FIELD OF THE INVENTION
The present invention relates to a non-invasive system for generating constant and stable alternating current output for calibrating energy meters. The technique is in full conformance with the tenets of lean manufacturing principles facilitating single ptete flow, quick turn-around times and Single Minute exchange of Dies (SMED).
BACKGROUND OF THE INVENTION
In existing systems one approach is using an ordinary current source controlled |y manually tunable variacs in series with a reference standard meter. The operator makes an approximate adjustment to generate the required loads from the current source and feeds it to the meter under test (MUT) which is connected in series with the reference standard meter. The test output of the meter is then used to compare the reference standard meter and the error detected is then fed back to the MUT for correction. After a few iterations the MUT is calibrated successfully.
The technique has an obvious drawback. As the current is not stable, the homogeneity of the test output is a function of the variance of the current and the gating time. As the response time for the reference standard meter and the MUT is different, process variation is large.
Another approach to the same end is to use a sophisticated and stable current source.. Although these sources give stable and precise current outputs, they are expensive arid cost upwards of a million Rupees. As these are based on complex power electronics, the maintainability and serviceability is also a question. Also, being expensive and large,, these are not conducive to the low cost, high volume manufacturing.
In order to produce low cost energy meters in high volume quantities using lean principles, a radical approach to generate a constant and stable current output was exigent, which could not be fulfilled using the existing methods described above.
There was therefore, a need for a system that gives an extremely stable current output with minimal circuitry, is reliable and low cost.
SUMMARY Of THE INVENTION
The main object of the present invention therefore, to provide a constant and stable alternating current output for reliable calibration of low cost energy meters with minimum circuitry and low cost. The system should also be serviceable and small in size. As the variability of the current is reduced to a minimum, the output is independent of the gating time, thus reducing process variability.
In the non-invasive current control of the present invention, the current output Is regulated and stabilized by modifying the load side impedance and impedance reflection on the primary side due to secondary side impedance variation. The secondary side' variation is governed using a simple ladder network controlled by a microcontroller and a current measurement IC connected in a feedback loop.
As the primary side circuit is not altered or modified or even tapped, and the secondary side is also acted upon using induction techniques only, the system is highly reliable and does not suffer from the maintainability problems caused due to heating and breakdown of current handling power electronics.
In a preferred embodiment the present invention provides a non-invasive system for generating constant and stable alternating current output for calibrating energy meters, comprising: a constant AC voltage source; a transformer for obtaining required output current at predetermined voltage; a manual range selector switch for selecting required calibration current; a loading transformer connected to the output of said selector switch; a current transformer arranged between the output of said loading transformer and a resister ladder for reflecting the resistance of said resister ladder in the primary of said loading transformer; and a micro controller for generating a feedback signal from the measured calibration current and for feeding said signal to said resister ladder to digially' control the effective resistance to change, causing a change in the current flowing in the current loop, which can be measured by a current sensing circuit.
BREEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the figures of the accompanying drawings in which
Figure 1 illustrates the concept of equivalent resistance seen from the primary side of a transformer.
Figure 2 shows in block diagram form the system of the present invention. Figure 3 shows the system of the invention in operation.
The invention, non-invasive current control, is a technique to regulate and stabilize the current output by modifying the load side impedance using Lenz's Law and impedance reflection on the primary side due to secondary side impedance variation. The secondary side variation is governed using a simple ladder network controlled by a microcontroller and a current measurement IC connected in a feedback loop.
When a resister is connected to a transformer secondary side, an equivalent resister is "seen" from the primary side. Figure 1 shows this typical example, where the turn ratio is normalized to the primary, thereby reducing the number of primary turns to 1 arid secondary turns reduced by a factor of the primary turns.

(Formula Removed)
Using this phenomenon, we can regulate RL and get a very fine grained adjustment; of the primary current using the secondary side, as the least count is reduced by tie division by the square of the number of secondary windings normalized to the nurnberiof primary windings.
The various functional blocks of the system illustrated in Figure 2 can now be descrlbed as follows.
The constant AC voltage source 1 is a common off-the shelf UPS and a CVT to generate a constant voltage at a required frequency (50 or 60HZ). This voltage is fine tuned iif required, using an AUTO transformer 2 it is then stepped down using a transformer 3 with a multi-taped secondary to get the outputs of optionally 0-42V, 0-52V, 0-62V or 0-72V and 2 Amps / l0Amps.
A phase compensation network 4 is provided which consists of an RC ladder t0 compensate for the phase errors introduced by the two transformers described in the earlier section. This is followed by a manual range selector switch 5 to select the calibration current required, for multi point calibration.
The currents after the range selector, although fairly in range now, have a very poor regulation and stability. The currents are given to a loading transformer 6 of IP: 0-240VAC and OP:0-3VAC, 100 Amps.
The current from the output of the loading transformer 6 is then looped through the primary of a small current transformer CT as shown in Figure 2. The secondary of the CT has a resister ladder 7 the resistance of which is reflected in the primary as shown in Equation 1. The current is measured using a current sensing circuit 8 comprising a current measurement IC with a 24 bit SA ADC connected in series with the current loop. The output of the IC is then used by the microcontroller 9 to generate a feedback signal FS to the resister ladder 7, the effective resistance of which is digitally controlled. The changed resistance causes a change in the current flowing in the current loop which is again measured by the current measurement IC. The closed loop with a feedback entails that the currents are stabilised and controlled.
The resister ladder 7 is standard DAC controlled using 9 I / Os to have the adjustment granularity of 512 states within the variation range, giving excellent stability.
The invention as described promises substantial financial impact on the low cost high volume manufacturing process of energy meters. The financial benefit is addition to the quality benefits derived from lean manufacturing principle adapted in the present invention.

WE CLAIM
1. A non-invasive system for generating constant and stable alternating currteit output for calibrating energy meters, comprising:
- a constant AC voltage source;
- a transformer for obtaining required output current at predeterrnif^d voltage;
- a manual range selector switch for selecting required calibration current;
- a loading transformer connected to the output of said selector switch;
- a current transformer arranged between the output of said loadirfg transformer and a resister ladder for reflecting the resistance of said resistir ladder in the primary of said loading transformer; and
- a micro controller for generating a feedback signal from the measured calibration current and for feeding said signal to said resister ladder to digitally control the effective resistance to change, causing a change in the current flowing in the current loop, which can be measured by a current sensing circuit.

2. The system as claimed in claim 1, wherein a phase compensation network- is provided between said transformer and said manual range selector switch to compensate for the phase errors introduced by the transformer and the auto-transformer.
3. The system as claimed in claim 1, wherein said current sensing circuit comprises a current measurement IC with a 24 bit SA ADC connected in series with the current loop.
4. The system as claimed in claim 1, wherein said resistor ladder is standard DAC controlled using 9 I / Os to have the adjustment granularity of 512 states wthin the variation range.
5. A non-invasive system for generating constant and stable alternating current output for calibrating energy meters, substantially as herein described and illustrated in the figures of the accompanying drawings.