ELECTRODYNAMOMETER INSTRUMENTS

The electrodynamometer-type mechanism, shown in Figure 6-6, is adaptable to a wider variety of measurements than any of the instruments previously described and is especially useful in AC measurement and as a DC-to-AC transfer instrument.

In this instrument, both stator and rotor are coils. Current flowing through the stationary or field coil winding produces a field in proportion to the current. As current is applied to the moving coil, the coil moves because of the reaction on a current-carrying conductor in a magnetic field. The torque actuating the moving element is a function of the product of the two magnetic fields and their angular displacement.

This instrument can be used for measurement of volts, amperes, watts, either alternating or direct current, as well as power factor and frequency. Figure 6-7 shows the coil arrangements for various applications.

Figure 6-6. Mechanism of Electrodynamometer Instrument.

Application of Electrodynamometer Instrument Measurement of Power

The most important use of the electrodynamometer mechanism is as a wattmeter.

In this construction the moving coil is in series with a resistance and is connected across the circuit as a voltmeter, while the field coil is connected in series with the load as an ammeter coil. The torque between these coils is proportional for DC to the product of volts and amperes, or watts. For AC, the instrument recognizes the phase difference between volts and amperes, which is the power factor. Its readings then are proportional to the product of volts, amperes, and power factor.

When used as a wattmeter, the moving coil is wound with fine wire, while the field coil may be wound with large-size wire, the nominal rating of the latter being usually 5 amperes.

By superimposing two complete wattmeter elements with the two moving coils on the same shaft, power in a three-wire circuit may be measured by one instrument. The torques developed by the two elements add algebraically to give an indication of total power. By using phasing transformers to shift the voltages to the moving coils 90°, a three-phase VARmeter is obtained.

Measurement of Current

Electrodynamometer ammeters have the field and moving coils connected in series. Since the moving coil is connected to the circuit by rather fragile lead-in spirals, it is evident that the current-carrying capacity of that part of

Figure 6-7. Circuits of Electrodynamometer Instruments.

the instrument is limited. For this reason the moving coil is shunted in instru-ments above l00 milliamperes capacity. The full line current passes through the field coil and the shunt.

Measurement of Voltage

In the electrodynamometer voltmeter, the field coil is connected in series with the moving coil and a resistance across the line. The sensitivity of this instrument is less than that of a DC voltmeter because of the greater current required by the dynamometer mechanism. It is, however, more accurate than the moving-iron voltmeter and is better adapted to precise voltage measurements.

Measurement of Power Factor and Phase Angle

A variation of the fundamental electrodynamometer instrument is used to meas-ure power factor or the phase angle, and is called the crossed-coil type. See Figmeas-ure 6-7. In this design the moving element consists of two separate coils, instead of one which are mounted on the same shaft and set at an angle to each other.

The lead-in springs or spirals to the crossed coils are made as light or weak as pos-sible so as to exert practically no torque. In the single-phase instrument, one of the crossed moving coils is connected in series with a resistor across the line while the other is connected in series with a reactor across the line. The current flowing through the reactor-connected coil is approximately 90 degrees out of phase with the line voltage. The field coil is connected in series with the line as an ammeter coil.

In operation, the moving system assumes a position dependent upon the phase relationship between the line current and the line voltage. If the line current is in phase with the line voltage, the reactor-connected moving coil will exert no torque and the resistor-connected coil will align its polarities with those of the fixed-coil field. If the line current is out of phase with the line voltage, the reactor-connected moving coil will exert a restraining or counter torque and the moving element will assume a position in the field of the fixed coil where the two torques are in balance.

This instrument may be calibrated to indicate either power factor or the phase angle between the line voltage and current.

In the three-phase power factor instrument, the crossed moving coils are con-nected to opposite legs of a three-phase system. The fixed coils are concon-nected in series with the line used as a common for the moving-coil connection. This instru-ment will give correct indication on balanced load only.

When these instruments are not energized, the pointer has no definite zero or rest position as do instruments whose restraining torque is a spring. They are therefore known as free-balance instruments.

Power factor meters may also be of the induction type. In one such type for single-phase use, the fixed element consists of three stationary coils and the mov-ing element comprises an indicator shaft bearmov-ing an iron armature. As in the elec-trodynamometer type, operation is based on the interaction of a rotating and an alternating magnetic field.

Measurement of Frequency

Another variation of the electrodynamometer instrument, the crossed-field type, is used to measure frequency. Crossed field or stationary coils are connected to the line through inductive and capacitive circuit elements so that the relative strengths of the fields become a function of the frequency. See Figure 6-7. An iron vane attached to a freely rotatable pointer shaft will align itself with the direction of the resultant field and the instrument will indicate the frequency. This fre-quency meter is also a free-balance instrument.