Dynamometer type wattmeter – Construction, operation and working principle

Hello friends, in this post we will see construction and working principle & classification of dynamometer type wattmeter. We will also see advantages and disadvantages of dynamometer type wattmeter.

Construction of Dynamometer Type Wattmeter:

Following figure shows the dynamometer wattmeter for measuring the power. If two coils are connected such that, current proportional to the load voltage, flows through one coil and current proportional to the load current, flows through other coil, the meter can be calibrated directly in watts. This is true because the indication depends upon the product of the two magnetic fields. The strength of the magnetic fields depends upon the values of the current flowing through the coils.

dynamometer type wattmeter
dynamometer type wattmeter

Working of Dynamometer Type Wattmeter:

Let

  • v=supply voltage
  • i=load current and
  • R=resistance of the moving coil circuit
  • Current through fixed coils, i(f)=I
  • Current through the moving coil, i(m)=v/R

Deflecting torque,

1

  • For a DC circuit the deflecting torque is thus proportional to the power.
  • For any circuit with fluctuating torque, the instantaneous torque is proportional to instantaneous power. In this case due to inertia of moving parts, the deflection will be proportional to the average power. For sinusoidal alternating quantities the average power is VI COS?, where
  • V= r.m.s. value of voltage,
  • I=r.m.s. value of current, and
  • ?= phase angle between V and I

Hence an electrodynamic instrument, when connected as shown in figure, indicates the power, irrespective of the fact it is connected in an AC or DC circuit.

Ranges:

  1. Current circuit: 0.25 A to 100 A with employing current transformers (CTs).
  2. Potential circuit: 5V to 750 V without employing potential transformers (PTs).

Types of Dynamometer Wattmeter:

Dynamometer wattmeters may be divided into two classes:

  • Suspended-coil torsion instruments.
  • Pivoted-coil, direct indicating instruments.

1. Suspended-coil Torsion Wattmeters:

These instruments are used largely as standard wattmeters.

  • The moving, or voltage, coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil. This coil is situated entirely inside the current or fixed coils and the winding in such that the system is a static. Errors due to external magnetic fields are thus avoided.
  • The torsion heads carries a scale, and when in use, the moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power.
  • Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument.
  • The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero.
  • The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern.

2. Pivoted-coil Direct-indicating Wattmeters:

These instruments are commonly used as switchboard or portable instruments.

  • In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained. The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position.
  • The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil.
  • The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box.
  • The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale.
  • The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material.

Advantages and disadvantages of dynamometer type wattmeter:

The advantages and disadvantages of dynamometer type wattmeters are as under:

Advantages:

1)    In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.)

2)    High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes.

Disadvantages :

1)    The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect)

2)    In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.

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