Electrical question?
 

OK so most of you know that we live and work in Prague, Czech Republic. I am about to begin an office renovation project which includes installing kitchen style cabinets, and countertops to create storage space and a solid work station in two seperate areas. One of the things that we decided to do was to install a Sharp combo microwave/convection oven that mounts above/over the countertop. I will not know until it arrives, but I believe the unit we ordered is an american 220volt unit. The problem is that we run on 220volt power over here, and this means one leg of 220volts, not two legs of 120v. If the unit does comes in as a 120v system problem solved, because we can simply hook it up to one 2000 watt step down (220v-120v) transformer. Easy, peasy, Japaneasy.

If it is in fact a 220v unit I would assume that it will have two 120v feeds, one neutral, and a ground. If so, is it possible that I can run this thing using two step down transformers (one to feed each leg), and simply tie the neutral legs and the grounds together? Sounds simple enough, but will this in fact work without blowing it up? The only other worry is that USA runs 60 cycle, and Europe runs on 50 cycles. So I think this will effect the timing features of the system making it run slower, but should have little effect on the actual running of the microwave oven. I can live with that.

SO what do you guys think? Who is the resident electrical professional with the knowledge to answer this one? Thanks in advance for the assistance.

I know you can tie the grounds together, because a ground is a ground is a ground and they are all happy being tied together. I'm not sure you can tie the 2 output neutrals together though as they serve as the return path for the hot leg. Maybe just get a neutral from the panel directly and use two transformers to step down each 220v leg?

On second thought if the neutrals are fed from different phases then they can be shared, I'm just not sure what would happen since they are on the output of a transformer.

Technically
 

Correct me if I'm wrong, but technically all neutrals are tied to the ground in the distribution cabinet anyway. The neutral is an internal ground, and the ground is an external ground in case anything shorts to the casing of the appliance. But both are still tied to the gound in the end.

If it is a 220V American oven, then you will need to provide two 120V sources that are out of phase (180 degrees) with each other to simulate how it would be powered in America. Simply using two 220V to 120V transformers connected to the same source would not work since the outputs would be in phase (and the microwave would only ever see 120V at a time). Without knowing the internal wiring of the microwave, I'm not sure how you could hook it up to the single phase 220V.

As far as the ground and neutrals go, from my understanding, the neutral wires are connected to ground at the breaker box. I believe this is only to allow the use of GFCI's. Ground Fault Circuit Interrupters measure the current on the hot wire and the current on the neutral wire and will open the circuit if those currents are not within about 20mA of each other. This happens if the current from the hot happens to flow through the ground wire instead of neutral. If you get in between the hot and the ground, the GFCI will see that not all the current is flowing back through the neutral wire and will trip, hopefully preventing you from getting badly electrocuted. I'm not sure if your office is fitted with GFCI breakers (located in the breaker box instead of right behind the wall plug), but if it is and you connect the microwave's neutral to the ground, the GFCI part would not be able to function properly if electrical current finds a path through ground instead of neutral. The breaker would still trip if there is a really high current. But it would be better to connect the microwave ground to the ground from the wall. What you do with the neutral depends on how the microwave is wired as well, I think. You may have to leave it disconnected and then use both the hot wires from the microwave with the hot and neutral from the wall to get the proper 220V into the microwave, if it just has a large center tapped transformer inside.

Not sure about the effect of using a 60Hz device with 50Hz power, but Google seems to think it is usually not good. Best to call the manufacturer of the microwave and ask them, I think.

Good luck!

call a prague electrician, we don't want you frying yourself Jerry:lol:

Confused:

USA is 110v and 60hz
Europe is 220v and 50hz
Uk is 240v and 50hz
Japan is 100v

I've never seen a USA 220v Microwave - Mine came as a 110v model.

If you do get a 220v model I think it just comes with a normal 3 prong plug which should work just fine.

The problem may come if the microwave wants to see two 120v legs (to make the 220v) with a ground (no neutral) like what the US would use, but where he is, the line voltage is 220v on a single leg with a "0v" neutral and ground (been talking to him via PM). If that's the case, then some kind of conversion transformer will be necessary to turn a single 220v line into dual 120v lines that are 180* out of phase. He'll just have to read the detailed electrical specs for the microwave when it arrives.

As far as 50/60Hz goes; I can see it being a problem if any timing circuits use the line frequency to determine the clock. But in today's world, that's not the most reliable method, so systems typically use a crystal or other IC-derived clocking mechanism.

Would it really be that much cheaper to buy a conversion transformer than returning it? I'd think something that can put out two 110 outputs 180 out of phase with each other wouldn't be all that cheap.

Btw, it's not too common here in US, but 220v single phase is available and our electrician called it a "high leg". Made a bad day for him since it wasn't marked right and he thought it was just a normal third phase. We didn't know either since the equipment it powered was long gone. So after installing the new breaker panel and turning it on, things got 220v on a single line and he ended up popping some bulbs and a few pieces of equipment in the shop.

220-240v
 

Below is a LINK that shows a similar appliance which in the specs shows 208-240v power supply.

http://www.curtos.com/products/Gener...sa2201rss.html

The microwave that was ordered may be this one or one very similar. The out of phase legs sound a bit strange to me. So from what some of you are sharing if you want to hook up a new welding machine in your garage you have to have two 120v legs to supply power and both legs have to be out of phase, correct? I have never heard this before. The second part of this question is how do you get or even know that these legs are out of phase when you hook it up? And why would they have to be out of phase?

I dont get the out of phase comment either... I have 220v in my house and it just comes off of a single line into the house.

Agree that if the appliance needs 2 X 110v to get 220v then you are hosed. A transformer will be a PITA. Honestly just buy a cheap local microwave :)

More info
 

Well not for sure of exactly how your 220v power might be supplied, but all USA housing power is supplied by two seperate 120v lines that run to each house. If you open your power panel you will see that there are three lines entering your distribution box. The center line is the ground. The lines on the right and the left are seperate 120v lines that feed each side of your buss. When the elctrician wires your house he tries to balance the loads by deviding your homes amperage equally with less than 100 amps per side. When you require a 220-240v feed they use a breaker that uses one leg from each side of your buss.

In most of Europe the feed is one single line of 220-240v. using transformers is no big deal and quite common for most Americans living and working in Europe so we can continue to use USA elecrical products. Our Embassy probably has at least a couple hundred of them throughout our mission. Finally European versions of the microwave are nowhere near as esthetically pleasing as some of the models sold in the USA. The whole idea and convenience of mounting a microwave above the counter for whatever reason has not really caught on here as well as it has in America. The closest thing they have over here are kitchens that look like one huge flat wall unit where you only see the face of each appliance. SO having one of these appliances will be a huge novelty/draw of interest for our office. Plus the renovation I am doing is to create a 2012 modern looking work station that is inovative and uses the most advanced technologies available today. not some cheap looking junk picked up at the nearest local swap meet. The appliance is already on its way, so I would like to make this work. Not just turn my back in defeat and give up.

To answer the out of phase question, the AC power that comes into your American house (or at least all of the houses in which I have seen the breaker box) has, like you said, two live wires and one neutral/ground wire. The two live wires have 120VAC 60Hz on them relative to the neutral wire. Relative to each other, however, they would measure 240V on a multimeter. 120VAC voltage is always switching between +120V and -120V. The two wires being out of phase means that when one wire is at its positive peak (+120V), the other wire is at its negative peak (-120V). A multimeter connected to each of the live wires would measure the difference between them ((+120) - (-120)) = 240. This picture might help some. The red and yellow lines are what the voltage of each live line looks like over time.

http://imageshack.us/a/img405/2373/240explain.png

Not sure if the microwave will be expecting two out of phase 120V inputs or just one 240V input.

Jerry - my 240V home wall-unit air conditioner only has three plugs - leading me to conclude it does not need a connection to neutral - it is two-phase 120V (plus a ground plug). I would guess your microwave would have same plug (for household use).

HIRE A LOCAL ELECTRICIAN FOR 5 MINUTES TO HOOK UP YOUR MICROWAVE.:yes:

Air conditioners are generally rated for less than 15amps. An electrician can chime in on this but I'm pretty sure it's not required to have both a neutral and a ground conductor with this amp draw. Although 4 conductor plugs and cords with separate neutral and grounds are available.

For higher amp draw, homes built before 1996 (or any additional wiring installed before then) are also allowed to use only 3 conductors combining ground and neutral. But newer homes and wiring have 4 conductors (separate ground and neutral) and 4 prong plugs for 220/240v use in circuits 30amps and above such as dryers and electric stoves.