NI-9242 Getting Started

NI-9242 Block Diagram

Each channel on the NI-9242 provides an independent signal path and ADC. Each terminal has the same input impedance to ground.
The NI-9242 returns the voltage between each AI terminal and the Neutral terminal as well as the voltage between the Neutral terminal and the chassis ground.

Filtering

The NI-9242 uses a combination of analog and digital filtering to provide an accurate representation of in-band signals and reject out-of-band signals. The filters discriminate between signals based on the frequency range, or bandwidth, of the signal. The three important bandwidths to consider are the passband, the stopband, and the anti-imaging bandwidth.

The NI-9242 represents signals within the passband, as quantified primarily by passband ripple and phase nonlinearity. All signals that appear in the alias-free bandwidth are either unaliased signals or signals that have been filtered by at least the amount of the stopband rejection.

Passband

The signals within the passband have frequency-dependent gain or attenuation. The small amount of variation in gain with respect to frequency is called the passband flatness. The digital filters of the NI-9242 adjust the frequency range of the passband to match the data rate. Therefore, the amount of gain or attenuation at a given frequency depends on the data rate.

Stopband

The filter significantly attenuates all signals above the stopband frequency. The primary goal of the filter is to prevent aliasing. Therefore, the stopband frequency scales precisely with the data rate. The stopband rejection is the minimum amount of attenuation applied by the filter to all signals with frequencies within the stopband.

Alias-Free Bandwidth

Any signals that appear in the alias-free bandwidth are not aliased artifacts of signals at a higher frequency. The alias-free bandwidth is defined by the ability of the filter to reject frequencies above the stopband frequency. The alias-free bandwidth is equal to the data rate minus the stopband frequency.

Data Rates

The frequency of a master timebase (f_M) controls the data rate (f_s) of the NI-9242. The NI-9242 includes an internal master timebase with a frequency of 12.8 MHz, but the module also can accept an external master timebase or export its own master timebase. To synchronize the data rate of an NI-9242 with other modules that use master timebases to control sampling, all of the modules must share a single master timebase source.

The following equation provides the available data rates of the NI-9242:

f_{s} = \frac{f_{M} \div 256}{n}

where n is any integer from 1 to 31.

However, the data rate must remain within the appropriate data rate range. When using the internal master timebase of 12.8 MHz, the result is data rates of 50 kS/s, 25 kS/s, 16.667 kS/s, and so on down to 1.613 kS/s depending on the value of n. When using an external timebase with a frequency other than 12.8 MHz, the NI-9242 has a different set of data rates.

Note The NI-9151 R Series Expansion chassis does not support sharing timebases between modules.

NI-9242 Pinout

Table 1. Signal Descriptions
Signal	Description
AI	Analog input signal connection referenced to the Neutral signal
Neutral	Referenced, single-ended analog input connection

Connecting Phase Measurements

The NI-9242 accepts three-phase and single-phase measurement configuration.

Three-Phase Measurement Configurations

NI recommends the following three-phase connection types.

4-Wire WYE Measurement Configuration

High-Leg Delta Measurement Configuration

3-Wire Delta Measurement Configuration

Corner Grounded 2-Wire Delta Measurement Configuration

Corner grounded 2-wire delta measurement configurations support only standard service levels up to 240 V RMS L-L.

Connecting Single-Phase Measurement Configurations

NI recommends the following single-phase connection types.

3-Wire Measurement (Split Phase) Configuration

2-Wire Measurement

Potential Transformer Connections

The NI-9242 accepts the following potential transformer connections.

4-Wire WYE-to-WYE (Full)

In a 4-wire WYE-to-WYE (full) configuration, the Neutral terminal on the NI-9242 measures the neutral-to-ground voltage through the bottom transformer. You can use a lower ratio transformer due to the typically low voltages on the Neutral terminal. Ensure that you scale each NI-9242 channel reading with the corresponding transformer ratio.

4-Wire WYE-to-WYE (Partial)

Delta-to-Delta

Note You can use the Neutral channel for any other measurement if the measurement does not exceed the Neutral-to-Earth and L-N input range.

Delta-to-WYE

3-Wire WYE-to-Delta

Note You can use the Neutral channel for any other measurement if the measurement does not exceed the Neutral-to-Earth and L-N input range.

3-Wire WYE-to-WYE

Potential Transformer Ratio Scaling

When measuring voltages using potential transformers, scale the NI-9242 readings with your application software using the transformer ratio to get readings that refer to the primary of the potential transformers.

Tip If using a 320:1 ratio potential transformer, multiply the readings by 320.

Connection Guidelines

You must use 2-wire ferrules to create a secure connection when connecting more than one wire to a single terminal on the NI-9242.
Make sure that devices you connect to the NI-9242 are compatible with the module specifications.

To ensure that measurements to chassis ground are correct, NI recommends connecting the chassis to earth ground using the chassis grounding screw. Refer to your chassis manual for information about connecting the chassis to earth ground.

For 4-wire WYE-to-WYE (partial) configurations, delta-to-WYE configurations, and 3-wire WYE-to-WYE configurations, follow these guidelines for the best accuracy results.

Connect the Neutral terminal of the NI-9242 to the isolated ground of the potential transformer to reduce noise between the potential transformer ground and the chassis ground.
Connect the Neutral terminal of the NI-9242 as close as possible to the isolated ground of the potential transformer.
Use the L-N voltage measurements the NI-9242 returns as the default value.

Note You can convert L-N measurements to channel-to-earth ground by adding the Neutral terminal measurements to each of the AI channels.

High-Vibration Application Connections

If your application is subject to high vibration, NI recommends that you follow these guidelines to protect connections to the NI-9242:

Use ferrules to terminate wires to the detachable connector.
Use the NI-9967 connector backshell kit.

Wiring the NI-9967

Caution For safe operation with hazardous voltages, you must use the NI-9967 Connector Backshell with the 4-terminal connector on the NI-9242.

Complete the appropriate procedure for each wire gauge.

Installing the NI-9967 Using 12 AWG to 14 AWG Wire

What to Use

NI-9967 backshell
12 AWG to 14 AWG wire
Smallest strain-relief piece
Screwdriver

What to Do

Route wires under the smallest strain-relief piece.
Secure the smallest strain-relief piece and the backshell in place using captive screws.

Installing the NI-9967 Using 16 AWG Wire

What to Use

NI-9967 backshell
16 AWG wire
Small and large strain-relief pieces
Screwdriver

What to Do

Route wires between the two strain-relief pieces, with the small strain-relief piece on top of the wires and the large strain-relief piece underneath the wires.
Secure the strain-relief pieces and the backshell in place using captive screws.

Installing the NI-9967 Using 18 AWG to 24 AWG Wire

What to Use

NI-9967 backshell
18 AWG to 24 AWG wire
Largest strain-relief piece
Screwdriver

What to Do

Route wires under the largest strain-relief piece.
Secure the largest strain-relief piece and the backshell in place using captive screws.

Converting L-N Measurements to L-L

To convert L-N measurements to L-L values, calculate the voltage difference between the AI channels using your application software.

Refer to the following equation for an example of converting L-N measurements to L-L.

Phase A to Phase B Voltage = AI0 - AI1

where

AI0 is the reading from Phase A
AI1 is the reading from Phase B

Converting L-N Measurements to L-Earth

To convert L-N measurements to L-Earth values, add the neutral channel reading to each AI channel reading.

Refer to the following equation for an example of converting L-N measurements to L-Earth.

Line to Earth = AIx + Neutral

where

AIx is the analog input channel reading
Neutral is the Neutral channel reading