Switching from AC grids to DC grids

[krish2487]

Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.


PFC, I agree is a problem, but then again we could dispense with the PFC altogether if the transmission is DC. No more leading and lagging vectors, no real and apparent power.


As I said before, it is as good an idea as any (also as dumb).


Also, we are not really trading anything here in the name of better,faster,cheaper at all. We are just moving AC generation downstream from the power station to the Point of Load. Each type of transmission comes with it own pros and cons.


Quote from: sync on Today at 09:36:24 AM

>Quote from: krish2487 on Today at 09:06:48 AM

Since most of the household appliances are designed for 340V+ DC (in case of 220V AC).
Dispense with the input bridge rectifier (or keep it) and switch to DC grid.

My central heating, dish washer, washing machine, refrigerator, fans, audio amp, turn tables, alarm clocks and most of my test gear is definitive AC only. That's way over 20000€ of devices. And probably the SMPS powered gear doesn't work on DC as well because the PFC doesn't like it.
That's the real problem for a transition to DC only. Backwards compatibility. It doesn't make sense to invest tens of thousands for the installation and new devices to save a few hundreds.

[djacobow]

In my proprietary system, I use bi-directional communication to determine the voltage/current that will be delivered. It takes all human decisions out, they just plug in and go. My design is for a specific commercial use, but the concept could work in a house.

This is also how EVs charge, J1772 and similar.

Once you've gone to this point, you might as well take it a bit further, like letting someone plug in a generator or PV panel and that, too will negotiate a voltage and current (and, obviously, a direction). This is part of what the idea of "nano grids" are about: http://nordman.lbl.gov/docs/icdcm2015nordmanLPD.pdf. Basically, letting loads and sources work out between each other what's going to happen over a circuit.

And once you have that negotiation, it becomes interesting to put network control on all the outlets, and maybe even allow devices to work cooperatively to maximize the use of a circuit. For example, if you plug in a blow dryer and a toaster on the same circuit, they will negotiate who gets to run and who doesn't. (That was a particularly bad example, as I don't think many people make toast in the bathroom, but some might dry their hair in the kitchen.)

But my point is that intelligence is nice, but it would be semi-tragic in my mind to come up with a ubiquitous intelligent power transfer negotiation standard that did not allow for more features.

Of course, I don't see how any of this militates for either DC or AC, as it can be done either way, though arbitrary voltage conversions for AC are a bit more expense (capital and efficiency costs).

In my mind, the decision for AC or DC is not so much about "what is best" but about "winners and losers." On a DC system devices with power supplies will save a few pennies by eliminating a diode bridge and some filtering (winners). Resistive heating devices won't care (neutral). Large induction motors will require VFDs to synthesize their local AC drive (losers). Whether such a conversion looks appealing to you has a lot to do with who you are and what your main loads are. And of course, if you make stuff that would need to be replaced, you probably would be *thrilled*. If you own stuff that needed to be replaced, much less thrilling.

That said, I'm not happy with the idea of low voltage DC like 48V. Any savings you make in devices and maybe conversion efficiency will be paid back in i2c losses in the wall and/or more copper.

[sync]

Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.

My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.

Also, we are not really trading anything here in the name of better,faster,cheaper at all. We are just moving AC generation downstream from the power station to the Point of Load. Each type of transmission comes with it own pros and cons.

The generators on the big power plans are AC and I think they will stay AC. The same for wind and water energy. I think only PV is DC at the source. For long transmission lines DC makes sense. But that's the problem of the companies which runs them. They can do the calculation.

[djacobow]

Again, most of them (modernish in the last 5-7 years) are all inverter driven which means that you have AC-DC-AC conversion happening internally,
Dishwashers, AC, washing machine, refrigerators. The exceptions I see to it are fans.

My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.

Yes, this is total rubbish. My late model dishwasher has 12VAC motors for all pumps. The last top loading washing machine also had an 120VAC induction motor. Our new front-loading washing machine has a VFD-driven motor, but the accompanying gas dryer does not.

I would not be surprised to see some high-end models using VFD driven motors, to be quieter or whatever, but it's def not mainstream in, in North America, at least.

[rx8pilot]

Large induction motors will require VFDs to synthesize their local AC drive (losers).

VFD do have some excellent benefits which is why they are so common now. My AC units have them, my previous business had VFD air compressors, CNC machines, etc. There is no inrush current as they slowly ramp to speed. They can also respond only the demand at that moment. For example, had a 20HP air compressor with an AC 3ph motor, it ran continuously because it was so hard on the motor and drive train to start/stop. When the air was not needed, it unloads the pump but keeps turning all day long.

That was replaced with a VFD based compressor that turned only as fast as was needed to keep up with demand. We saved something like $600/mo on electricity just from that change. My AC units are the same, distributed VFD blowers that only move what is needed which maintains a constant temp and is very easy on the mechanics and associated electric supply. Again, my washing machine is VFD as well. Dishwasher too.

It's not necessarily an argument for an all DC infrastructure, but more and more VFD's are coming to the most normal of appliances.

[sync]

My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.

Yes, this is total rubbish.

Why is that rubbish?

[Brumby]

Like we save energy by boiling kettles slower?

No by using low power kettles.

Energy is a function of power and time. Using a lower power kettle will use at least as much (probably more) energy as a higher power kettle.

It will be more.

Using lower powered kettles isn't good thinking.  Unless the kettle contains the water within a environment that is much better insulated than present designs, the longer heating time will result in greater losses through heat escaping into it's surrounds.  A far, FAR better idea is to stop people boiling a litre of water to make a single cuppa.


I also have deep reservations about the conductor size required for any low voltage supply.  Cost is the main one - but I also wonder how much fun it would be to wrangle in a domestic wall outlet.

As for the power negotiation scheme idea, to truly solve the power balancing issue, each outlet would ideally need to be individually wired back to the controller.

[djacobow]

My dishwasher and washing machine are not that old but using 230V AC motors for the pumps. Also the relay for the heating element will destroyed by DC mains. And if they don't use a relay for it then they will using a triac.

Yes, this is total rubbish.

Why is that rubbish?

Sorry, I must have quoted the wrong thing. What I was rejecting was previous poster's idea that modern appliances all are moving to inverter driven motors. Some are, most aren't. At least here.

[djacobow]

Large induction motors will require VFDs to synthesize their local AC drive (losers).

VFD do have some excellent benefits which is why they are so common now.

This is interesting. I suspect there is an inflection point where VFD becomes more attractive. For example, a motor that runs 24/7 and needs to adjust smoothly to varying loads: very attractive. A motor that runs for 20 minutes once a week (eg, household vacuum) crappy but cheap universal motor (another bad example as those will run on DC -- I'm a fountain of bad examples today) will remain attractive for awhile. But over time, as the controls become cheap, maybe the cutoff point will move to more and more "brushless dc" as they used to say.

The only VFD in my house at this time is our front-loading washer, which certainly does make good use of it, asking quite a bit of the motor: low and high speeds and everything in between, lots of torque at times, etc.

[NiHaoMike]

The new heat pump dryers (in the US, of course) all seem to run on 120V.

I think that for electronics, pulsed DC at 170-340V would be a good compromise between AC and DC. It solves the arcing issue with DC and conversion from DC is much simpler. For large motor loads, PoL inverters (VFDs) are a good solution.

[vk6zgo]

 Industrial 3 phase motors are highly efficient,need hardly any servicing,& are quite likely to remain the device of choice for heavy duty applications where fine speed control is not necessary.

Such motors may well be in use for 30 plus years,so many now in service have no need of replacement with totally new systems for decades..

Even if a motor fails,usually it is only the motor which needs to be replaced (or repaired)---all the other circuitry can be re-used.
Replacement with a VFD controlled motor,driven by a dedicated DC line makes no economic sense,either in direct costs.or in Environmental impact.

In use,losses in a single DC feed  are also going to be greater than a 3 phase system,where resistive loss is distributed over 3 lines.


Our front loader washing machine probably has a VFD motor.

It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.

The stupid thing uses a resistive element to heat the cold water.
It saves water,though!

Brainless design like this cast large doubts in my mind about the abilities of the current generation of appliance designers to address the possibilities of totally new household power supplies.
 

[vk6zgo]

Large induction motors will require VFDs to synthesize their local AC drive (losers).

VFD do have some excellent benefits which is why they are so common now.

This is interesting. I suspect there is an inflection point where VFD becomes more attractive. For example, a motor that runs 24/7 and needs to adjust smoothly to varying loads: very attractive. A motor that runs for 20 minutes once a week (eg, household vacuum) crappy but cheap universal motor (another bad example as those will run on DC -- I'm a fountain of bad examples today) will remain attractive for awhile. But over time, as the controls become cheap, maybe the cutoff point will move to more and more "brushless dc" as they used to say.

The only VFD in my house at this time is our front-loading washer, which certainly does make good use of it, asking quite a bit of the motor: low and high speeds and everything in between, lots of torque at times, etc.

The place where VFD motors have in fact,made inroads in home appliances is in just those that normally used "universal" motors.

[Monkeh]

It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.

The stupid thing uses a resistive element to heat the cold water.
It saves water,though!

That's done for the ability to reheat water and more reliably control temperature.

It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.

[vk6zgo]

It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.

The stupid thing uses a resistive element to heat the cold water.
It saves water,though!

That's done for the ability to reheat water and more reliably control temperature.

It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.

No, it's done just be cheap,& because many countries don't have hot water on tap in the laundry.

[NiHaoMike]

It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.

The stupid thing uses a resistive element to heat the cold water.
It saves water,though!

That's done for the ability to reheat water and more reliably control temperature.

It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.

Install a thermostatic mixing valve and set it to whatever temperature is optimal for the detergent you're using.

[HAL-42b]

Motors below 5~10 HP can run perfectly fine from DC connected VFDs. In that sense there is little reason to retain AC in domestic settings. Actually a DC VFD could be even cheaper compared to an AC counterpart.

In industrial settings everyone is installing VFDs anyway because they like the control and the efficiency.

[mamalala]

A 30A dryer uses the same plug as a 10A kettle?

In the US, we have 15A, 20A 120v plugs in houses but mainly 15A. Then there are 30A, 40A 220v types for dryers and stoves. At least those are the common ones seen in houses. 3-4 different types.

Our dryers are not 30A. Keep in mind that we have 230V instead of only 115V, so to get the same wattage, we need less current. Stoves/ovens usually are connected to a 3-phase supply. Although the heating elements run on 230V, using 3-phase the load is shared amongs the phases (if more than one element is turned on).

Speaking of 3-phase, this another issue with the idea of a multi-line DC system. When you have a 3-phase supply, and connect eqal loads between all three phases and neutral, there is no current flowing on the neutral anymore. Only if there are unequal loads, a bit of current will flow there. Due to that the neutral conductor can have the same size as the phase conductors. On a DC system, you would need a much larger return conductor if you want to distribute several voltages. Or each rail gets it's own return.

Greetings,

Chris

[Someone]

It cost more to run than the old top loader------not because of the motor control system,but because the idiots didn't include a hot water inlet to use cheap solar heated water.

The stupid thing uses a resistive element to heat the cold water.
It saves water,though!

That's done for the ability to reheat water and more reliably control temperature.

It is a bit of a downside on electricity costs though. Win some, lose some. I'd like to see BOTH - take hot water in and mix it, then reheat.

No, it's done just be cheap,& because many countries don't have hot water on tap in the laundry.

As I recall having no hot input also improves the water "star" rating, but thats all hidden behind paywalls around Australian standards so I cant check easily.

[sync]

I DO like to see distribution voltage become DC SELV for lighting, electronics, plus conventional AC 100~240V for motors and heating elements. AC lines are deeply buried in walls and semi-permanently connected to prevent shocking, while DC lines comes from sockets, poses virtually no hazards if properly fused.

Here the AC lines are buried in the wall for appearance. It's ok to put the cables on the wall. But people don't like to see them. It would be the same for DC lines. They will be buried too. It adds additional costs with very little gain. I see a niche for DC lighting in large non-domestic buildings. But not at home.

[Jeroen3]

DC would be better for lighting. Since LED's are the best when run on DC. No annoying flickering or tricky drivers pumping harmonics.

[sync]

DC would be better for lighting. Since LED's are the best when run on DC. No annoying flickering or tricky drivers pumping harmonics.

Sure. But people won't invest lot of money for a domestic lighting DC distribution. They just buying LED lamps which don't have that problem.

[NiHaoMike]

Here the AC lines are buried in the wall for appearance. It's ok to put the cables on the wall. But people don't like to see them. It would be the same for DC lines. They will be buried too. It adds additional costs with very little gain. I see a niche for DC lighting in large non-domestic buildings. But not at home.

Thin film cable (similar to flex PCB type stuff) can be made the same color as the wall so it's nearly invisible. Great for adding lights where there isn't existing wiring. They use low voltage as the insulation is very thin.

One place where a "nanogrid" would be very easy to implement is a home network center. Most home network equipment already run on 12V so it's just a matter of wiring them to a 12V battery, then add a float charger and possibly some alternative energy sources.

[Papal_Stick]

The old yellow lights bulbs are lone gone

No they're not. I do all my work - soldering, design or even reading a book in a workspace illuminated by a good old incandescent. If you prefer LEDs or CFLs - fine but don't force your flawed solutions onto others.

[SteveyG]

eFuses are the same thing, solid state and easy. Much easier than the mechanical breakers we use today. At household and most commercial current levels, switching and managing DC is easy.

The wiring regulations mandate physical separation. eFuses will never be suitable - how would an electrician lock off such a device for safe isolation?

[Cerebus]

The wiring regulations mandate physical separation. eFuses will never be suitable - how would an electrician lock off such a device for safe isolation?

Based on most of the sparks I've known - with a 4lb club hammer. 

I'm kind of amazed that this whole thread has run so far without anyone saying "I²R losses" along with "Have you seen the proce of copper recently?".