What is Deflecting Torque?
Deflecting torque is the force or energy needed to move the pointer on a measuring instrument, such as a voltmeter or ammeter. This movement is caused by an electric current passing through the deflecting system, which is the part of the instrument that converts electrical energy into mechanical energy.
When we use a measuring instrument to measure an electrical quantity, like voltage or current, the pointer will move to a certain position on the scale to indicate the amount of that quantity. The movement of the pointer is caused by the deflecting torque produced by the deflecting system.
In order for the pointer to move accurately, the deflecting torque must overcome three main factors:
- Inertia of the moving system: The pointer and any other moving parts of the instrument have their own inertia, or resistance to motion. The deflecting torque must be strong enough to overcome this inertia and move the pointer.
- Controlling torque produced by springs: Many instruments use springs to control the movement of the pointer. These springs can apply a force that opposes the movement of the pointer. The deflecting torque must be strong enough to overcome this controlling torque and move the pointer.
- Damping torque produced by the damping system: Damping is a mechanism that is used to slow down the movement of the pointer and prevent it from oscillating. Damping systems can apply a torque that opposes the movement of the pointer. The deflecting torque must be strong enough to overcome this damping torque and move the pointer.
In simple terms, deflecting torque is the force that causes the pointer on a measuring instrument to move, and it is created by the deflecting system converting electric energy into mechanical energy. It has to overcome other forces and torque in order to move the pointer correctly.
Effects Used to Produce Deflecting Torque :
- Magnetic Effect
- Electromagnetic Induction Effect
- Heating Effect
- Chemical Effect
- Electrostatic Effect
- Hall Effect
1. Magnetic Effect:
The magnetic effect is a principle used in many electrical measuring instruments to produce a deflecting torque. This torque is the force that causes the pointer of the instrument to move and indicate the value of the electrical quantity being measured.
One type of instrument that uses the magnetic effect is the moving coil instrument, also known as a PMMC (permanent magnet moving coil) instrument. These instruments use a permanent magnet to create a magnetic field, and a current-carrying coil is placed within this field. When current flows through the coil, it experiences a force due to the interaction between the magnetic field and the current. This force causes the coil to move and deflect the pointer attached to it.
Another type of instrument that uses the magnetic effect is the moving iron instrument. In these instruments, there is a movable piece of soft iron that is placed within a magnetic field created by stationary electromagnets. When an electrical quantity is applied to the instrument, the magnetic field causes the iron piece to either be attracted or repelled, depending on the type of instrument. This movement of the iron piece causes the pointer to deflect and indicate the value of the electrical quantity. There are two main types of moving iron instruments: attraction type and repulsion type.
2. Electromagnetic Induction Effect:
Electromagnetic induction is a principle that states that a current can be induced in a conductor when it is placed in a changing magnetic field. This principle is used in many electrical instruments, such as voltmeters, ammeters, wattmeters, and energy meters, to produce the deflecting torque required to measure electrical quantities.
In these types of instruments, an aluminum disc is placed between two magnets (shunt and series) that are magnetized in proportion to the supply voltage and current, respectively. When the load current flows through the circuit, it creates a changing magnetic field that induces eddy currents in the aluminum disc. These eddy currents create a force on the disc, which produces a torque that deflects the pointer of the instrument.
Energy meters are a common application of the principle of electromagnetic induction. They are used to measure the amount of energy consumed by a particular load. The energy meter consists of a driving system that includes two magnets (shunt and series) facing each other, and a movable aluminum disc is placed in between the two magnets. The eddy current flows in the disc due to the flux produced by the load current and this torque produces a deflecting force on the pointer which indicates the energy consumed.
3. Heating Effect:
In heating effect based instruments, a current to be measured is passed through a coil, causing a rise in temperature of the coil. The heat produced can be converted into an electrical voltage with the help of a thermocouple, which is a device that converts temperature changes into an electrical signal.
A common example of a heating effect based instrument is a hot wire ammeter. It consists of a thin wire that is heated by the current to be measured, and the resulting temperature change causes a change in the electrical resistance of the wire. The change in resistance results in a change in voltage which can be measured and used to calculate the current flowing through the wire.
Another type of heating effect based instrument is the thermocouple ammeter. It utilizes the principle of thermoelectricity, which is the phenomenon of generating an electrical voltage when two dissimilar metals are joined together and heated. In this instrument, the current to be measured flows through one of the dissimilar metals, causing a change in temperature, and resulting in an electrical voltage that can be measured and used to calculate the current.
4. Chemical Effect:
In integrating instruments that utilize the chemical effect, a chemical reaction is used to measure the total energy supplied over a period of time. A common example of this is the watt-hour meter, which is used to measure the amount of electrical energy used by a particular device or system.
The principle behind these instruments is that a chemical reaction occurs when a current is passed through a specific type of material, such as an electrolyte. As the current flows, the chemical reaction causes a change in the material, which can be measured and used to determine the amount of energy that has been used.
For example, in a watt-hour meter, the current flowing through the electrolyte causes a chemical reaction that results in the formation of a certain amount of a chemical compound. The amount of this compound that is formed is directly proportional to the amount of energy that has been used. This chemical compound can be measured and used to determine the total energy used over a period of time.
It is important to note that this type of instrument is mainly used for measuring DC ampere-hours, which is the amount of current flow over time in a DC circuit. This type of instrument is not suitable for measuring AC current, as the chemical reaction is not consistent with the alternating nature of AC current.
5. Electrostatic Effect:
The electrostatic effect is used in voltmeters, which are instruments that measure the potential difference or voltage between two points in an electrical circuit. The principle of operation of electrostatic instruments is based on the force of attraction or repulsion between two charged bodies.
The instrument consists of two plates, one fixed and one movable. When a potential is applied across the plates, an electrostatic field is created and this causes a force to be exerted between the plates. This force results in a deflecting torque which causes the movable plate to move, and this movement is proportional to the potential difference being measured.
The electrostatic effect is well suited for measuring very high voltages, as the plates can be widely separated to reduce the force required to move the movable plate. This is important because high voltages can be dangerous and it is necessary to ensure that the instrument can safely measure them. Additionally, electrostatic voltmeters are relatively simple and inexpensive to construct. The principle of electrostatic effect is widely used in high voltage measuring instruments.
6. Hall effect:
The Hall Effect is a phenomenon that occurs in certain materials, particularly semiconductors, when they are placed in a uniform magnetic field and a current is passed through them. When this happens, an electric field is generated perpendicular to both the magnetic field and the current, which results in a voltage difference between two points on the material. This voltage difference is known as the Hall voltage.
The Hall Effect can be used in a variety of applications, one of which is in flux meters. Flux-meters are instruments that measure the strength of a magnetic field. They make use of the Hall Effect by passing a current through a semiconducting material and then measuring the Hall voltage generated. By knowing the material's Hall coefficient and the magnetic field strength, the flux density can be calculated.