Cirrus Logic AN337 User Manual

CS5463: Determining No-load Condition Without

Violating Starting Condition Standards

1. INTRODUCTION

AN337

When the CS5463 is used in power meter application s, some customers use the I threshold for the external microprocesso r to decide if the meter is under the no-load condition, and then find the meter can not pass the starting test. This applications note gives the reason and the correct solution.

2. NO-LOAD AND STARTING CONDITIONS

According to the standard IEC 62053-22, the definition of a no -load condition is defined as: "Wh en the voltage is applied with no current flowing in the current circuit, the test output of the meter shall not produce more than one pulse.". The test of starting condition is defined as: "The meter shall start and continue to register at 0.001In and unity power factor.".

To meet these requirements, a power meter has to use the corre ct no-load threshold in order to restrain the pulse output and energy accumulation when there is no load current but resume pulse output and energy accumulation when the load is above the starting current (I

3. DETERMINING A NO-LOAD CONDITION CORRECTLY

The CS5463 has three energy pulse outputs (E1, E2, and E3). The pulse outputs un der no -loa d o r starting condition has been well manipulated by the CS5463. In addition, the CS5463 has a no-load threshold register (LOAD magnitude of the P

Different from the energy pulses, the energy register, either mechanical or electronic, is normally manipulated by a microcontroller. The microcontroller needs to use a variable and a thre shold value to decide if the meter is under a no-load condition, in which case the values in the power registers should be discarded.

) to help disable the active energy pulse output under some specified condition. When the

Min

Active

, the active energy pulse output is disabled.

Min

residual value as the

RMS

There are two options to select the variable and the threshold.

Variable

Used

RMS

Active

Threshol d Value

R es idu a l va lue (A C of fs et) wi th

inputs of U

Less than the value with inputs of U

and starting current

an d no cu r re nt

Table 1: Variable & Threshold Values Comparison

Because I to determine the no-load condition.

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exhibits much more system noise than P

RMS

Active

Com pare

Result

BAD

GOOD

, I

should not be used as a variable

RMS

DEC '08

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For example, assume a 10(60)A Class 0.5 watt-hour meter using a 200 uΩ shunt as the current sensor. The voltage on the CS5463 I-channel input (IIN±) with the starting current (I well below the I-channel noise level. Therefore, if I

was used to decide the no-load condition, the meter

RMS

will not pass the no-load and starting tests. Even though the meter has lower than 2 uV noise level and passes the no-load test, this application can not be used as a 5(60)A meter because the noise level is fixed within the application, however the starting current would be 50% lower.

=0.1%Ib) will be 2 uV which is

In the next example, P

is used as the variable.

Active

Assume the following:

• A 5(60)A Class 0.5 watt-hour meter, AC gain calibrated, with inputs of U

•P

•P Typically, the residual value in the P

_Imax = 0.36 (decimal register value, same below) with inputs of Un and I

Active

_Ib = 0.03 with inputs of Un and I

Active

_Starting = 0.00003 with inputs of Un and starting current (Ist = 0.1% Ib)

Active

_Residual is less than 0.000002 (with 150 mVrms

Active

and I

max

on VIN± and no signal on IIN±, HPFs enabled, N = 4000). Because P

_Residual and P

Active

_Starting >> P

Active

_Starting (a threshold of 0.00001, for example) to allow th e meter pa ss the n o-

Active

_Residual, the threshold can easily be selected between

Active

load and starting tests. Furthermore, if the meter is well designed and has very low system noise, the threshold can be set even

lower (e.g. 0.000005) to allow much smaller starting current which is more welcomed by electric utilities. The meter current dynamic range (I

max/Ib

) can also be extended.

Contacting Cirrus Logic Support

For all product questions and inquiries contact a Cirrus Logic Sales Representative.

To find one nearest you go to http://www.cirrus.com

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