240V Oven Question

[bostonman]

Seems lots of discussion about whether to call standard household electricity single or two phase is taking place. Personally I think if one leg is a sine wave, and the other leg is an inverted sine wave, that would be two phase because they are 180 degrees apart. Maybe technically from the viewpoint of an electrician, it's single phase because a two phase system is wired differently.

From the description about how the burners (and oven) temp is controlled, the current is constant (except when it's cycling off once temp is reached). This would match what I measured because the current remained constant for the short time I had the oven on.

If the spring pushes against a heating element, then why do the plastic knobs not get hot?

Unfortunately I'm still uncertain why the oven current measured the same on each leg, however, the top burners had different current on each leg. I may need to move the stove to see if it's a three or four prong outlet. From reading posts, it seems it must be four because it has a clock and light indicating it also needs 120V.

If you read my original post, one of the burners (the largest one) had 15A on one "phase" and the other "phase" had 8.3A (almost half). Keep in mind, I didn't stop and smell the roses while in the electrical box, so maybe the heating element caused the current to change, so that 15A may have increased a bit to 16.6A from the time I switched the clamp on meter to the other "phase", or I measured the 8.3A first, went to switch "phases", and during that time the current decreased to 15A from 16.6A.

Had I measured the other "phase" again, maybe it would have been 7.5A.

Just throwing that out there so less confusion exists.

[bdunham7]

from the viewpoint of an electrician, it's single phase because a two phase system is wired differently.

If the electrician only does residential in the US, then it would be convenient to think of it as two phases.  The reason most electricians would use the term 'split phase' is to distinguish it from other systems.

As for your stove, you need to measure both legs at once and try to find a wiring diagram.  As previously mentioned, the two common methods used to control burners are a thermostatically-cycled control and applying 120 or 240 volts to various combinations of elements.  Burners with continuous variability typically used the thermostatic cycling and those with fixed settings, like the old pushbutton models, used fixed combinations of voltages, usually with two elements per burner.  There are many possible combinations and permutations of these schemes.  The design goals for the manufacturer are to provide the widest range of settings (High/Med/Lo might not be good enough) and to minimize variation in heat output. 

[Zero999]

Two phase is 90° out of phase. Split phase is a centre tapped transformer.

Your oven will be wired as per the drawing on the right of the picture below. If it has a neutral connection, it will go to the centre tap, joining the earth/ground symbol on the drawing, but should be a separate conductor.

The picture on the left is a three phase 230V system, which is common in some parts of Europe, but not in the US, although 208V three phase, 120V to neutral, is and will work with some 240V appliances.

Note: do not wire loads between phase (hot) and earth. The drawing is just an illustration showing the voltages with respect to earth. Always connect loads between phase and neutral, or phase-to-phase!

[james_s]

If the spring pushes against a heating element, then why do the plastic knobs not get hot?

The heating element is just a watt or two and is wrapped around a bimetal strip. It doesn't have to get very hot, just enough for to actuate the bimetal strip that moves the contacts. It's been about 25 years since I've taken one apart so I don't recall exactly how it's arranged internally but the whole thing is encased in a bakelite housing and the control shaft is not directly connected to anything that heats up. Old photocontrols for outside lights work similarly, a CdS cell controls current through a little heater wrapped around a bimetal strip, when it gets dark, the resistance of the CdS rises which reduces the current through the heater and the strip snaps the contacts closed. The photocell housing doesn't get hot, just slightly warm to the touch.

[james_s]

Note: do not wire loads between phase (hot) and earth. The drawing is just an illustration showing the voltages with respect to earth. Always connect loads between phase and neutral, or phase-to-phase!

For many years it was common for large appliances like clothes dryers and kitchen stoves to use a 3 wire circuit with no neutral. Small 120V auxiliary loads like the interior lamp and timer motor were connected between live and earth ground. The code was changed in I believe 1996 to ban this however older homes are grandfathered in so replacement appliances can still be wired this way.

[bostonman]

Two phase is 90° out of phase. Split phase is a centre tapped transformer.

How is two phase 90° and not 180°?

[BrokenYugo]

https://en.wikipedia.org/wiki/Two-phase_electric_power

[bostonman]

I remeasured the current.

It seems the difference in current is when the oven is set to 'bake', I get (rounding off) 8A on one "phase" and 15A on the other "phase".

The numbers are as follows (I'll include the number on each phase separated by a comma):

Top largest burner set to 'high': 9.9A, 9.6A

All four top external burners set to 'high': 26.3A, 25.9A

Oven set to 'bake' at 500°F: 15A, 8.3A

Oven set to 'broil' at 400°F: 13.4A, 13.4A

All four top burners set to 'high' and oven set to 'bake' at 400°F: 39.8A, 32.7A

[bdunham7]

Can you post photos of the heating elements?   That seems a bit weird, but I suspect that the oven may be doing something like putting 120VAC on the broil element during the bake cycle.

[bostonman]

See attached.

[james_s]

Nothing unusual about those elements. It's impossible to say how they're wired just by looking at them, but they are definitely just standard single elements.

[bostonman]

I zoomed in on the top coil and looks like it's engraved with 108V / 140V.

After some research, I read both top and bottom turn on during 'bake' to heat the oven, and then the top turns off once temp is reached.

Maybe this accounts for the difference in current since the top may be going through the 120V leg and the bottom through the 240 leg?

[james_s]

I zoomed in on the top coil and looks like it's engraved with 108V / 140V.

After some research, I read both top and bottom turn on during 'bake' to heat the oven, and then the top turns off once temp is reached.

Maybe this accounts for the difference in current since the top may be going through the 120V leg and the bottom through the 240 leg?

I *strongly* suspect that is 208/240V, because 208V is typical in apartment buildings that are fed with 3 phase, and 240V is the standard residential voltage. It would make perfect sense for a stove to be rated 208/240V, while neither 108V or 140V are used anywhere to my knowledge.

[bdunham7]

Maybe this accounts for the difference in current since the top may be going through the 120V leg and the bottom through the 240 leg?

Yes, and if you do the math it adds up perfectly. 

[bostonman]

Yes, and if you do the math it adds up perfectly.

I need to look at what is after the slash (looks like a wattage value), but how does it add perfectly?

I'll take a closer look at that writing later and post a picture.

[bdunham7]

Your broiler element apparently draws 13.4A at 240 volts, so it would draw 6.7A at 120V.  Your lower element draws 8.3 A, so on bake, one leg has 8.3A and the other has 8.3+6.7 = 15A, which is what you measure.  For all I know this may be a common setup in electric ovens.

[james_s]

That would make perfect sense now that you mention it. I found the manual for one newer oven so far that states the top and bottom elements both operate in bake mode, but the top element must not operate at full power because I remember using electric ovens to broil and when doing that the top element glows orange and puts out a LOT of heat, it would scorch anything you were trying to bake. I never sat down to think about how they were actually wired but I bet you hit the nail on the head, the broil element is run on 120V which is 25% power.

[richard.cs]

When the supply has two voltages available then switching between them is a cheap method for power control. The only downside is it limits the appliance to the US market only.

It's somewhat common to do the same on three phase supplies, switching loads between star and delta changes the power by a factor of three. I don't imagine it's on domestic stoves though as 3 phase models are normally able to have all phases paralleled for single phase use.

[james_s]

Aren't the typical American 30" wide freestanding ranges pretty much a North America thing anyway? Seems like most of the similar appliances I saw in England were a bit smaller, and so were the dishwashers, and clothes washers and refrigerators too for that matter, although a friend of my friend did have an American made refrigerator which looked somewhat conspicuously out of place in a British house. Speaking of that, I remember he said something about the defrost heater being problematic in some way. I've never seen a 3 phase range over here, even in buildings that have 3 phase power the range will only use 2 phases. I'm fairly sure that's a European market only thing.

[richard.cs]

A normal UK range (we'd normally call a freestanding one a "cooker") is usually 24" wide, a bigger one normally 36" (actually 600/900 mm), my parents have an 800 mm one (31.5") but that's considered really oddball. Many European models have the elements grouped to allow 3 phase use (unbalanced star) with some links that can be fitted for single phase. The same appliance might be connected as 3x16A in Germany and 1x32A or 1x45A in the UK.

Most kitchen appliances here are 600 mm wide for the standard models, 450 mm for a "slimline" one like a dishwasher for 2 people. Big fridges with double doors, icemakers, etc. are usually advertised as "American style" but usually not imported from the USA.

[bostonman]

I removed the racks and got deep inside the oven to take a closer picture.

It looks like 240V and 3410W (if I understand the plate correctly).

If this is the case, then maybe the bottom element works off the 120V line?


<edit> I sent an email to GE about a wiring diagram. They sent an email a day or two ago stating they passed along my message to someone and wait a few days.

[bdunham7]

No, it would make more sense that this element would be at 120V on bake and then 240V for broil. 

[james_s]

The bottom element will always be on 240, it is the main element that does most of the work when baking. The top element it seems is run on 120V while baking, but when you want to broil something it is fed 240 so it gets very hot. The bottom element is likely not run at all while broiling.

[bostonman]

One (hopefully) last question.

Since I measured 15A in one leg and 8.3A in the other, while on 'bake', and we determined the top element must be on from 120V, does this mean it's 23.3A draw total?

[richard.cs]

One (hopefully) last question.

Since I measured 15A in one leg and 8.3A in the other, while on 'bake', and we determined the top element must be on from 120V, does this mean it's 23.3A draw total?

No, it's not really meaningful or useful to add them like that. It's simply 15A one side, 8.3 A the other (and 6.7 A neutral current). I guess if you were trying to normalise to 120V to calculate power consumption adding them has some meaning but there is no conductor where 23.3 A ever flows.