Measuring Instruments

  1. Introduction and classification

Measuring instruments are those which are used for measuring the electrical measurements.   These include: ammeters, voltmeters, energy meters, wattmeter and many more.

The electrical instruments are broadly classified as:

Absolute instruments:  in these instruments which indicate quantity which you want to measure when one quantity is constant.  The deflections are needed to be multiplied with the instruments constants. There is need of the previous comparisons.  The most popular example of this type of instrument is tangent galvanometer.

Secondary instruments:  in this type of instruments, the   value of the quantity which is needed to be measured is measured from the deflections seen on the instrument on making variations.   The quantity which is needed to be measured is obtained directly.

The secondary instruments are further divided into the following categories:

  • Indicating instruments: These instruments indicate the values of the quantities being measured at a time. Pointers are used to give the indications.
  • Integrating instruments: These types of instrument the total quantity after integration in a particular time period.
  • Recording instruments: These instruments record the variations of an electrical quantity which is being measured over a particular time period.

Some of the instruments include:

  1. Ammeter: for measuring current
  2. Voltmeter: for measuring voltage
  3. Wattmeter: for measuring power
  4. Ohmmeter: for measuring resistance
  5. Power factor meter: for measuring power factor
  6. Frequency meter: for measuring frequency

Instruments are classified as AC, DC and AC/DC on the basis of kind of current.  And, on the basis of kind of indication they are classified as indicating type and recording type. According to the application, instruments are either switch board based or portable. There are many more types of classification within these broad categories.

  1. Electrical principles of operation

There are so many physical effects of the electric current or potential thus all the electrical instruments depend n these effects for their actions.

The common effects which are used in the manufacturing include:

  • Magnetic effect
  • Chemical effect
  • Thermal effect
  • Electrostatic effect
  • Electromagnetic induction effect
  1. Electrical Indicating Instruments

The indicating instruments have the pointer that indicates the value of the quantity which is being measured on the scale.  Spindle of hardened steel is carried by the moving system is of the instrument. It has its tapered ends with highly polished to form the pivots. These pivots rest in hollow ground bearing that are set in steel screws.  The moving system is attached to the thin ribbons in the spring which is held by the tension springs mounted on the movement frame.  The arrangement eliminates friction of the pivots and there is a less chance of damage due to the shock or vibration.

3.1 Essential features

 The indicating instruments have three important features:

  • Deflecting devices: There are the devices in which the electric current, voltage or power are responsible for producing mechanical force.
  • Controlling devices: In these devices the value of deflection is highly depended on the magnitude of the quantity which is being measured
  • Damping devices:These are the devices which are responsible for preventing the oscillations and help the pointer to reach final position

3.2 Deflecting devices:

It produces a deflecting torque which is produced due to the effects like thermal effects, electrostatic effects or chemical effects.   This deflecting torque moves the position of the pointer from zero to the final position.

3.3 Controlling devices

There are two types of controlling devices spring contact and gravity control.

In spring control device, there are two spiral hair springs, the inner ends which are attached to the spindle. The outer spring end is fixed and is attached to the lever which gives zero on adjustment. Two springs which are wounded in the opposite direction gets deflected when one spring windsup while another spring unwinds up.   The controlling torque is also due to the combined torsion by the spring.

 The torsion toque is in proportion with the angle of twist thus the controlling torque is directly proportion to the angular defection caused by the pointer.

Tcα θ

 The spring used should be non magnetic, should not get fatigued and have low temperature co-efficient and specific resistance.

Gravity control

In this method, the weights are directly attached to the spindles. The function of one weight is being balanced with the weight of the pointer of the controlling device.  The second weight offers the controlling torque and the pointer is at zero when this weight hangs vertically downwards.   The controlling torque is equal to the weight of the second load and distance d, when pointer‘s deflection is θ.

Tcα sin θ

Benefits of gravity control method

This method s inexpensive than the spring control instrument and is not affected by temperature or gets fatigue.

Disadvantages of gravity control method

It gives the cramped scale and the instrument is needed to be leveled before usage.

3.4 Damping devices

Due to the inertia of the moving system, the controlling devices are subjected to the deflection and restoration of the torque and several vibrations will occur before resting at a place.  A damping torque is used to avoid this type of vibrations because it opposes the motion and stops when the pointer comes to rest. Instrument is called as the deadbeat when the pointer rises quickly but does not overshoots.

 There are three types of damping:

  • Air damping
  • Eddy current damping
  • Fluid Friction damping

In the air damping, a thin metallic vane is attached to the spindle.  This vane moves while forming the sector. The oscillation of the moving system damps with the due to action of air on vane.

In eddy current damping, when the conducting non magnetic material is kept in the magnetic field, and ane.m.f is induced which gives rise to current. A force arises between the nonmagnetic material and the field.  This type of damping is based on the Lenz’s Law according to which this type of force is always in opposition to the force which causes rotation hence it gives the necessary damping.

In the Fluid Friction damping, the light vanes are attached to the Spindle of the moving system and these are dipped in pot which contains the damping oil.  The vanes are completely submerged into the damping oil and this oil on the vanes r assist in creating friction which opposes the motion. When the vanes are stationary no damping occurs.

 The damping oil should be a good insulator and non-evaporating. It should also be non corrosive and resistant to change in temperature.


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