Contact - electrical thermometers

Contact - electrical thermometers

1. Resistance thermometers

Principle is that the electrical resistance of the sensors is strongly temperature dependent, and changes with temperature in a predictable way.

Platinum Resistance

Glass coated 100 ohm Pt100 platinum resistance sensor

Standard Platinum Resistance Thermometers (SPRTs) are the most accurate.
However, they are only suitable for laboratory use, and in industry more rugged industrial platinum resistance thermometers are used, variously known as IPRTs, Pt100s, RTDs (resistance temperature detectors).
In a thermometer, the high purity platinum wire sensor is located near the tip of a closed protective tube, to make a probe which can be inserted into the measurement environment.
Most sensors are made with two wires emerging from the instrument, the resistance of these wires is included in the measurement and errors of a few °C may result.
Some compensation for the lead resistances can be achieved by connecting a third wire to one side of the sensor (3-wire connection), but best accuracy requires four wires, two for passing the current and two for sensing the voltage across the Pt100 resistance.
Good sensitivity can be achieved: measurements routinely made with a precision of better than a thousandth part of 1°C.

Some Pt100/RTD sensors suitable for insertion in steel protective tubes or on flat surfaces. The smallest are about 1 mm in diameter.

Thermistors

Thermistors for use in current limiting circuits 

Semiconducting materials such as thermistors are very temperature sensitive and resistance increases very strongly as the temperature falls.Well suited for use in small probes with fast response, e.g. as current limiters in electronic circuits and in medical thermometry, where good sensitivity is achieved over limited temperature ranges.
Most common are negative temperature coefficient (NTC) types.
Since resistances are large, generally several kilohms, 2-wire connections can usually be used without significant error. Thermistors are not standardised, and the manufacturer’s specification must be referred to.

DC Generator working principle

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Generator Construction:

Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional.

Components of a generator:

Rotor: In its simplest form, the rotor consists of a single loop of wire made to rotate within a magnetic field. In practice, the rotor usually consists of several coils of wire wound on an armature.

Armature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.

Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).

Stator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.

Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.

Brushes:The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.

How DC generator works?

The commutator rotates with the loop of wire just as the slip rings do with the rotor of an AC generator. Each half of the commutator ring is called a commutator segment and is insulated from the other half. Each end of the rotating loop of wire is connected to a commutator segment. Two carbon brushes connected to the outside circuit rest against the rotating commutator. One brush conducts the current out of the generator, and the other brush feeds it in. The commutator is designed so that, no matter how the current in the loop alternates, the commutator segment containing the outward-going current is always against the "out" brush at the proper time. The armature in a large DC generator has many coils of wire and commutator segments. Because of the commutator, engineers have found it necessary to have the armature serve as the rotor(the rotating part of an apparatus) and the field structure as the stator (a stationary portion enclosing rotating parts)

Operating principle of ammeters and voltmeters

Introduction:

Ammeter

Voltmeter

In our day today life, many times we require to measure different electrical quantities like current, voltage, resistance, etc. While doing experiment, there is necessity of multimeter. As we have already discussed about multimeter, how it measures different electrical quantities like electrical current, voltage, resistance, etc. But the basic instruments for the measurement of electric current and voltage are ammeters and voltmeters respectively.
Let us discuss these instruments one by one, operating principle (working principle) of ammeters and voltmeters, finally major differences between ammeters and voltmeters.

Operating Principle:

Analog ammeters and voltmeters are classed together as there are no fundamental differences in their operating principles. The action of all ammeters and voltmeters, with the exception of electrostatic type of instruments, depends upon a deflecting torque produced by an electric current .in an ammeter this torque is produced by a current to be measured or by a fraction of it. In a voltmeter this torque is produced by a current which is proportional to the voltage to be measured. Thus all analog voltmeters and ammeters are essentially current measuring devices.
The essential requirement of measuring instruments are (i) that its introduction into the circuit, where measurements are to be made, does not alter the circuit conditions ;(ii)the power consumed by them for their operation is small.

Ammeters:

Ammeters are connected in the series with the circuit whose current is to be measured. The power loss in an ammeter is (I^2.Ra) where I is the current to be measured Ra is the resistance of the ammeter therefore ammeter should have low electrical resistance so that they cause a small voltage drop and consequently absorb small power.

Voltmeters:

Voltmeters are connected in parallel with the circuit whose voltage is to be measured .the power loss in voltmeter is (V^2/Rv), where V is the voltage to be measured and Rv is the resistance of the voltmeter. Therefore voltmeters should have a high electrical resistance, in order that the current drawn by them is small and consequently the power consumed is small.

Difference between Ammeters and voltmeters:

Parameters	Ammeter	Voltmeter
Connection	It is to be connected in series mode	It is to be connected in parallel mode
Resistance	It has comparatively low resistance	It has high resistance
Uses	It is used to find the amount of current flowing in the circuit	It is used to find the potential difference in the circuit
Circuit	Circuit must be disconnected in order to attach the ammeter	Circuit does not need to be disconnected
Accuracy	Considered as less accurate	Considered as more accurate compared to ammeter

Working principal of DC Motor

                            Construction of DC Motor

           A DC Motor like we all know is a device that deals in the conversion of Electrical energy to mechanical energy and this is essentially brought about by two major parts required for the construction of dc motor, namely.

                 1)       Stator – The static part that houses the field windings and receives the supply and

                 2)      Rotor – The rotating part that brings about the mechanical rotations.

  Other than that there are several subsidiary parts namely the

                 3)      Yoke or casting of dc motor.

                 4)      Magnetic poles of dc motor.

                 5)      Field winding of dc motor.

                 6)      Armature winding of dc motor.

                 7)      Commutator of dc motor.

                 8)      Brushes of dc motor.

     All these parts put together configures the total construction of a dc motor\, that has been pictorially represented in the diagram below.

Now let’s do a detailed discussion about all the essencial parts of dc motor.

                                   Construction of yoke of dc motor

             The magnetic frame or the yoke of dc machine made up of cast iron or steel and forms an integral part of the stator or the static part of the motor. Its main function is to form a protective covering over the inner sophisticated parts of the motor and provide support to the armature. It also supports the field system by housing the magnetic poles and field windings of the dc motor.

                              Construction of Magnetic poles of dc motor

                   The magnetic poles are structures fitted onto the inner wall of the yoke with screws. The construction of magnetic poles basically comprises of two parts namely, the pole core and the pole shoe stacked together under hydraulic pressure and then attached to the yoke. These two structures are assigned for different purposes,  the pole core is of small cross sectional area and its function is to just hold the pole shoe over the yoke, whereas the pole shoe having a relatively larger cross-sectional area spreads the flux produced over the air gap between the stator and rotor to reduce the loss due to reluctance. The pole shoe also carries slots for the field windings that produce the field flux.

                                    Construction of field winding of dc motor

                   The field windings of the dc motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when field current flows through it, then adjacent poles have opposite polarity are produced.

The field windings basically form an electromagnet, that produces field flux within which the rotor armature of the dc motor rotates, and results in the effective flux cutting.

                         Construction of Armature winding of dc motor

              The armature of the dc motor is attached to the rotor, or the rotating part of the machine, and as a result is subjected to altering magnetic field in the path of its rotation which directly results in magnetic losses.  For this reason the rotor is made of armature core, that’s made with several low-hysteresis silicon steel laminations, to reduce the magnetic losses like hysteresis and eddy current loss respectively. These laminated steel sheets are stacked together to form the cylindrical structure of the armature core.  

              The armature core are provided with slots made of the same material as the core to which the armature windings made with several turns of copper wire distributed uniformly over the entire periphery of the core. The slot openings a shut with fibrous wedges to prevent the conductor from plying out due to the high centrifugal force produced during the rotation of the armature, in presence of supply current and field.

The construction of armature winding of dc motor can be of two types:-

     i)      Lap Wnding-

            In this case the number of parallel paths between conductors A is equal to the number of poles P.

i.e A = P

***An easy way of remembering it is by remembering the word LAP-----à L A=P

  ii)         Wave Winding– 

            Here in this case, the number of parallel paths between conductors A is always equal to 2 irrespective of the number of poles. <br/>Hence the machine designs are made accordingly.

                               Construction of commutator of dc motor

The commutator of a dc motor is a cylindrical structure made up of copper segments stacked together, but insulated from each other by mica. Its main function as far as the dc motor is concerned is to commute or relay the supply current from the mains to the armature windings housed over a rotating structure through the brushes of dc motor.

                                  Construction of Brushes of DC Motor

The brushes of dc motor are made with carbon or graphite structures, making sliding contact over the rotating commutator. The brushes are used to relay the current from external circuit to the rotating commutator form where it flows into the armature windings. &there4; the commutator and brush unit of the dc motor is concernied with transmitting the power from the static electrical circult to the mechanically rotating region or the rotor.

DC Generator working principle

An electrical generator is a device that converts mechanical energy to electrical energy, generally using electromagnetic induction. The source of mechanical energy may be a reciprocating or turbine steam engine, water falling through a turbine or waterwheel, an internal combustion engine, a wind turbine, a hand crank, or any other source of mechanical energy.

The Dynamo was the first electrical generator capable of delivering power for industry. The dynamo uses electromagnetic principles to convert mechanical rotation into an alternating electric current. A dynamo machine consists of a stationary structure which generates a strong magnetic field, and a set of rotating windings which turn within that field. On small machines the magnetic field may be provided by a permanent magnet; larger machines have the magnetic field created by electromagnets.

The energy conversion in generator is based on the principle of the production of dynamically induced e.m.f. Whenever a conductor cuts magneticic flux , dynamically induced e.m.f is produced in it according to Faraday's Laws of Electromagnetic induction.This e.m.f causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux.

Generator Construction:

Simple loop generator is having a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnet or electro magnets.In case of without commutator the two ends of the coil are joined to slip rings which are insulated from each other and from the central shaft.Two collecting brushes ( of carbon or copper) press against the slip rings.Their function is to collect the current induced in the coil. In this case the current waveform we obtain is alternating current ( you can see in fig). In case of with commutator the slip rings are replaced by split rings.In this case the current is unidirectional.

Components of a generator:

Rotor: In its simplest form, the rotor consists of a single loop of wire made to rotate within a magnetic field. In practice, the rotor usually consists of several coils of wire wound on an armature.

Armature: The armature is a cylinder of laminated iron mounted on an axle. The axle is carried in bearings mounted in the external structure of the generator. Torque is applied to the axle to make the rotor spin.

Coil: Each coil usually consists of many turns of copper wire wound on the armature. The two ends of each coil are connected either to two slip rings (AC) or two opposite bars of a split-ring commutator (DC).

Stator: The stator is the fixed part of the generator that supplies the magnetic field in which the coils rotate. It may consist of two permanent magnets with opposite poles facing and shaped to fit around the rotor. Alternatively, the magnetic field may be provided by two electromagnets.

Field electromagnets: Each electromagnet consists of a coil of many turns of copper wire wound on a soft iron core. The electromagnets are wound, mounted and shaped in such a way that opposite poles face each other and wrap around the rotor.

Brushes:The brushes are carbon blocks that maintain contact with the ends of the coils via the slip rings (AC) or the split-ring commutator (DC), and conduct electric current from the coils to the external circuit.

How DC generator works?

The commutator rotates with the loop of wire just as the slip rings do with the rotor of an AC generator. Each half of the commutator ring is called a commutator segment and is insulated from the other half. Each end of the rotating loop of wire is connected to a commutator segment. Two carbon brushes connected to the outside circuit rest against the rotating commutator. One brush conducts the current out of the generator, and the other brush feeds it in. The commutator is designed so that, no matter how the current in the loop alternates, the commutator segment containing the outward-going current is always against the "out" brush at the proper time. The armature in a large DC generator has many coils of wire and commutator segments. Because of the commutator, engineers have found it necessary to have the armature serve as the rotor(the rotating part of an apparatus) and the field structure as the stator (a stationary portion enclosing rotating parts)

The basic working principle of transformer

Transformer refers to the static electromagnetic setting which can transfer power from one circuit to another one. In AC circuits, AC voltage, current and waveform can be transformed with the help of Transformers. Each transformation is usually to transfer from one circuit to another one by the way of electromagnetism, but it has no direct relation with this circuit. It also can be transformed through electromagnetism (electrical manner). This electromagnetism is known as auto-transformer.
　　Transformer plays an important role in electronic equipment. AC and DC voltage in Power supply equipment are almost achieved by transformer’s transformation and commutation. At the same time the electrical parameters transformed by transformer are not one but a few ones.
　　Most of the isolation, matching and impedance in the circuit carry out by transformer.
　　Most of isolation, matching and impedance in the circuit carry out by transformer.
　 　Simple schematic diagram of the transformer is shown in (1-1). It is connected by closed-magnet (iron cores), two windings and AC power supply. The winding is called the primary winding; another winding is connected with load, and it is called secondary windings.

 Figure 1-1 structural schematic diagram of transformer

　　No-load state of Transformer: viz. the disconnecting state between he secondary winding and load (Figure 1-2). Connect the primary winding and the power supply of AC voltageU1, and then it will produce alternating current I0, this current is called no-load currents. This current set up alternating magnetic flowφ0 which is closed along iron core magnetic circuit. At the same time, it traverses the primary winding and secondary winding, and then produces inducting electromotive forceE2 (secondary no-load voltage).