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3 phase vs single phase

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sean logie08/12/2018 11:01:05
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588 forum posts
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I'm having to rethink the single phase 2hp motor on my lathe, it's too powerful and eats electricity, 85 more last month than normal . I have two other motors both 1/2hp one is 3 phase and the other is single phase . I have a vfd for the 240v 3 phase . Which one would have more torque .

Sean
Simon Williams 308/12/2018 11:07:24
415 forum posts
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85 quid is about 300 hours running, so that's a month of 12 hour days. You don't need a smaller motor you need w holiday
David Jupp08/12/2018 11:14:47
699 forum posts
17 photos

If rated speeds are similar, then toque will be similar at same power. Power is proportional to torque x speed.

If one motor has significantly lower rated speed (e.g. a 4 pole motor at half the speed of a 2 pole motor) it'll produce more torque.

The 3 phase motor will have smoother torque delivery, and the VFD brings other benefits.

Brian H08/12/2018 11:27:06
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1269 forum posts
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Agree with above, 3 ph every time.

Brian

Mike Poole08/12/2018 11:38:16
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2147 forum posts
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Powerwise they are both 1/2hp so not a lot to choose between them. To go from 2hp to 1/2hp is a big change, what would the standard motor size be for the lathe? If the 1/2hp motor will drive the lathe and cut a large diameter at slow speed then you may be in luck. The VFD solution will give a more flexible drive but remember even if the VFD has a sensorless flux vector mode that can maintain 100% torque at lower speeds it does not give full power so at half rpm you will have about half power. The usefulness of belts and gears is that power is maintained at low rpm so torque is multiplied. Using the VFD and the mechanical speed reduction is probably the best of both worlds.

Mike

SillyOldDuffer08/12/2018 11:52:01
4783 forum posts
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Posted by sean logie on 08/12/2018 11:01:05:
I'm having to rethink the single phase 2hp motor on my lathe, it's too powerful and eats electricity, �85 more last month than normal . I have two other motors both 1/2hp one is 3 phase and the other is single phase . I have a vfd for the 240v 3 phase . Which one would have more torque .

Sean

Um, look for a fault or some other cause unless you've been been working the lathe really hard. At 20p per kwh £85 would buy you 212 hours continuous operation under full load. That's about 3 working weeks of 10 hour days cutting metal flat out. Depends on what you're doing of course but few hobby lathes are used like that.

Also motors shouldn't consume full power unless actually doing strenuous work. Unlikely on a lathe, partly because most cuts aren't that deep in tough metal with blunt tools and partly because full power is only consumed while actually cutting, which is intermittent. My lathe has an 1500W output motor but I rarely consume more than 600W during a typical cut. I'm not particularly delicate, and have hit 1400W cutting with power traverse and carbide going 1.5mm into mild steel. From memory, idling consumes about 150W, and driving the lead-screw & gearbox costs another 150W. Even so the lighting in my workshop uses more electricity than the tools.

If you're really using that much power to cut metal, switching to a smaller motor is a bad idea. It won't save money and cutting the same amount of metal will take proportionally longer.

As we're heading into winter have you been using an electric fire, perhaps leaving it on overnight to defeat condensation? Or is the lathe unusually stiff, you might have some bad bearings or a lube failure?

Dave

sean logie08/12/2018 12:05:41
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I tried to tell er indoors that the lathe isn't what's chewing the power it's the bloody condenser tumble dryer 😂🤣
Phil Whitley08/12/2018 12:16:14
909 forum posts
125 photos

I agree with Simon Williams, that seems like a lot of power useage. assuming 17p per kW hr thats 10.6 hours per day running and loaded! Are you really working those hours, or have you got some other electrical problem, like for instance, a faulty immersion heater which is not RCD protected? To answer your question always go for three phase, they are more efficient and deliver smoother torque than single, and with the vfd you can do soft start and speed control too. Do you have a smart meter ? there has been talk about the smart meters ability to apply higher usage to inductive loads like motors which have poor power factor, but don't know if this is really happening (yet) or not. With a VFD in circuit, this may not be recognised by the meter as actually being an inductive load. Are you on the end of a long supply line in a remote area? If so, volts drop is a possibility too. Try the 3 phase/vfd motor for a month, and see if there is a marked drop in your consumption, if not, look for another problem!

Phil

Phil Whitley08/12/2018 12:19:06
909 forum posts
125 photos

well you said it Sean!!! a condenser tumble dryer with a blocked or partially blocked condenser spray will chew away for hours and not dry clothes at all! they are a fundamentally crap idea! chuck it out and get a ducted outlet one.

Phil

Neil Wyatt08/12/2018 12:46:43
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16655 forum posts
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Ha! You are probably the person who needs a smart meter - I knew there was one!

Wouldn't hurt to get a smart plug and test various appliances to see how much they really use.

Neil

Phil Whitley08/12/2018 12:49:17
909 forum posts
125 photos

Good idea Neil!

Clive Foster08/12/2018 13:20:56
1867 forum posts
59 photos

Plenty of smart plugs on Amazon, and probably E-Bay too, for around £12 to £20. I shall get one if and when I have an issue that simple maths and specification data won't resolve.

Before buying tho' do verify how much space and where is needed. Most of them look as if they will at least partially obscure the switch. CPC (Farnell) sell a nice looking one with the display and control buttons below the plug part so the cable hangs down over it. D'oH. Supposed to be professional outfit yet sell summit with such an obvious design blooper. A mistake that obvious makes you wonder how good the innards are.

Clive

Phil Whitley08/12/2018 13:31:20
909 forum posts
125 photos

Read this Sean!

https://www.mumsnet.com/Talk/products/214863-tumble-dryer-condenser-or-vented

I do not make the claim for the crapness of condenser dryers lightly, it is based on the experience as a repairer of vastly extended drying times and large electricity bills. some very expensive ones may have a slight edge over ducted when they are new and working correctly, but that doesnt last for long as the condensing system spray tends to block up with limescale, fluff or grit from the water supply, and of course the only way you know it is not working efficiently, or indeed at all, is by the drying time getting longer and longer, This seems to apply even more to the combined washer/dryers which are an absolute pig to repair, and usually fail again in pretty short order. they are a good idea IN THEORY, but in practice they take much longer to dry clothes, and therefore use more power. Try convincing a customer who bought the all singing and dancing expensive wash/dry machine that it is working correctly and then get called back when she finds she can only get one wash dry cycle in a day, and the clothes are still damp. The latest ones use heat pump technology (refrigeration) to condense the water andalso have "vigorous condenser cleaning" which flushes all the lint out of the condenser, which is another problem that most suffer from but this has an energy cost too., This is why among the top ten efficient tumble dryers you only find 2 condensing ones, and one of those is at number ten! For all us mortals who cannot afford to buy Siemens ar Miele, stick to ducted, and avoid washer dryers like the plague!

https://www.sust-it.net/energy-saving/tumble-dryers

Simon Williams 308/12/2018 13:52:52
415 forum posts
67 photos

All joking about tumble driers aside, the OP has raised an interesting question regarding the relative efficiency of single and three phase motors.

Given that a single phase motor is a fairly nasty electrical load, but a three phase motor on a single phase supply suffers the conversion inefficiency of the VFD, there's likely not much to choose between them in terms of converting cost of electricity into work done.

Does anyone have any in situ estimates of the actual efficiency of power out over power in for the sort of size motor (1 - 3 HP say) we are likely to encounter in a home workshop?

Best rgds Simon

Phil Whitley08/12/2018 14:01:13
909 forum posts
125 photos

Short answer is No Simon, I have only installed a couple of VFD's for a friend, as I have three phase in my workshop. You would have thought that the soft start would avoid the sudden loading of a motor starting, but I dont really know. Time to use what my old boss called the "tong testers"< A clamp meter, and do the maths!

Phil Whitley08/12/2018 14:16:04
909 forum posts
125 photos

or look at this?winkhttp://ijcee.org/papers/010.pdf

Mark Rand08/12/2018 14:39:10
785 forum posts
Posted by Phil Whitley on 08/12/2018 13:31:20:

Read this Sean!

https://www.mumsnet.com/Talk/products/214863-tumble-dryer-condenser-or-vented

I do not make the claim for the crapness of condenser dryers lightly, it is based on the experience as a repairer of vastly extended drying times and large electricity bills. some very expensive ones may have a slight edge over ducted when they are new and working correctly, but that doesnt last for long as the condensing system spray tends to block up with limescale, fluff or grit from the water supply, and of course the only way you know it is not working efficiently, or indeed at all, is by the drying time getting longer and longer.

What water supply? who buys a condenser tumble dryer without a heat pump these days? Ditto, don't people clean the filters?

Mike Poole08/12/2018 14:44:11
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2147 forum posts
52 photos

As a machine tool motor in a home workshop is most likely to spend much of its time only lightly loaded then its efficiency is going to be poor. Rather like the optimistic fuel consumption figures for a motor car are only realised under ideal conditions an electric motor will only reach its maximum efficiency under optimum conditions. I think cutting the plug off the tumble dryer would have the the most impact on your electricity bill (and mine).

Mike

Tim Stevens08/12/2018 15:13:22
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1095 forum posts

Don't forget that when any motor is running idle its efficiency is ZERO.

Tim

Phil Whitley08/12/2018 15:59:23
909 forum posts
125 photos

mark rand, that is exactly what it says in my post! their poor showing in the efficiency stakes, and the extremely high retail cost (£825.00 for the Siemens) shows how efficient they actually are! heat pumps may work, but add to complexity,. A ducted TD takes already warmed air from your house, heats it some more, it absorbs the moisture in the clothes, and then gets thrown out the duct, a condenser one has to cool the air down to its dew point every time it is cycled, then reheat it the actual efficiency gain is very marginal at best, as is witnessed by the poor results in the league table. I wonder if it would last long enough for you to save the difference in retail price, I very much doubt it!!

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