3-phase

[altec]

I have a few questions regarding 3 phase electricity. I would like to think i have a reasonable basis in domestic wiring but know very little about 3 phase. I am fully aware that putting into practice some of what i say here would be very dangerous. I am asking purely out of curiosity.

1. I understand that in single-phase systems the current on the live should be very similar to the neutral current. But in three-phase you have only one neutral and if the load on each phase is balanced there will be very little current on the neutral. How does this work? Lets say i had 3 phases installed in my home and hence have 3 individual fuse boards. Is only one neutral still required? I just don't understand.

2.IF somebody were to have a single-phase service and want to power a 3-phase saw for example, what would happen IF they were to connect the phase to phase 1 connection and loop it into the connections for phase2 and 3.

3. Are all phases in a 3-phase system at the same voltage? (i.e. 230V) If you were to short two of these together, would you see an arc and have large current flowing (like a live/neutral short in domestic) If so why?

4. This question refers to industrial premises where 'emergency stop' switches are used in conjunction with a solenoid.

If one of these switches were to break lets say. Would it be possible to remove the switch and connect the 2? Wires (i believe these switches are single phase) in order to have power again.


These questions show a great lack of knowledge with regard to these systems. I am aware of this, and as stated above am asking these questions purely out of interest.

I would be very grateful if somebody took the time to enlighten me.
Thanking You,
Declan.

 

[plugwash]

1: yep there is only 1 neutral comeing in usually the same size as the phases usually 3 phase is terminated in a proper 3 phase distribution board in which either single pole mcbs for single phase cuircuits or 3 pole mcbs for 3 phase cuircuits can be fitted if you were going to use sinlge phase boards you'd have to split the neutral in a henly block

2: with a moter driven appliance you'd destro the moter at the very least possiblly worse. however 3 phase german cooekrs are i believe designed so you can wire them like this

3: the phases have the same rms voltage line-neutral but because of the phase difference (they are 120 degrees out of phase with each other) between any 2 of them you get an rms voltage sqrt(3) times bigger ie on a 240V phase-neutral system you have roughly 415V phase-phase! watch out!

4: i belive so they are just ordinary switches driveing contactors but i wouldn't advise actually doing this emergency stop buttons are there for a reason

 

[Stod]

Here's my way of saying what plugwash said:

1: The current changes direction 50 times a second, but the alternation occurs at different times for each phase. That means that at any instant in time, the current can flow from one phase wire and back through the other two (and vice-versa). Provided the load is balanced over the three phases so that the average current in each phase is the same, there is no current in the neutral and it’s not needed.
However, if the load is not balanced, you need a neutral to conduct the ‘spare’ current, otherwise the voltage fluctuates.

2: The motor wouldn’t start and the circuit would overload because you can’t (normally) set up a rotating magnetic field with only one phase.
Single phase motors have a starting winding to set up an initial partly rotating magnetic field. Small motors (as in electric drills) use a commutator to achieve motion.

3: The average (over 20 milliseconds) voltage of all three phases is the same: 230 volts to neutral. At any instant it is not the same because the altenations occur at different times in each of the three phases. So if you measure the voltage between phases you get 400 volts. And a fault if you connect the two!

4: The solenoid is operated by a single phase supply to switch all three phases to the load at the same time.

 

[plugwash]

it's not the average volatge it's the rms voltage


a rms voltage of 240V will deliver an average power of 57600W into a 1 ohm load

an avereage voltage of 240V is quite different from a rms voltage of 240V
rms stands for root mean square than is sqrt(average(voltage^2))

the reason rms is used is because the average power from a 240v rms power supply is the same as the power from a 240V dc supply into a particular resistance of load

 

[Stod]

Plugwash, you are splitting hairs. Most people use the word ‘average’ instead of ‘mean’ or ‘median’ or ‘mode’, each of which has a distinct meaning. You yourself used ‘average’ when you meant ‘mean’ in your definition of rms.

I used average instead of rms because I don’t know if altec understands rms. Average in this context is near enough.

If I split hairs, then all your statements are wrong.

The power into the 1 ohm load is only 57600W in special conditions, viz: 1. it must be a pure resistance and 2. the average must be measured over a period that is exactly divisible by 20mS. On average, it's less.

The average of a pure (i.e. without harmonics) wave is zero, so your definition of rms will result in zero volts.

The reason rms is used is to make the power flow positive whichever direction the current flows.

A parting thought – the average person has fewer than two arms…

 

[Big_Spark]

Altec, would I be correct in thinking that you are asking these questions as you intend to put the answers into practice? I say this because I cannot think of any earthly reason why a non-industrial employed technician/engineer/electrician would ask such questions. If that is the case, it is clear you understanding of three phase is dangerously limited and if you are, despite what you say, planning to undertake such work then I hope you reconsider.

One question I will answer though is the motor question.

Three phase motors CAN be connected to single phase supplies and made to work, in fact most three phase motors are designed with this option in ind. Whether they operate on single or three phase is decided how the motors are connected in their terminal block. Equally, these motors, when used in three phase, are capable of being connected in STAR or DELTA depending on use. Such connections should only be made by a competent person though.

Stod said
2: The motor wouldn’t start and the circuit would overload because you can’t (normally) set up a rotating magnetic field with only one phase.
Single phase motors have a starting winding to set up an initial partly rotating magnetic field. Small motors (as in electric drills) use a commutator to achieve motion.

If the motor has been electrically designed to start without the use of a capacitor, then it will, single phase or not. That is the idea of the so called "squirrel cage" induction motor. The supply is fed through the stator which sets up a rotating magnetic field, the rotor, made up of conductors that are shorted out at either end, has a voltage induced into it, this then creates a magnetic field that begins to rotate in conjunction with the stator field. These motors always "lag" the supply voltage and their control is very limited, but they do work.

They can be made more efficient by the introduction of capacitors on either the Start, the run, or on both. This improves the overall operating charactoristics of the motor.

 

[plugwash]

sorry should have said mean not average
sqrt(mean(voltage^2)) is the formulare for rms
note the voltage^2 as the innermost part this means that voltage readings are squared before averageing and the square of any real number is positive

so the result won't be 0

 

[Stod]

Three phase motors CAN be connected to single phase supplies and made to work, in fact most three phase motors are designed with this option in ind. Whether they operate on single or three phase is decided how the motors are connected in their terminal block.

FWL – I think I’ve been around a bit but I’ve never ever seen an induction motor designed to run on single or three phase by how they are connected. I can’t find such on the net either: they are either single or three phase. Could you give a link?

The star – delta issue. Generally, small motors (less than around 3 h.p.) are designed to run star connected. You can’t go wrong because they only have three terminals for the three phases, the star connection being made internally. Larger motors are designed to run in delta, with 400V over their windings. They have six terminals to connect the three phases either in star or delta. For a delta connected motor, there is a starting transient of six times full load current. This transient can be reduced by starting the motor connected in star (externally). When the motor nears its running speed, the starter switches the connection to delta.

The squirrel cage rotor has nothing whatsoever to do with starting. To start a single phase induction motor, you either have to have a winding that carries a current that’s not in phase with the main winding or (only on tiny motors) have a copper ring around part of one of the poles of the stator. The phase change for the starter winding can be made (expensively) with an inductor or (much cheaper) with a capacitor or even (highly inefficiently) with a resistor.

 

[kendor]

Three phase motors CAN be connected to single phase supplies and made to work

Wrong! three phase motors are designed for three phase.
although you can get a three phase motor to run on two phases by manually cranking it to start and it will run although not happily
or if already running and you lose a phase it will continue to run if the load isn't heavy enough to stall it.

in fact most three phase motors are designed with this option in ind. Whether they operate on single or three phase is decided how the motors are connected in their terminal block

Are you suggesting that it can run on single phase or three phase depending on whether you connect the windings in STAR or DELTA?
Wrong! the reason for the option of STAR or DELTA for larger motors with heavy starting currents is so that you can wire them up to a STAR/DELTA Starter to limit the starting current until it reaches sufficient speed for the back EMF to limit the running current, it starts in STAR which reduces the starting voltage across the windings by allowing single phase voltage across each respective phase winding thus reducing the voltage and therefore the current, these single phase voltages would still be out of phase with each other and therefore you will get your rotating magnetic field (notice i mentioned single phase voltage and not what that voltage would be as you can get three phase systems that don't necessarily run at 415 volts, I have personally worked on 230v three phase systems and motors), Once up to speed the starter would move to the DELTA position allowing full running volts, the motor would run at it's designed speed not slower as would happen if you left it wired in STAR.

Equally, these motors, when used in three phase, are capable of being connected in STAR or DELTA depending on use.

Wrong again! you would be running the windings at a lower than designed voltage which will increase the current flow through the windings from their manufactured spec and the motor wouldn't achieve it's designed running speed, the STAR configuration as explained above should only be made by a starting device for a short period of time(only enough so that the motor doesn't stall when put into DELTA mode).

Stod said
2: The motor wouldn’t start and the circuit would overload because you can’t (normally) set up a rotating magnetic field with only one phase.
Single phase motors have a starting winding to set up an initial partly rotating magnetic field.

Correct! this start winding arrangement could be a capacitor start (a capacitor is incorporated through a centrifugal switch arrangement that disconnects the capacitor and start winding once the motor is up and running(rotating field) or capacitor run and start where the capacitor and start winding stay in circuit, usually small motors like the drill example below.

Small motors (as in electric drills) use a commutator to achieve motion.

If the motor has been electrically designed to start without the use of a capacitor, then it will, single phase or not. That is the idea of the so called "squirrel cage" induction motor. The supply is fed through the stator which sets up a rotating magnetic field, the rotor, made up of conductors that are shorted out at either end, has a voltage induced into it, this then creates a magnetic field that begins to rotate in conjunction with the stator field.

The term "Squirrel Cage"merely refers to the type of motor and doesn't suggest that it will run on single phase alone, it would need to incorporate a method like the above of starting the rotating field on single phase.

They can be made more efficient by the introduction of capacitors on either the Start, the run, or on both. This improves the overall operating charactoristics of the motor.

A Capacitor start and run motor is more efficient than a capacitor start motor, although the capacitor is in the start winding only.

 

[kendor]

sorry stod,my reply will have repeated a lot of what you were saying because i was still in the process of composing my reply when you posted yours

 

[Stod]

sorry stod

Apology not needed, Kendor. The same information expressed in different words is always helpful because it aids understanding (and exposes bulls**t). I have six different translations of the bible for this reason.

 

[Big_Spark]

Three phase motors CAN be connected to single phase supplies and made to work

Wrong! three phase motors are designed for three phase.
although you can get a three phase motor to run on two phases by manually cranking it to start and it will run although not happily
or if already running and you lose a phase it will continue to run if the load isn't heavy enough to stall it.

Kendor...YOUR WRONG

If you doubt that, then please by all means phone the technical departments of Vent-Axia, Woods, Siemans, H&G Fire pumps or any other large motor manufacturer.

I recently connected up a Vent-Axia TP450-14 Turboprop cased Axial extract fan, that can be connected as single phase or three phase..just like every other motor they manufacture that can run on three phase supplies.

Wrong again! you would be running the windings at a lower than designed voltage which will increase the current flow through the windings from their manufactured spec and the motor wouldn't achieve it's designed running speed, the STAR configuration as explained above should only be made by a starting device for a short period of time(only enough so that the motor doesn't stall when put into DELTA mode).

Oh Yeah...I think you need to read up on three phase motors Kendor, that is not a dig, but you clearly do not understand the workings of them fully. I have installed a shed load of motors over the years, delta connected, star connected and using Star-Delta and DOL starters, again I suggest you speak to the poeple that make them, they will tell you the same as I. The instructions I have in front of me for a Large H&G Fire Pump, 128Kw, show that it can be connected in star or delta configuration, and also that is can be used with a Star-Delta or a Delta-Star starter, although they do recommend that the most efficient method method is with a Star-Delta Starter and a standard Star/delta connection for the motor.

The term "Squirrel Cage"merely refers to the type of motor and doesn't suggest that it will run on single phase alone, it would need to incorporate a method like the above of starting the rotating field on single phase.

I worded that sentence badly, you are correct, but I believe my commment also said the same, except for the starter. It is possible, though rare these days, to have single phase motors that have no capacitor in them at all. These are not common as they have have very high start-up currents and are not particularly efficent. Personally I have not seen one in about 10 years, and I think if you wanted one it would be a special order job. But it is possible.

 

[Big_Spark]

(and exposes bulls**t).

I hope that comment is not aimed in my direction.

 

[Studders]

Aint no expert on three phase but, I've certainly seen three phase compressors running from 240v outlet. So it must be possible.

 

[securespark]

Moi, aussi, Studders!