Before studying about dynamometer type single phase power factor meter, we have to know about power factor.

Power factor of an electrical circuit can be determined from the wattmeter, ammeter and voltmeter readings suitable connected in circuit.

This method involves mathematical calculations. Sometimes it is required to measure the power factor of the circuit instantaneously when the power factor of the load is varying continuously. This purpose is served by connecting a power factor meter in the circuit. Power factor meter directly indicates, by a single reading, the power factor of the circuit to which they are connected. In this article, we will study about the dynamometer type single phase power factor meter.

Principle

The basic principle of operation of this instrument is similar to that of dynamometer type wattmeter i.e. when the filed produced by moving system tries to come in line with the field produced by the fixed coil, deflecting torque torque is exerted on the moving system which deflects the pointer attached to it.

Construction

It  consists of a fixed coil which acts as the current coil. The coil is split into two parts and carries the current of the circuit under test. Therefore, the magnetic field produced by the current coil is directly proportional to the  load current. Two identical moving coils A and B pivoted on a spindle constitute the moving system. Moving coil A has a non-inductive resistance R connected in series with it, and coil B has a highly inductive choke coil L connected in series with it. The values of non-inductive resistance R and inductance L are so chosen that for the normal frequency, the current in the two moving coils are same. Thus the field produced by the two coils are of same strength. The field produced by the coil B lags behind the field produced by the coil A slightly less than 900 and same angle between the planes of the coils.

dynamometer type single phase power factor meter

There are no controlling torque provided in this instrument. Connections to moving coils are made through thin silver or gold ligments which are extremely flexible and thus give a minimum control effect on the moving system.

Working

When the power factor meter is connected in the load circuit, current flows through the fixed coil FF and moving coils A and B, flux is set by the fixed coils and moving coils. By the alignment of field produces by fixed coil and moving coils, torque develops i.e. the resultant field produced by the moving coils tries to come in line with the field produced by the fixed coils and torque develops till both of them come in line with each other. There are three extreme conditions in which this instrument is connected in the circuit.

When power factor of the circuit is unity:

In this case, current is in phase with circuit voltage. The current flowing through potential coil A is in phase with voltage which is also in phase with the current flowing through fixed coil FF. At the same time current flowing through potential coil B lags behind voltage as well as the current flowing through current coil FF by 90°. Thus pressure coil A will experience a turning moment so its plane will come in position parallel to the plane of the current coil FF. The torque acting on the pressure coil B is zero. Thus, the pointer indicates unity power factor on the scale.

When power factor of the circuit is zero lagging:

In this case, current lags behind the circuit voltage by 90°. Therefore, the current flowing through pressure coil B will be in phase with current in fixed coil FF, both being lagging behind the circuit voltage by 90°. Thus a turning moment acts on the pressure coil B and bring its plane parallel to the plane of fixed FF and pointer indicates zero power factor lagging.

When power factor of the circuit is zero leading:

In this case, current leads the Circuit voltage by 90°. Therefore, the current flowing through moving coil A lags the current in fixed coil FF by 90° and current flowing through pressure coil B lags the current in fixed coil FF by 180°. Thus, field produced by the moving system is just reversed to that in case (ii). Hence, an opposite turning moment acts on the pressme coil B and bring its plane parallel to the plane of current coil FF and pointer indicates zero power factor leading.