Moving Iron Power Factor Meters

There are two varieties of moving iron type power factor meter. These are rotating field type and alternating field type.

Rotating Field Type Power Factor Meter


In this type of moving iron power factor meter, three fixed coils of are mechanically fixed in such a way when these three coils will be supplied from three different phases, there will be a rotating magnetic field in the space between the coils. Three similar current transformers connected to three different phases supply the current to the three different coils.

There is another coil at the center of the space between the current coils. This coil has resistance in series. We connect this coil across any two phases of the system. This is the voltage coil of the rotating field type moving iron power factor meter.

There is a small iron rod pivoted axially at the center of the voltage coil. There are two sector-shaped iron pieces directed to the opposite of each other. One of such iron pieces is connected to the front end of the small iron rod and another in the back end. In other words, one iron piece will be in front of the voltage coil and the other is in the back.

rotating field type moving iron type power factor meter

The damping vanes and the pointer are fitted on the extended part of this small iron rod.

Working Principal of Rotating Field Type Three Phase Power Factor Meter

There is no controlling torque acting on the system. The voltage coil that means central coil connected across any of the two phases produces an alternating flux. But the moving iron pieces do not rotate continuously but rest at a definite position. This position depends on the relative phases of the current in the central coil and the current in the current coils. Since we have connected the central coil across the two phases of the system; the current through it is in phase with the system voltage. Again as we told the angular position of the pointer that is moving iron system depends on the phase difference between the current of the central coil and the current coils. Hence the deflection of the pointer equals the angle of a phase difference between the current and the voltage in the three-phase system.

Since it is a kind of moving iron instruments it has been moving iron pieces. Also, the reactance of the central coil affects the performance of the instrument. Therefore, we should calibrate the instrument at normal power frequency (50Hz to 60Hz).

Alternating Field Type Power Meter

An alternating field type moving iron power factor meter is also known as Lipman moving iron power factor meter.


It consists of three numbers of moving iron pieces. Each of them is mounted on a single spindle. These are separated from each other by a nonmagnetic separating piece. The shape of the iron pieces is similar to that of rotating field power factor meter. But instead of using a single sector shaped set of iron pieces we use here three. Each of these iron pieces is physically directed 120° apart from each other. Each of the iron pieces is magnetized with a voltage coil connected to it from one of the three different phases of the system.

alternating field type moving iron power factor meter

In the alternating field type power factor meter, we split the current coils into two halves like dynamometer type power factor meter. Then we place the entire moving system in between the two halves of the current coil. We connect this current coil to any of the phases in the series. The spindle also carries the pointer damping vanes. Also, this instrument does not consist of any control device.

Working Principal of Alternating Field Type Three Phase Power Factor Meter

When currents flow through the current coil and the voltage coils, the moving system starts rotating and rests on a position that the main torque upon one iron piece is equal and opposite to the resultant of torques of other two iron pieces. Therefore, the net torque acting on the moving system becomes zero at the steady position of the moving system.

From the constructional details of this moving iron power factor meter, there is one iron piece connected to the voltage coil of that phase which is also in series with the current coil. Therefore the deflection of this iron piece is equal to the phase angle between the voltages and the currents of the three-phase supply system.

The voltage coils connected to different phases are placed in the same alignment but at different levels. Therefore, they cannot produce any rotating magnetic field.

An alternating field moving iron power factor meter can be suitably designed for measurement of power factor of an unbalanced three phase system. Also, it can be modified for two-phase and single phase system.


  1. The scale of this instrument can extend up to 360°.
  2. Constructional of this instrument is robust and simple.
  3. They are cheap.
  4. The deflecting torque of this instrument is more than that of a dynamo-meter type power factor meter.
  5. No ligament is required since all the coils of the instrument including current and voltage coils are stationary.


  1. The instrument is highly affected frequency, voltage, and waveform of the power signal.
  2. Due to the presence of iron pieces in the instrument, there is always be an iron loss. As a result, the reading of the instrument becomes less accurate than that of a dynamo-meter type.


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