October 13, 2021

The working principle of the Electrodynamometer Wattmeter is very simple disturbs the main magnetic field of the Electrodynamic Wattmeter. For example, in using an electrodynamic wattmeter on direct current, two readings are taken, the direction of current flow through the instrument being reversed. Dynamometer type wattmeter works on very simple principle and this principle We know that instantaneous torque in electrodynamic type.

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This allows any shifts in the mechanical zero to be readily detected. The computed phase angle, 9 C9 For the derivation of this relation see appendix II. Thus it appears that most of the deflec- tion due to eddy currents in the moving system is caused by eddy currents in the damping vane. By passing current through the fixed coils of the wattmeter and a capacitor, connected in series, and applying the voltage drop across the capacitor to the voltage circuit of the wattmeter, the current and voltage of the wattmeter will be very nearly 90 degrees out of phase and the wattmeter deflection should be about zero.

Such transfer instruments are used in two separate and distinct kinds of tests: Diode detectors are either directly connected to the source, or used with a sampling system that diverts only a portion of the RF power through the detector.

The deflection of the pointer is directly proportional to the power flows through it.

Attached to upper part of stationary glass panel Details of the movable head to which the upper end of the suspension is attached. The traditional analog wattmeter is an electrodynamic instrument.

Full text of “Standard electrodynamic wattmeter and ac-dc transfer instrument”

The d-c supply was used to calibrate the wattmeter. Capacitance currents in the fixed coils.

Also, on analog wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement. December Learn how and when to remove this template message.

Electrodynamometer Wattmeter

The cables of the four individual coils were connected in series in such a manner that the flux of the upper coil is in the electgodynamic opposite Journal of Research of the National Bureau of Standards Research Paper Figure 1.

By using an instrument-reading camera focused directly on the pointer and ad- justing current until the image of the pointer edge coincides with a fine line on the ground-glass screen of the camera, settings can be repeated to within 0. The “shunt resistance” elextrodynamic connected in parallel with the moving coil and the ohm resistor. Any torque pro- duced in the present wattmeter by the eddy currents in the moving coils may be combined with the torques produced by wattmmeter currents in other moving parts of the wattmeter, since the torques due to all these eddy currents are similar functions of the same variables.


Sources of error 1 and 2 would cause a phase defect directly proportional to frequency.

Wattmeter – Wikipedia

This transfer instrument must be such that its reading or indication is the same with direct current as with alter- nating currentor differs by a known amount. The two circuits of a wattmeter can be damaged by excessive current.

A current electordynamic through the current coil generates an electromagnetic field around the coil.

This deflection was measured with 60 ma the maximum value to be used through the moving coils for various positions of the mov- ing system near the point of zero mutual inductance between the fixed and moving coils.

The remainder of the wattmeter phase defect must be due to “skin effects” in the fixed coils. The position of zero mutual induc- tance is readily determined by passing an alternating current through the fixed coils and wathmeter across the moving-coil terminals. The possibility of warping was reduced to electodynamic minimum by carefully varnishing the mahogany inside and out.

These current ratings are conservative on a self-heating basis and might be exceeded by moder- ate amounts with wagtmeter sacrifice in accuracy if necessary. In constructing another wattmeter, it would be advisable either to increase the distance between the damping vane and wattmeter coils or to use as a damping vane some nonconducting material, coated with a very thin conducting film to eliminate accumulation of static charge.

These cables were then wound into tight coils 32 turns of cable per coil on a coil-winding machine. For readings outside this range cor- rections 6 can be applied. The absolute wattmeter sensitivity is equal to the relative sensitivity to within 0.

Eddy current or skin effects in the fixed coils. Style B coils were permanently mounted in the wattmeter. Thus it was concluded that the error due to magnetic impurities would be negligible with rated current through the fixed coils. The sensitivity of the wattmeter can be derived from the following quantities: Sufficient space is left to allow a moving coil to be sus- pended in the center of each of the two fixed coils so formed.


Thermistors and thermocouples are used to measure heat produced by RF power and can be calibrated either directly or by comparison with a known reference source of power. With switch S 2 see fig. The pointer is fixed on the moving coil which is placed between the fixed coils.

These coil holders were in turn mounted in the glass panels as previ- ously indicated see fig. The plotted points on this figure represent the values of ac-dc errors of the wattmeter at unity power-factor obtained by the tests just described. General features of design 3. The drift in deflection was found to be very nearly a linear function of time and equal to 0. Scale is uniform upto a certain limit.

This plot shows that there is a range of 10 cm near the point of zero mutual within which the sensitivity is constant to within 0. Any warping of the wooden frames can result only in a displacement of the entire second panel with respect to the first panel. Air damping is provided by mounting a rectangular damping vane on the moving system and having it rotate within a fixed damping box. Two different types of fixed coil were made see fig. With a constant value of current in the fixed coils and a constant voltage across the moving-coil circuit, the deflection of the wattmeter was noted during a period of 2 hours.

Imperfection of the astatic construction of the wattmeter results- in a small deflection of the moving system with direct current through the moving coils only, because of interaction with the earth’s magnetic field.

The three effects causing the deflections noted above may now be considered as two: The lowest measurable current is given electrodynxmic 0.

A switching arrangement was placed in the current and voltage supply such that the alternating current and voltage could be disconnected and a direct current and voltage substituted.

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