Non-contact current and voltage sensor

This invention was made with government support under DE-EE0002897 awarded by the Department of Energy. The government has certain rights to this invention.

1. Field of the Invention

The present invention is related to sensors providing input to power measurement systems, and more specifically to a non-contact sensor that includes an electrostatic voltage sensor and an electromagnetic current sensor that can be used to detect the voltage and current at a wire of a power distribution system.

2. Description of Related Art

A need to measure power consumption in AC line powered systems is increasing due to a focus on energy efficiency for both commercial and residential locations. In order to provide accurate measurements, the characteristics of the load must be taken into account along with the current drawn by the load.

In order to determine current delivered to loads in an AC power distribution system, and in particular in installations already in place, current sensors are needed that provide for easy coupling to the high voltage wiring used to supply the loads, and proper isolation is needed between the power distribution circuits/loads and the measurement circuitry.

Therefore, it would be desirable to provide a sensor that can provide isolated current draw information and permit load characteristics to be taken into account using outputs of a single sensor in an AC power distribution circuit.

The invention is embodied in a current and voltage sensing device and its method of operation. The current sensing device includes a current sensor and a voltage sensor both integrated in a housing that can be detachably coupled to a wire and provides outputs indicative of the current passing through the wire, as well as an electric potential on the wire.

The housing may be a clamshell containing portions of a current sensor formed from a ferrite cylinder, which when closed around the wire, form either a complete ferrite cylinder, or one with a gap along the circumference. A semiconductor magnetic field sensor may be included in the gap and used to measure the current passing through the wire, or a winding may be provided around the ferrite cylinder along its axis. The voltage sensor may be a separate cylindrical plate, another wire or other suitable conductor either offset from the current sensor along the length of the wire, or may be a foil located inside of the ferrite sensor or a film deposited on an inside surface of the ferrite.

The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and:

FIG. 1A and FIG. 1B are isometric views and FIG. 1C is a cross-section view of a sensor according to an embodiment of the present invention.

FIG. 2A is an isometric view and FIG. 2B is a cross-section view of a sensor according to another embodiment of the present invention.

FIG. 3A is an isometric view and FIG. 3B is a cross-section view of a sensor according to yet another embodiment of the present invention.

FIG. 4A is an isometric view and FIG. 4B is a cross-section view of a sensor according to still another embodiment of the present invention.

FIG. 5 is an electrical block diagram illustrating circuits for receiving inputs from sensors according to embodiments of the present invention.

The present invention encompasses sensors for current and voltage sensing features for providing input to power measurement systems. For example, the present invention can provide input to power monitoring equipment in computer server rooms, in which branch circuits distribute power to various electronic chassis power supplies, and in which it is beneficial to provide power usage information for the various branch circuits to power monitoring and/or system control utilities within a computer operating environment. Other applications include power monitoring for commercial and/or residential energy management.

Referring now to FIGS. 1A-1C , a sensor 10 in accordance with an embodiment of the present invention is shown. A plastic sensor body 12 encloses a current sensor and a voltage sensor, that provide information about a magnitude and phase of a current passing through a wire 3 around which sensor body is detachably secured as shown in FIG. 1B . A latch 13 secures a top portion and a bottom portion of sensor body 12 together, along with a hinge formed on sensor body 12 at an opposite side from latch 13 . A current sensing portion of sensor 10 is formed by three ferrite pieces 14 A, 14 B that form a ferrite cylinder around wire 3 , when sensor body 12 is closed. Top ferrite piece 14 A forms a half-cylinder, while ferrite pieces 14 B define a gap between ferrite pieces 14 B and in the circumference of the ferrite cylinder, in which current sensing element 17 , which is generally a semiconductor magnetic field sensor, such as a Hall effect sensor, is disposed. Current sensing element 17 is shown as having interface wires 15 extending from its body, but other types of terminals may be used as an alternative manner of providing connections to current sensing element 17 . An aperture is formed through sensor body 12 to receive current sensing element 17 . A voltage sensor is formed by metal plates 18 A, 18 B, which provide capacitive coupling to branch circuit wire 3 and provide an output via interface wire 15 A, which may also alternatively be replaced with a terminal or other suitable electrical connector. The voltage sensor provides an AC waveform that is at least indicative of the phase of the voltage on wire 3 and may be calibrated to provide an indication of the magnitude of the voltage if needed. Electrical connection to metal plate 18 B is provided by interface wire 15 A and electrical connection to metal plate 18 A is provided by contact between metal plates 18 A and 18 B when sensor body 12 is latched closed. Metal plate 18 A includes a contact 27 and metal plate 18 B includes a mating recess 29 to improve electrical contact between metal plates 18 A and 18 B, so that connection of one of metal plates 18 A and 18 B to the measurement system is needed to provide voltage sensing. Contacts 27 and mating recesses 29 are optional and may be omitted in other embodiments of the invention, and electrical connection may be provided only by contact between metal places 18 A and 18 B, or alternatively by other suitable connection improvement techniques. FIG. 1C illustrates such an embodiment so that metal plates 18 A and 18 B making contact when sensor body 12 is closed, and shows the connection of interface wire 15 A to metal plate 18 B.

Referring now to FIGS. 2A and 2B , a sensor 10 A in accordance with another embodiment of the invention is shown. Sensor 10 A is similar to sensor 10 of FIGS. 1A-1C , so only differences between them will be described below. Rather than including current sensing and voltage sensing elements that are laterally displaced along the axis of the cylinder formed by sensor body 12 as in sensor 10 shown in FIG. 1A , in sensor 10 A, the voltage sensor and current sensors are concentrically arranged, reducing the length of sensor 10 A over that of sensor 10 , while providing similar capacitive area for the voltage sensing and ferrite volume for the current sensing. Therefore, sensor 10 A includes metal plates 18 C and 18 D having shapes differing from that of than metal plates 18 A- 18 B in sensor 10 , and ferrite pieces 14 C- 14 D differ from ferrite pieces 14 A- 14 B of sensor 10 , as well. Metal plates 18 C and 18 D may be inserts mechanically secured by sensor shell 12 A, or metal films bonded to or deposited on the interior surfaces of ferrite pieces 14 C- 14 D. In the illustrated example, metal plates 18 C and 18 D include jogs at their ends in order to provide electrical contact between them and ferrite pieces 14 C- 14 D do not make contact as in sensor 10 of FIGS. 1A-1C , and therefore the total circumferential gap in the ferrite cylinder is increased slightly. However, in alternative embodiments, the jogs may be omitted from metal plates 18 C and 18 D and alternative electrical connection techniques may be employed, by including a second interface wire 15 A bonded to metal plate 18 C and/or additional interface metal along the edges of sensor body 12 outside of the ends of ferrite pieces 14 C- 14 D, which can then be extended to make contact as in sensor 10 of FIGS. 1A-1C .

Referring now to FIGS. 3A and 3B , a sensor 10 B in accordance with yet another embodiment of the invention is shown. Sensor 10 B is similar to sensor 10 A of FIGS. 2A-2B , so only differences between them will be described below. Rather than locating current sensing element 17 in a gap between two ferrite pieces 14 B as in sensor 10 A of FIGS. 2A-2B , in sensor 10 B, current sensing element is located between two ferrite pieces 14 E and 14 F that extend around the entire circumference of sensor 10 B, excepting the thickness of current sensing element 17 , and therefore only one circumferential gap is formed provided that ferrite pieces 14 E and 14 F are in contact when sensor 10 B is closed at the area opposite the hinge in sensor body 12 B. Recesses are formed in sensor body 12 B to accept current sensing element 17 , which may be bonded to, or molded within sensor body 12 B, as may also be performed for any of the integration of current sensing element 17 in the present application. Metal plates 18 E and 18 F are shown as having jogs only opposite of the hinged portion of sensor body 12 B, to provide for ferrite pieces 14 E and 14 F extending all of the circumferential distance to the body of current sensing element 17 and since ferrite pieces 14 E and 14 F are not in contact along the hinged portion of sensor body 12 B. However, in accordance with an alternative embodiment of the invention, metal plates 18 E and 18 F may include features within the gap formed between ferrite pieces 14 E and 14 F along the hinged portion of sensor body 12 B to provide additional electrical contact between metal plates 18 E and 18 F. Further, in accordance with another embodiment of the invention, if sensor body 12 B is made of a sufficiently flexible material and/or the hinged portion of sensor body 12 B is sufficiently elastic, ferrite pieces 14 E, 14 F may extend all of the way to the inside faces of sensor body 12 B on both sides of sensor body 12 B. In such an embodiment, sensing element 17 is inserted in either the hinged side or the latching side of sensor body 12 B between the faces of ferrite pieces 14 E, 14 F to form the gap and make contact with ferrite pieces 14 E, 14 F.

Referring now to FIGS. 4A and 4B , a sensor 10 C in accordance with yet another embodiment of the invention is shown. Sensor 10 C is similar to sensor 10 of FIGS. 1A-1C , so only differences between them will be described below. Rather than including metal plates 18 A and 18 B and the portion of sensor body 12 that extends to provide the voltage sensing portion of sensor 10 in FIG. 1A , interface wire 15 A extends within the cylindrical cavity formed by sensor body 12 C and ferrite pieces 14 A- 14 B to provide voltage sensing, which can provide sufficient coupling to perform voltage sensing, in particular when only the phase of the voltage on wire 3 is to be measured.

Referring now to FIG. 5 , a circuit for receiving input from the current/voltage sensors of FIGS. 1A-1C , 2 A- 2 B, 3 A- 3 B and 4 A- 4 B is shown in a block diagram. Interface wires 15 from current sensing element 17 provide input to a current measurement circuit 108 A, which is an analog circuit that appropriately scales and filters the current channel output of the sensor. The output of current measurement circuit 108 A is provided as an input to an analog-to-digital converter (ADC) 106 , which converts the current output waveform generated by current measurement circuit 108 A to sampled values provided to a central processing unit (CPU) 100 that performs power calculations in accordance with program instruction stored in a memory 104 coupled to CPU 104 . Alternatively, current measurement circuit 108 A may be omitted and current sensing element 17 may be connected directly to ADC 106 . The power usage by the circuit associated with a particular sensor can be determined by assuming that the circuit voltage is constant (e.g., 115 Vrms for electrical branch circuits in the U.S.) and that the phase relationship between the voltage and current is aligned (i.e., in-phase). However, while the assumption of constant voltage is generally sufficient, as properly designed properly distribution systems do not let the line voltage sag more than a small amount, e.g., <3%, the phase relationship between voltage and current is dependent on the power factor of the load, and can vary widely and dynamically by load and over time. Therefore, it is generally desirable to at least know the phase relationship between the branch circuit voltage and current in order to accurately determine power usage by the branch circuit.

Interface wire 15 A from the voltage channel of the sensor is provided to a voltage measurement circuit 108 B, which is an analog circuit that appropriately scales and filters the voltage channel output of the sensor. A zero-crossing detector 109 may be used to provide phase-only information to a central processing unit 100 that performs power calculations, alternatively or in combination with providing an output of voltage measurement circuit to an input of ADC 106 . Alternatively, voltage measurement circuit 108 B may be omitted and interface wire 15 A connected directly to ADC 106 . An input/output (I/O) interface 102 provides either a wireless or wired connection to a local or external monitoring system. When power factor is not taken into account, the instantaneous power used by each branch circuit can be computed as:

P BRANCH =V rms *I meas

where V rms is a constant value, e.g. 115V, and I meas is a measured rms current value. Power value P BRANCH may be integrated over time to yield the energy use. When the phase of the voltage is known, then the power may be computed more accurately as:

P BRANCH =V rms *I meas *cos(Φ)

where (Φ) is a difference in phase angle between the voltage and current waveforms. The output of zero-crossing detector 109 may be compared with the position of the zero crossings in the current waveform generated by current measurement circuit 108 A and the time ΔT between the zero crossings in the current and voltage used to generate phase difference Φ from the line frequency (assuming the line frequency is 60 Hz):

Φ=2Π*60*Δ T

In general, the current waveform is not truly sinusoidal and the above approximation may not yield sufficiently accurate results. A more accurate method is to multiply current and voltage samples measured at a sampling rate much higher than the line frequency. The sampled values thus approximate instantaneous values of the current and voltage waveforms and the energy may be computed as:

Σ( V n *I n )

A variety of arithmetic methods may be used to determine power, energy and phase relationships from the sampled current and voltage measurements.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.