Fault induced delayed voltage recovery from Air conditoning units?

[mbrooke]

This is more of a simple question rather than a complex one- Why do we use single phase power for residential AC units? Why were residential AC units in the past 3 phase only to go to single phase?

The way I see it is that 3 phase ACs would be vastly more efficient, last longer, generate fewer calls from failed capacitors and greatly reduce the risk of black outs. In its current state residential units may go the inverter route or a redesign to lower the point at which voltage dips cause stalling.




https://www.nerc.com/comm/PC/System...01/FIDVR Conference Presentations 9-29-09.pdf

 

[gar]

180911-1951 EDT

mbrooke:

Many residential neighborhoods only have single phase distribution.

If air conditioners are made with single phase capacitor run motors, then power factor is lower than for a three phase motor.

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[mbrooke]

180911-1951 EDT

mbrooke:

Many residential neighborhoods only have single phase distribution.

If air conditioners are made with single phase capacitor run motors, then power factor is lower than for a three phase motor.

.

I recall at one point homes having a high leg service, with a feed through delta breaker. Of course that stopped at least 40 years ago.

 

[gar]

180911-2117 EDT

In my neighborhood we have three phase primary distribution with no neutral. Probably a wye source at the substation. Several homes in this area have a wild leg transformer and a three phase air conditioner.

At our shop we have three phase wye at the street, and in the building three phase high led delta. Half a mile south our substation is being rebulit. Not sure if this supplies Michigan Stadium.

We are getting a large new substation on the U of M North Campus. The underground conduit is now in place, and wire is being pulled. I saw a trailer the other day with a huge roll of wire for this project. Roll might have been 12 ft in diameter.

The addition of the substation on North Campus will greatly improve power reliability on the Campus. This is extremely critical. Two different substations will feed North Campus to provide redundancy.

On North Campus there are several megawatt diesel backup generators. Medical research and semiconductor fabrication can not tolerate loss of power for more than a short time.

News articles indicate about 70 megawatts of new capacity is being added in the area.

.

 

[mbrooke]

180911-2117 EDT

In my neighborhood we have three phase primary distribution with no neutral. Probably a wye source at the substation. Several homes in this area have a wild leg transformer and a three phase air conditioner.

At our shop we have three phase wye at the street, and in the building three phase high led delta. Half a mile south our substation is being rebulit. Not sure if this supplies Michigan Stadium.

We are getting a large new substation on the U of M North Campus. The underground conduit is now in place, and wire is being pulled. I saw a trailer the other day with a huge roll of wire for this project. Roll might have been 12 ft in diameter.

The addition of the substation on North Campus will greatly improve power reliability on the Campus. This is extremely critical. Two different substations will feed North Campus to provide redundancy.

On North Campus there are several megawatt diesel backup generators. Medical research and semiconductor fabrication can not tolerate loss of power for more than a short time.

News articles indicate about 70 megawatts of new capacity is being added in the area.

.

Thats good to hear- we need more upgrades.


Anyone know why AC units started out as 3 phase for resi?

 

[kwired]

Thats good to hear- we need more upgrades.


Anyone know why AC units started out as 3 phase for resi?

just a guess, but because not many had them at their homes and they were primarily used for commercial applications at that time?

If you still have one of those originals, the energy efficiency alone would be worth upgrading, unless you hardly ever use it.

 

[mbrooke]

just a guess, but because not many had them at their homes and they were primarily used for commercial applications at that time?

If you still have one of those originals, the energy efficiency alone would be worth upgrading, unless you hardly ever use it.

True, but a new 3 phase motor would be more energy efficient then a new single phase. However I like your explanation- perhaps thats it. They were always commercial and central air in resi wasn't a "thing" yet.

 

[victor.cherkashi]

I believe that assumption that Single phase less energy efficient than 3 phase is wrong. keep in mind that efficiency is amount of energy that goes to motor body heat. a motor with better material and bigger wire inside will be more efficient. am I right?

Sent from my Nexus 5X using Tapatalk

 

[Julius Right]

In Europe, the standard electrical installation of residential units is provided with a single phase[230 V].
A 4 kW[1.37 ton-RT] HVAC unit draws about 27 A.
An usual utility fuse is 40/30 A. In order to start this unit you have to stop washing machine, electric oven and all high current other equipment.
In new buildings there are three phases from the beginning.
Passing the installation from a single phase to three involves separating the circuits by phases and this means placing new pipes through the walls.
If 3 phases are provided [400/231 V] and only 25/20 A fuse you may start the A-C unit without stopping anything [only 9 A rated ac unit]:weeping:.

 

[kwired]

True, but a new 3 phase motor would be more energy efficient then a new single phase. However I like your explanation- perhaps thats it. They were always commercial and central air in resi wasn't a "thing" yet.

Watts is watts, current is lower on three phase motor, but we pay for kW hours. Single or three phase motor with same output and efficiency rating will use same watts to drive same load.

Newer motor may very well be more efficient than an older motor - both three phase and single phase.

My comment on efficiency of the old unit was focused more on the efficiency rating of the refrigeration system and not so much the prime mover of the system.

 

[mbrooke]

I believe that assumption that Single phase less energy efficient than 3 phase is wrong. keep in mind that efficiency is amount of energy that goes to motor body heat. a motor with better material and bigger wire inside will be more efficient. am I right?

Sent from my Nexus 5X using Tapatalk

Yes- but I think its easier to do that with a 3 phase motor than a single phase motor. Remember that a single phase motor is basically delivering power in pulses- to a load the does its job in pulses.

In Europe, the standard electrical installation of residential units is provided with a single phase[230 V].
A 4 kW[1.37 ton-RT] HVAC unit draws about 27 A.
An usual utility fuse is 40/30 A. In order to start this unit you have to stop washing machine, electric oven and all high current other equipment.
In new buildings there are three phases from the beginning.
Passing the installation from a single phase to three involves separating the circuits by phases and this means placing new pipes through the walls.
If 3 phases are provided [400/231 V] and only 25/20 A fuse you may start the A-C unit without stopping anything [only 9 A rated ac unit]:weeping:.

Aren't most Euro AC's inverter type? Or am I thinking mini splits in China, India and Asia?

 

[mbrooke]

Watts is watts, current is lower on three phase motor, but we pay for kW hours. Single or three phase motor with same output and efficiency rating will use same watts to drive same load.

Newer motor may very well be more efficient than an older motor - both three phase and single phase.

My comment on efficiency of the old unit was focused more on the efficiency rating of the refrigeration system and not so much the prime mover of the system.

But efficiency for efficiency- would not one have more iron?

 

[romex jockey]

this>>>

Measurements confirm stalling voltage increases as function of
temperature; even higher stalling voltage for overcharged units -> 2x
increase in real power; 7x increase in reactive power
• Retrofit devices expensive: capital cost $80-100 + $120 installation
cost – currently available models may not be fully effective in
addressing issue
• Currently, protection based on thermal criteria (3-15 sec);
overcharge or loss of coolant add time to trip – but cannot trip faster
on thermal criteria
• Electronics-based solutions, which can trip faster, are currently used
in larger machines
• Higher current during restart is a secondary consideration – can be
addressed with existing solutions
• Short-cycle controls can trip rapidly, but may be “insulated” from line
voltages

what are they talking about?

~RJ~

 

[kwired]

But efficiency for efficiency- would not one have more iron?

If you put 1000 watts into a motor and regardless if it is single phase, two phase, three phase, DC ...you end up getting 900 watts of useful power out it, then it is 90 percent efficient. Such a motor would be rated as a 900 watt motor not a 1000 watt motor.

Thing with air conditioners and heat pumps is they have an efficiency rating on the entire unit, not just the compressor motor itself. Changes in design over the years have made them more efficient, some maybe in the compressor motor itself, but most the changes are more likely in the refrigeration circuit and compressor design as well.

 

[mbrooke]

this>>>



what are they talking about?

~RJ~

Basically saying this: A moderate drop in voltage for only a few cycles can cause the compressor to stall. The higher the temperature outdoors, the lower the stall voltage. This voltage sag can come from a transmission line, distribution line or other piece of equipment short circuiting.

When the compressor stalls, current draw increases dramatically. It takes 3-15 seconds for the compressor's overload relay to break this current and de-energize the compressor. Electronics could trip faster than 3 seconds. 3 phase motors take a much lower voltage to stall in the first place.


In summary a fault in the POCO's system can cause millions of compressors to stall each placing 10-15x current on the grid which puts the system as risk for voltage collapse (black out).

But the double whammy doesn't stop there...

If the system survives the initial 15 second current draw, millions of compressors dropping out all at once creates a load vs generation mismatch. Generators may go offline to compensate for the drop in load, and capacitor banks will trip off line because the voltage through out the system will soar due to the reduced current.

The grid can usually stabilize and "correct" itself at this point, but once 5-10 minutes go by compressors will start up again. Within minutes load goes up again, but this time there may be a lack of generation- in addition to voltage dropping because cap banks have tripped of line and voltage regulators have made adjustments to compensate for the drop in load prior.


If residential AC units were 3 phase and as such had a lower stall voltage, all of the above could avoided more often.

 

[mbrooke]

If you put 1000 watts into a motor and regardless if it is single phase, two phase, three phase, DC ...you end up getting 900 watts of useful power out it, then it is 90 percent efficient. Such a motor would be rated as a 900 watt motor not a 1000 watt motor.

Thing with air conditioners and heat pumps is they have an efficiency rating on the entire unit, not just the compressor motor itself. Changes in design over the years have made them more efficient, some maybe in the compressor motor itself, but most the changes are more likely in the refrigeration circuit and compressor design as well.

But doesn't a 900 watt output single phase motor weigh more than a 900 watt output 3 phase motor? I'm just curious which takes more iron.

 

[kwired]

Remember that a single phase motor is basically delivering power in pulses- to a load the does its job in pulses.

A VFD is also delivering pulses - of DC current, but they are controlled in a manner that it looks like a three phase AC wave form to the motor.

A single phase motor with a run capacitor is basically a two phase motor. That run capacitor establishes the second phase. Most single phase refrigerant compressors have a run capacitor, even the small ones like you find in a refrigerator or water cooler usually have a run capacitor.

 

[romex jockey]

Basically saying this: A moderate drop in voltage for only a few cycles can cause the compressor to stall. The higher the temperature outdoors, the lower the stall voltage. This voltage sag can come from a transmission line, distribution line or other piece of equipment short circuiting.

When the compressor stalls, current draw increases dramatically. It takes 3-15 seconds for the compressor's overload relay to break this current and de-energize the compressor. Electronics could trip faster than 3 seconds. 3 phase motors take a much lower voltage to stall in the first place.


In summary a fault in the POCO's system can cause millions of compressors to stall each placing 10-15x current on the grid which puts the system as risk for voltage collapse (black out).

But the double whammy doesn't stop there...

If the system survives the initial 15 second current draw, millions of compressors dropping out all at once creates a load vs generation mismatch. Generators may go offline to compensate for the drop in load, and capacitor banks will trip off line because the voltage through out the system will soar due to the reduced current.

The grid can usually stabilize and "correct" itself at this point, but once 5-10 minutes go by compressors will start up again. Within minutes load goes up again, but this time there may be a lack of generation- in addition to voltage dropping because cap banks have tripped of line and voltage regulators have made adjustments to compensate for the drop in load prior.


If residential AC units were 3 phase and as such had a lower stall voltage, all of the above could avoided more often.

oh ok, 'inrush' , forgive me if i'm not up on the latest trade jargon

i actually had a day here where multiple calls for compressors down came in all at once

at first i suspected the servive pole pig, only to find other structures with the same problem fed from different poles

turns out the substation 'auto' tap, went down a few notches too much

i went through a lotta cartdige fuses that day, but sold a boatload of phase/voltage protective relays



~RJ

 

[mbrooke]

oh ok, 'inrush' , forgive me if i'm not up on the latest trade jargon

Inrush is just one part of it, the big issue is locked rotor current. A locked rotor for 3-15 seconds... If you have a RLA of say 15, one could easily have 65-125+ amps. Picture how lights dim when an AC starts. And code lets you up-size AC breakers.

i actually had a day here where multiple calls for compressors down came in all at once

at first i suspected the servive pole pig, only to find other structures with the same problem fed from different poles

turns out the substation 'auto' tap, went down a few notches too much

i went through a lotta cartdige fuses that day, but sold a boatload of phase/voltage protective relays



~RJ

Sounds like the tap changer malfunctioned leading to low voltage. Most likely all those compressors could not start. Cycling on their overloads or blowing fuses. Even those that would start, current draw might have been excessive.