Clamp Meter

Clamp Meter - a device for contactless measurement of the current of any circuit without the need to break, as happens with an ordinary ammeter. This technical solution ensures the safety of service personnel. Especially useful for working substations and other sources of increased voltage.

The history of the development of measuring devices

Rogowski belt

The Rogowski belt is considered the prototype of the clamp meter. For the first time such a construction was described in 1887 by the professor of the University of Bristol Chattok. He proposed to wound a coil in a special way on a flexible rod made of Indian rubber so that the induced emf in the circuit would be proportional to the difference of the magnetic potential at its ends. The scientist covered the wire coil and engaged in research. For example, Chattok tried to determine the magnetic resistance of iron, using the law of Ampere.

Rogowski and Steijus were also interested in the mentioned field of physics. A review of Die Messung der magnetischen Spannung( 1912) provided a description of this magnetic potentiometer, but without reference to the activities of Chattok. It is not known for certain whether these two knew that their device has been known for a quarter of a century. Sample coil design:

1. A coil is laid along a flexible rod.
2. Its end is wrapped around and with each turn approaches the beginning.

Simple design gives a number of advantages:

1. avoids the influence of the device on the result of measurements;
2. , unlike modern ticks on magnetic cores, does not have the phenomenon of magnetic saturation, it becomes possible to measure currents of any magnitude;
3. , due to the previous cause, calibration is simplified: the dependence is linear and easily approximated over the entire range;
4. simply changes the sensitivity of the device, which makes it suitable for solving various problems;The
5. Rogowski belt becomes an ideal impulse current receiver( small inertia), is used in digital technology, suitable for estimating transient process values, where typical measuring transformers flow:

• current control of precision soldering stations and welding machines;
• laboratory plasma current measurement;
• arc parameters in smelting furnaces;
• devices for automatic control of electrical circuit parameters;
• evaluation of railgun starting currents;
• simulation of short circuit mode in electrical engineering( see transformation ratio);
• due to the inertia-free Rogowski belt is acceptable for accurate evaluation of complex waveforms with harmonic components( thyristor currents);
• are used on the railways to work with signals in real time;
• are used to evaluate the currents induced by the magnetic field in the metal parts of transformers;
• consistent with the objectives of the educational process to illustrate the law of Ampere.

The Rogowski Belt is an ideal tool for studying circuits with previously unknown currents. An additional advantage is recognized by ease of manufacture, it is possible to obtain a measuring tool for any structures, regardless of small or large geometric dimensions. The voltage received at the output of the Rogowski belt is passed through an operational amplifier for a pronounced effect. The resulting current controls a switch or digital ammeter.

Due to the specificity of Ampere's law, the path of coverage of a conductor with a current coil, curves or direct hands of a master is not important. In the worst case, this will slightly affect the accuracy, but the result will by and large remain the same. However, the current clamp meter is similarly said: there is no difference, the conductor is in the center or on the periphery.

Despite such a long-standing discovery, the use of the Rogowski belt was not. The first patent in this area is the Austrian, after the name of Werner, number 191015( 1957), which describes the possibility of studying the density of magnetic flux using a steel needle. A similar type of device is considered in US2644135 A, declared March 20, 1950.And it is impossible to find a patent for the Rogowski belt. Apparently, the device is not declared to the bureau.

But the Rogowski belt shows a tangible flaw, an inventive thought must move on. It implies the need for tight closure of the ends of the coil. Otherwise, the contour cannot be considered as satisfying the conditions for the applicability of Ampere's law. The homogeneity of the contour is also important: the number of reverse turns is provided to be constant per unit length, even in the bent state. What is not always convenient from a technological point of view. The complexity is solved with current clamp pliers.

Frequency becomes the second limitation of the Rogowski belt. The lower limit is 0.1 Hz, and the upper is not all right. The measurements on ultrasound are quite tolerable up to 1 MHz. And then you need to apply the integration on a low-resistance resistor using its own inductance circuit. The process differs from that described above( the current is already measured, not the EMF).This principle extends the applicability of the Rogowski belt to 100 MHz.

Clamp Meter

In a clamp meter, a ferromagnetic core is used. Flexibility is absent, it is assumed to provide a mechanism for opening the circuit. Using a special pen, pressing it, the master covers the wire with current. The first such construction is found in patent US1924039 A, filed December 7, 1932.William Hockley from Crompton Parkinson Ltd.writes about his own invention:

1. The magnetic core is broken in two places: at the bend and the disconnect point to cover the circuit under study.
2. The tool contains the component parts of modern clamp meters. Even the shape of the case is similar: rectangular with a side button for opening the spring-loaded magnetic circuit.
3. A number of measurement limits are distinguished, and each uses its own removable movable type sensor. Thus, the universality of the device is claimed.
4. A measuring coil is wound on the back side of the core. The indicator becomes an ordinary ammeter.

The patent claims the first( in the world) use of a movable core, asking for the conclusion that there are current clamp meters in their original form. Then repeats the ideas, this is happening in the patent US2494206 A, declared April 23, 1946.However, in the latter case, there is a clarification that the DC force is measured by changing its field by the permeability of the magnetic circuit. Indeed, it is something new.

. In patent US1924039 A, the talk was about direct current, but the method proposed for parameter estimation was not specified. As is known, only a variable magnetic flux induces an emf in the circuit; it cannot be used in this case.

The phenomenon of direct-current bias has been known in technology since 1921, thanks to patent US1640881 A. The application has been reviewed for six years before it was accepted in 1927.This means that the specialists of the bureau did not consider the invention to be applied on an industrial scale or did not perceive novelty. Indeed, the magnetism for adjusting the speed of asynchronous motors is not widely used even today, and the first devices for recording sound using this effect appeared only during the Second World War. Patent US2494206 A is considered a separate idea, designed to solve the difficulties that were missed in earlier versions of current-measuring tongs( the ability to measure currents with a frequency of 0.1 Hz).

In other cases, to measure direct current, the sensitive coil is rotated at a known frequency. Then the magnetic flux is constantly changing, it is possible to calculate the desired value. The rotational speed is stabilized, for example, using a quartz oscillator. Sometimes the movement of the coil is translational. Such vibration devices for measuring direct current were first described by Groshkovsky in 1937.The translational motion was carried out using an eccentric wheel. Today, constructions on piezoelements are known, where electrical energy is converted into mechanical oscillations. Due to this, it is possible to achieve speeds of tens of kHz, which greatly increases the response of the EMF in the circuit, which means that the measurements are simplified.

calculations

Practically implemented piezoelectric structures contain a miniature coil of 10 turns of wire with a length of 30 and a width of 0.8 microns. With a frequency of 2 kHz, the sensor provides a sensitivity of 18 µV / 100 µT.The technique of the coil inserted into the field is widely encountered. In the latter case, the result is calculated using the formula shown in the figure. By V is meant the EMF taken from the circuit, A is a certain constant, n is the number of turns in the circuit, and the difference in brackets characterizes the rate of change of the magnetic field. From this, it is clear that the original patent US2494206 A is not considered the worst solution for measuring the parameters of a constant magnetic field with a clamp meter.

Current Clamp Meters in Modern Electrical Engineering

Sensory coils with an air gap show low sensitivity for obvious reasons: the field strength is approximately equal to induction. This causes a small response in the form of EMF.When designing miniature applications, it is more profitable to use a ferromagnetic core, which multiplies the induction value many times over. Modern materials have a permeability coefficient of hundreds of thousands of units, according to which there is an increase in the response in the form of EMF.Air sensors are linear. Even the best ferromagnet gives distortions depending on frequency, temperature, magnetic flux density, etc.

The Barkhausen effect makes the signal of the core stepwise. This is an insurmountable defect today, called quantization noise. Often for this reason, current clamp can not be considered an ideal tool for measuring currents. Although they are suitable for assessing the processes occurring in the chain. For example, it is easy to detect the starting current of the electric motor, although due to the inertia of the indicator, experience will have to be done several times.

The latest modifications combine their own role with the usual tester. Favorably differ in the ability to measure high currents without the need to touch the network, but suffer a large error. Here it is necessary to make a reservation that for the extension of the functional three inputs are added that are standard for multimeters:

1. Common - common( ground, minus).
2. V - for measuring voltages and resistances.
3. Ext - connection of an external source in order to assess the insulation resistance of electrical circuits.

There are known variations that estimate temperature using a special probe with a sensor. It is easy to ring the diode. It is proposed to choose the device on the counter, based on the ratio of price and quality. It is necessary to inspect the clamp meter before purchase for errors. Many, when shorting the contacts, give non-zero voltage readings. This is inconvenient in a practical sense, especially for low voltage circuits. The power source is the usual Crohn, as in most testers. In this regard, the clamp meter does not offer any advantages or disadvantages.