question Clarification by alienintelligence-ga on 07 Dec 2004 21:31 PST

I'm curious, can the solution be a 
mechanical fix, or does it have to 
be an electrical fix?

Does it happen all the time, or only
under load?

-AI

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Clarification of Question by andythekid-ga on 08 Dec 2004 06:02 PST

It happens all the time, load or no load.  It even happens when the
motor is just turning over not even attached to anything.

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Clarification of Question by andythekid-ga on 09 Dec 2004 13:33 PST

The Exact piece of emuipment I have is this: http://www.finepcr.com/index-2.html

I once ran the fuse of a (now very old, but then state of the art)
water heater to the outside of the box.

It made it easier changing the blown fuses

In my case it was a design fault (I believe) 
- in your case, if the appliance is moderately small, take it take to
the store and be a nuisance.

I cant take the appliance to a store as my lab recently made me in
charge of fixing all electrical appliances and I am supposed to fix
whatever breaks right there.  I think someone in my lab is trying to
make me look foolish, so I need to show him up!

At your own risk you could try a ~slightly~ higher current rated fuse.

The device may need Slow-Blow Fuses.  The metal element in Slow-Blow
Fuses is thicker and can take current for a longer time period before
opening.  Slow-Blow Fuses are common in equipment that have motors and
heaters that require an intial high current draw to start.

Can you say what the appliance is -- not something embarrassing is it?
If you can provide full details including the voltage and current (or
VA or power) on the maker?s plate, it should be possible to diagnose
the problem. However, in the meantime, since (I take it) the fuses
blow immediately, a likely candidate is the interference suppression
capacitors across the mains input -- after the mains switch. The
function is often minimal and many times they are just snipped off,
though this does of course render the machine in non-compliance of
noise regs.

Best

So it *is* a vibrator.

What is the motor configuration? Is it powered directly from the mains
or is there a speed controller? Or is there a transformer / low
voltage power supply and other stuff like a timer?

If it?s a straight mains voltage motor, does it have brushes or is it
squirrel cage incorporating a start / run capacitor -- a quite bulky,
usually cylindrical capacitor connected to the motor ?

Will look again tomorrow night -- I?m on GMT. But we?ll get there :-)

Best

Still watching if you want more input.... 

"I think someone in my lab is trying to
make me look foolish, so I need to show him up!"

.... it's personal!

Best

Could you say which unit it is on the website?

http://www.finepcr.com/index-2.html

Can?t have these dirty (superiority?) looks -- blow him a kiss. From
what I can see on the Finepcr product range they utilise DC motors of
3.5 amp rating. That means that it is a low voltage motor and your
description of "Finepcr model 110x110x11, freq:50/60Hz, Capacity:
40VA." sounds like a transformer. From there it goes to the speed
switch -- how many wires? If there are several, the switch most likely
just connects different tappings on the transformer thereby changing
the voltage. The next black box would be a bridge rectifier which
crudely converts the AC to DC. If you measure this voltage with a DVM
it won?t give you exactly the right reading because by your
description there is no smoothing capacitor.

So it doesn?t blow immediately after all? The more detail you can give
the better. In all the notes below, make sure the plug is out of the
wall when you are connecting and disconnecting leads. To diagnose the
fault, follow the numbers. Best if you can power the thing from an
isolating transformer but you may not have one available. If not, make
a request that one be bought because it is not reasonable (and perhaps
illegal) to be tasked to repair mains powered equipment without one.
(UK is 240 volts -- it smarts).

1) Obvious question first, is the voltage selector on the correct setting?
2) Does the motor spin by hand without graunching? (Need to
mechanically disconnect). It won?t spin absolutely freely because of
brush friction.
3) Is the rest of the mechanism free?
4) Electrically disconnect the motor and see if the fuse blow after
the normal period.
5) With a DVM set on ?DC Volts?, check the voltage on the leads which
would have been connected to the motor and change the speed settings.
You should see significant changes in voltage, higher for higher speed
-- in the region of 5 to 12 volts
6) If the fuse is still blowing, buy shares in a fuse manufacturer. Is
the rectifier getting hot? Any nasty smells from the switch or the
transformer?
7) Disconnect the rectifier and try again.
8) Disconnect the wires going to the switch if it?s reasonably easy.
If not, try to examine the switch and see if there are any mechanical
faults or burning on the contacts.
9) DANGEROUS TEST. Transformers can short to chassis. Remove the earth
wire from the plug or the case and switch on by plugging the plug into
the socket. DO NOT TOUCH THE CASE AND MAKE SURE NOTHING ELSE IS
TOUCHING THE CASE. If the fuse doesn?t blow now, the transformer is
faulty.
10) If the fuse is still blowing, you will have to isolate the leads
to the switch and try again so that the only component still in
circuit is the transformer.

You may of course do these tests in a different order but the process
is one of isolation. Does your company have a web site -- perhaps you
could post pictures of the internals. Let us know how it?s going or
any further details or clarifications.

There is another small problem here though. When (and I mean when) you
fix this, they?ll give you something *really* horrid to repair. Can?t
win eh :-)

Best

Hi andy (namesake) I didn?t notice that all Finepcr pages have the
same url. Won?t bore you with how and why but it?s not best practice.
Anyway, could you quote the exact model and I shall look it up.

How?s it going in the meantime?

Best

Hello Andy,

Call these guys and your problems are solved: http://www.dnagroup.com
They had everything relating to appliances, fuses, switches, walk them
thru the problem on the phone and your fus problems will disappear in
hours.

Good luck