UDC 613.647:621.791.75]-07

N. V. Maksimenko

Measurement of Magnetic Field Strength in Resistance Welding

Kharkiv Research Institute of Occupational Hygiene and Occupational Diseases

Resistance welding machines of various types and applications, operating at a frequency of 50 Hz, are now widely used in mechanical engineering. Under industrial operating conditions, they produce pulsed magnetic fields (MFs) in the form of short, repeating pulses consisting of one or more periods of distorted sinusoidal oscillations. The sources of the MFs are the elements of the welding circuit of resistance welding machines.

The need to assess MFs in the work area arises when determining the appropriate placement of welders’ workstations and monitoring and measuring equipment.

At present, instruments designed to measure the peak MF strength for pulse durations ranging from 0.02 to several tens of seconds are not commercially manufactured. The task was therefore set to develop a method for measuring MFs under industrial conditions using commercially manufactured magnetometric transducers and recording instruments.

Devices based on galvanomagnetic phenomena in semiconductors, induction coils, and other devices can be used as MF transducers. The former, however, have a number of disadvantages compared with induction transducers: their operation is significantly affected by ambient temperature; they require a special bias-current source; they have insufficient mechanical strength; and their output signal voltage is low. The latter provide a high output voltage amplitude, have high mechanical strength and temperature stability, and are easy to manufacture [1, 3].

Thus, induction transducers with recording instruments are the most suitable for measuring MF strength under industrial conditions.

Studies have shown that in various types of resistance welding, the MF contains higher harmonic components. Therefore, to eliminate the transducer’s frequency dependence, a correction filter (integrating circuit) is connected to its output. With this correction, the transducer was found to have a sufficiently linear frequency response over the range from 0.05 to 1 kHz.

The transducer consists of a rigid insulating frame carrying a winding whose number of turns is selected according to the sensitivity of the recording instrument at the output of the integrating circuit. The measurement error is then determined by the intrinsic error of the recording instrument. When an electronic oscilloscope is used as the recording instrument, the vertical size of the trace must fall within the working area of the screen, with an accuracy of ±5–10%; if an analog pulse voltmeter is used, the pointer deflection must be in the second half of the scale; if a digital pulse voltmeter is used, the reading must have at least three digits.

To reduce measurement error due to interference, the measuring coil is connected to the integrating circuit and recording instrument by a shielded cable. For this purpose, its leads are twisted together and drawn into a shielding braid, to which the outer lead of the coil is soldered. The transducer is mounted on an extension rod at least 0.5 m long, made of nonferromagnetic materials.

Calibration of the induction transducer consists of determining its constant (\(k\)) by measuring the voltage across it when it is placed in the magnetic field of Helmholtz coils [2].

To reduce the error in measuring the peak MF strength, the integrating circuit must meet the following requirements: the capacitor must have a low leakage current; the resistor must be stable and non-wirewound; the temperature coefficients of the capacitor and resistor must be small; the value of

The resistance of the integrating circuit should be 5–10 times smaller than the input resistance of the recording device, and the time constant should be at least 10 times longer than the duration of the magnetic-field pulse [1]. The output signal of the induction transducer with an integrating circuit is a voltage pulse that reproduces the waveform of the magnetic field being measured. A pulse voltmeter (analog or digital) or an electronic oscilloscope with an input resistance of at least 1 MΩ may therefore be used as the recording device. The latter also makes it possible to measure the duration and shape of the pulse. Voltmeters must be capable of measuring pulses consisting of one or more periods of a 50 Hz sinusoidal voltage (a distorted sinusoidal waveform with a harmonic coefficient of 10–15%), while oscilloscopes must operate in triggered sweep mode with a frequency bandwidth from a few kilohertz to several hundred kilohertz.

The strength of the magnetic field being measured, \(H\) (A/m), is determined from the measured peak voltage \(U\) (V) across the capacitor of the integrating circuit (\(\tau=RC\), Ω·F) and the measuring coil constant \(k\) (m²), i.e., \(H=RCU/1.2k\).

Under industrial conditions, it is convenient to use a graph of magnetic-field strength versus capacitor voltage, plotted for constant \(\tau\) and \(k\).

Using a measuring coil 0.11 m in diameter with 500 turns and \(k=0.06\ \mathrm{m^2}\), an integrating circuit with \(R=51\ \mathrm{k\Omega}\) and \(C=1\ \mathrm{\mu F}\), and an oscilloscope with a sensitivity of 1 mV/cm makes it possible to measure peak magnetic-field strengths from 0.34 to 200 kA/m. The total measurement error does not exceed ±15%.

During operation of alternating-current resistance welding machines, personnel may be at any point near the equipment. Measurements must therefore be made at various locations in the work area to determine the maximum possible field strength. For measurement, the induction transducer is placed in the field under investigation and oriented in space to obtain the maximum reading. Measurements are made at several heights above the floor and at various distances from the field source, in accordance with the methodological guidelines [2]. For fields with long intervals between pulses, three measurements are made in three mutually perpendicular planes; the reading in each plane is recorded, and the resultant magnetic-field strength vector is determined.

References

1. Babenko, N. S. Pribory i tekhnika eksperimenta. 1970. No. 4. Pp. 7–16.
2. Methodological Guidelines for the Hygienic Assessment of the Principal Parameters of Magnetic Fields Generated by Resistance Welding Machines Operating at an Alternating-Current Frequency of 50 Hz. Research Institute of Occupational Hygiene and Occupational Diseases, Ministry of Health of the Ukrainian SSR / Evtushenko, G. I., et al. Kharkov, 1986.
3. Chernyshev, E. T. Magnetic Measurements. Moscow; Leningrad, 1972.

Received September 16, 1988.