Rotating field measuring device
A rotating field measuring device is used to display the direction of rotation of the rotating field with three-phase alternating current , colloquially "three-phase current". It is used in particular in the field of electrical installation in low-voltage networks to check the cabling, since a clockwise rotating field is specified in relevant standards for electrical installation such as VDE 0100, which is valid in Germany .
Despite its name, it is not a measuring device , but a test device for the verification of electrical safety, as it does not meet the necessary characteristics of a measuring device: It does not provide a measured value of a physical quantity as the product of numerical value and unit, but only shows a binary state.
The direction of the rotating field is essential for three-phase motors such as the asynchronous motor , because if the rotating field is "wrong", the motor will turn in the opposite direction. This can lead to consequential damage, for example in the case of electric pumps or other machine parts driven by the motor. A clockwise rotating field must also be ensured when using industrial and multi-phase plugs, such as the plugs according to the IEC 60309 specification .
With three-phase alternating current, the rotating field can be reversed by swapping any two external conductors . If a reversal of the direction of rotation is provided for individual electrical machines, this reversal can be achieved selectively for individual motors, for example by means of the reversing contactor circuit .
construction
Electromechanical rotating field measuring devices consist of a small three-phase motor with a gearbox that drives a rotating disc with an arrow symbol printed on it. The connection to the individual outer conductors, formerly labeled R , S and T , today usually L 1 , L 2 and L 3 , has to be made according to the labeling on the measuring device.
Electronic measuring devices consist of an electrical circuit that is connected to the three outer conductors and indicates the direction of rotation without any mechanical movement. By vectorial addition of the external conductor voltages, with the help of two reactive resistors in the form of capacitors for phase shifting as in the adjacent circuit, the display takes place by means of light emitting diodes . Usually, the right-hand direction of rotation is signaled with the color green and the left-hand direction with the color red.