Why are all single phase machines < 3 Kw?

[flanajb]

Anyone able to explain why all 240V machines only have motors < 3Kw where as the equiv 3 phase machine motors are more powerful. Surely a 240V could supply enough power to run a 5Kw motor if it was designed for a dedicated 32 amp supply ?

 

[theartfulbodger]

If I remember correctly (and I've been known not to!) commercial dishwashers ran off a 32 amp supply not a 3 phase. Not sure what wattage they were but they'd do a full load of dishwashing in about 3 minutes flat.

EDIT
it seems with some machines you get the choice.

Requires hardwiring to mains.
Power Type 13A, 30A and 16A 3 phase available.


forgive the non-woody machine but you did say "all" machines 8)

 

[siggy_7]

240V supplies could easily supply sufficient power with larger cables designed to carry the required currents. However, 3kW is adequate power for a hobbyist grade machine, above that and you're definitely into machines deisgned for professional and industrial use (although no reason why some very serious hobbyists wouldn't consider these types of machines also, especially second hand). Single phase motors are less efficient than three phase, require additional systems like separate starting circuits (three phase motors are self-starting) and the motors can be smaller for an equivalent power output. Dishwashers the primary power requirement is heating rather than big motors.

 

[Jacob]

Anyone able to explain why all 240V machines only have motors < 3Kw where as the equiv 3 phase machine motors are more powerful. Surely a 240V could supply enough power to run a 5Kw motor if it was designed for a dedicated 32 amp supply ?

Simply so that they can be plugged in to a normal domestic supply which is standardised to operate at 13amp .
13amp x 240 volt = 3.120kw which means the realistic limit for a motor will be lower as most need more power at start up.
Anything more and a special circuit with heavier cable is required e.g. 16amp with blue plugs, or things like showers wired in on their own circuit and rated at 9kw
3 phase gives more power with less cable and is more practical for bigger motors for a number of reasons, so you don't find many 5kw single phase motors.

 

[9fingers]

The starting currents for motors over 3kW would be excessive for most domestic mains.
Machines needing that sort of power can hardly be regarded as domestic and so are designed for an industrial environment which would have a 3phase supply available.
If you are serious about setting up a pro workshop then you can always get a 3phase supply connected to your house.

Bob

 

[flanajb]

Makes sense, but given that most people buying a large planer or table saw which is 240V rated would most likely have put a decent power supply into their workshop and could therefore make use of it.

I could get a 3 phase installed, but I know it would be super costly, due to the fact that there isn't an overhead supply nearby and I am surrounded by domestic buildings.

Informative answers. Thanks

 

[WoodMangler]

If you are serious about setting up a pro workshop then you can always get a 3phase supply connected to your house.

Can cost an arm and a leg - a farmer friend of mine had the 3-phase supply actually running past his barn, but they still wanted 5 grand to hook it up.

 

[TheTiddles]

3kW is an awfull lot of power, how often do you find a sharp saw or similar just hasn't got enough power with a 3kW motor on it?

Aidan

 

[9fingers]

Aidan, Elsewhere, the op has been talking of a spindle moulder with 100mm deep cutter block

Megalomaniac tendencies methinks!!

Bob

 

[Jacob]

3kW is an awfull lot of power, how often do you find a sharp saw or similar just hasn't got enough power with a 3kW motor on it?

Aidan

Only for fairly big saws yes, but a normal sized spindle needs 5kw or more ideally. You can work with a smaller one - I wouldn't want to put anybody off!

 

[tool613]

wadkin made a 4 hp motor is that why? over here in Canada we have 1/2, 1, 1.5,2,3,5,7.5.10 hp motors as standard. 4 and 6 hp motor are odd here yet wadkin made them.

for consumer grade machines i would say yes that 3 kw is plenty of power, but everyone one knows that real tools are 3 phase :mrgreen:

jack

 

[Dibs-h]

that there isn't an overhead supply nearby and I am surrounded by domestic buildings.

Not necessarily. You want to speak to whoever maintains the network where you are and ask for a "network map" of your immediate vicinity - i.e. your house, street\road, etc. It will show where the different phases are and you can get an idea of how expensive, super expensive, etc. or not, it might be.

Most roads\streets will have all 3 phases - they don't run all the houses off 1 phase.

HIH

Dibs

 

[siggy_7]

I could get a 3 phase installed, but I know it would be super costly, due to the fact that there isn't an overhead supply nearby and I am surrounded by domestic buildings.

Have you looked at getting a digital 3 phase converter? Not cheap, but almost certainly cheaper than a dedicated 3-phase supply from the utility companies - which will run into thousands to install and extra costs to run as well (separate meter, second standing charge, reduced number of suppliers/tarrifs). A 7.5HP converter you should be able to get from somewhere like drives direct for the £1,000 mark, and a 10HP one not a great deal more. Is this just for your own use or a commercial multi-user workshop?

 

[Dibs-h]

Not cheap, but almost certainly cheaper than a dedicated 3-phase supply from the utility companies - which will run into thousands to install and extra costs to run as well (separate meter, second standing charge, reduced number of suppliers/tarrifs).

I disagree. Most if not all companies will replace the 1Ph meter that you have, with a 3Ph meter - so still 1 lot of standing charge, etc. or not, if the tariff doesn't have one. I've shopped around over the years for the leccy supply (domestic) and everyone I've used has stated that they would still supply on the same tariff if the meter was 3 Ph.

If the property is a very large one - a single 100A supply might not cut it. Do you think they'll up it to 150A or state that you need a 3Ph supply (obviously balance the phases internally)?

This 3Ph stuff is myth laden - speak to the network company and get a network map - costs nothing and will tell you how far the 3 phases are. If there are street lamps on the other side of your boundary - you are bound to have 3 ph very close. A line of street lamps are on alternating phases, not the same phase. So will have all 3 phases running down the street.

Then ask for a quote for the connection. Again costs nothing to get one.

HIH

Dibs

 

[Hudson Carpentry]

Each house is wired in alternative phases now as well. The sparky I was talking to last year said that there are 3 phase in most roads now. I have a 3 phase 230v supply??? Although the sparky just split the single phase coming in into 3 fuses then a tail each to the 3 in's on the meter. Apparently the meter required 3 phase to operate a single phases supply??

 

[Dibs-h]

Each house is wired in alternative phases now as well. The sparky I was talking to last year said that there are 3 phase in most roads now. I have a 3 phase 230v supply??? Although the sparky just split the single phase coming in into 3 fuses then a tail each to the 3 in's on the meter. Apparently the meter required 3 phase to operate a single phases supply??

Yes houses may now be connected to alternate phases. When a fuse pops in the substation on a single phase - that's why all the houses don't go out. Also balances the loads.

You might not have 3ph coming into the house. You would have 4 tails coming in - 3 phases and 1 neutral (most likely). If you have 3 fuses before the meter then you do have 3ph, but only 1 phase in use, i.e. fed to meter).

Dibs

p.s. To get all 3 phases in use - you just need to have the meter changed, assuming there are 3 supply fuses before the meter. These aren't fitted by the sparky, but are the property of the network operator and are usually sealed with crimp seals.

 

[Hudson Carpentry]

Each house is wired in alternative phases now as well. The sparky I was talking to last year said that there are 3 phase in most roads now. I have a 3 phase 230v supply??? Although the sparky just split the single phase coming in into 3 fuses then a tail each to the 3 in's on the meter. Apparently the meter required 3 phase to operate a single phases supply??

Yes houses may now be connected to alternate phases. When a fuse pops in the substation on a single phase - that's why all the houses don't go out. Also balances the loads.

You might not have 3ph coming into the house. You would have 4 tails coming in - 3 phases and 1 neutral (most likely). If you have 3 fuses before the meter then you do have 3ph, but only 1 phase in use, i.e. fed to meter).

Dibs

p.s. To get all 3 phases in use - you just need to have the meter changed, assuming there are 3 supply fuses before the meter. These aren't fitted by the sparky, but are the property of the network operator and are usually sealed with crimp seals.

It was an Eon sparky that did it Dibs. He came to change my meter. There is only 1 phase coming into the shop, then split into 3 (each with fuse) to accommodate the new meter which is a 230v 3 phase meter although it does support 415v. I asked why and does this mean I can have 415v 3 phase and he said no, the meter for some strange reason needed all its 3 phases energized, but the result will be single phase 230v out. If I wanted 3 phase, ring and pay thousands. I think he said the meter would support 415v 3 phase out as long as I had 3 phases coming into the shop.

 

[kirkpoore1]

Interesting discussion. I wish I had the option of 3 phase from the pole at my house. I gotta make do with my rotary phase converter. On the other hand, I have 30 amp 240v circuits in my shop and my largest single phase motor is 5 hp, so things aren't all bad.

Kirk

 

[9fingers]

Hi Kirk,

Is there an online description of how the US power grid is wired up? I'm aware of the 240-120 transformers but surely the feed to those is three phase in the street?

We used to have 5 parallel wires on the pole outside my house. 3 phases, neutral and a separate wire for street lighting that used to switched on and off years ago. but all modern street lights here have their own daylight sensors so come on when each wants to.
Last year they ran new over head cables that have three coaxial cables twisted together. Each of the three has 1 phase core and an outer which is neutral. Each house has one of those coaxial cables going to the meter.
Presumably if I requested 3 phase, I'd get all three of the coaxes?
Like you I roll my own three phase as I want it for each machine.

Bob

 

[kirkpoore1]

Bob:

The typical residential arrangement in the US is to have 3ph high voltage connected to a neighborhood transformer that steps it down to 240V. At that point it gets distributed to a few streets in the local area using two hot (non-neutral) wires, with different pairs run on different streets (or just to different houses) for load balancing. (This can result in some odd situations, where you may have power on one side of the street while the other side of the street can be out. A few years ago this happened in my neighborhood after a lightning strike. After about a day, some people were running extension cords across the street to get power from their neighbors for their refrigerators and freezers.) Each house gets hooked in at an electric meter, then the lines run into a distribution panel in the house. Today, the panel is usually sized to handle 200 amps, but some big houses have 400 amp panels and older houses have 100 amp panels. Really old houses might have a 40 amp fuse box.

Each house is separately grounded, and while the two incoming lines are 240V relative to each other, they are 120V to the ground. At the panel, a neutral line is created with a direct tie in to the ground, while a real ground wire is also hooked in (the ground wire is usually uninsulated and is there strictly for safety). For 120V circuits, you have one hot line and the neutral line coming from one circuit breaker (plus the uninsulated ground wire). These lines are either 15 or 20 amps, and are branch circuits (no ring mains). For 240V circuits, you have the two hots and the ground wire, and (usually) no neutral. These can be wired for any size, but are typically 20 amps for general use and 40 or 50 amps for things like electric clothes dryers, electric ovens, and air conditioners. I have a 100 amp circuit to a sub panel in my shop, along with a 60 amp circuit to my garage that also feeds my phase converter. Within my shop I have 120V 20 amp circuits and 240V 30 amp single phase circuits for my single phase machines, lights, and heat pump, and 240V 20 amp circuits from a three phase panel fed by my phase converter.

Since there are only two wires going to each house, you can't have 3ph power from the power company unless you want to pay big bucks for them to run a separate line. In business areas, 3 phase is usually run to every building.

I hope this all makes sense. I'm not an electrician. I used to say that everything I knew about electricity I learned from Lionel trains, but since I put in my phase converter that's not really true anymore.

Kirk