CW140 Clamp-On Power Meter

KAWASAKI Makoto¹ FUNAKI Kazuo¹ MORITA Akio¹

We have developed the clamp-on power meter CW140, which is capable of measuring various data for energy saving and equipment maintenance in the field. Users can select from four measurement modes; Instant, Electric Energy, Demand, and Harmonics. Development focused, in particular, on measuring electric energy in the field and the CW140 can realize continuous sampling of 10 kHz. A single CW140 can serve as two measuring instruments by providing support for not only up to three-phase four-wire type systems but also dual load type systems. An accuracy of 1% and frequency range of 45 Hz to 1 kHz (including the clamp), required by the basic specifications for on-site measurement has been met. For simplicity and easy operation, a 5.9-inch LCD display and user-friendly design was adopted. This paper gives an overview of the CW140.

*1 Yokogawa M&C Corporation

INTRODUCTION

Figure 1 CW140 Clamp-on Power Meter

The recent increase in companies or plants that have obtained ISO14001 certification, an international environmental management system standard, and the latest revision of the Law concerning the Rational Use of Energy, has lead to significant demand for various power parameters. This situation often results in people who are inexperienced in power measurement or unfamiliar with operations of measuring instruments performing measurements. Measuring instruments are expected to be easier to use and more user friendly. In addition, personal computers have become a common tool for analyzing and storing measured data.

Meanwhile, users have valued live-line measurement that does not require cutting the measured object. Yokogawa M&C Corporation has an established track record in the development of clamp-on current testers and applied this technological know how to the development of a clamp-on sensor for the CW140.

Furthermore, as more users measure harmonics on-site for performing suppression, facility diagnostics, and preventive maintenance measures, we have equipped standard the CW140 with a harmonics measurement function.

The CW140 is a user-friendly measuring instrument with emphasis on operability. A single unit of this current-clamp- input-type digital power meter allows various power parameters to easily be measured. Figure 1 shows an external view of the CW140.

FEATURES

1. Main Specifications
   Measuring ranges:
   Voltage :150/300/600 V
   Current : 20/50/100/200/500/1000 A (in combination with current clamp sensors)
   Basic accuracy: Voltage : ± (0.1% of rdg + 0.2% of rng) at 45 Hz to 65 Hz
   Current: ± (0.2% of rdg + 0.4% of rng) at 65 Hz to 1 kHz
   ± (0.6% of rdg + 0.4% of rng) at 45 Hz to 65 Hz
   ± (1.0% of rdg + 0.8% of rng) at 65 Hz to 1 kHz (in combination with current clamp sensors)
2. Four Measurement Modes
   The CW140 has four basic measurement modes: Instant Measure mode, Electric Energy Measure mode, Demand Measure mode, and Harmonics Measure mode. The Instant Measure mode includes measurement of instantaneous values of RMS values (voltage and current), powers (active, reactive, and apparent), power factor, phase angle, frequency, and three-phase voltage unbalance factor. The Electric Energy Measure mode allows measurement of regenerative energy for a reverse power flow of elevators, co-generation systems, and etc. In the Energy Measure and Demand Measure modes, the instantaneous values are measured at the same time. In addition, harmonics analysis can be made for up to 13-th order components (at commercial frequency of 45 to 65 Hz), which is the required level on-site.
3. Dual-load System Measurement
   The CW140 has four channels for current clamp inputs that allow measurement of dual-load systems in a single-phase two-wire, single-phase three-wire, or three-phase three-wire configuration at the common voltage. This means the CW140 serves as two measuring instruments when measuring various power parameters for each load at the same voltage, greatly contributing to possible simplification of wiring and installation, and reduction of costs of measuring instruments.
4. Large LCD and Operability
   The CW140 is designed with consideration of operability, portability, and installation. We placed special emphasis on the display and used a 5.9-inch dot matrix LCD. As an LCD is a semi transparent display, it ensures reduction of power consumption by achieving sufficient viewability in a bright place without a backlight. We also concentrated input and output terminals on one side to facilitate wiring and routing for installation.
   The CW140 has the minimal number of operation keys for better operability. Settings are made via the interactive display, Function keys, Cursor key, Enter key, and Escape key. The display language can be selected from Japanese, English, German, French, Italian, and Spanish. There is also a TOP MENU key that changes any display to the TOP MENU screen and thus is very useful for switching measurement modes and recovering from operation errors.
5. Quick Selection of Electric Energy Measurement with Watt- hour Key
   Before measurement can be performed settings must be made for each measurement mode display. Thus for electric energy measurement, which is likely used most often, we provided a Watt-hour (Wh) key for direct selection from four preset settings and the last measurement setting. Use of the Watt- hour key improves installation efficiency and allows users who are not used to measuring instruments to start measurement of electric energy simply by selecting a setting.
6. Power Saving and Multi Power Supply Compatibility
   Applicable power supplies include an accessory AC adapter, alkaline batteries, as well as a nickel-hydrogen (NiMH) battery pack that can be recharged within the main unit for use at various sites.
   We used a single chip micro controller with built-in DSP (digital signal processor) to simplify the peripheral circuits. We lowered power consumption to minimum by stopping operation of unused modules and incorporating the power- saving mode. The CW140 can operate about nine hours or longer with an NiMH battery pack, and about five hours or longer with alkaline batteries (times measured under normal temperature, with the LCD backlight turned off and without a floppy disk drive connected).

CONFIGURATION

 

Figure 2 Block Diagram

Figure 2 shows the basic configuration of the CW140. It consists of the input block, CPU block, memory block, drivers block, display and operation block, and power supply block (rechargeable NiMH battery).

Input Block

Figure 3 Linearity of Current Input

Figure 4 Linearity of Active Power

The voltage-input block is isolated from other circuits. The voltage signal is divided by the voltage-dividing resistor, normalized by a range amplifier, and then input to the isolated amplifier circuit. The isolated amplifier consists of an analog photocoupler and an operational amplifier.

The current clamp sensor converts a current to a voltage and outputs it. The voltage is normalized by the range amplifier of the current input block and input to the multiplexer before the A/D converter. Figure 3 shows the linearity of current input including the current clamp sensor, and figure 4 shows the linearity of active power.

A/D Converter Block

There are two A/D converters provided: one for voltage input and one for current input. While maintaining synchronism of sampling, the A/D converter processes multi-inputs by switching them with a multiplexer, achieving the sampling rate of 10 kHz/ channel. The selected signal of a multiplexer is input to the zero- cross detecting circuit to measure its frequency.

Harmonic components are sampled through PLL (phase lock loop) synchronization. The PLL circuit after the zero-cross detecting circuit generates the synchronizing signal and provides it for the A/D converter as the start of conversion signal and to the CPU.

DSP and CPU Block

To measure electric energy accurately, it is essential to measure power continuously. Thus, the CW140 carries out various power computations while performing continuous data collection. Figure 5 shows a timing chart of sampling at an input frequency of 50 Hz in instant, electric energy, and demand measurement modes.

The start of a conversion signal from the CPU generates a clock for serial communication that is fed to the A/D converter and CPU. A switch of input signal is generated at the same time.

Figure 5 Timing Chart of Sampling

Each digital voltage and current data from A/D converters is transferred to a FIFO (first-in first-out) buffer of SCSI (serial communication interface within the CPU) based on the zero-cross signal of the selected frequency source at a maximum frequency of 40 kHz. It is then transferred to CPU memory using the DMA (direct memory access) technique.

The CPU reads the data in memory in units of 1 block (1024 bytes) and integrates RMS values of voltage and current, active power, and reactive power (when using the reactive power method) with the DSP for each input. The integration cycle in which the CPU repeats the same process is from a start of zero- cross signal to the next zero-cross immediately after 100 ms elapses. The CPU completes the integration during the last zero- cross period of cycle to obtain the instantaneous value for voltage and current from the RMS values and each power value over the entire integration cycle. In electric energy measurement and demand measurement modes, the CPU measures the integration time to obtain the electric energy or demand values from measured power values.

Simultaneously, the CPU computes the three-phase unbalance factor, checks overranges (overflow and underflow) of voltage and current, and performs the scaling function with VT (voltage transformer) ratio and CT (current transformer) ratio settings.

This series of computations mentioned above is repeated without interrupting data transfer of the A/D converter.

FUNCTIONS

Miswiring Check

Table 1 Conditions for Miswiring Check-up

Check Item	Error Condition
Presence of voltage input	10% or less of range
Presence of current input	1% or less of range
Voltage phase sequence	· Voltage input of 10% or less of range · For three-phase three-wire line: V3 is not about 60° ± 20° above V1. · For three-phase four-wire line: V2 is not about 120° ± 20° below V1 or V3 is not about 120° ± 20° above V1.
Clamp direction error	· The active power is 0.17% or less of the rated voltage. · Power for one or more phases is negative (Or the power for the whole three-phase three-wire line is negative).
Frequency measurement	· Frequency source is not stable enough for proper measurement. · Input frequency is 40 Hz or less, or 1.2 kHz or more.

The CW140 has a checking function to avoid miswiring when performing measurements on-site. Performing the check after wiring work checks and judges wiring with regards to the conditions of table 1. As the wiring diagram checked is graphically displayed as shown in figure 6, users can confirm the check results on-site without the instruction manual.

Data Saving

Figure 6 Wiring Diagram

Electric energy is often continuously measured over one month to one year. So once the measuring instrument is installed, it is left at the site for a long time, which causes a problem of reliability for data saving. Measurement environments are so dusty and littered that the reliability of floppy disks cannot be used ensured. For this reason, we equipped the CW140 with 1 MB of memory to improve data saving reliability.

Meanwhile, in response to a request from many users for easy data transfer to a personal computer without a communication interface, an external floppy disk drive can be connected to the CW140. Duplicating data is also made possible by using internal memory and the floppy disk drive simultaneously.

In addition, connection of a printer to an RS-232 communication port of the CW140 allows users to directly print measurement results. Furthermore, optional D/A output enables the provision of the optimal measurement system for each site.

Power Failure Handling

If a power failure occurs during logging in the instant mode or harmonics mode, or integration in the electric energy mode or demand mode, the CW140 automatically saves the time of power failure occurrence. When the power recovers, it saves the time of recovery as well. If a power failure has occurred when the CW140 is performing integration, it holds the integrated value and resumes integration with the value after the power recovers.

Event Input

The event input function saves high/low logic signals as data. For example, when inputting an on/off signal indicating the operating status of equipment connected to the load, users can receive power parameters and data informing them of the operation status of the equipment simultaneously. The event input can be useful for leveling the load or grasping power consumption.

Three-phase Unbalance Factor Measurement

When a three-phase voltage becomes unbalanced, it can damage equipment. Especially with a three-phase induction motor, temperature rises, lower efficiency, or increased vibration or noise can cause an accident. Yokogawa M&C Corporation previously released the 270/01 three-phase voltage unbalance meter, and we adopted some of its functions for the CW140. The unbalance factor was conventionally obtained by measuring voltages and using computational expressions or diagrams. The CW140 allows it to be directly measured in the instant measurement mode.

CONCLUSION

Essential design concepts for the CW140 included a wealth of functions such as on-site type clamp input, four measurement modes, dual-load measurement, and data saving together with simplicity and user-friendliness. We expect that taking full advantage of these functions will allow the CW140 to be useful in the ever-expanding market for on-site measurement of power parameters.

We would like to take this opportunity to acknowledge and thank T&M Business Division of Yokogawa Electric Corporation, which has field-proven technology for digital power meters, for its contributions to the development of the CW140.

REFERENCES

1. Masahiro Kazumi et al. WT110/WT130 Digital Power Meters. Yokogawa Technical Report. No. 22. 1996. p. 19-22
2. Masahiro Kazumi et al. PZ4000 Power Analyzer, Yokogawa Technical Report, No. 30. 2000. p. 15-18