Huh, wait. I had misgivings about the reasons behind 3-phase, now I can articulate them:
3-phase delivers more power
Yes, but you can also get more power by delivering 3 times the single-phase power, either by 3 sets of wiring or triple the amps. If you only use single-phase, you simplify wiring. (are those high-voltage transmission lines 3-phase or single-phase?) If you want more water, you use bigger pipes, instead of installing more small pipes.
3-phase uses less wiring
Ok so in post 5, 3-phase only needs 3 big + 1 little wires. But wait! 3-phase can only provide 1.72 times the power of single-phase. To deliver this much power, single-phase would require only 1.72 × 2 = 3.44 times the wiring. I’m not sure how big the “little” wire is, so I’m not clear on this.
3-phase is used to start big motors
True, and using capacitors adds complexity, points of failure, etc. But it seems the proper solution to provide starting torque is to use gears, not to add more power.
Bolding size mine
Dammit** Snnipe 70E**!
You’ve exposed the sacred hidden secret of all power engineers everywhere!
The Power Factor Guild will be furious.
I guess this is why electrical engineering is a four year degree and not something granted on a message board.
If you bring three single phase wires to a motor, you are not tripling the power, it’s still just one single phase supply.
It is the load that determines the power, not the source. You can determine how much power is being used to run the motor by looking at the source (actually measuring voltage and amps on the wires between the source and the motor). This is what the calculations provided are doing, but the motor will determine how many amps it needs based on its mechanical load. Further, you can size your power source by knowing the horsepower of the motor you are using and what load that motor will be driving.
Perhaps it’s better stated this way; you don’t push power to the motor, the motor draws power from the source.
I don’t want to give you too much in fear of exposing more of the “hidden secrets of all power engineers everywhere.”
Well, yes, but the source still has a limit on how much power it can safely provide. Triple the amount of wiring, and you do in fact triple the power safely available, just as you’d expect.
What does that have to do with anything? I can halve the amount of wire and increase the amount of power that is safely available.
I don’t care if you bring 50 wires to the motor, the power to the motor will be the same.
This maybe true for small motors connected to the grid but as a general statement it is wrong. TheMaximum power transfer theorem states that the combined resistance (or impedance in case of AC) of the load (wires + motors) must equal the internal resistance (impedance) of the generator.
No - see MPT above. That depends on how much power the motor needs versus the generator.
No you cannot measure real power - you need a VAR meter or a Wattmeter. Measuring voltage and amps wont give you the power factor.
A good engineer should be able to explain things clearly in plain language.
Also - there are different types of AC motors for different horsepower ratings (induction, 3-phase, synchronous, etc.). Single phase AC motors are a lot like Diesel engines - their low speed torque sucks.
Most AC motors suck on their startup current too - it can be as high as 3-5 times the normal current. The high current for startup actually does very little work (low power factor) but as the wires have to carry it - they need to be sized for this current. For 3 phase motor starting - a common arrangement is to use a Wye-Stararrangement.
Another thing to note is that the speed of AC motors primarily depends on the frequency (fixed by the grid) of the supply and the number of poles (roughly like magnetic poles) in the motor. Its a nightmare (very expensive - since you need variable frequency generators) to control the speed of AC motors. For DC motors speed is easily controlled by voltage.
For motors that require a high startup torque (say a subway or crane ) - a series DC motor was typically preferred.
Sometimes generator + DC motor combination is used in place of a gear box because it takes up less space - this is usually the case for diesel locomotives.
Sorry, I was not trying to give a four year degree in electrical engineering. I was responding to the points being made and questions being asked. You know, the KISS rule.
From the point of view of what was being asked/proposed I provided what was needed as others have tried to also keep it simple……….i.e. the post requesting to neglect power factor in calculating the power for a motor.
So, good engineer, how does your explanation help the person asking if he can just hook up three single phase circuits to give the motor more power?
I certainly didn’t want to confuse the issue further by introducing imaginary power.
Well of course I don’t mean plug in a motor 3 times and BOOM! a more powerful motor. I mean, it seems you could design a single-phase motor to be as powerful as a 3-phase. Such a single-phase motor would use, at the most, twice the wiring of a normal motor.
Sorry am77494 but I had to laugh at this. I schooled and worked with brilliant engineers who couldn’t explain anything about engineering to anyone other than a fellow engineer. Something about the ability to visualize complex math and mechanics in their heads must have taken up all the room used for effective communication.
In school I was an ever so slightly above average engineering student. At times I thought about leaving the program till one Prof told me that I would be very successful in the trade not so much for my innovation but more for the fact that I could effectively communicate engineering subjects to non-engineers.
So while all those “good engineers” went on to lucrative careers many of them ended up working for me, the guy who could explain what they were doing to the corporate honchos.
If they used gears properly, they wouldn’t need such a high starting torque, would they? Surely they’re not all direct-driven. Gears would also vastly increase the range of useful RPMs of the motor.
Well what did you mean then? If my mechanical engineer says we need a 1 hp motor to complete this task, I can use a single phase 1 hp motor or a three phase 1 hp motor. Usually there is more design criteria than this but let’s not complicate things for now.
Is your question? Why would I pick one over the other?
If so, that is simple. Our facility is serviced with three phase 13.8 kV which is transformed down to 480 V and distributed throughout the plant. With everything else being equal, if I bring three phase 480 to each of my machines and use three phase motors and balance all my loads at the machine level, then the three phase distribution in my entire plant is balanced without a whole lot of extra work. If I use single phase motors at the machine level, I will have a very difficult time keeping the distribution in my plant balanced.
Maybe you can answer that for yourself. What is it you sacrifice by using gears……….you are correct most loads are geared for one reason or another but there is a give and take.
A single phase motor uses a capacitor to start. Bigger motors will require a bigger capacitor. Gears will not change that. And adding a gear box makes the system more complex requiring more maintenance.
