The electrostatic instrument works on the principle of mechanical interaction of the electrodes that consists the opposite electrical charge. The quantity which is measured by the electrostatic instrument is converted into either AC or DC voltage.

There are two ways of constructions of electrostatic instruments;

1. In the electrostatic instrument, the charge is stored between the plates. The electrostatic instrument consists the charges of two opposite polarity and force of attraction occurs between these two plates. Because of the force of attraction, the movable plates move towards the fixed plates and store the maximum electrostatic energy.
2. In this type of instruments, there are forces of attraction or repulsion occur between the rotary plate.

Force and Torque Equations of Electrostatic Instruments

Linear Type Electrostatic Instrument

The figure below shows the linear electrostatic type instrument. The plates A become positively charged, and the plate B becomes negatively charged. The positive charge plates become fixed, and the negative plates become movable. The spring is connected to the negatively charged plates for controlling the movement.

When the voltage is applied to the plate, then the force of attraction induces between them. The plate tries to moves towards A until the force becomes maximum. The C is the capacitance (in farad) between the plate. The expression gives the total energy stores between the plates.

Rotary type Electrostatic Instrument

This type of instrument carries the rotary plates. Because of the movement of the rotary plate, the force of attraction or repulsion occurs between them.

Advantages of Electrostatic Instrument

1. Both the AC and DC voltage can be measured by using the electrostatic instrument.
2. The electrostatic type instrument consumes very less power.
3. The high value of voltage can be measured by using the instrument.
4. In the rotary type electrostatic instrument, in spite of linear displacement, the angular displacement occurs between the fixed and the moving plates.
5. The instrument has less Waveform and frequency error.
6. No error occurs because of the stray magnetic field.
7. The instrument is designed for large voltage.

Disadvantages of Electrostatic type instrument

1. The non-uniform scale is used in the instrument.
2. The force of very small magnitude involves in the instrument.
3. The instrument is quite costly as compared to the other instrument.
4. The size of the instrument is also very large.

The reading of the electrostatic instrument is free from frequency.

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The following are the characteristic of the temperature transducer.

1. The input is always a thermal quantity.
2. Transducer mostly converts the thermal quantity into an alternating quantity.
3. It is used for measuring the temperature and heat flow of the devices.

Sensing Element

The sensing element which is used in the temperature transducer must have the properties of changing the characteristics with the variation of temperature. For examples – In resistance thermometer, the platinum metal is used as the sensing element.

1. The temperature sensing element converts the temperature into the heat.
2. The change in temperature concerning resistance should be large.
3. The sensing element must have high resistivity.

Types of Temperature Transducer

The temperature transducer is mainly classified into two types.

Contact Temperature Sensor Device

In such type of transducers, the sensing element is directly connecting to the thermal source. And the heat is transferred by the phenomenon of conduction. The conduction is the process through which the heat is transferred from one substance to another without the movement of the substance.

Non-contact Type Temperature Sensor Device

The sensing element is directly not contacting the thermal source. They use convection phenomenon for the transfer of heat. The convection is the process in which the heat is transfer by the movement of the substance. The non-contact type transducer is sub-categorised into the following categories.

Thermistor – The thermistor is a type of resistor whose resistance varies with the temperature. The resistance is measured by passing the small measured direct current, and this current causes the voltage drop across the resistance.

 The resistance thermometer is categorized into two types.

• Negative Temperature Coefficient – Used for sensing the temperature.
• Position Temperature Coefficient – Used for controlling the current.

Resistance Thermometer – The resistance of metal varies with the temperature. And this property of the metal is used for measuring the temperature. The resistance thermometer uses the platinum as the sensing element and hence measures the surrounding temperature.

Thermocouples – The thermocouple converts the temperature into the electrical energy at the point of the contact. It works on the principle that the metals have different temperature coefficient and when these two metals join together then the voltage induces, and this voltage is directly proportional to the temperature.

Integrated Circuit Temperature Transducer – The IC temperature transducer uses the combination of temperature sensing element and an electronic circuit for measuring the temperature. The integrated temperature transducer has a linear characteristic. The operating range of the transducer is very less. It lies between 0ᵒ to 200ºC which is one of the major disadvantages of the transducer.

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Symbolic Representation of voltmeter

The voltmeter is represented by the alphabet V inside the circle along with the two terminals.

Why is Voltmeter connected in Parallel?

The voltmeter constructs in such a manner that their internal resistance always remains high. If it connects in series with the circuit, it minimises the current which flows because of the measurand voltage. Thus, disturb the reading of the voltmeter.

The voltmeter always connects in parallel with the circuit so that the same voltage drop occurs across it. The high resistance of the voltmeter combines with the impedance of the element across which it is connected. And the overall impedance of the system is equal to the impedance that the element had. Thus, no obstruction occurs in the circuit because of the voltmeter, and the meter gives the correct reading.

Why Voltmeter has High Resistance?

The voltmeter is constructed with very high internal resistance because it measures the potential difference between the two points of the circuit. The voltmeter does not change the current of the measuring device.

If the voltmeter has low resistance, the current passes through it, and the voltmeter gives the incorrect result. The high resistance of the voltmeter does not allow the current to pass through it and thus the correct reading is obtained.

Types of Voltmeter

The voltmeter is classified into three ways. The classification of the voltmeter is shown in the figure below.

On the basis of the construction, the voltmeter is of the following types.

PMMC Voltmeter

It works on the principle that the current carrying conductor placed in the magnetic field and because of the current the force acting on the conductor. The current induces in the PMMC instrument because of the measurand voltage, and this current deflects the pointer of the meter.

The PMMC voltmeter uses for DC measurement. The accuracy of the instrument is very high and having low power consumption. The only disadvantage of the instrument is that it is very costly. The range of the PMMC voltmeter increases by connecting the resistance in series with it.

MI Voltmeter

The MI instrument means moving iron instrument. This instrument uses for the measurement of both the AC and DC voltage. In this type of instrument, the deflection is directly proportional to the voltage of the coil. The moving iron instrument is classified into two types.

• Attraction Type Moving Iron Instrument
• Repulsion Type Moving Iron Instrument

Electro-dynamometer Voltmeter

The electro-dynamometer voltmeter is used for measuring the voltage of both AC and DC circuit. In this type of instruments, the calibration is same both for the AC and DC measurement.

Rectifier voltmeter

Such type of instrument is used in AC circuits for voltage measurement. The rectifier instrument converts the AC quantity into the DC quantity by the help of the rectifier. And then the DC signal is measured by the PMMC instrument.

The following are the classification of instruments regarding the displays of output reading.

Analogue Voltmeter

The analogue voltmeter uses for measuring the AC voltage. It displays the reading through the pointer which is fixed on the calibrated scale. The deflection of the pointer depends on the torque acting on it. The magnitude of the develops torque is directly proportional to the measuring voltage.

Digital Voltmeter

The voltmeter which displays the reading in the numeric form is known as the digital voltmeter. The digital voltmeter gives the accurate result.

The instrument which measures the direct current is known as the DC voltmeter, and the AC voltmeter is used in the AC circuit for alternating voltage measurement.

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Types of Electric Motor

The classification of an electric motor is shown in the figure below.

AC Motor

The AC motor converts the alternating current into mechanical power. It is classified into three types; they are the induction motor, synchronous motor, the linear motor. The detail explanation of the motor is expressed below.

1. Induction Motor

The machine which never runs at synchronous speed is called the induction or asynchronous motor. This motor uses electromagnetic induction phenomenon for transforming the electric power into mechanical power. According to the construction of rotor, there are two types of an induction motor. Namely squirrel cage induction motor and phase wound induction motor.

• Squirrel Cage Rotor – The motor which consists squirrel cage type rotor is known as a squirrel cage induction motor.The squirrel cage rotor decreases the humming sound and the magnetic locking of the rotor.
• Phase Wound Rotor – This rotor is also known as the slip ring rotor, and the motor using this type of rotor is known as the phase wound rotor.

By the phases, the induction motor is classified into two types. They are the single phase induction motor and the three phase induction motor.

• Single phase induction motor – The machine which changes1-phase AC electric power into mechanical power by using an electromagnetic induction phenomenon is known as a single phase induction motor.
• Three-phase Induction Motor – The motor which converts 3-phase AC electric power into mechanical power, such type of motor is known as a three-phase induction motor.

2. Linear Motor

The motor which produces the linear force instead of the rotational force is known as a linear motor. This motor has unrolled rotor and stator. Such type of motor is used on sliding doors and in actuators.

3. Synchronous Motor

The machine that changes the alternating current into mechanical power at the desired frequency is known as the synchronous motor. In the synchronous motor, the speed of the motor is synchronised with the supply current frequency.

The synchronous speed is measured regarding the rotation of the magnetic field, and it depends on the frequency and the poles of the motor. The synchronous motor is classified into two types they are reluctance and the hysteresis motor.

• Reluctance Motor – The motor whose starting process is similar to an induction motor and which runs like a synchronous motor is known as the reluctance motor.
• Hysteresis Motor – The hysteresis motor is the type of a synchronous motor which has the uniform air gap and does not have any DC excitation system. The torque in the motor is produced by the hysteresis and the eddy current of the motor.

DC Motor

A machine that converts the DC electrical power into mechanical power is known as DC motor. Its work depends on the basic principle that when a current carrying conductor is placed in a magnetic field, then a force exerted on it, and torque develops. The DC motor is classified into two types, i.e., the self-excited motor and separately excited.

1. Separately Excited Motor

The motor in which the DC winding is excited by the separate DC source is known as the separately excited dc motor. With the help of the separate source, the armature winding of the motor is energised and produce flux.

2. Self-Excited Motor

By the connection of field winding the Self-excited DC motor is further classified into three types. They are the series, shunt and compound wound DC motor.

• Shunt Motor – The motor in which field winding is placed parallel with the armature, such type of motor is known as shunt motor. 
• Series Motor – In this motor the field winding is connected in series with the armature of the motor.
• Compound Wound Motor – The DC motor which has both the parallel and series connection of the field winding is known as the compound wound rotor. The compound wound motor is further categorised into short-shunt and long-shunt motor.
  
  • Short Shunt Motor  – If the shunt field winding is only parallel to an armature of the motor and not the series field, then it is known as the short shunt connection of the motor.
  • Long Shunt Motor – If the shunt field winding is parallel to both the armature and the series field winding then the motor is known as the long shunt motor.

Apart from the above mention motors, there are various other types of the special machine which have additional features like stepper motor, AC and DC servo motor, etc.

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The energy conversion takes place via the medium of an electrical and magnetic field. When the conversion takes place from electrical energy to mechanical energy, then the device is called motor. When the mechanical energy is converted into electrical energy, the device is known as a generator. The conversion of energy takes place from following two electromagnetic phenomena.

1. When the current carrying conductor moves in a magnetic field, the voltage induces in the conductor.
2. When a current carrying conductor placed in a magnetic field, the conductor experiences a magnetic field.

Usually, the converter uses a magnetic field as a coupling medium between the electric and magnetic system because their energy storing capacity is much greater than that of the electric field.

The energy balance equation for motor action is given as

In motor action, the electric power is taken as the input from the main supply. The mechanical energy obtained from the output is not fully utilised. The fraction of the energy is dissipated in frictional losses (friction and windage).

For generator action, the energy balance equation is written as

The total energy stored in any electromechanical devices is equal to the sum of the energy stored in the magnetic field, the energy stored in the mechanical system, and potential or kinetic energy.

The energy dissipated in an electric circuit is equal to the sum of the energy dissipated in an electrical circuit as an ohmic loss, energy dissipated in a magnetic circuit as hysteresis and eddy current losses and the energy consumed in the mechanical system as friction and windage loss, etc.

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The three phase AC motors are mostly applied in the industry for bulk power conversion from electrical to mechanical. For small power conversion, the single phase AC motors are mostly used.The single phase AC motor is nearly small in size, and it provides a variety of services in the home, office, business concerns, factories, etc. Almost all the domestic appliances such as refrigerators, fans, washing machine, hair dryers, mixers, etc., use single phase AC motor.

The AC motor is mainly classified into two types. They are the synchronous motor and the induction motor.

Synchronous Motor

The motor that converts the AC electrical power into mechanical power and is operated only at the synchronous speed is known as a synchronous motor.

Working Principle of a Synchronous Motor

When supply is given to synchronous motor, a revolving field is set up. This field tries to drag the rotor with it, but could not do so because of rotor inertia. Hence, no starting torque is produced. Thus, inherently synchronous motor is not a self-starting the motor.

Induction Motor or Asynchronous Motor

The machine which converts the AC electric power into mechanical power by using an electromagnetic induction phenomenon in called an induction motor. The induction motor is mainly classified into two types., i.e., the single phase induction motor and the three phase induction motors.

Working Principle of an Induction Motor

In an induction machine the armature winding serve as both the armature winding and field winding. When the stator windings are connected to an AC supply flux is produced in the air gap. The flux rotates at a fixed speed called synchronous speed. This rotating flux induces voltages in the stator and rotor winding.If the rotor circuit is closed, the current flows through the rotor winding and react with the rotating flux and a torque is produced. In the steady state, the rotor rotates at speed very close to synchronous speed.

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Lap Winding

In lap winding, the conductors are joined in such a way that their parallel paths and poles are equal in number. The end of each armature coil is connected to the adjacent segment on the commutator. The number of brushes in the lap winding is equal to the number of parallel paths, and these brushes are equally divided into negative and positive polarity.The lap winding is mainly used in low voltage, high current machine applications. They are three types

1. Simplex Lap Winding
2. Duplex Lap Winding
3. Triplex Lap Winding

1. Simplex Lap Winding: In simplex lap winding, the terminating end of one coil is joined to the commutator segment and the starting end of the next coil is placed under the same pole. Also, the number of parallel paths is similar to the number of poles of the windings.

2. Duplex Winding: In duplex winding the number of parallel paths between the pole is twice the number of poles. The duplex lap winding is mainly used for heavy current applications. Such type of winding is obtained by placing the two similar winding on the same armature and connecting the even number commutator bars to one winding and the odd number to the second winding.

3. Triplex Lap Winding: In triplex lap winding the windings are connected to the one-third of the commutator bars.

The lap winding has many paths and hence it is used for the larger current applications. The only disadvantage of the lap winding is that it requires many conductors which increase the cost of the winding.

Wave Winding

In wave winding, only two parallel paths are provided between the positive and negative brushes. The finishing end of the one armature coil is connected to the starting end of the other armature coil commutator segment at some distance apart.In this winding, the conductors are connected to two parallel paths irrespective of the number of poles of the machine. The number of brushes is equal to the number of parallel paths. The wave winding is mainly used in high voltage, low current machines.

If after passing one round, the armature winding falls into a slot to the left of its initial point, then the winding is said to be retrogressive.And if the armature windings fall on one slot to the right then it is called progressive winding.Assume the two layers winding and suppose that the conductor AB must be at the upper layer half of the slot on the left or right. Consider that the Y_B is the back pitch and Y_F is the front pitch. The sum of the back pitch and the front pitch is nearly equal to the pole pitch of the winding.The equation gives average pitch of the winding

If Z_A is the total number of conductor or coil side, then the average pitch is expressed by the equation, Where, P – number of poles
Since P is always even, then Z = PY_A ± 2, will always be considered as an even integer.

For progressive winding plus sign will use and for retrogressive winding negative winding will be used.

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The chopper was operated at high frequency due to which it upgrade the motor performances by decreasing the ripple and removing the discontinuous conduction. The most important feature of chopper control is that the regenerative braking is carried out at very low generating speed when the drive is fed from a fixed voltage to low DC voltage.

Motoring Control

The transistor chopper controlled separately excited DC motor is shown in the figure below. The transistor T_r is operated periodically with period T_r and remains open for a duration T_on.The waveforms of motor terminal voltage and armature current are shown in the figure below. During on the motor terminal voltage is V and operation of the motor is described as

In this interval, the armature current raises from i_a1 to i_a2. This interval is called duty interval because the motor is directly connected to the source.

At t = t_on, T_r is turned off. Motor current freewheels through diode D_f and motor terminal voltage is zero during interval t_on≤ t ≤ T. Motor operation during this interval is known as freewheeling interval and is described by

Motor current decreases from i_a2 to i_a1 during this interval.The ratio of duty interval t_on to chopper period T is called duty cycle.

Regenerative Braking

Chopper for regenerative braking operation is shown in the figure below. The transistor T_r is operated periodically with a period T and on-period of t_on. The waveform of motor terminal voltage v_a and armature current i_a for continuous conduction is shown in the figure below. The external inductance is added to increase the value of L_a. When the transistor is on, i_a increased from i_a1 to i_a2.

The mechanical energy is converted into electrical energy by the motor, now working as a generator, partly increased the stored magnetic energy in the armature circuit inductance and the remainder is dissipated in armature and transistors.

When the transistor is turned off, the armature current flows through diode D and the source V and reduces from i_a2 to i_a1. The stored electromagnetic energy and the energy supplied by the machine are fed to the source. The interval  0  ≤ t  ≤ t_on is called energy storage interval and the interval t_on ≤ t ≤ T called the duty interval.

Forward Motoring and Braking Control

The forward motoring operation of the chopper is obtained by the transistor T_r1 with the diode D₁.The transistor T_r2 and diode D₂ provide the control for forward regenerative braking operation.

For the motoring operation, transistor T_r1 is controlled, and for braking operation, the transistor T_r2 is controlled. Shifting of control from T_r1 to T_r2 shift the operation from motoring to braking and vice versa.

Dynamic Control

The dynamic braking circuit and its waveform are shown in the figure below. During the interval between 0 ≤ t ≤T_on, i_a increases from i_a1 to i_a2. The part of the energy is stored in inductance and rest is dissipated in R_a and T_R.

During the interval T_on≤ t ≤ T, i_a decreases from i_a2 to i_a1.The energies generated and stored in inductances are dissipated in braking resistance R_B, R_a and diode D.Transistor T_r control the magnitude of energy dissipated in R_B and therefore control its effective value.

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Three phase transformer is divided into four main groups according to the phase difference between the corresponding line voltage on the high voltage side and the low voltage sides. The phase difference is the angle by which the low voltage line lags the high line voltage, and is measured in units of 30° in clockwise direction. These groups are

• Group number 1 – no phase displacement
• Group number 2 – 180° phase displacement.
• Group number 3 – (-30°) phase displacement.
• Group number 4 – (+ 30°) phase displacement.

The connection Y d 11 gives the following information – Y indicates that the high voltage is connected to star and d indicates the low voltage is connected in delta. The 11 indicates that the low line voltage lag, high line voltage by 11 Χ 30° = 330° measured from higher voltage phasor in a clockwise direction.

The phasor differences can also be measured by using the clock methods. Consider the minute hand of the clock shown the high voltage and the low voltage winding is represented by the hour hand. The angle of 30° is the angle between two adjacent figures on the clock dial and is taken as the unit of dial shift.

When the hour hand of the clock is at 12, then the phase displacement is zero. When the hour hand is at 1 then the phase shift -30° degree. At 6 the phase shift is 6 Χ 30º = 180º. Similarly, when the hour hand is at 11 the phase shift is 11 Χ 30º  = 330º.

The number 0, 6, 1, and 11 in the group reference number indicates the primary to secondary phase shift regarding the hours of the clock. The connection designated by D y 11 is the delta-star transformer in which the low voltage line phasor is at 11 and is a phase advanced of +30° on the corresponding line voltage on the high voltage side.

Note: The only transformer in the same group may be connected in parallel. For example, a star-star, 3-phase transformers can be parallel with another three phase transformer whose windings are either connected in Y-Y or ∆-∆. The ∆-∆ transformer cannot be parallel with Y-∆ transformer.

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1. Three separate single phase transformer is suitably connected for three phase operation.
2. A single three-phase transformer in which the cores and windings for all the three phases are merged into a single structure.

The three single-phase transformer can be used as a three-phase transformer when their primary and secondary winding are connected to each other. The three phase transformer supply has many advantages as compared to three single phase units like it requires very less space and also very lighter smaller and cheaper in size. The three phase transformer is mainly classified into two types, i.e., the core type transformer and the shell type transformer.

Core Type Three Phase Transformer

Consider a three single phase core type transformer positioned at 120° to each other as shown in the figure below. If the balanced three-phase sinusoidal voltages are applied to the windings, the fluxes φ_a, φ_b and φ_c will also be sinusoidal and balanced. If the three legs carrying these fluxes are combined, the total flux in the merged leg becomes zero. This leg can, therefore, be removed because it carries the no flux. This structure is not convenient for the core.

The core of the three phase transformer is usually made up of three limbs in the same plane. This can be built using stack lamination. The each leg of this core carries the low voltage and high voltage winding. The low voltage windings are insulated from the core than the high voltage windings.

The low windings are placed next to the core with suitable insulation between the core and the low voltage windings. The high voltage windings are placed over the low voltage windings with suitable insulation between them. The magnetic paths of the leg a and c are greater than that of leg b, the construction is not symmetrical, and there is a resultant imbalance in the magnetising current.

Shell type Three Phase Transformer

The shell type 3-phase transformer can be constructed by stacking three single phase shell transformer as shown in the figure below. The winding direction of the central unit b is made opposite to that of units a and c. If the system is balanced with phase sequence a-b-c, the flux will also be balanced

The magnitude of this combined flux is equal to the magnitude of each of its components. The cross section area of the combined yoke is same as that of the outer leg and top and bottom section of the yoke. The imbalance in the magnetic path has very little effect on the performance of the three shell-type transformers. The windings of the shell type three phase transformer are either connected in delta or star as desired.

Also see; Three-Transformer Connections.

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