MAXQ3183-RAN+ datasheet(67/102 Pages) MAXIM

67 / 102 page

Low-Power, Multifunction, Polyphase AFE

with Harmonics and Tamper Detect

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• Take the inverse tangent of this value. You get -0.5

°;

that is, 0.5

° capacitive.

• Subtract the UUT phase offset from the reference

meter phase offset. In this case, the phase needs to

move 1

° toward the capacitive. Convert this value to

radians: 1

° x π/180° = 0.0175 radians.

• Multiply this value by 65,536. The result is 572

(0x023C).

• Because the phase correction is toward the capaci-

tive, the value must be complemented. The two’s

complement of 0x023C is 0xFDC4. This is the value

that should be written to the PA0 phase compensa-

tion register.

At this point, the meter is compensated for a single

phase offset. If the phase offset were perfectly flat over

all current levels, that would be sufficient (and for many

current sensors, particularly current shunts, one point is

usually good enough.)

Interfacing the MAXQ3183 to

External Hardware

The MAXQ3183 has all the internal circuitry that is

needed for a sophisticated electricity meter, but specif-

ic external hardware is required when configuring the

meter for a particular application. The most critical

decision that must be made is how the load will be con-

nected to the power source, and how the meter will be

connected to measure power consumed in the load.

This section covers how to select hardware compo-

nents for a MAXQ3183 electricity meter.

Connections to the Power Source

Generally, three-phase power as delivered from the util-

ity consists of four wires: three voltage phases and a

neutral wire. In one typical three-phase delivery system,

measuring from neutral to any phase would read 120V,

while measuring from any phase to any other phase

would read 208V. Connecting a load so that load cur-

rent is taken from phase lines and returned to neutral is

called a wye-connected load. Connecting a load so

that load current is provided by one phase and

returned on another phase is called a delta-connected

load. The MAXQ3183 can measure power consumed in

either a wye-connected or a delta-connected load.

If the load is connected in a wye fashion, the voltage is

measured from the neutral lead to each of the phases,

and the current measuring device is placed in series

with the load, most often in the hot lead. The sensor is

not placed in the neutral lead to prevent a customer

from defrauding the utility by returning the current to

ground rather than neutral. A current sensor placed in

the hot lead makes fraud even more difficult.

A delta-connected load can have current measured in

two possible ways. If it is primarily desirable to know

how much power is delivered to the load, one can place

the current sensor in the load circuit between two phas-

es. But if it is more important to know how much current

is being drawn from each supply phase, each current

sensor is placed in the line circuit of each single phase.

Most utilities are only concerned with the total amount

of energy being consumed. If individually accounting

for the power delivered by each phase is not a require-

ment, it is not necessary to measure all three voltages.

Instead, knowing only two voltages and the three cur-

rents is all that is necessary to measure total energy

usage.

There are several ways of doing this. In a wye arrange-

ment, one of the phases—usually phase B—–can be

considered the voltage reference point instead of neu-

tral. Then the voltage measurements can be made from

phase A to phase B and from phase C to phase B. By

using some simple arithmetic, the power delivered by

phase A, phase B, and phase C can be calculated

even though only two voltages are available.

A second mechanism is to have a delta-connected

load, but with one leg—usually the BC leg—split into

two equal loads. The point where the load is split is

defined as the reference. In this arrangement, it is only

necessary to know the voltage between phase C and

the split and phase A and the split, since VC = -VA.

Finally, there is the connection arrangement in which

the load is in a delta configuration with the current sen-

sor at each load, but it is still desired to determine how

much current is in each supply branch. The MAXQ3183

supports all of these connection arrangements.

Sensor Selection

The MAXQ3183 supports a variety of voltage and cur-

rent sense elements. This section describes the proper-

ties of many of these sensing devices.

Voltage Sensors

Voltage-Divider

A voltage-divider is an ideal voltage-sensing element

when there is no need for voltage isolation. Modern

resistors have virtually no parasitic capacitance or

inductance at the frequencies of interest in an electrici-

ty meter and have extremely low variation with tempera-

ture. When selecting resistors for a voltage-divider,

keep the division ratio high enough so that the peak

voltage value cannot exceed the maximum allowable

input voltage. In the MAXQ3183, the peak input voltage

is about 1V; consequently, a divider in the range of

400:1 to 600:1 is ideal.

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