MAXQ3181-RAN+ datasheet(61/84 Pages) MAXIM

61 / 84 page

Low-Power, Active Energy, Polyphase AFE

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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 MAXQ3181 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 MAXQ3181

supports all of these connection arrangements.

Sensor Selection

The MAXQ3181 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 MAXQ3181, the peak input voltage

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

400:1 to 600:1 is ideal.

The second consideration is the total power dissipation

and voltage hold-off requirements of the resistor. It is

tempting to design a 400:1 divider with a 400k

Ω resis-

tor in series with a 1k

Ω resistor, but that would force the

400k

Ω resistor to dissipate about 140mW. This is not an

excessive amount of power, but if the design is to use

small SMT parts, it can handle greater than a 1/10W

SMT resistor. It is better to use a series of several small-

er components to improve system reliability.

Voltage Transformer

If isolation is required between the meter electronics

and the line, a voltage transformer is required. A volt-

age transformer is designed to faithfully transfer an AC

voltage applied on the primary side to a sensor on the

secondary side. On the primary side, a voltage-divider

is used to reduce the voltage to a workable level. On

the secondary side, a load resistor is selected so that

the current in the transformer windings is safely within

the transformer’s linear operating region.

Because the impedance seen in the primary side of the

transformer is equal to the impedance of the load resis-

tor in the secondary circuit plus impedance of the

transformer secondary winding at the operating fre-

quency, it is easy to calculate the value of the required

voltage-divider resistors in the primary side. For exam-

ple, assume we want a 500:1 divider ratio and assume

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