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Why DC May Replace AC (2019)

electricalindustry.ca

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Re: Why DC May Replace AC (2019)

#21

Post by naikrovek »

DoingIsLearning wrote: post #2

> In China and Europe, new cities and villages are being envisioned that will be entirely DC powered. Citation needed? Other then HVDC links or micro-generation I can't see a practical use for DC unless you are entirely off-grid.

lots of things are envisioned every day. everyone has ideas; calling it "envisioned" doesn't make those things any more feasible or realistic.

article author is either sole owner of a huge copper deposit or isn't articulating themselves very well. DC makes no sense for distribution at all.

Re: Why DC May Replace AC (2019)

#22

Post by jacquesm »

kimpeek wrote: post #3

> Direct Current (DC) electric power is an emerging disruptive technological area that has the potential to stimulate economic growth, inspire innovation, increase research and development opportunities, create jobs, and simultaneously advance environmental sustainability. Was this published in the early 1900s? There is no date and DC is definitely not emerging nor disruptive. DC won't replace AC for those who rely o…

For grid transmission over longer hauls it will definitely be the standard, for shorter runs and local distribution we will likely be using AC for a long time to come, possibly forever.

Re: Why DC May Replace AC (2019)

#23

Post by jbay808 »

kevin_thibedeau wrote: post #15
castratikron wrote: post #11

Earlier quoted context omitted.

Question is whether those devices would be better off using brushless DC or some other DC motor? Tesla uses a switched reluctance motor (basically stepper motor) instead of induction motor on their low end vehicles for example.

No. AC motors are more reliable simply by having fewer parts that can fail.

The trend is towards DC motors that are electromechanically AC motors, but supplied with AC current generated by an inverter from a DC power source. The inverter can control the motor more precisely than a fixed-frequency grid voltage can, and the DC source can be supplied by a battery.

In terms of reliability, the inverter is still an extra part that can fail, but on the other hand, it's also much less likely to blow a fuse when your motor shaft stalls on startup.

Re: Why DC May Replace AC (2019)

#24

Post by jacquesm »

bsder wrote: post #10
kimpeek wrote: post #3

> Direct Current (DC) electric power is an emerging disruptive technological area that has the potential to stimulate economic growth, inspire innovation, increase research and development opportunities, create jobs, and simultaneously advance environmental sustainability. Was this published in the early 1900s? There is no date and DC is definitely not emerging nor disruptive. DC won't replace AC for those who rely o…

This is, in fact, precisely backward. DC is great for transmitting power. You crank the voltage, use all of the copper wire (no pesky skin effect), and sync to the grid at the DC-AC conversion point. The limiting factor to DC was conversion losses. The Pacific DC Intertie needed to use gigantic, toxic mercury vapor tube diodes for the conversion for a very long time. Now that we use high voltage semiconductors, that'…

At your typical power distribution frequencies (50 Hz, 60 Hz) the skin effect is negligible.

Re: Why DC May Replace AC (2019)

#25

Post by bsder »

jacquesm wrote: post #24
bsder wrote: post #10

Earlier quoted context omitted.

This is, in fact, precisely backward. DC is great for transmitting power. You crank the voltage, use all of the copper wire (no pesky skin effect), and sync to the grid at the DC-AC conversion point. The limiting factor to DC was conversion losses. The Pacific DC Intertie needed to use gigantic, toxic mercury vapor tube diodes for the conversion for a very long time. Now that we use high voltage semiconductors, that'…

At your typical power distribution frequencies (50 Hz, 60 Hz) the skin effect is negligible.

You are simply wrong.

Skin effect at 60Hz is about 8mm. Power transmission (especially the long distance ones) conductors are normally quite a lot larger than that. Even the wires coming into your house are probably pretty close to that so there will be some effect even if it's not huge.

Re: Why DC May Replace AC (2019)

#26

Post by xxpor »

WorldMaker wrote: post #14
castratikron wrote: post #11

Earlier quoted context omitted.

Question is whether those devices would be better off using brushless DC or some other DC motor? Tesla uses a switched reluctance motor (basically stepper motor) instead of induction motor on their low end vehicles for example.

Just about every electric vehicle at this point uses DC induction motors. Many support AC fast charging with various sorts of battery hacks, but Lithium Ion batteries are kind of inherently DC when it comes to applying power to the motors. At this point DC motors generally seem to out-class their AC counterparts other than the efficiency of using the same current as walled outlets in homes. The article puts it this w…

> AC fast charging

I believe you meant DC fast charging, unless you were referring to level 2 charging.

Re: Why DC May Replace AC (2019)

#27

Post by Mizza »

InTheArena wrote: post #18

I keep wondering if there might be some value in a derivative high-efficiently USB-C PD standard (since distance, and other factors come in) for whole house. IE, could you add DC power via USB connections to a bunch of different devices with a high-efficiency power supply for all of the different connections, rather then having low-efficiency power supplies in lots of other devices?

I've always wanted something like this, and I imagine the global efficiency benefit would be monumental in the long term, despite the enormous cost of enforcing a change. Though I also have trouble wrapping my head around a USB washing machine.

Re: Why DC May Replace AC (2019)

#28

Post by WorldMaker »

xxpor wrote: post #26
WorldMaker wrote: post #14

Earlier quoted context omitted.

Just about every electric vehicle at this point uses DC induction motors. Many support AC fast charging with various sorts of battery hacks, but Lithium Ion batteries are kind of inherently DC when it comes to applying power to the motors. At this point DC motors generally seem to out-class their AC counterparts other than the efficiency of using the same current as walled outlets in homes. The article puts it this w…

> AC fast charging I believe you meant DC fast charging, unless you were referring to level 2 charging.

Yes, I was referring to Level 2+ charging. Some EVs can charge surprisingly efficiently that way through some interesting engineering hacks, but yes overall the industry has moved on to DC fast charging standards with "AC fast charging" a fallback.

Re: Why DC May Replace AC (2019)

#29

Post by candiodari »

watermelon0 wrote: post #20
kevin_thibedeau wrote: post #15

Earlier quoted context omitted.

No. AC motors are more reliable simply by having fewer parts that can fail.

I thought newer devices (washing machines, ACs, etc.) mostly used brushless DC / ECM motors, since they are more efficient and quieter? They do need controllers that use AC, so I don't think that existing devices would work on DC.

Yes but they work on feedback (if the washing machine has 5kg load, use frequency X, voltage Y, if machine has 10kg load ... they don't measure the load they measure how the motor reacts to their first guess voltage (this is called startup) and then adjust). Even comes with mechanical advantages: instead of using brakes, you just use the same motor and reverse the feedback.

Now if you want the ability to adjust frequency and voltage, at large power levels, you're talking about changing the parameters of an inverter. So what it's going to do with AC input voltage is AC -> DC -> AC* (* with different frequency and voltage, synchronized to the rotation angle changes of the drum of your washing machine). This comes with a second advantage: it's easier (and cheaper) to be tolerant to frequency and voltage changes in the wall plug, maybe even tolerant enough to have one device that works in US and EU (and ...)

You're doing this because the power plant is not going to change frequency or voltage based on how fast your washing machine is turning, but doing that makes the washing machine much more efficient.

Re: Why DC May Replace AC (2019)

#30

Post by jacquesm »

bsder wrote: post #25
jacquesm wrote: post #24

Earlier quoted context omitted.

At your typical power distribution frequencies (50 Hz, 60 Hz) the skin effect is negligible.

You are simply wrong. Skin effect at 60Hz is about 8mm. Power transmission (especially the long distance ones) conductors are normally quite a lot larger than that. Even the wires coming into your house are probably pretty close to that so there will be some effect even if it's not huge.

8 mm (in copper, which is rarely used for powerlines, if at all, it is super expensive and heavy) is huge for a single conductor, and your typical overhead powerline is concentric shells of tens of conductors. Negligible: has no practical effect on the construction. It's in the 4th significant decimal or so for a typical powerline segment, dwarfed by plain resistive losses.

You want those multiple conductor arrangements anyway to reduce the corona discharge.

If you go up to multiple KHz then it will become a problem.

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