How do you calculate the amps of a three phase generator from the amps of the three single phase of the generator?

Answer 1

87 amps three phase means 87 amps on each phase.

Additional Answer

It's only 87 A per phase for a wye (star) connected load; for a delta connected load, the current per phase will be 50 A, while each line current will be 87 A. That's assuming a balanced load, of course. Line currents pass through the supply conductors to the load, whereas phase currents pass through the loads themselves.

Next answer: No, 87 amps three-phase means 87 amps on each phase wire, so you can put an ammeter in each wire and it registers 87 amps.

It doesn't matter whether the load is connected delta or star :) But the amount of power is different because the line-to-line voltage is sqrt(3) times as much as the line-to neutral voltage. It isn't the current that is different, it's the voltage.

Another Answer

This question is rather confusing as we don't really know what the original '87' A is referring to. Is it a line current or a phase current? But it's important that we use the correct terminology if we are to sort it out.

First of all, the 'hot' conductors that connect the generator to its load are called line conductors (or, simply, 'lines'), and the voltages between any pair of line conductors are termed 'line voltages', while the currents flowing through each of them are termed a 'line currents'. (Confusion exists when people call these three interconnecting conductors 'phases', which is common but completely incorrect! Remember, 'line' voltages and currents are so-called because they refer to 'lines', not phases!)

Next, the three windings within the generator, which may be connected either in delta or in star (wye) represent the machine's 'phases'. So the voltage appearing across any one of these internal windings is called a 'phase voltage', while the current through any one of these windings is termed a 'phase current'.

Generally speaking, while its very easy to place a voltmeter and ammeter to measure line voltages and line currents, it's usually very much more difficult to place these instruments so that they can measure phase voltages and currents, This is because the phases are inaccessible, because they are enclosed insidethe generator.

So, in practise, whenever we work with a generator (and the same goes for a motor), we usually measure line values notphase values.

So, returning to the question, if we assume the '87 A' quoted by the questioner is a phase current (that is, as measured in any winding inside the generator) then, for a balanced load, the value of the line current (as measured in any one of the external line conductors) will depend on whether the windings are connected in delta or in star. If the windings are connected in star, then the line currents (i.e. outside the generator) will have exactly the same magnitude as the winding (phase) currents -i.e. they will also be 87 A. If, on the other hand, the windings (phases) are connected in delta, then each line current will be 1.732 times the phase current -that is, about 151 A.

On the other hand, if the '87 A' quoted is a line current (that is, as measured outside the generator) then, for a delta-connected generator, the winding (phase) current will be 0.577 times the line current: i.e. about 50 Awhereas, for a star-connected generator, the winding (phase) current will be exactly the same as the line current, i.e. 87 A.

Next Answer: the longer it gets, the more confusing it gets :) However the wires carrying the three-phase supply can correctly be called 'lines'. Then you can have a 'line-to-line' voltage and a 'line-to-neutral' voltage and those terms are unambiguous. At the generator or load, if it's star-connected, the line current equals the generator or load current. When the generator or load are delta-connected, the line current splits two ways and the generator/load current is 1/sqrt(3) times as much as the line current.

The confusion is based in calling one condition 'line' and the other 'phase'.

All this assumes a perfectly balanced system.

Answer 2

In three-phase systems, we have to measure the current in each of the three line conductors individually. There is no 'overall' current for a three-phase system. So if you know the current in each phase, then that's it! There's nothing else to calculate.

Answer 3

depend upon load .