Electrical Theory - Interaction between loads

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

simonplights

Member
Joined
Jan 20, 2015
Messages
5
Reaction score
0
Location
Newmarket, Suffolk
Hello,

Sorry if this isn't the right forum - I had a look around and this seemed to be the closest match. Apologies, it's a bit of an essay but may be of use to others...maybe!

Haven't been here long - I posted more about me in the introduction forum, but in short I've been working as a touring lighting technician aka 'roadie' for 10-odd years and want to further improve my knowledge of electrical theory now that I've stopped touring and work at a rental company. Hoping to embark on the 2365 soon, so I have no actual qualifications but a lot of hands-on experience.

So, on to my question. I'm trying to get straight in my head the mechanisms behind which some loads can affect the power delivered to other loads, and where in the electrical system the waveforms are affected moreso and less so, and why.

A scenario - a supply (Vs) feeding a household with a SMPS laptop PSU upstairs (let's say voltage across input terminals 'V1') and a hifi audio amplifier in the lounge 'V2'. I often hear that things like SMPS power supplies 'send rubbish back down the mains' etc. The concept of this is intuitive on a simple level - the choppy switching operation of the power supply is vastly different to linear loads, and the strange harmonics and power factors that result aren't a surprise. What I can't fully understand is how these 'go back down the mains' and affect other devices, such as the hifi amplifier. It doesn't seem right that the power delivered to the system from the actual mains is affected by a small load nearby, especially when the affected loads are connected directly to the mains by (relatively) low impedance wiring. Because the voltage should come in from the supply as a (near enough) decent sine wave, because that point is electrically common with the mains input of our hifi amplifier - why should the voltage waveform look anything different? I understand that reactive power will transfer power back to the supply from reactive loads, but why would that affect other loads in the system? I.E. why should 'V2' be affected by 'V1'?

Quite often in our industry - venues will have a separate 'sound power' supply. There will be a 400A Powerlock supply on Stage Right for Lighting, and a 63A 1 PH CEE supply on Stage Left for Audio. However, both of these often go back to a single mains supply at the entrance to the venue. It is assumed that if the Audio power was to be taken from the same supply as the lighting dimmers, the Thyristor switching in the high power dimmers affecting the power factor would have an impact on the performance of the audio amplifiers. Moving the audio equipment on to it's own supply, a guess what you would call a radial circuit, is taken to remove this interaction. Similarly, I don't expect my next door neighbour's potentially inductive white goods to affect my audio amplifier in the lounge, even though we both take a supply from the substation behind the houses.

I understand in some of those examples that earthing is a big thing, particularly where audio and ground loops are involved, but the scope of my question solely relates to the actual voltage/current waveforms being distorted throughout a system and exactly how it happens.

Solution

So, rather than just ask for help straight away, I've drawn the attached picture and tried to have a think through this and come up with some logical idea to explain it to myself. I have this possible scenario, which may be partially or totally wrong:

Because the system isn't actually made up of 'common' connections between supply and loads, and instead made up of multiple resistive wire elements, the system acts like one big network of voltage dividers. Noisy loads draw irregular current waveforms, and ohms law dictates that these will drop a corresponding irregular voltage across the impedances they are in series with (the L and N lines feeding the device). IF an affected load has that wire impedance in common with the offending load, the irregular voltage drop over the wire impedance will subsequently affect the voltage delivered to the affected device.

Therefore, if I wanted a clean feed to my audio amplifier (or sensitive medical instrument etc etc) that is less affected by a SMPS in the same network, I could run a direct radial circuit from the supply. This means that of all the different impedances in the system supplying the SMPS (supply cable from substation, cable in to house, ring main etc), only the ones common to both the amplifier and the SMPS would drop a voltage that affect the amplifier.

Therefore also, because the mains wiring feeding the house must have some form of impedance, if there is a given load in the house with a poor power factor or inducing harmonics etc, because that distorted current is being drawn through the common impedance of the supply cable, some of that current waveform will drop a corresponding voltage over the cable supplying the house and thus affect every load in the house in a minor way.

Therefore again, because the wiring in modern installations is (hopefully) as low impedance as possible for safety and reduction of losses, interaction between loads is reduced because the voltage drop over the wiring in the system is smaller - therefore imparts less of an effect on the mains delivered to other loads, because the distorted current waveforms are 'dropping' less distorted voltage?

Any distorted current waveforms from noisy loads are dropping corresponding voltages over various system impedances, which are then superimposed on the mains waveform coming from the supply. Moving to a different house means that the only thing common is the cabling within the substation, which is much lower impedance compared to the smaller cable (dropping more voltage) in the walls of a different house.

Am I on the right track here?

Much appreciate any thoughts anyone has.

Thanks,

Simon

Another thought...reading up on power factor, I'm aware of lagging and leading current waveforms and the physical reasons why. Am I right in thinking there is no way a load could offset the phase of the voltage waveform? Voltage will always be 0deg and the current waveform will just shift back and forth depending on flavour and magnitude of reactance?

View attachment Multiple loads.tiff

 
Last edited by a moderator:


Thanks Tony,

I did actually spend several hours Googling power quality, power factor in systems etc, but couldn't find an article that answered my question in the detail I wanted. Articles often went to great lengths to discuss a supply supplying a given single load - with all of the resulting reactance, chopping, harmonics etc that appear over the input terminals of that device, which all made sense, but then stopping short of discussing the actual mechanism by which this affected other loads within the system, often saying something like "such harmonics can affect the power delivered to other sensitive medical/instrumentation equipment in the electrical system" - yes, but how? And rather than just make an assumption as to what I believe is the case, I like to check with those more experienced that I'm on the right track.

I thought it would be somewhat more appreciated that someone, especially an electrical newbie, had tried to research and understand a possible mechanism before posting, rather than just shout 'help' without trying to think first. I did have a read of some of the links from your search and I'm fairly settled that I've got this about right.

Thanks,

Simon

 
When you mentioned superimposing other waveforms on the mains supply this is the principle by which certain pieces of equipment are used, 2 examples are the plug in units that send the broadband down the mains, and an even older one, the plug in baby monitors, we had a set of these and it led to an interesting if embarrassing incident!

At the time my then partner and I lived in a terraced house, we had 2 small children and used one of these plug in intercoms when the kids were in bed. One night we were sitting watching TV, the kids were asleep, over the intercom we'd hear the occasional cough, or other small noise from one of the kids as they slept.

Later on we began to hear a strange noise, a sort of panting, gasping sound, she crept upstairs to check on the kids, they were fine.the strange sounds continued, then we heard a voice, a woman's voice, "oh yes. oh yes,don't stop, keep going, followed by her calling out a mans name, suddenly the penny dropped, we were hearing one of our neighbours, they were obviously in the throes of passion!

We turned the unit off, and the following morning I was talking to them on the front, "did you have a good night last night" I asked, with a knowing grin, "oh yes", she replied "my mum had the baby overnight"

I asked where the baby normally slept, and she said in a cot in their bedroom, I then asked if they had a baby monitor "oh yes, we've got one of those new plug in one's" she replied.

"oh, that will explain the strange noises last night" I chuckled.

The woman was mortified, I invited her in, switched on our monitor, and you could hear her husband, I think he was singing to himself in their bedroom! "it's ok I unplugged it as soon as I heard the noises last night" I explained. It turned out they were on the same phase as us and the unit was so powerful it's signal was being picked up by our unit, I think they got rid of theirs after that.

I've seen similar things happen in hospital too, a hospital I was working in was being refurbed, they had a new scanner installed and it worked fine, then one day it started to play up, they tested it but couldn't find a fault, the manufacturers engineers came out, they drew a blank too, it was completely random.

In the end it was eventually traced to a piece of welding equipment that was being used in the refurb,they swapped it and the problem disappeared.

It's interesting if you connect an oscilloscope to a supply, then watch the waveform as certain pieces of equipment are switched on

 
Top