Transformer for US inductive load appliance

[dmccormick]

This is a bit of a saga so I'll start with the short question and then for those who would like the background carry on a bit:

I have a Vacmaster VP112 vacuum chamber sealer US version 110V/60Hz 600W. I have blown it up using a voltage converter that I now realise was crap.

I am hoping to replace the damaged parts and if successful would then get a proper (more expensive) transformer to use it.

However, as it literally blew something up on the circuit board I want to check my repairs have worked before shelling out on a beefier transformer. I was therefore thinking to hire a 110V yellow box site transformer to test it with.

Will this work, and is it just a case of wiring the centre tap to appliance earth and the other 2 pins to the other 2 appliance pins ?

Ok that is the question. More background follows:
I bought this while in the US as there it was $699 whilst the EU model is £780. Obviously I knew the voltage differences and frequency, but thought using a transformer would be OK and knew I also already had one like
this

When I turned it on it seemed OK, but as soon as the pump started to operate it just made a buzzing noise and then after a second or 2 the display went off.

I 5A fuse had gone on the PCB that I took to be a power supply circuit. I thought perhaps the 50Hz/60Hx difference had caused the pump to overload. To check if the rest of the electronics were OK I replaced the fuse and disconnected the pump wiring.

When I powered it up again it was OK, but then a buzzing noise and one of the large capacitors literally exploded.

I have now gathered that the 50Hz/60Hz is probably no big deal and much more likely is that the output from the crap power converted was not well received by the PCB which I have since guessed is a 12V high power switched mode power supply - (the sealing bar is 12V 1 ohm so I reckoing 124 Watts).

I am planning the replace the obviously exploded capacitor in the hope that it is the only compnent that has been fried. I would like to test with a clean 110V source before I then buy a permanent 110V transformer.

Here is the PCB with the blown capacitor:

 

[Taylortwocities]

The transformer will not change the frequency, So you'll have 50HZ, not 60HZ. Do not know if that is significant.

Otherwise a site transformer will give you 110vish so should be OK.

 

[dmccormick]

Pushed submit to early when meant to prview - have now added a bit more detail.

Re the Hz - well it does have a motor with a capacitor. The motor plate says 110 V 60Hz 310 W 1.5 A and so does not mention 50Hz compatibility. However, the start capacitor is 38uF and does have 50/60Hz on it. The issue would be if it can't start, but I would be surprised as initially the load would be low until the vacuum had started to build up.

From what I have read the motor "should" probably be OK - it will run slower and draw more current and run hotter, but the duty cycle time for the machine is only 30 seconds so I can't imagine the motor getting too hot in that time is an issue.

I really think that what has messed it up is the pwer adapter. It has a switch for 60W or 1600W. The first time I turned it on I had forgotten to set the switch to 1600W so it was on 60W.

When I turned the machine on the control panel lights came on fine, but when I pressed the button to operate it it didn't work. Then I realised - oh I need to put it on 1600W. That's when it buzzed and went off.

After replacing the 5A fuse, disconnecting the motor and powering up again that is when the capacitor exploded.

 

[ban-all-sheds]

£ to a p that on the 1600W setting it's some kind of switched-mode PSU that's not putting out anything remotely resembling a sine wave, and that's what did the damage (note that they say it's suitable for heat-related appliances).

 

[dmccormick]

Yes, I should have realised that when you look at other 1000+ W voltage converters they are much bigger and cost more than £15. Problem was we already had this cheapo one and I had forgotten/lost the instruction manual.

I am hoping to just solder in a new 220 uF capacitor tomorrow from RS components and plug it in to a yellow site transformer and all will come back to life...........

 

[Monkeh]

It indeed looks like you've just lost the SMPS, which won't care in the least what frequency you feed it (.. to a point, anyway).

Replace all three large electrolytics (input and output) while you're at it, and please use the right type.

That SMPS might even take 240V directly without complaint.

While you're working on it, check the bleeder resistor hasn't failed (crowbar those caps before you touch them.. with a resistor, not a screwdriver).

 

[dmccormick]

Thanks for the tips. Pardon my lack of knowledge but which would the bleeder resistor be ?

 

[Monkeh]

Thanks for the tips. Pardon my lack of knowledge but which would the bleeder resistor be ?

Probably the large vertical one between the transformers, but I'd have to see the other side of the board.

 

[Spark123]

(the sealing bar is 12V 1 ohm so I reckoing 124 Watts).

12V x 12A was 144W when I was at school
(However the cold measurement of a heating element isn't going to work as resistance increases with temperature in a normal positive temperature coefficient material)

 

[dmccormick]

That's why its 124 not 144 - I was taking that effect into account (honest )

This is the reverse:

Another thought I had - the mains input only powers this SMPS and then via a 12V relay the pump. If the SMPS board could handle 240V then I was thinking I could maybe insert a 500W transformer between the input and the relay to supply the motor and thereby avoid any external transformer. Would it be that simple to use something like this

 

[dmccormick]

Changed the capacitor (I'd already ordered from RS so just had the one) and it lives !

Does anyone know what the white stuff is that looks like silicon sealant and is between the capacitors - should I refill the gap ?

 

[Monkeh]

It's just glue. No need to replace it.

 

[dmccormick]

Having tested OK with a yellow site transformer that I hired I wanted to set up a more permanent solution. Saw on Farnell a toroidal transformer 650VA for £33 which seemed a good deal VTX-146-625-155.
http://www.farnell.com/datasheets/1661839.pdf

Plan to put this in a metal box and wanted to check wiring and earth situation.
I believe I need to connect the yellow and black wires together so that overall secondary voltage is 110V.

To supply the earth connection to the appliance I am not sure whether to:
a) connect mains supply earth to appliance earth only
b) connect mains supply earth to appliance earth and yellow/black leads
c) don't connect any earth to the appliance

I am thinking (b), but advice appreciated.

 

[Monkeh]

Yes, connect yellow and black together and you will get 110VAC between orange and red.

Secure it in a metal box with the appropriate mounting kit, connect mains earth securely to the box and through to the device. Do not connect any lead of the transformer to mains earth. You can use a normal connector strip for the terminations, secured to the box. Don't forget strain relief (stuffing/IP65 glands are easiest).

You may want to step up to 800VA to reduce the operating temperature of the transformer (that one will get a tad warm if loaded for a long duration).

 

[dmccormick]

Thanks Monkeh.
I was trying to to work out the safest option. I guess with that setup the earth is protecting against the UK mains getting in contact with the box; if one of the appliance phases touches the case then nothing much happens as there is no earth route back to the secondary, but then that also means if I touch the box it won't do much either ?

The only thing I'd thought was that by connecting earth to the centre of the transformer it would mean I'd get a short if one of the phases came in contact with the appliance case and so blow the fuse I am going to fit and so I'd know there was something wrong. If the fuse didn't blow however I'd be looking at 55 V AC with a return to the secondary via earth and me. The only thing then I'd thought was to fit an RCBO instead of a fuse but that might be overkill (not literally I hope).

So overall the safest is to leave the transformer isolated from earth completely I guess ?

Re the rating - the motor is 310 VA and the sealing strip I am guessing 144W. The appliance rating sticker says 600W, but the duty cycle is:
30 seconds pump, 5 seconds seal, then you have to take the sealed bag out and position the next one if you are using it again so I think there is enough rest time.
In fact not really sure why they put 600W on it unless there is a period of overlap when the pump is running and the seal is pre-heating.

Anyway thanks very much for your advice.