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Questions and Answers
What principle is utilized in Permanent Magnet Moving Coil instruments?
What principle is utilized in Permanent Magnet Moving Coil instruments?
What happens when a fixed coil interacts with a free coil carrying current?
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?
What is the primary function of the controlling system in an instrument?
Which materials are most commonly used for spring control in instruments?
Which materials are most commonly used for spring control in instruments?
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How do Hot Wire instruments operate to measure current?
How do Hot Wire instruments operate to measure current?
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What characterizes electrostatic instruments in current measurement?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
What characteristic defines an analogue instrument?
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What is a primary classification of analogue instruments based on their operation?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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?
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°?
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?
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?
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?
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?
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?
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?
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?
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.
