Why have three phase power?

Factual Questions
21

casdave and MachineElf kind of got to this, a bit, but I think we’re all missing the most important thing.

Three phase power ROTATES! The power itself is pumping in a cyclic manner on those three wires. To run an induction motor, you have to have a magnetic field that rotates. If you have three phase power, you can get that field with three electromagnets at 120 degree angles to one another (or six at 60 degree angles, et cetera). Three phase motors have coils in them, and nothing else.

With single phase power, you still need the field to rotate, so there are various cheesy ways of making it. You can run one of two sets of coils through a capacitor. You can do something funny with the windings to make part of the magnetic field lag behind the rest. But these tricks require centrifugal switches or other troublesome parts, and they’re not that efficient.

Notice that the way we run power, we use two hots and a neutral. We are running three wires anyway. Seems kind of a shame we didn’t spend those three on three phases. I think three phase power is cool, and wish I had it in my house. Of course, I think a lot of off-kilter stuff…

22

You can also make a simple, no-switches AC motor with a single phase of power, but it’ll rotate backwards as readily as it does forwards. This is acceptable in a few applications (such as the turntable in a microwave), but not in most.

23

It’s a joke, you see, crafted to give you a smile.

24

We get 3 phase to the end of the street, where there is a small transformer. It used to be quite big, but is now a small box in a fenced off space with “DANGER OF DEATH” SIGNS ON IT.

From that point, the phases are split and the cable under the street carries 240 volts, single phase to all the houses.

Way back in the 60s I used to deliver catering machinery. In Sheffield, a large industrial town, there were still shops that ran on DC. This was because it came from the nearby steel works which was yet to modernise. We exported to countries all over the world with all kinds of voltages. In Australia we had motors specially wound for strange voltages like 160 volts or 339/3 phase, because the place was so far away from the generator that the voltage dropped in the line.

25

Meh. That’s what giant caps are for. :stuck_out_tongue:

26

Lots of places in the world still use 50 Hz power. I work in entertainment lighting and traveling shows have to be aware of that because some systems use the 60 Hz frequency to establish time. If you don’t convert over, you might find your moving lights moving at 5/6 speed (or 6/5, I suppose). Japan is half-and-half; one side of the country uses 50 Hz, one side 60 Hz.

27

Here’s the deal:

If power is delivered using a single sine wave at 60 Hz – like the 120 VAC at your wall receptacle – then the voltage is at exactly 0 V every 8.3 milliseconds. That’s a bummer, because 0 V = 0 power. So every 8.3 milliseconds, the power delivered by the receptacle is exactly zero. And this obviously means the power is very very low when the voltage is near zero.

This sucks all around, because you expect anything you plug into the outlet to deliver constant, uninterrupted power. So how is an appliance supposed to do this if the damn power delivered by the outlet is at or close to zero much of the time?

The most common solution is to incorporate devices into the appliance that store energy. These devices will deliver energy when the power delivered by the receptacle is at zero or close to zero. The most common devices are capacitors and inductors, and they “take up the slack” so to speak. Other appliances use thermal heat capacity (e.g. electric heater) or momentum (e.g. motor) as energy storage devices.

For some appliances, the situation sucks supremely when you initially try to start the appliance. Such as a motor. It’s *really *difficult to start a big honkin motor when the power delivered by the receptacle is at zero or close to zero much of the time. So motor designers are forced to employ clever tricks in order to get the motor spinning from a standstill.

The solution to this problem, of course, is to use a voltage at the receptacle that has *lots *of power available 100% of the time.

The most obvious solution is to use DC. But DC has a lot of drawbacks. Large DC motors really suck, since they must employ a commutator. And it’s difficult to bump up/down DC, which leads to problems when you’re trying to deliver power over long distances.

So that’s where three phase power comes in. With three phase power, there will always be a healthy voltage (in one of the three wires) that can deliver lots of power, right now. And when the power on that wire starts to decrease, you can switch over to another wire where the power is increasing. And so on and so on. In other words, when an appliance runs on three phase power, there will always be lots of good power available to the appliance, 100% of the time, at every nanosecond. This is especially advantageous with motors, since the designer doesn’t have to pull any tricks to get it started. Since power is always available, it’s a piece of cake to start.

28

With you till you got to that: are you saying that in America dryers and ovens are installed with the same heating elements in 208V installations and 240V installations?

Heating elements are normally replaceable parts, and in AUS we get appliances from all over the world: you just specify the heating element and the mains plug to make them work at local voltages.

Same question for 240V dryer motors used in 208V dryers: I would have expected the same model to be available in 208/240V versions, with different motor windings?

29

Nikola Tesla’s biggest contributions to electrical engineering were his brilliant independent invention of the rotating magnetic field, his use of that concept to invent efficient polyphase motors and generators, and his work on transformers and AC power distribution systems. He got rich off these inventions, but went bankrupt in later years pursuing the ideas that have made him so popular recently with conspiracy theorists and pseudo scientists.

30

You can order commercial equipment with elements for either 208 or 240, but outside of apartment buildings 208 is uncommon for residential use so electric ranges and clothes dryers and the like are all rated for 240 only. It will take longer to heat up, but usually you have a thermostat turning it on and off in a duty cycle, so a 240 unit run at 208 will just have the heating element on longer.

31

how is the 208 created if not from a split in the transformer?

32

You’ve all done amazing explanations of a complex que.You guys are all brlilliant electricians[ Though, much more than that title explains].You are setting yourselves up for ques. like why nuetral can be shared on two differant circuits & thousands of other questions. But, a great and learned thread ! Thanks , realmarine

33

208 is created by vector-addition of two out of the three 120-volt signals which are 120-degrees out of phase. Here’s a picture. On the right-hand side of the diagram, the red, black, and blue dots are 120-degrees apart. Imagine each of them is the output of an independent step-down transformer which converts a high-voltage signal from the generator to a 120v signal. Remember, the generator outputs three signals that are 120-degrees apart, so these three transformers will give you three 120v signals 120-degrees apart.

Measure from any phase to the center and you get a distance of 120. Apply a little Pythagoras and you’ll find the distance between any two adjacent hot wires is 208.

The moral of the story is that complex numbers are just a way of screwing around with right triangles. Hooray!

34

208V in the U.S. is pretty rare, at least for residential use. Parts of New York City still have it and there are a few other areas using it scattered across the U.S., and larger apartment buildings are usually fed by 3 phase power, but most homes have split phase 120/240V. You can usually buy 208V heating elements from the manufacturer but an off the shelf oven or dryer is going to be designed for 240V.

Commercial ovens (like for restaurants) on the other hand are often specified for 208V or 240V.

Since 208 is also common in office buildings, large office photocopiers and printers are ordered with the voltage specified as well. With some of them you just connect the electrical cord to different terminals inside the unit. Others actually have a different power transformer installed.

35

There are two types of 3 phase transformers, Y and Delta. In a Y transformer the 3 coils are arranged in the shape of the letter Y, and the center of the Y becomes your “neutral” connection. In a delta transformer, the coils are arranged in a triangle and there is no center neutral connection. The primary side of the transformer and the secondary side don’t have the be connected the same way, so you can have Y-Y, Y-Delta, Delta-Y and Delta Delta transformers. The important thing out of this is that if you want a neutral, you need a Y transformer.

This picture should help if you are having trouble picturing what a Y and a Delta transformer looks like:

In a 3 phase Y transformer, the voltage from any line to neutral is 120 volts, and the voltage from any line to any other line is 208 volts.

Y transformers are also called wye transformers or star transformers. Googling these three terms will give you lots of reading material.

36

That’s why some apartment laundry room dryers are slow to dry. S cheapskate landlord got a deal on 240V dryers, and installed them rather than getting 208V ones at a higher price.

37

Liberia probably would have been for a while. The entire electrical infrastructure got looted and destroyed so the only electricity was from people with generators. They’re now rebuilding it to European specs.

38

HOLY SHIT! You’re right! I never thought of that! Mind blown!

:head asplode smilie:

39

I run a machine shop and took it as a matter of faith that most of my lathes, mills and punches have 3 phase because it delivers more power for the bigger motors. I’m a machinist and only think about electricity when the magic smoke is leaking out. I’ve sort of wondered how it worked but never took the time to find out. Now I understand and thank you all.

So real world application, Two of the lathes are a similar size, one is 3 phase, the other is 2 phase. Now I understand why the 2 phase one takes longer to get up to speed from a dead stop.

As an aside question into electric motors, I’ve had to buy replacements over the years. For my big 8 foot engine lathe ( not relevant but a cool fact that it runs great and was made in 1906 ) I needed a reversable motor so it can run it in forward and reverse. Why did that motor cost so much more than an otherwise 3hp motor that only goes one way?

40

Was this a three phase motor? Because, as far as I know, a three phase induction motor is ALWAYS reversible. Swap any two of the three leads, and it runs the other way.

If it’s a single phase motor, in that horsepower range, it would have a starting capacitor and possibly a run capacitor, and a centrifugal switch to control the starting capacitor. You’d have to have a means to switch which of the windings the capacitors worked with. Also, single phase induction motors would have to be bigger to have the same starting torque, if starting torque was the thing necessarily constrained to be some given value.