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Power meter oddities?

Power meter oddities?

Power meter oddities?

(OP)
I'm doing some charity work in the Philippines that involved cleaning up some wiring in a small warehouse. The local L.V. supply is an odd hybrid, loosely based on the U.S.A. with 220v 60hz two wire incoming, center tapped to ground at the pole transformer.

The first surprise was to find several appliances running between pole and local ground - I assumed a cheap and cheerful way to get 110v appliances working. The next surprise was to find an autotransformer between pole and local ground, providing a 220v output for a 1 H.P. well pump.

The autotransformer is now re-cycled to provide a clean 110v supply and a decent RCB is in place. I'm still curious though, was the transformer there from general ignorance or some attempt to scam the conventional moving disk meter by presenting an asymmetric load?

Thanks

RE: Power meter oddities?

It wouldn't make any difference to a properly functioning meter. Probably just a result of ignorance or convenience.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Power meter oddities?

(OP)
Alan,

Thanks for the fast response, sorry to display my ignorance on the workings of the Wh meter. I was puzzled how the meter could handle unbalanced currents with only a two wire connection. Presumably the voltage vector is derived across the poles and the current from series winding(s) - in fact won't the meter overread in this case since it is multiplying the excess current in one leg by the full line to line voltage?

I'll draw a veil over the rat's nest of underated cables and yards of decaying black pvc taped twisted splices. No wonder electrical fires are so common here.

Regards

RE: Power meter oddities?

It is funny that you brought this up now. I have been doing work to find out why modern kWh meters (some of them) record very high consumption when certain loads are used.

I have then seen that the good old rotating disk (Ferraris) is very difficult to fool, simply because it is built to react to the instantaneous product of current and voltage. It doesn't matter whatever you do to the load, the meter will still faithfully record power. The only way to fool it seems to be to have a DC component in the load so that a braking DC field slows down the disk.

The modern meters are NOT tested with all types of loads and I think that is because the standards still reflect the Ferraris meter's use of basic natural laws for its functioning (which made them work well with thyristor control and other very non-linear loads) and the standards people are a bit slow to react to new technology.

Modern meters do not rely on natural laws at all, but on the designer's thinking - and I am not so sure he knows about all loads there are in everyday life. Have a look at thread238-266145: Energy meter with fast cycling load to learn a little more about a meter's daily life.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Power meter oddities?

North American revolving disk meters, for 120:240 Volt service (and the electronic meters that are replacing them) have a current coil in each line (240V). The potential coil is across the lines at 240 volts. The current coils are each 50% coils so any line to line load current is measured coming and going. There is no connection to the neutral conductor. The assumption is made that the line to neutral voltages are balanced.
Suppose that we have a heavy, line to neutral load (120V) that is causing a 8V voltage drop between the transformer and the meter. If we assume equal impedance line and neutral conductors, that will be 4 Volts drop on the line and 4 Volts drop on the neutral.
The potential coil of the meter will see 240V - 4V = 236V The meter will assume that the load voltage is 50% of 236 or 118 Volts.
The actual voltage at the load will be (240/2) - 8V = 112V
In this instance the meter will read 118V/112V = 1.0536 or about 5% high.
If the neutral conductor is undersized the error will be greater.
If you don't like it just balance your load.
By the way, there are a number of three phase metering schemes that make voltage assumptions.  

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Power meter oddities?

The meters we see (northern Europe) are "three phase with neutral" or "four wire", which I think is more realistic.

Whatever meter type there is, it is a lot better than other arrangements. We once paid the utility for each lamp installed. No metering at all.
 

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Power meter oddities?

(OP)
Waross,

Thank you for the clear explanation.

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