DYNAMOMETER POWER FACTOR AND PHASE ANGLE MEASUREMENT

Measurement of Power Factor and Phase Angle

A variation of the fundamental electrodynamometer instrument is used to measure power factor or the phase angle, and is called the crossed-coil type. See Figure 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 possible 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 connected to opposite legs of a three-phase system.

The fixed coils are connected in series with the line used as a common for the moving-coil connection. This instrument 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 moving element comprises an indicator shaft bearing an iron armature. As in the electrodynamometer type, operation is based on the interaction of a rotating and an
alternating magnetic field.