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Questions and Answers
What principle is utilized in Permanent Magnet Moving Coil instruments?
What happens when a fixed coil interacts with a free coil carrying current?
What is the primary function of the controlling system in an instrument?
Which materials are most commonly used for spring control in instruments?
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How do Hot Wire instruments operate to measure current?
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What characterizes electrostatic instruments in current measurement?
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In gravity control systems, what condition is required for the control torque to be zero?
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What is a significant disadvantage of using gravity control in measuring instruments?
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In which scenario is the induction effect primarily utilized?
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What is one requirement for the spring materials used in spring control?
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What occurs when a conducting strip carrying current is in a transverse magnetic field?
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Why are thermocouple instruments effective for measuring current at high frequencies?
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What is the consequence of stressing a spring beyond its elastic limit?
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Which of the following statements is true regarding temperature effects on controlling systems?
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What is the primary function of the electrostatic effect in measurement instruments?
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The deflection angle θ in a spring control system corresponds to what aspect of the instrument?
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What characteristic defines an analogue instrument?
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What is a primary classification of analogue instruments based on their operation?
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What type of mechanical effect is utilized in the repulsion-type moving iron instrument?
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Which of the following is NOT a type of analogue instrument?
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Which principle is NOT a basis for classifying analogue instruments?
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When the direction of current in a coil is reversed, what is the effect on the force it generates?
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Which feature is commonly associated with the Hall effect in analogue instruments?
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What interaction occurs when a current-carrying coil attracts a piece of soft iron?
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What is the controlling torque in a gravity-controlled instrument related to when the deflection is at an angle θ?
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What characterizes an underdamped system in terms of deflecting and controlling torque?
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If an instrument experiences critical damping, what is the relationship between deflecting and controlling torque?
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Which damping method does NOT require the use of a permanent magnet?
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In the context of springs used for controlling torque, which of the following describes the tension in the spring material?
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What is a significant drawback of fluid friction damping?
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What force describes the interaction of a damping system working against the movement of the moving system?
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Which type of damping is considered the most efficient?
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When considering the weight of a 5 g controlling weight, what is the distance from the spindle if the deflecting torque is 1.13 * 10-3 Nm and deflection is 60°?
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In which type of instruments can eddy current damping not be used due to its requirements?
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The maximum allowable stress for the control springs is what value in MN/m2?
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What characteristic should pointers in instruments have to minimize damping torque requirements?
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What is the purpose of buffer stops in instrument pointers?
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Which of the following is NOT an application of eddy current damping?
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What type of oil is recommended for fluid friction damping?
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How are pointers designed to enhance the precision of instrument readings?
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Study Notes
Force Between A Current Carrying Coil And A Permanent Magnet
- A permanent magnet brought near a current-carrying coil creates a force of attraction or repulsion.
- This effect is used in permanent-magnet moving coil instruments.
Force Between Two Current Carrying Coils
- Two current-carrying can be considered as two magnets, with the direction of magnetic field as in the figure.
- If one coil is fixed and the other is free, there will be an attraction force between them.
- The free coil will move towards the fixed coil.
- This effect is used in dynamometer-type instruments.
Heating Effect
- Current is passed through a small element, which heats up and expands.
- This principle is used in hot-wire instruments.
- A thermocouple produces an electromotive force (EMF) when the junction of two dissimilar electric conductors is heated by passing a current through it.
- Thermocouple instruments are free from errors due to frequency, waveform and external magnetic fields when used on AC.
- They can be used for measuring currents at extremely high frequencies.
Electrostatic Effect
- When two plates are charged, there is a force exerted between them.
- This force can be used to move one of the plates.
- Instruments working on this principle are called electrostatic instruments.
- They can be used to measure current and power with the help of external components.
Induction Effect
- If a non-magnetic conducting pivoted disc or drum is placed in a magnetic field by a system of electromagnets excited by alternating currents, an EMF is induced in the disc or drum.
- If a closed path is provided, the EMF forces a current to flow in the disc or drum.
- The force produced by the interaction of induced currents and the alternating magnetic fields makes the disc move.
- The Induction Effect is mainly utilized for AC energy meters.
Hall Effect
- If a strip of conducting material carries current in the presence of a transverse magnetic field, an EMF is produced between two edges of the conductor.
Analogue Instruments
- An analogue device is one in which the output or display is a continuous function of time and bears a constant relation to its input.
- Measuring instruments are classified according to the quantity measured and the principle of operation.
- Analogue instruments depend on one of the many effects produced by current and voltage.
- The most common effects used in analogue instruments are: Magnetic, Hall, Heating, Electromagnetic, and Electrostatic.
Classification of Analogue Instruments
- Analogue instruments can be categorized as Electromechanical, Indicating, Electronic, Recording, and Integrating.
Classification According to Principle of Operation
- Instruments can be broadly classified by their principle of operation: Magnetic effect, Hall effect, Heating effect, Electromagnetic effect, and Electrostatic effect.
Magnetic Effect
- Used in: Moving coil, Moving iron, Electrodynamic
Magnetic Effect
- Placing a current-carrying conductor in a uniform magnetic field results in a force acting on the conductor.
- The force's direction depends on the current's direction and the magnetic field's direction.
Force of Attraction or Repulsion
- A current-carrying coil produces an imaginary bar magnet.
- A piece of soft iron placed near the end of the coil will be attracted.
- This effect is used in attraction-type moving iron instruments.
- If there are two pieces of soft iron magnetized with the same current-carrying coil, they will repel each other.
- This effect is utilized in repulsion type moving iron instruments.
Controlling System
- Purpose of the controlling system:
- To produce a torque equal and opposite to the deflecting torque at the final steady position of the pointer, making the deflection definite for a specific current.
- To bring the moving system back to its zero position when the force causing the deflection is removed.
Controlling Force
- Controlling force can be obtained by using either spring control or gravity control.
Spring Control
- Two phosphor bronze spiral hair springs coiled in opposite directions create the controlling force.
- Spring material should be non-magnetic, resistant to mechanical fatigue, and have low resistivity and a low temperature coefficient.
- Common spring materials include silicon-bronze, hard-rolled silver or copper, platinum-silver, platinum-iridium, or German silver. Phosphor-bronze is used for most applications, except in low resistance instruments such as millivoltmeters.
- The controlling torque is proportional to the deflection angle.
- Springs should be stressed well below their elastic limit to prevent a permanent set or zero shift.
Gravity Control
- Used to compensate for the effect of temperature on the stiffness of the spring.
- A small weight attached to the moving system produces a controlling torque when the system is deflected.
- Advantages: Cheap, unaffected by temperature changes, free from fatigue or deterioration.
- Disadvantages: Cramped scale, instrument must be kept in a vertical position.
- The controlling torque is proportional to the sine of the deflection angle.
Damping System
- A damping force opposes the movement of the moving system, bringing it to rest at the final deflected position quickly.
- Underdamped system: The deflecting torque is much greater than the controlling torque.
- Critically damped system: The deflecting torque is equal to the controlling torque.
- Overdamped system: The deflecting torque is much less than the controlling torque.
- Ideally, damping should be adjusted to a value slightly less than the critical value.
Methods of Damping Torque
- Air Friction Damping
- Fluid Friction Damping
- Eddy Current Damping
Air Friction Damping
- Simple and cheap method.
- Does not require a permanent magnet, avoiding distortion in the operating field.
- Used in moving iron and dynamometer instruments.
Fluid Friction Damping
- Consists of a van or disc immersed in a damping oil.
- Oil should be a good insulator, non-evaporating, non-corrosive, and have a viscosity that does not change with temperature.
- Advantages: Oil can be used for insulation, reduces friction errors.
- Disadvantages: Creeping of oil, instrument must be kept in a vertical position.
- Used in electrostatic laboratory instruments.
Eddy Current Damping
- Most efficient damping method.
- Based on the induction of eddy currents in a conducting, non-magnetic material moving in a magnetic field.
- Used in instruments where a metallic disc or former and permanent magnet already exist, such as moving coil, hot wire, and induction instruments.
- Cannot be used in instruments that require a permanent magnet for eddy current generation, as it would distort the magnetic field.
Pointers and Scales
- Scales and pointers fall into two categories: those for quick reading and those for accurate reading.
- Pointers should have low weight and inertia to reduce load on bearings and minimize the need for excessive damping torque.
- Pointer motion is limited by buffer stops, which are springs that prevent damage if the pointer hits them due to overload or current reversal.
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Description
Explore the dynamics of forces between current-carrying coils and permanent magnets. Understand how these principles are applied in various instruments like dynamometers and thermocouples. Dive into the heating effects of current and its real-world applications.
