Microwave oven current draw is off

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
I recently bought a new microwave oven from a popular warehouse store. As I began using it I noticed it wasn't heating the food as well as the previouse oven, even though the new one is rated at 1200 watts "cooking power". The spec in the owners manual says that the new oven should be drawing 12 amps from the line while running at full power. I wondered how it could be undercooking with 1200 watts "cooking power". I decided to measure the power consumption of the new oven while operating. Was I surprised to discover that while cooking at full power, the oven was drawing 17 amps! That's 5 amps more than expected. Where are the extra 600 (5 A * 120 V) watts going? What is getting heated up by that 600 extra watts?

It's drawing 600 watts more than the spec says, yet it's not cooking as well as it should given its 1200 "cooking watts" rating. Are those extra 600 watts a fire hazard?

Edit: Here's the spec showing where I got the 12 amp figure.

Microwave.jpg
 
Last edited:

Papabravo

Joined Feb 24, 2006
14,678
Your numbers seem a bit off. 1200 Watts from the power line with a nominal voltage of 120 VAC would be drawing 10 amperes. So this oven is drawing 17 amperes from the 120V line which is 2040 watts. From this I conclude that the efficiency of the oven is 1200/2040 = 59%. Not as good as an SMPS, but better than a linear regulator. Were you expecting the oven to be 100% efficient. An alternative way to check the performance is to measure the temperature rise of say a liter of water to see if that correlates with the alleged cooking power. Knowing the specific heat of water will allow you to cross check your the claim about cooking power. Losses due to inefficiency are just gone, and the entropy of the universe keeps increasing.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
The number "1200 watts" refers to what they call "cooking power". That means power delivered to the food.

Of course I'm not expecting the oven to be 100% efficient. The spec says that the oven will be drawing 1440 watts (12 amps) from the line while delivering 1200 watts "cooking power". That would be an efficiency of 1200/1440, or 83%.

But in fact the oven is drawing 17 amps while delivering less than 1200 watts "cooking power". I say it's delivering less than 1200 watts "cooking power" based on the observation that it is not cooking the food as it should based on the performance of the previous oven which was also rated at 1200 watts "cooking power".

The question I have is where is the extra 600 watts (17 amps measured vs. 12 amps it should draw) going?
 

Papabravo

Joined Feb 24, 2006
14,678
The number "1200 watts" refers to what they call "cooking power". That means power delivered to the food.

Of course I'm not expecting the oven to be 100% efficient. The spec says that the oven will be drawing 1440 watts (12 amps) from the line while delivering 1200 watts "cooking power". That would be an efficiency of 1200/1440, or 83%.

But in fact the oven is drawing 17 amps while delivering less than 1200 watts "cooking power". I say it's delivering less than 1200 watts "cooking power" based on the observation that it is not cooking the food as it should based on the performance of the previous oven which was also rated at 1200 watts "cooking power".

The question I have is where is the extra 600 watts (17 amps measured vs. 12 amps it should draw) going?
It may have escaped your notice but 17 amperes(peak) * .707 = 12.02 amperes (rms)
Does your ammeter measure RMS current or peak current?
cuz ya can't multiply peak current by rms voltage and get watts -- it don't work that way.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
Surely I have enough cred on this board that it should go without saying that I wouldn't make that mistake. This is the meter I used to make the current measurement: https://hiokiusa.com/product/acdc-clamp-meter-cm4372/

I also checked with a wattmeter and the power factor is .98, so the difference between VA and Watts is negligible.

Do you really think I don't know that VA and watts are not exactly the same, but that when the power factor is nearly 1, they are close enough that reporting the power draw of the microwave oven in terms of its current draw is good enough?

Rather than continuing to question my competence, address the question of the 600 extra watts.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
Sounds a little high to me.
Do the water heating test to see how much power it's actually delivering to the load.
What sounds high? The oven is drawing 5 amps more that its spec; that's 600 watts it shouldn't be drawing.

I'm not going to bother with calorimetry. It left my chicken pot pie all gooey after 7 minutes, compared to a nicely browned pie produced by the previous "1200 cooking watts" oven this new one is replacing. Other examples, such as failing to heat water for tea as quickly as the previous oven. The box the new oven came in says in large print "1200 WATTS COOKING POWER", but it isn't living up to that claim.

But, notwithstanding the obvious deficiency in "cooking power", there is the fact that the oven is drawing 17 amps while cooking rather than the 12 amps the spec says it should draw. Where is the extra 600 watts going? It certainly isn't going into cooking.
 

Papabravo

Joined Feb 24, 2006
14,678
Surely I have enough cred on this board that it should go without saying that I wouldn't make that mistake. This is the meter I used to make the current measurement: https://hiokiusa.com/product/acdc-clamp-meter-cm4372/

I also checked with a wattmeter and the power factor is .98, so the difference between VA and Watts is negligible.

Do you really think I don't know that VA and watts are not exactly the same, but that when the power factor is nearly 1, they are close enough that reporting the power draw of the microwave oven in terms of its current draw is good enough?

Rather than continuing to question my competence, address the question of the 600 extra watts.
I guess you're free to solve your own problem.
 

ericgibbs

Joined Jan 29, 2010
11,111
hi Elect,
Recently bought a Russell Hobbs combination Oven/microwave unit.
It too under cooked the food when the instruction book times/temperatures were entered.
I now have to set it 20C higher and 5 minutes longer than the expected settings.

Checked it with a temperature probe and it is close to 30C under the temperature indicated on the oven display.
I suspect their heat sensor is incorrectly located in the oven

Complained to the supplier, still no response.

E
 

Tonyr1084

Joined Sep 24, 2015
5,236
Too weak cooking? Too much power being drawn? Take it back and get something else.

Imagine what would happen if the main transformer was defective. More power draw and less output. NO???
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
I took it back days ago. I'm just asking people to speculate about where the extra power is going. There was no smell of overheated electrical parts after it had been running at full power for a while.
 

MisterBill2

Joined Jan 23, 2018
7,299
It is also possible that the oven is a bit defective. That happens. Or it may have a resistive heater element. It may also be that there is a problem in the energy path from the magnetron to the cooking compartment, and some energy is escaping. 600 watts is a lot of power and so it should be making a lot of extra heat someplace. It should be possible to find that heat with some investigation. With that much excess power plus weak cooking, the problem has to be in that magnetron system area.

One of my friend had a microwave oven and she called and said that it was getting hotter inside even though it was switched off. So I made a fairly quick service visit because that should not be happening. She was sort of right: the bottom of the cooking compartment was too hot to touch by the time that I arrived.
The explanation is interesting. Her previous microwave oven had been a countertop model installed on a shelf above her stove, by a previous resident. That one failed, and so she went to a store and purchased another micriowave oven intended to mount on the wall above the stove. That model included two small 75 watt lights built into the bottom, directly under the cooking compartment. In a standard installation they were well vented. Sitting on the shelf they had no venting, and they did not show when they were on. A non-vented 75 watt bulb gets very hot.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
Did you get a replacement? If so does the current match the specs?
I didn't get a replacement at that store. Several aspects of it seem like it is built to be a low cost oven. I'm going to get a model that is less a low cost version of the usually very good Panasonic ovens.
 

MisterBill2

Joined Jan 23, 2018
7,299
I have a Panasonic "Inverter"microwave oven, and while it cooks quite well the mechanical quality is not great. I have had a lot of problems with the door latch mechanism and the door hinges. The first symptom started as the latch would not return to the ready position after opening the door, so closing the door would not make it latch. That required disassembly and lubricating areas that had not been adequately lubricated when it was built. And all of this started shortly after the short warranty ran out.
What is novel is that it does not have the big heavy transformer, it has an inverter with a much lighter transformer, all encapsulated. So if it ever fails it will be time for a replacement, while usually I can repair them.
 

Thread Starter

The Electrician

Joined Oct 9, 2007
2,801
I bought another Panasonic oven from Best Buy. They have a much better selection. The previous unacceptable oven was a 1.3 cubic foot item, and the one just purchased is a 1.6 cubic foot version. The first one was rated 1200 watts cooking power, and the new one is rated 1250 watts cooking power. The spec for the new one says 12.3 amps power consumption, slightly more than the 12.0 amps of the first one.

This new oven has much better apparent quality. The touch pad is not erratic like the first oven, the door closes with a better feel and sound, etc.

The startup current for this oven is 17.5 amps. Apparently this large startup current is typical of Panasonic microwaves. I wish I had another brand to compare; if anyone feels like making a measurement on their microwave, report your results here.
 
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