| An electrician was telling me the other day that three phase equipment is | more efficient than single phase. If you have a single phase 208V device, | and it only draws energy on a single phase of a three phase circuit, why | should that be any less efficient than a three phase implementation of the | same device, which draws lower amps on three separate phases?
Certain equipment can be more efficient in three phase versions. Motors are the big thing. Efficiency is measured in total cost, too, so you do factor in the manufacturing and maintenance costs, as well as the energy and delivery costs.
Power supplies that convert AC to DC can also be more efficient in terms of simpler less costly design, and some reduction in heat loss, when they operate from three phase. This is generally only realized on largs scale designs.
In terms of total power with a given set of conductors (cost, weight, or whatever), three phase can come out ahead, depending on the way things are wired. Lets say you have 21600 watts of incandescent lighting. You could wire this up a number of ways:
- A single pole 120 volt circuit totalling 180 amps. The single phase wire is 180 amps, and the neutral is 180 amps.
- A single phase two pole (2 phase angles at 180 degrees are considered to be single phase) circuit with each pole being 120 volts to ground, with 90 amps on each pole. The neutral only needs to be 90 amps.
- A three phase three pole circuit with each pole being 120 volts to ground, with 60 amps on each pole. The neutral only needs to be 60 amps.
So the total cost of wiring, expressed in the sum of current capacity, is:
- 180x2 = 360
- 90x3 = 270
- 60x4 = 240
While light bulbs typically cannot be connected phase to phase, many other loads can be. That can eliminate the neutral being connected at all (the separate equipment ground would stay for the ground fault protection).
Suppose we have heating elements for a water heater which we can choose the voltage for. I'll use 21600 watts again.
- 240 volt elements connected to opposite poles on single phase. The two poles have 90 amps.
- Three separate 208 volt elements connected phase to phase over the three phases equally. The current in each element will be 34.641 amps. But the wires on each phase will be serving two elements each, with currents 120 degrees out of phase. The total current is NOT 69.282 amps. Since some of the current can run between elements across the point of connection due to the phase angle difference, the end result is 60 amps of current in each phase wire.
So now we have wiring costs of:
- 90x2 = 180
- 60x3 = 180
Some of these savings comes from reducing the size of the neutral, or by eliminating it entirely. But some also comes from the fact that currents are lower. Since the loss in the wire is proportional to the square of the current, reducing the current is a substantial savings if the size of the wire remains constant. But even if we cut the size of the wire in half, there is still some savings. The loss is also affected by the resistance, which goes up as we cut the size of the wire. But that only compensates for half the loss due to increased current.
Note that in example 5 I used only 208 volts instead of 240. This might not seem fair, but in both cases the voltage relative to ground is 120 volts. If you did this with a 240 volte delta load, the current would be reduced by 15.47 percent. But a power source providing that is going to have at least some conductors at a higher voltage relative to ground. To power such a load with a WYE source, the voltage to ground would be 139 volts.
When a comparison is made between say, 240 volts single phase and
240 volts three phase (or 480 or whatever), you're really dealing with a voltage increase, relative to ground (1.1547 times as much). That changes the picture.
Basically, if you double the voltage, halve the current, and halve the current capacity (approximately double the resistance), you still have cut the loss approximately in half. This works because while the wire resistance goes up by about 2 times, the total system impedance goes up 4 times for the same watts or volt-amps. The wire then becomes a smaller proportion of the circuit impedance. The cross section of the wire is half as much and the current is half as much. The heat produced per cross section is the same, but this is only half as much, and smaller wire has more surface for a given cross section. So it stays cooler, or can be shaved in size even more. Utility transmission lines can get very very hot (because they cut the size of the wire as much as they can for many reasons, such as its weight and supporting requirements) and not really be losing all that much power. The "magic" is that the overall system impedance, compared to the resistance in the wire, is very high at transmission voltages.
Your electrician as basically correct, but it's not so much that three phase is somehow better (it is for certain uses), but rather the advantage is in how we use three phase, or the voltages that three phase comes in, such as 400 (Europe), 480 (USA), 600 (Canada),
690 (Europe), 1000 (mining services where electrical equipment is run over cables extended sometimes many miles underground), and even higher for distribution and transmission. Even single phase loads like lights are often run at voltages like 277 (the voltage to ground of 480Y/277) and 346 (of 600Y/346).