Why do many appliances, even ones that aren't cheap, have such poor duty cycles? 4

[LMF5000]

I figured this is probably the correct subforum since it relates to electrical motors.

I noticed that paper shredders and stand mixers, to name a few, have abysmally low duty cycles. Every one of the 50-dollar shredders in the office will overheat and enter protective shutdown after 5 minutes of non-stop running, and need a 20-minute cooldown. Was it really cheaper to integrate the temperature sensor, overheat light, and resettable shutoff switch than fitting a bigger fan to the motor and molding an external exhaust port in the housing? All these different shredder models lack any air exhaust outside.

I'm aware that continuous-duty shredders exist, but the cost jumps up massively for them, by a factor of 2x to 4x. Is it that hard to make a cheap, continuous-duty shredder? Ours draw 300W from the wall socket. A 20-dollar electric corded drill with a 500W motor can run for at least 30 minutes without overheating while drawing 200-300W (I had the misfortune of testing it myself trying to drill porcelain tile with a cheap worn out bit), so it seems the technology to manufacture a motor of the required size and power level cheaply exists - is there something I'm missing?

Secondly, stand mixers. Our 1980s Kenwood stand mixer (from the Thorn EMI vintage) would run nonstop for up to an hour at a time making batch after batch of cake batter or frosting for parties (ultimately it died when the capacitors exploded after approximately 20 years of service). I'm looking at modern stand mixers and every single one says not to exceed 10 minutes of running and allow 20 minutes of cooling afterwards. Not just the 150-dollar budget models, but even the 500-to-700-dollar KitchenAid models. What happened? Is it that hard for a 500-dollar appliance to be able to run continuously at rated power? Have they sacrificed cooling for a sleek exterior devoid of air intake and exhaust ports, or is there something else I'm missing?

 

[IRstuff]

We all have to do our part to keep the factory workers employed. ;-)

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Compositepro]

Low duty cycles are due to the need to keep cost down. An industrial one hp motor and gear box can run continuously at full rated power, and is heavy and costly. A consumer appliance rated at one hp and 10% duty cycle actually has only a 0.1 hp motor, using the same rating criteria. It's a lot cheaper.

 

[cranky108]

Anne Rand Keep everything somewhat good so everyone can keep a job.

 

[LMF5000]

Well, FWIW I gave my cheap "10 minute" stand mixer its first good run this weekend, making homemade sausage with the meat grinder and stuffer attachment.

For starters, although the box says "1900W motor", my inline power meter actually measured 30W from the plug running at the lowest speed with no load, about 60-80W mincing meat (speeds 1-3 out of 6), and about 120W mixing 3kg (6lb) of meat in the bowl with the beater attachment.

Secondly, I kept it running non-stop for close to an hour to process 3kg (6lb) of poultry into sausage meat and stuff the natural sausage casings with the meat grinder's horn attachment. The mixer didn't burst into flames or melt or stop due to overheating, nor did it get excessively hot to the touch. At most some oil appeared at the attachment point of the meat grinder where the splines of the mixer mate with the splines of the auger of the meat grinder (above the bowl), but I'm not sure if that was meat juices or the oil I added to the mix, or grease from the gearbox, and at any rate this was the mixer's first run ever so some grease drips are probably to be expected until pressures equalize.

So in short, looks like the 10 minutes only applies for harder jobs than me grinding meat and stuffing sausage. Or maybe some other worst-case scenario. Or maybe just to cover their behinds in case someone makes a warranty claim.

Maybe making pastry will strain the motor more. I will measure the power consumption next time we feel like making bread and see how much of the "1900W" nameplate value we can come close to. Maybe speed will do it. I will whisk eggs or cream at full speed to check too.

 

[cranky108]

i am guessing you are outside the US. Most US devices rated at 115V ac, can not draw more than 12 amps.

So why are you using a power monitor anyway?

 

[LMF5000]

Yes, I'm in Malta. We copy the UK system (plugs and all) - each outlet is 230V nominal (though it swings between 220 and 240V) and fused for 13A. The kitchen has a dedicated circuit for large consumers like induction hobs which is rated for 30A (no plug in that case, the wire connects straight to a screw terminal via what's called a flex outlet).

I'm using an inline power meter (your equivalent in the US would be a "kill-a-watt" device). Reason being mostly curiosity. This thread made me wonder how much thermal energy that stand mixer is trying to dissipate to limit its runtime to just 10 minutes. Turns out it might have been grossly exaggerated because a mere 30 watts at no load and 110 watts at high load isn't a massive amount of heat load.

 

[cranky108]

Try making bread. That is what I used years ago to burn up a mixer, and why I purchased the one I did.

Our 30A and 50A 220V plugs for big things like a range and dryer, do have plugs, and outlets.
Not very many of them needed, unless you do welding, arts with a big kiln etc.
But that might change with electric cars.

Which makes me wonder why the prior owner of my home had an outlet like that put in the basement.

 

[Gr8blu]

LMF5000: An electric motor only draws the power demanded by the process, regardless of the actual nameplate rating. Your car engine is no different: you don't run flat out at 150 kW through the city (although you might on a really open country road, at least for a while). You throttle back to meet the local requirements (city speed limits, traffic patterns, pedestrians, etc.).

That being said - 1900 W is almost guaranteed to not be a "continuous" rating. It's much more likely to be a "peak" rating, probably in what could be considered an instantaneous (or at least intermittent) time frame. The reason for this would be fairly simple: how often do you run into a chunk of frozen dough that needs to be broken up? Much more likely the material being processed has a "softer" consistency - which would require far less power to move. Also consider that thermal load on the mixer motor is proportional to the square of the current - while power is directly proportional to the current. If the maximum load capability is 1900 W, and you only loaded it to 120 W while holding the voltage relatively constant, you could theoretically run it for a period (1900/120)*(1900/120) times as long ... which works out to be something like a factor of 250x.

If the system can withstand 1900 W for 10 minutes, it could therefore withstand your 120 W load for something around 41.5 hours.
If it was more instantaneous than that (say 1900 W for 1 minute), you'd still be able to run 120 W for over 4 hours.

Similarly, if we buck that load up to the 200 W level (should be slightly above your "standard" fuse panel limit), you'd still be able to run 1.5 hours at that level, assuming the "nameplate" was actually written around 1900 W for 60 seconds.

Converting energy to motion
for more than half a century

 

[LMF5000]

Awesome post Gr8blu! I'm aware that motors consume power in proportion to load, and my suspiction was already that the 1900W is probably a fanciful number added to impress buyers who are swayed by the biggest "power" number on the box. It does say "1900W max" and I didn't spot a continuous rating (see for yourself on the website page here). I will try and remember to get a look at the sticker on the motor next time I'm at the house to see if it's listed as 1900W there as well, but I'm guessing the 10-minute thermal limit is probably specced at whatever the load is with a full load of dough in real life - much less than 1900 watts. One thing - I'm in Malta so all plugs follow the UK system (230V @ 13A = 3200W max).

Thanks for the thermal calculations, that's a handy way to estimate thermal effects which I will use in future.

cranky108 - very interesting, I never imagined 50A plugs in a residence. When our appliances get that big we switch to the red three-phase plugs and install a 3-phase supply

 

[Hoxton123]

I don’t have to read the above.

in the 1950's in the UK, we had manufacturers of domestic appliances who made products to last a lifetime. They were blown away by upstart companies who in the absence of regulation sold cheap sh***.

If you visit any industrial kitchen, you will see bulky expensive kit, designed to work continuously,
Or you can buy the cheap short-rated stuff, consumers choice.

I am on my third electric carving knife in 30 years. I paid ten dollars for each one. Why should I pay 100 dollars for one which lasts 50 years.? I might die next year of whatever.

 

[Palletjack]

I haven’t read every post above.
Just want to add.

Planned obsolescence.

If everything worked forever where would all the people work that build replacement parts?

Plus no one wants to pay premium prices for things that last forever.

Apple wanted to sell more phones. Send an update that reduces battery life…

It worked pretty good until they were caught.

 

[cranky108]

Just sent an update with an App that wastes battery life.
Think Bitcoin calculations.