Three phase versus single phase

dcom

Hi everyone,

Just a quick question from a complete novice....

What is the difference between single phase and three phase electricity??
We are just starting our self build and are about to apply to the esb for a connection.

We are installing a geo-thermal heat pump(not yet decided on the make) some of the companies are advising us to go for a three phase connection if economically possible.

Our site is about 500m from a brand new hotel which I expect has three phase supply.Is it difficult to obtain 3 phase from the esb??

Would it work out cheaper to run our heatpump on a 3 phase system as opposed to single phase?

The esb new connection form requires us to fill in technical details for our heatpump when we apply but we dont know it yet so we are in a catch 22 situation. is this normal?

curraghyid

it depends what kw rating pump you require single phase avalable up to about 1.75 kw really looking at 3 phase after that .another possibility is to use a single to 3 phase convertor to power your pump

Stoner

three phase power is very efficient, balanced three phase loads like motors where the windings on each phase match, make the most of a three phase supplies. the ESB will charge you a hefty sum for getting a three phase supply.

there are other advantages, as the voltage can be 220Vac and 400Vac depending on the configuration you choose. The higher voltage will require less current to produce the same power, this means that the copper loss in the cable will be lower, so if you have long cable runs that supply at 400V your electrical loss and voltage drop will be lower than if it was 220V.

Also with three phase voltage you can generate rotational magnetic field with no physical moving parts or components, there is no need to alter an electric field to create a difference between two Fields so that you can get so meting moving. i.e a 3 phase induction motor is very simple and efficient and requires less parts and reduced maintenance. But you'd have to work out the cost vs savings etc. The ESB will give you a cost very quickly so you could find this out very quickly. the next thing is the loadings of your devices and the distances the cables run etc.

dcom

Thanks curraghyid and stoner, I think I have it now.......:D

fishdog

it depends what kw rating pump you require single phase avalable up to about 1.75 kw really looking at 3 phase after that

I have drills bigger than that and they are single phase!!

another possibility is to use a single to 3 phase convertor to power your pump

This will not alter the size of your load, so it would not resolve any issues. It is also expensive
and can cause harmonics (= problems!)

A 3 phase machine is cheaper to buy, run, maintain and is more reliable. In fact there are many other advantages to 3 phase.

However it may cost afortune to get it installed and the standing charges will be alot more. How much more?? I dont know, ring and see. I think the price will put you off. You can go for an increased single phase supply, 16kVA (I think).

It is not normal to have a 3 phase supply to a house. Perhaps it would make sense if you had a large workshop or milking parlour.

sunnysoutheast

Hi dcom

Basically a street etc. will be supplied with three phases, each house will be on one of them in a repeating pattern - no1 on P1, no3 on P2, no5 on P3, no7 on P1 again etc. The voltage between each phase and neutral is the nominal 240v. Larger buildings, such as commercial premises, flats, etc. will be supplied with all 3 phases so that the larger loads can be shared between them.

Anyway, we went through some rigmarole with the ESB as the heat pumps we were looking at required 3 phase supply ( I think this is because many are German/Austrian-sourced and three phase supply is common there). We couldn't even order 3 phase supply however hard we tried - whether by accident or design I don't know. We were also told that, even if we could, we'd have three meters, 3 standing charges etc and would need a very expensive 3 phase distribution board.

We eventually decided not to bother with a heatpump (not only for these reasons) and just got the standard 12Kva supply (no electric showers etc.).

Good luck!
SSE

curraghyid

fishdog wrote: »

I have drills bigger than that and they are single phase!!


This will not alter the size of your load, so it would not resolve any issues. It is also expensive
and can cause harmonics (= problems!)

A 3 phase machine is cheaper to buy, run, maintain and is more reliable. In fact there are many other advantages to 3 phase.

However it may cost afortune to get it installed and the standing charges will be alot more. How much more?? I dont know, ring and see. I think the price will put you off. You can go for an increased single phase supply, 16kVA (I think).

the convertor is a relatively inexpensive option and is widely available from a number of manufacturers.especially if you have no need for athree phase supply other than one pump, also single phase iduction motors not drills are fairly difficult to source above 1.75kw (they are available from some manufacturers) but are expensive and physically much larger than a corresponding thre phase motor giving you the same output.

It is not normal to have a 3 phase supply to a house. Perhaps it would make sense if you had a large workshop or milking parlour.

the necessaary rating and supply to a premises is determined by load which is determined by the supply board

RoundyMooney

However, you can opt for a three phase supply as far as I'm aware.

Generally such a decision is made in conjunction with the Engineering Officer (or in smaller areas the guy dealing with design) who conducts the site visit.

@sunnysoutheast, it would seem to me that three phase supply was not available to you at your location. In many rural instances, customers are only served by a single phase 10/20kV spur, which rules out three phases at the meter, without what may be a considerably expensive spur upgrade.

I can't grasp why someone would insist on three seperately metered single phase supplies, particularly when they are probably fed from the one pair of conducters anyway, and regardless of whether a conventional three phase supply was available or not.

fishdog

the necessaary rating and supply to a premises is determined by load which is determined by the supply board

Which will be based on the information supplied by the electrical contractor

sunnysoutheast

hi Roundy

In theory we could get three phase - just that we were given the right runaround actually trying to order it so, for this and other reasons, we didn't bother.

Could be that the ESB is more aware of supplying 3P to single domestic premises now (we were looking in late 2006) but the only options at the time were the standard 12KVa or enhanced 16KVa unless you wanted a "farm supply".

We're in Waterford BTW. We have friends in Kildare who faced similar problems ordering 3P and in the end had to go with the 16KVa supply for their GSHP which is barely adequate.

Cheers
SSE

RoundyMooney

SSE, I'll make an inquiry about this, for my own info, and that of the forum, as much as anything.

Bruthal

yes single phase motors above 1.5 to 2 kw`s are rare. You might get a big single phase universal motor which is the type in drills, but induction motors dont have high torque in single phase configuration, single phase is only useful in small induction motors, as once its running its just magnetic poles swapping from one side to opposite side, unlike 3 phase which is in a trianglular rotating configuration creating very powerful torque.

You can make your own single to 3 phase inverter with a small 3 phase motor, some capacitors and contactors, and some luck

To get 3 phase supply from ESB is not a big amount once its for a heating unit.
Its when its for commercial use such as a workshop that it gets expensive.

We put an invertor in a workshop during the year and that invertor was 2 and half grand, because 3 phase supply was expensive for the commercial
aspect.

We also put an air source 3 phase heat exchanger in a house in donegal, it was not much extra for the 3 phase supply for it as far as i remember.

davelerave

Hi ive connected a few of these mostly in 4-5 kilowat range. for these its normally enhanced 16 kva supply with nightsaver (80amp main mcb 25 sq tails ,depemding on the rest of the load in the house) for geothermal u need to know the specs of the heat punp b4 u apply .in any event its normaly dual tariff (nightsaver).

V6DEC

Hi dcom, I am in the same situation - Have spoke to a few Geothermal providers and they all advise that I should go for 3-Phase supply.
They have mentioned that I need a Heatpump in the region of 17 kW.
There is 3-Phase about 300M from the site but I can't seem to find forms for supplying 3-phase to a domestic site anywhere on the esb.ie website. I am not even sure of the cost of Single phase V's 3-Phase although I have been told that it is expensive......So I will be following this post with interest.

davelerave

hey V6DEC i dont believe there is a 3-phase application as such.i think you would fill out the regular domestic application and as you're well over the 16kva enhanced limit you'd have to specify your 'MIC'(total supply needed in kva) and also the specs for the heat pump(rating,starts per hour,and starting current) and you'll prob need dual tariff as well.anything in the 17kw range will be 3-phase for sure .regular (wholecurrent )3-phase nightsaver would be the simplest and cheapest if you're under the max limit for this .they should be able to tell you.anyhow .

fishdog

Hi V6DEC. I installed (well I did the electrical end) of a heat pump in a 3,500 square foot house. I used an increased single phase supply for the house and had no issues.

Now if your electrical input for the heat pump is 17 kW then davelerave is correct, a 3 phase supply is required.

My point is that the output of a heat pump is greater than the electrical input. The short fall is supplied by the water in the ground. You may be reading the output of the heat pump, not the required electrical input. I would be no harm to check and make sure!

I would find it hard to believe that your heat pump requires an input of 17 kW, unless your house if far larger than the one I did (which is possible, perhaps you are a loto winner!).

As davelerave said

Hi ive connected a few of these mostly in 4-5 kilowat range

His figures sound far more reasonable and in the region of what I would expect.


At 17 kW electrical load if it was single phase would draw almost 74 amps on full load. The standard main switch fuse in a house is rated at 63 amps. On start up a heat pump such as this would draw in excess of 100 amps. For this reason I do not believe such a heat pump of this size would ever be single phase.

Also a unit now costs about €0.18 so 17 kW would cost over €3 per hour to run!! Hardly the way you want to go.

BTW, I think heat pumps are a great idea and if installed properly they will save you a lot of money!

Bruthal

the heat exchange unit will be 17kw, as in it supplies 17kw of heat, or more acurately it transfers the heat from the evaporator to the condensor, the 3 phase motor in it wont be anything near 17kw electrical load, probably 3 or 4 kw motor or somewhere around that, theres no shortfall really, as the motor is driving a compressor to transfer heat from the source to the supply end (evaporator to the condensor), and an induction motor over 2 kw wont be single phase, they are useless compared to a 3 phase one,

Bruthal

well i doubt a 17 kw 3 phase motor would be seen anywhere, not just because of the huge starting current as ye said, as that can be solved with a VSD or soft starter, but single phase has no real power in an induction motor compared to equal rated 3 phase one, so single phase ones are only usually used in small sizes.

even a 3kw single phase one would be around 14 amps and possibly 80 to start, 3kw 3 phase would be just over 4 amps per phase and 25 amps to start. And 0.7 pf would have the running currents up to 19 or 20 amps in the single phase one and around 6 in 3 phase one, but the 3 phase one would be far more powerful, far higher starting torque and running torque as well

fishdog

robbie7730, I think we are in agreement that 17 kW is not the required electrical input.

As you say:

as in it supplies 17kw of heat

A 3 phase 17 kW motor would draw about 29 amps at full load. A motor of this size would often be started using a star delta starter. VSDs are great, but very expensive.

Here is a nice big 30 kW motor for you!

http://www.edmrstore.co.uk/siemens-three-phase-motors-188-p.asp

Bruthal

yes for sure, its only a 3 or 4 kw motor in them 17kw heat exchangers,
i connected an air source one earlier this year, i cant remember the exact sizes, it will need a high volume of air through it id say,

and yes VSD`s are expensive, i was mentioning them because i was saying any size single phase motor can be made, and as you rightly say it would be a monster starting current on a big one, but single phase insuction motors are not uses much after a kw or 2 in size, they just dont have the torque a 3 phase one has.

17kw star delta would take the 29 amps and so starting at 6 times that would be 170 or so, but starting in star would be around 60, still hefty enough but certainly has been done, but the starting current if it was starting with a load on motor would last a lot longer, although still be the same as starting current with no load on the motor but with no load its very short duration.

I connected a few and most were on VSD`s, even fairly small ones, as you rightly say though, they are not cheap, a few were in intel, so they`d probably put VSD`s on everything that had a motor as much for the control as the starting currents.

Thats some money for a motor is`t it, 1700 sterling for 30kw,, i connected a 55kw one in baldonnel aerodrome about 10 years ago, imagine the price of that one

Bruthal

i have a couple of radio control helicopters, they have little 3 phase motors in them, and a small electronic speed drive which is actually a miniature VSD, the power of the motors is unbelievable for the size of them,

Bruthal

the heat exchangers are just the same as a fridge freezer, the freezer part (evaporator) is in the ground for a geothermal one, and the condensor which is the metal tubing at the back of your fridge is the part in the house that supplies the heat to the house, the motor and pump compresses the refrigerant into the condensor, and as the gas condenses it gives off the heat that it absorbed in the evaporator.

After it leaves the condensor it goes through expansion valve, drops in pressure, and so evaporates absorbing heat from the ground, or ground water,

This is the 17kw rating it has, its capasity to transfer heat, the electrical power input is what the motor needs which is probably 3-4kw so as fishdog said, the 17kw rating is the output, where as the electrical power input is much lower,

The motor and pump are just transfering heat, and its possible they can take heat from 10 degree`s c ground water and heat the house up to 20 by taking heat from a large ground area and transfering to a smaller house area, but usually they are in a deep core well and the temp is higher and more constant throughout the year. The higher the source temp the more efficient

davelerave

ya i should have copped that alright .thats the 2nd time ive got that mixed up recently.its likely only 17kw heat output so its prob only a few kilowatt motor which is probably the 16kva supply.

fishdog

i have a couple of radio control helicopters, they have little 3 phase motors in them, and a small electronic speed drive which is actually a miniature VSD, the power of the motors is unbelievable for the size of them,

I have been studying them recently, they use hall effect sensors and convert DC to 3 phase AC. We are about to use them in robots (sounds exotic, but they are very basic robots). Fantastic motors with great control and as you say they can be very small. I am told they start at about €100, is that correct??? We were using PWM to control the speed. Is that what you use?

Bruthal

i did`t really look too in depth at their method of controlling the speed but as far as i know it varies the voltage along with the pulses to the motor, in a sort of on verses off cycle so pwm would be an accurate description of that, if you reverse 2 of the 3 wires to the motor it reverses and all,

The price of the motor to get from england i use in the large size helicopter is very near to 100 euro alright. Cheaper from usa at 100 dollars, but bigger postage then. They are unbelievable motors for sure, i was shocked at the power of them. The one in the larger chopper draws about 40 amps hovering, from a 22v lithium polymer battery, and the motor is about the size of a red bull can cut in half.

i dont know if you looked at video of it, you get an idea of the power from one of these motors in first video

http://www.youtube.com/user/robbie77300

I started the motor up once on it and accidently bypassed the soft start of the speed controller, it instantly stripped most of the teeth off the large main drive gear,
robots sounds like an interesting application for them

fishdog

pulses to the motor, in a sort of on verses off cycle

yeah, that is PWM in a nutshell.
Good link! Looks like fun.

Bruthal

yes its good crack, i was flyin the engine powered planes since being an apprentice, and then seen about the electric powered stuff so thought it would be interesting, so started with the electric choppers last november, its interesting to put the cameras on them, and another fella filmed the first one from the ground a few weeks ago or around sept i think

Michael Collins

fishdog wrote: »

...I am told they start at about €100, is that correct??? We were using PWM to control the speed. Is that what you use?

That price is probably a bit out-of-date now. You can get an outrunner (the rotor is an outer magnetic shell, with the stator being the field coils in the centre) on ebay here for €42. This price includes the ESC (electronic speed controller) - bascially a few power MOSFETs used to drive the large current required.

If it's just the motor itself you're after, you can get them seperately even cheaper, here for example. Not much use unless you have a three phase power supply handy though!

Bruthal

well i`d do well to get a trex600 motor for that price, but they are getting cheaper, 100 dollars for them in usa, the smaller trex 450 motor is only about 35 sterling,