Measurement & Control - short answer type questions from AMIE exams (Winter 2020)

Answer the following in brief (2x10)

What do you understand by magnetic potential and how it is measured?

the scalar quantity characteristic of a point in a magnetic field whose negative gradient equals the intensity or strength of the magnetic field and which represents the work required to bring a unit north pole from a point infinitely remote up to the point in question

A tangent galvanometer is an absolute instrument, justify your answer.

Absolute instruments provide the quantity to be measured in terms of the instrument constant and its deflection. For example, a tangent galvanometer gives the value of the current to be measured in terms of the tangent of the angle of deflection produced, the horizontal component of the earth’s magnetic field, the radius, and the number of turns of the wire used. Such instruments are used in laboratories and in similar institutions as standardizing instruments.

Write the restoring torque equation for the gravity control instrument.

When the coil is deflected through an angle θ°, if T Nm is the torque required to twist the restoring springs through 1º, the total restoring torque is T θ. The pointer comes to rest at the position where the deflecting and restoring torques are balanced, i.e. when


Is there any relation between deflection angle and torque for the spring control system?

  • This torque is produced by the spring action and opposes the deflection torque so that the pointer can come to rest at the point where these two torques are equal (Electromagnetic torque = control spring torque). The value of control torque depends on the mechanical design of spiral springs and strip suspensions.
  • The controlling torque is directly proportional to the angle of deflection of the coil.

PMMC instrument is preferred for the measurement of DC current and voltage, why?

PMMC type instrument uses two permanent magnets in order to create a stationary magnetic field. These types of instruments are only used for measuring the DC quantities as if we apply AC current to these types of instruments the direction of current will be reversed during the negative half cycle and hence the direction of torque will also be reversed which gives the average value of torque zero. The pointer will not deflect due to high frequency from its mean position showing zero reading. However, it can measure the direct current very accurately.

Draw the block diagram of components of the control system.

The following figure (a) shows an open-loop control system and figure (b) shows a closed-loop system.

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