Submetering 3 phase electric motors/ovens/compressors

Calahonda52

What sort of errors might I expect if instead of sub metering each of the 3 phases, just one is done and save on the cost of the meters and CT's which are typically 100 and 20 euros each, before installation costs.

All I need is kWhs, don't need PF, Amps, etc

Thanks

Supplementary questions if I may.
How is single phase extracted from an incoming 3 phase supply: am told the esb meter is on the main incomer, MIC kVA on bill is 2,500

Whats the difference between a current transducer and a current transformer?

ShamFeen

Not too sure why you just want to meter one phase, surely for practical reasons, billing, tariff and monitoring you are better of with all monitored?

I'll have a go at the transducer and transformer one. CT metering uses a small current to measure a much larger one. The meters come in a wide range of ratios and can be located quite a distance away from the mains.

I've only heard of a transducer a few times. I believe these are solid state devices with some for of circuitry, electronics pcbs perhaps?, which convery the input ac to dc mA. I'm sure someone will correct me on this but I believ the transducer to be a more expensive item as they are not as popular in Ireland

2011

ShamFeen wrote: »

I've only heard of a transducer a few times. I believe these are solid state devices with some for of circuitry, electronics pcbs perhaps?

Correct.

I am using these current transducers at the moment. They provide a 4 to 20mA output that can be connected to a control system.

I'm sure someone will correct me on this but I believ the transducer to be a more expensive item as they are not as popular in Ireland

I would think that you are correct. Current transformers are just that a transformer with no intelligence so they are quite cheap. Where as a current transducer is a more complex animal that has a level of intelligence with offset and span adjust so it is likely to be more expensive.

Calahonda52

Thanks for the replies.
The (perhaps daft) thinking re measuring the single phase on say an oven was that using a shunt meter it "measures" each phase separately so I thought I could save on the meters which are c STG 80 for a basic 300 model from here:
http://www.ndmeter.co.uk/panel-mount-metering
or STG 130 for the 350 model
there is also the issue of either finding room on the DIN rail or cutting the doors for the panel versions.

It appears I am wrong, am told I need a separate meter for each phase.

Re the Current Transducer/transformers: if I use the abbreviation CT, how will the reader know which one I mean or is the transformer much more common?

ted1

Each phase may face completely different loads and so have no relationship with each other.
Transducers are quite common these days and are commonly called clip on CTs which are based on rogowski coils. The output is usually a mV signal.

They have loads of benefits over convential CTs the primary one being that they can be fitted much easier and safer. You can leave then open circuit and fit them live without the need to disconnect busbars etc to fit them.

ted1

Calahonda52 wrote: »

Thanks for the replies.
The (perhaps daft) thinking re measuring the single phase on say an oven was that using a shunt meter it "measures" each phase separately so I thought I could save on the meters which are c STG 80 for a basic 300 model from here:
http://www.ndmeter.co.uk/panel-mount-metering
or STG 130 for the 350 model
there is also the issue of either finding room on the DIN rail or cutting the doors for the panel versions.

It appears I am wrong, am told I need a separate meter for each phase.

Re the Current Transducer/transformers: if I use the abbreviation CT, how will the reader know which one I mean or is the transformer much more common?

You don't need a separate Meter.
An inline meter like one of the ones here may do the job without CTs.
http://www.dhmeters.com/Din-Rail-Electricity-Meter.php


Hanley energy also do open core CTs that work with Carla gavazzi meters.

Calahonda52

ted1 wrote: »

You don't need a separate Meter.
An inline meter like one of the ones here may do the job without CTs.
http://www.dhmeters.com/Din-Rail-Electricity-Meter.php


Hanley energy also do open core CTs that work with Carla gavazzi meters.

Agreed again except they are in line: the project has kit that draw 1,600 to 2,000 amps on start up so again I want to work with the real kit.
Thanks for the links and time taken to reply, both here and in other thread, its all learning for moi:D

Calahonda52

Calahonda52 wrote: »

Whats the difference between a current transducer and a current transformer?

The transducer uses the Hall Effect and so can measure DC.

The transformer uses inductance and can only see AC.

Neither are precision at the lower end of the scale and can pick up stray fields.

A shunt is a low impedance resistor in line and monitors current from the changing resistance across itself. I = V/R.
The datasheet will state the accuracy your link is class 1 which is 1%
https://en.wikipedia.org/wiki/Class_of_accuracy_in_electrical_measurements

Shunts can be precise but also can lose accuracy due to heat while operating at the load limit.

2000A shunt :eek:
I'd stick with a CT