Transistor as amplifier and switch

Use of transistor as amplifier

To use a transistor as an amplifier it is used in common emitter configuration. The operating collector current is set nearly half the max collector current with the help of collector load resistance and base biasing resistance. transistor never go in saturation or cutoff value of collector current. Some short of negative feedback is also given by providing emitter resistance and emitter by pass capacitor. This negative feed back improve the quality of signal amplified.

Use of transistor as oscillator

A transistor connected in common emitter configuration just like an amplifier circuit can be converted to a oscillator just by providing a positive feedback from output circuit to input circuit. In common emitter circuit the output wave form is always 180° out of phase with input wave form. if some part of output is fed back to input in additive phase  then it becomes oscillator. below is diagram of resistance capacitance phase shift oscillator.

Use of transistor as switch 

A transistor can be used as a switch to control small current load. Relay can  be controlled by a transistor to control heavy loads. In switch mode transistor is biased to either cutoff current (for switch off) or to saturation current (for switch on).Set reset flip flop can be used with transistor to get latching property.

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Impedance, resistance, inductive reactance and capacitive reactance

Introduction:- The current flowing in a DC circuit is basically calculated by ohm's law. The current is equal to voltage divided by resistance ie I =V/R. But in ac circuit effective resistance offered by an inductance or/and capacitance are also to be considered. The resistance to flow of ac current by an inductance is known as inductive reactance its value is given by 2πfL where f is frequency of supply and L is inductance of inductor coil. Inductive reactance is denoted by XL. Similarly the resistance offered by a capacitor is known as capacitive reactance given by 1/2πfC where f is frequency and C is capacitance of capacitor. Capacitive reactance is denoted by Xc. A circuit with two or all three components  present, the effective resistance offered by circuit is known as impedance of circuit denoted by Z. Then current I =V/Z amps. Where Z is given by formula

Resistance:- When ac supply is given to a pure resistance. The current flowing trough resistance is equal to V/R amps and it is in phase with applied voltage. The vector diagram is given below.

Pure inductance:- When ac supply is given to a pure inductance the magnitude of current is given by V/XL where XL is known as inductive reactance is equal to 2πf,L. The current is 90 degree lagging to the voltage. The vector diagram is given below.

Pure capacitance:- When ac supply is applied to a pure capacitance the current flowing through capacitor is given by V/Xc where Xc is known as capacitive reactance, given by 1/2πfC. The current is leading voltage by 90 degree. The vector diagram is given below.

Series R L circuit:- Any inductance can never be a pure inductance it has some internal resistance also. So any inductance can be considered as an inductance in series with a resistance. So for this effective resistance offered is given by Z = √R²+X_L². So the current through inductance is I = V/Z amp. Current is lagging voltage by by an angle Φ = Cos^-1 R/Z.

Series RC circuit:- When ac supply is applied to a series circuit of resistance and capacitance. In series combination of resistance and capacitance the resistance offered by circuit is given by impedance Z = √R²+ Xc². So that current through circuit is I = V/Z. The current is leading voltage by an angle Φ = Cos^-1 R/Z.

Series R L C circuit:- In series RLC circuit the current flowing through circuit is given by I = V/Z where Z is impedance of circuit given by formula

The current may be leading or lagging the voltage as per the value of capacitive / inductive reactance which ever is greater. The phase angle is given by Φ = Cos^-1 R/Z. The difference of XL and Xc taken. 

In this way the current and power factor of any series circuit can be calculated.

Effects of low power factor

Effects of low power factor

Phase angle lagging / leading:- AC (Alternating current) voltage or current is continuously varying in magnitude and direction with respect to time. Its instantenious magnitude varies according to sine function of angle from 0 to 360 degree or 0 to 2π radian. Voltage value is increases from 0 volt to maximum positive value Emax volt and then decreases back to 0 volt in positive half cycle. In negative half cycle voltage decreases from 0 volt to maximum negative value -Emax volt and then  increases to  0 volt. These are known as positive half cycle and negative half cycle. One positive half cycle and one negative half cycle makes a complete cycle. The alternating voltage cycles are repeated continuously. The number of cycles per second is known as frequency of supply. For 50 hz supply there are 50 cycles per second that means supply voltage will change its direction 50 times in a second. For one cycle it takes 1/50 second ie 20 mili second known as time period of ac cycle denoted by capital letter T. A cycle consist of one positive and one nrgative half cover 0 to 360 degree angle. 0 degree of this cycle is consider as reference point for angle measurement. For example in three phase system three alternating voltage are present whose staring point is 120 degree apart from each other. If we consider red phase as first phase then yellow phase 0 degree point is 120 degree after the red phase 0 degree starting poing. Similarly blue phase 0 degree point is 120 degree after the yellow phase 0 degree point. Again red phase will come after 120 degree of blue phase and this continue endlessly. This 120 degree is known as phase difference. As in this case yellow phase voltage come 120 degree after red phase it said like this  the yellow phase is 120 degree lagging red phase. Or red phase is leading yellow phase by 120 degree. The word leading means before and lagging means after.

Types of load :- there are mainly three types of load. Resistive load, inductive load and capacitive load.

Resistive load:-When  ac voltage is applied to a resistive load such as filament bulb the current flowing through resistance is in same page with voltage, means the 0 degree starting point of current is at the same 0 degree point of voltage. This is said as current is in phase with voltage. The power factor is cosine of angle between voltage vector and current vector (ac quantities are represented as vector)
Power factor = cos(0)
Power factor =1
So in resistive load power factor is always 1.

Inductive load:- Inductance always oppose to change of flow of current. Due to this property when ac voltage is applied to an inductive load ac current is start flowing through it but the current is some degree (say for example 30 degree) lagging the aoplied voltage. Now power factor is cosine of angle
Power factor = cos(30)
Power factor = .866
So here we say power factor is .866 lagging.

Capacitive load:- When ac supply is applied to a capacitive load the current flowing in pure capacitor is 90 degree leading with voltage. Hence the power factor is cos(90) = 0  leading.

Conclusion:- in resistive load power factor is always 1 also said unity power factor. In pure inductor power factor is 0 lagging. But inductor is always having some resistance so that power factor lies between 0 to less than 1 lagging. in case of capacitor power factor if 0 leading.

Power in ac circuit:- The power in ac circuit is product of voltage and component of current in phase with voltage. So if applied voltage is V and current flowing in circuit is I and the angle between voltage and current is phi. Then

Power = V x I x Cos(phi)
The equation show that cos(phi) is a multiplying  factor in power calculation, that,s why it is known as power factor.

Effect of power low factor:- It is clear from equation that if power factor is low then more current is required for the same load. Therefore any load which is running at low power factor will draw more current from power supply system.

Disadvantage of lower power factor

1 conductor size is line need to be of higher size as per increased demand of load current.
2 All switchgear sizes are to be of higher size.
3 Transformer will supply less power as compared to they could supply at good power factor.

Due to all these problems power supply authorities are always insists for improving power factor for industrial consumers,

Main causes of Motor burning

Introduction:- Number of motor burns  routinely and reason for motor burning is never very clearly find out. Motor can run without burning for life time of motor if all the reasons for motor burning are avoided.

Main reasons:- Motor winding burnt only because of temperature rise of winding. Reasons of high temperature of winding is heat produced in side of the motor. Heat is produced in winding of motor due to load current flowing in winding, Induced current flowing in rotor bars or winding..Heat generated in lamination due to  hysteresis loss and eddy current loss. Some heat is generated in bearings due to friction. All the sources of heat, generate heat but motor dissipate that heat efficiently as all are considered in design.If any of heat source start generating more heat, temperature of motor will increase and ultimately motor will burn.

Motor protection with overload relay- When a motor operates at a higher load than designed it gets heated. The load current increases and thermal overload relay starts working. It takes some time to trip as per current time characteristics of thermal overload relay. By tripping motor it protects motor from burning. But in this way motor gets overheat hence winding wire and stator core starts loosing their properties. After many numbers of tripping winding wire insulation is failed and motor burnt. A better practice of thermal over load setting may be use to set thermal at 90 % of motor full load current. Some person use thermal setting slightly higher then running load of motor.To avoid such failure motor must be operated within there load capacity. In machines like belt conveyor, elevators etc mechanical equipment health must be maintained to reduces unwanted frictional load. Jamming of material near moving parts to be removed to reduce load.

Motor cleaning:-Motor body, cooling fins and cooling ducts are kept clean, All heat generated in motor will dissipated and motor temperature  remains in safe limit This will improve motor cooling and hence the life of motor.

Bearings:- Bearing are maintained well lubricated so that bearings will run smoothly and their temperature lies within limit. If a motor run with dry bearing or some what jam or play in bearing, more heat will generate in bearing which will cause over heating of end shied and hence motor stator which will heat winding and ultimate motor burning takes place. Some times due to play in bearings stator core and rotor core starts rubbing each other which increases stator core temperature and hence causing winding burnt out.The load  increases because of bearing problems or core rubbing is not too much to operate overload relay that's why in these problems motor does't trips with overload before winding burnt out.

Vibration in motor:- Due to misalignment or unbalance in rotating part vibration generates in motor. Due to vibration a relative motion starts taking place between winding and stator core, between adjacent turns of coils near over hang and between soldered joint and turns of coil. This rubbing of wire results in insulation failure and causing internal short circuit.This problem is also not sense by over load relay. It can be saved by keeping operation of motor vibration free. If any vibration observed, should be rectified by alignment, balancing or some foundation defects correction.

This is what I have seen in my field experience. If some one like to give more reason please feel welcome to comment because I really like to avoid motor burning.
Thanks

Three phase induction motor reversible starter

Introduction :- Many application requires the direction reversing of induction motors.for example electric hoists, electrical overhead cranes, lifts used in industries. Electrical actuators for operating gates in pipe lines for flow control. now a days various door control mechanisms are also using reversible motors.

Basic:- direction of three phase motor is changed by inter changing the supply phase sequence by inter changing two phase connection.

Circuit diagram :- circuit diagram is given below.

 It uses two Contactor, main circuit bagrker , over load relay, and three push buttons for forward reverse and stop. Three indication lamps can be used to indicate status of running of motor. MCB is first switched on. Forward button is pressed to run in forward direction. When forward push button is pressed C1 contactor picks up and motor run in forward direction and L1 indication  lamp indicate forward running of motor. To stop motor stop button is to be pressed, it dropped out contactor and motor stops L3 indication lamp indicate the motor is stopped. Similarly to run in reverse direction reverse button is to be pressed, it picks up C2 contactor which run the motor in reverse direction by inter changing power supply two phases. This type of circuit is used in all reversing operation with minor changes. Some reversible dol starter use external ICTP in place of MCB. Some places indication lamps are not used. In this 230 volt control is used in some circuit 400 volt or 110 volt control supply is used. In case of overload of motor, thermal overload relay trips the motor and save from burning.

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no-load vector diagram of-transformer

THREE PHASE SYNCHRONOUS MOTOR CONSTRUCTION DETAILS AND OPERATION

    THREE PHASE SYNCHRONOUS MOTOR CONSTRUCTION DETAILS AND OPERATION

Three phase synchronous motor is normally comes in higher kilowatt and voltage rating. These motors are used for driving heavy loads such as Exhaustors fan, compressors etc. These motor are designed to run continuously for months together. In large industries these motors serve two fold purpose of driving mechanical load as well as power factor improvement. These motors can run at leading power factor with increase in  excitation current. The main feature of synchronous motor is its constant speed operation. The motor runs at constant synchronous speed given by supply frequency and number of poles in stator winding.

CONSTRUCTION

The motor is mainly consist of  a stator, rotor, bearing and cooling system for higher kilowatt motors.

Stator:- The stator is made of silicon steel lamination of desired diameter and stacked for suitable stator length. The motor kilowatt rating is depends on diameter of core at air gap and core length. Radial ventilation ducts are provided at regular distance of 70 to 80 mm for forced air cooling of motor. Stator is wound with a three phase  winding with suitable number of poles depending on motor speed. High voltage winding is used for motors above 500 kw rating. voltage is usually 6.6 kv or 11 kv depending on supply system standards. The stator is fixed in motor body. Body is made of steel with box type fabricated construction.

Rotor:- there are two types of rotors used in synchronous motors.

1 Smooth cylindrical rotor- It is used in machine with 3000 RPM in India for 50 hz. Smooth cylindrical rotor has slots on the periphery which holds distributed winding to produce sinusoidal magnetic field.

2 Salient pole rotor- It is used in motors with 1500 RPM and lower RPMs.

Salient pole rotor- The number of poles in a rotor depends on the number of poles in stator winding. For example if a motor has 4 poles in stator winding then rotor will also have 4 poles and must be wound for 4 pole dc winding. In Salient pole rotor a spider is mounted on shaft. Rotor poles are fixed   on spider with dovetails or T slots. The winding leads are connected to two slip rings mounted on shaft. A low voltage 24 volt to 50 volt high amperes DC supply is given to rotor for synchronizing the motor. This rotor current is normally 300 to 500 amp for 1 MW motor. In addition to this DC excitation winding there is a squirrel cage damper winding on the rotor poles face. This damper winding provide  the starting torque to the motor which makes motor self starting.

Pedestal bearing :-There are two pedestal bearings to support motor shaft. Bearings are made of babbitt metal. A oil cooling system is used for cooling of bearings.
Heat exchanger :-A heat exchanger is used in forced air cooling circuit to transfer heat from cooling air to cooling water.

Exciter:- Exciter is a low voltage high current DC generator coupled to the main motor shaft. The DC supply generated in exciter is used for excitation supply of main motor.

Operation:- Synchronous motors are started at no load so first of all damper gate of air line is completely closed. Three phase power is given to motor by closing main ocb (oil circuit breaker). As the three phase supply is switched on a rotating magnetic field is established in stator core which induces torque in damper winding and rotor starts rotating. This is just like a simple induction motor starting. Gradually motor speed increases and reaches near synchronous speed, DC contactor is automatically picks up and DC excitation supply is switched on to rotor circuit through carbon brushes and slip rings. This excitation supply magnetize the rotor poles which pulls up the rotor poles to stator poles and then  stator and rotor pole gets magnetically locked known as motor is synchronized. Then rotor excitation current is regulated as per load and power factor requirement. Now damper can be opened as per requirement.

Running parameter:- stator current, rotor current, winding temperature, bearing temperature, oil flow, motor vibration  etc are monitored and recorded on hourly basis.

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THREE PHASE WELDING RECTIFIER CIRCUIT WITH WORKING PRICIPLE

Three phase welding rectifier circuit working 

Introduction:- A three phase welding rectifier is used in places where very smooth and perfect welding work is required. It consist of three phase step down transformer, rectifier and current controlling saturable reactor.

Circuit description:- A three phase  transformer is used for step down three phase 400 volt to 3 phase 100 v ac for welding rectifier. In a three phase transformer there are three winding in primary connected in star/delta connection and three secondary winding also connected in star / delta connection. A three phase bridge configuration is used, as shown in diagram for ac to dc conversion.

for output current contolling saturable reactor is used.saturable reactor has two winding one is used in series with output dc line and other winding is connected dc power source with variable resistance to control output current.

A  MCB, Contactor and Over Load relay is used  in control circuit to switch on power to welding transformer and protection from over load. one cooling fan is also used for cooling of transformer not shown in diagram. Start and stop button is provided for on/off contactor.

Controlling Current:- When controlling current is less saturable reactor reactance is more causing low output current. As control current trough reactor is increases the reactor reactance decreases causing more output current.


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