Calculating Correct Step-Down Transformer Size

[NanLaew]

Here's one for those with fresher electrical knowledge than my 40 year-old City & Guilds... actually hoping Crossy gets here first!

I have a Dacor DCM24S combo microwave/convection oven. These were built locally for the US export market so internals are all 110V and this 'non-export' model had an internal step-down transformer fitted. This transformer has gone short-circuit. It has no external markings or ratings but was a heavy bugger about 10x10x12 cm. I would guesstimate maybe 1000W ?

The published ratings for the appliance are 120V, 15 A and a 'power load' of 13 A. NOTE this is for 120V.

Can someone calculate the wattage I would need for a replacement, external 240/120 V transformer so I can use this appliance on 240V here? Maybe need something nearer 1500W would be better?

[Crossy]

15A @ 120V is 1,800 Watts, you're looking at a 2KVA transformer.

You may be able to get the faulty transformer re-wound at a reasonable price in the Ban-mo area.

What makes you think it's gone short, transformers usually fail open? Remember it's almost certainly an auto-transformer.

[NanLaew]

15A @ 120V is 1,800 Watts, you're looking at a 2KVA transformer.

You may be able to get the faulty transformer re-wound at a reasonable price in the Ban-mo area.

What makes you think it's gone short, transformers usually fail open? Remember it's almost certainly an auto-transformer.

Cheers sir!

Dead short, blows the main breaker... since the bodgy transformer was installed ahead of the internal fuse! I disconnected the output side of the transformer and 'Bang!' still blows the breaker.

I was actually cooking a turkey when it blew and when I was in the kitchen earlier, there was that distinct 'brown smell' that only sparkies know is insulation melting. I am assuming that the internally mounted transformer that melted was under-sized as a bigger one would not fit inside the appliance. It looks like a regular, generic, unmarked transformer with primary and secondary windings. The 2000VA externally should hopefully solve the dilemma. I think I saw one at the excellent hardware haberdashery opposite Foodland on Klang (Pattaya).

[GinBoy2]

15A @ 120V is 1,800 Watts, you're looking at a 2KVA transformer.

You may be able to get the faulty transformer re-wound at a reasonable price in the Ban-mo area.

What makes you think it's gone short, transformers usually fail open? Remember it's almost certainly an auto-transformer.

This is correct, but what I would suggest is give yourself some headroom. Rather than buy a 2K Watt transformer buy the 3K. All of these a big buggers size wise, no difference between the 2K & 3K. I have a 3K which I use for my US cappuccino machine, gives some surge protection.

I purchased this one from Amazon couple of years back and has worked flawlessly:

Simran THG-3000 Step Down Voltage Transformer 3000

[h90]

If it is interesting for you Americans.

I have here a 15kVA 3 Phase transformer, which I would sell.

It is extreme heavy and big and was used to drive a European CNC lathe in USA. But it should be also possible to use it the other way around.