3-Phase Current Measuring

[jayw]

Hi,

I've spent hours trawling the internet to find the answer to what I thought was a pretty basic question but have failed thus far. So, I thought a trip to the "Horses Mouth" would yield a decent response.

I am NOT an electrician, only having covered basic principles in my Engineering degree many years ago!

So:

I work as the Maintenance Manager in a large (yet old) privately owned hotel, I have been trying to educate the owner on the benefits of reducing our power consumption in order to reduce our Electricity Bills and reduce any strain on our aging (but safe and certified) installation.

He only pays a Spark to come in when there is actually something needing repaired or replaced, so getting one to do this "2 minute job" isn't an option.

In order to emphasise my point, all I want to do is measure the load on the three phases. For the mostpart, the phases individually feed separate areas of the building, so by doing this I can demonstrate (to a degree) which are the cheapest (or least wasteful) areas to utilise.

I have access to the main supply which is a typical 4-wire "Y" supply. But, where I can access them, the insulations are not colour marked so I can't identify which is Neutral!

When measured today with my clampmeter the conductors read (approx.) 40A / 40A / 20A / 20A.

1. Do those figures make any sense to anyone?

2. Is it possible to identify Neutral? (I am NOT qualified to open anything up)

3. How do i ACTUALLY calculate the TOTAL current draw?

Thanks for your time, I appreciate i'm only coming in with a question and nothing to offer, but if you can't go to the organ-grinder who can you go to?

 

[Andy™]

if you test from the meter, the neutral will usually be at the far right

 

[Joseph Howarth]

Hi Jay, if I remember my power triangles correctly

one of the 20's will be the neutral, three phase likes to be balanced and the two 40's will induce current into the neutral

a good way to show it visually is to draw the triangle, draw a line that represents the 40A eg. 4cm then measure 45* and draw the next line 40A (4cm), 45* again then 20A (2cm). The length left between the end of the 3rd line and the start of the 1st will be 2cm = 20A that's the neutral current.

The only way to lower the neutral current is to balance the loads which is usually quite hard to do unless it's all motors (eg the UPS system in this factory has 48A, 36A and 16A respectively on each phase! due to each area of the shopfloor and offices having different amounts of computers).

As to telling which one is the neutral, it won't have a mains 'cut-out' fuse the 3 'Lines' will. :Salute

3 fuse carriers and the far right is the neutral ( no fuse carrier)

 

[binky]

might be worth getting some monitoring installed which records loads over the week. This will give a full picture of load in the hotel which will vary during the day, eg when kitchen is busy, or it gets dark and everyone turns the lights on. This in turn will give you a picture of how to reduce loads and potential energy savings. Shame your not in my neck of the woods

 

[jayw]

Thank you for the responses!

So, assuming the neutral is to the right (20A), is the total load 40+40+20=100 or would i include the 20A the Neutral is carrying? OR is there some other calculation?

I knew it would be a lot easier just asking here!

Thanks.

 

[Andy™]

total load is the total of the phases only, not the neutral

 

[Manator]

I would guess that you are after a calculation as to total usage and then alternative reduction methods?

A good start as with al,l is a measure over a period of time and not instant measurements, this will show the times that the power usage is at its optimum. Then you need to asses where this power has gone to, and how you can reduce this.

With Hotels the main power outage is always the lighting. Simply complying with HSE laws means that some lighting, recently badly designed, are still in use providing full lighting 24/7. New methods introduce dimming associated with motion detectors to provide a more efficient lighting system, some even use the computerised Dali system and are programmable from a simple remote.

The three phases should be balanced, asd the power quality should be monitored. A simple reduction from say 243v to 230v could save a hotel a fortune!

 

[ACR ELectrical]

Jay if its any help to you I regulary sub for a firm who specialise in energy monitoring and energy reduction, power quality analysis...etc, i've been involved in several reporting projects with hotels in london, many of those which we submit a report ends up saving them more than they paid for the montioring.

If your still stuck drop me a message. Joe, Binky and Man have given some good solid advice. What i will say is if the installation is old and has many improper and inapproipate alterations/additions you may find it easier to balance, however you may find many other areas where you can save a lot of energy, if you have a locally housed transformer you may be able to get the supplier to alter the windings, you may have high draw equipment running extremely inefficient, also have you got the right electrical tariff??? You'd be surprised with the areas you can make savings if investigated thoroughly.

Ultimately i think it depends on the size of your installation, the bigger the installation the bigger the potential savings. Hope this helps.