Archive for June 2014

UNIVERSAL MOTOR WORKING

UNIVERSAL MOTOR;

CONSTRUCTION;

Universal motor can be operated either on ac and dc supply.the construcation of universal is same as that of dc series motor.the stator and rotor are made up of laminated silicon steel, to reduce eddy current and hysteresis loss,when the motor is operated on ac supply.the field winding is placed in the stator and the armature winding is housed in the rotor. the field winding has a few number of turns of turns t reduce the reactance.

the field winding is connected in series with armature winding

OPERATION;

when the motor is connected to an ac supply,the same ac current fluxes through the field and armature winding.the field winding produces an alternating flux that reacts with the armature current and produces a torque.since both the armature current and field fluxes are reversed simultaneously,the torque always in the same direction.

CHARACTERISTICS;

the speed torque characteristics of universal motor are similar to that of dc series motor.

the speed increases with decreasing of load.at no load the speed of the motor is very high.however in a dc series motor the speed at no load is dangerously high value.

ADVANTAGES;

1.it is speed is high

2.ac and dc supply will be working

3.starting torque is high

APPLICATION;

1.vacuum cleaners

2.sewing machine

3.food mixers

4.machine tools

5.portable drilling machines

DISADVATAGES;

1.the speed at no load is dangerously high value.

Author : Unknown Comments : 0

Posted :

REPULSION INDUCTION MOTOR WORKING

REPULSION INDUCTION MOTOR

construction;

THIS motor is a combination of repulsion and induction motor.the stator of this is made with non salient pole with single phase winding.
THE rotor is provided with two separate windings in common slots.one winding is armature winding with commutator and short circuited brushes (just like dc motor) the other winding is of squirrel cage type.the inner winding is a squirrel cage winding and the outer winding is commutated winding.repulsion induction motor the reactance
of the squirrel cage winding is very high.

working;
at the time of starting the frequency of rotor emf is more
(f=sf at starting slip (s)=1).hence the reactance of inner cage winding is high XL.therfore very little current flows through
the inner cage winding.when the rotor picks up speed the frequency
of the rotor emf is reduced and therefore the reactance of the rotor
will decrease now the squirrel cage winding takes up a larger portion of the current.

the brushes are short circuited and ride on the commutator continuously.the speed torque characteristics of repulsion inducation motor.the starting torque is high.the speed regulation is also good.
it can develop torque sudden heavy loads.

APPLICATION;
1.it is used to drive compressors.
2.petrol pumps
3.air pump
4.mixing machines
5.machine tools
6.hoists

ADVANTAGES;
1.starting torque high
2.speed regulation good
3.it can develop torque sudden heavy loads
4.starting current reduced

Author : Unknown Comments : 0

Posted :

Bipolar Stepper Motor

Stepper motors are available in three basic configurations:

1) Universal Stepper Motor
A universal stepper motor can be connected as a bipolar or a uni-polar stepper motor.

2) Uni-polar Stepper Motor
A Uni-polar stepper motor can be used as uni-polar or a bipolar stepper motor.

3) Bipolar stepper Motor
A bipolar stepper motor can only be used as a bipolar.

Bipolar Stepper Basics

A bipolar stepper motor has one winding per stator phase. A two phase bipolar stepper motor will have 4 leads. In a bipolar stepper we don’t have a common lead like in a uni-polar stepper motor. Hence, there is no natural reversal of current direction through the winding.

A bipolar stepper motor has easy wiring arrangement but its operation is little complex. In order to drive a bipolar stepper, we need a driver IC with an internal H bridge circuit. This is because, in order to reverse the polarity of stator poles, the electric current needs to be reversed. This can only be done through a H bridge.

There are two other reasons to use an H Bridge IC

1) The current draw of a stepper motor is quite high. The micro-controller pin can only provide up to 15 mA at maximum. The stepper needs current which is around ten times this value. A external driver IC is capable of handling such high currents.

2) Another reason why H Bridge is used is because the stator coils are nothing but inductor. When coil current changes direction a spike is generated. A normal micro-controller pin cannot tolerate such high spikes without damaging itself. Hence to protect micro-controller pins, H bridge is necessary.

The most common H Bridge IC used in most Bipolar stepper interfacing projects is L293D.

Interfacing to Micro-Controller

4 micro-controller pins are required to control the motor. We need to provide the L293D with 5 V supply as well as the voltage at which the motor needs to operate. Since we will be using both the drivers of the IC, we will assert the enable pin for both of them.

Interfacing Diagram

There are three different ways in which we can drive the bipolar stepper motor-

1) Only one of the phase winding are energized at a time. That is, either AB or CD is energized. Of course the coils will be energized in such a way that we get correct polarity. But only one phase is energized. This type of stepping will give less holding torque because only one phase is energized.

2) In this method, both the phases are activated at the same time. The rotor will align itself between two poles. This arrangement will give higher holding torque than the previous method.

3) The third method is used for half stepping. This method is used generally to improve the stepping angle. Here, in step 1 only 1 phase is ON, then in step 2, 2 phases are ON, then again only one phase is ON and the sequence continues.

Bipolar Stepper Drives

Many companies have started assembling their own bipolar stepper drives. Care must be taken that you connect the stepper motor correctly to the drive. Also the drive must be able to supply sufficient current for you stepper. The micro-controller must only provide the step and direction signal to the drive. This method will occupy only two micro-controller pins and is very helpful in projects that require large number of micro-controller pins for other functions.

Unipolar Stepper v/s Bipolar Stepper

Both uni-polar and Bipolar steppers are used widely in projects. However, they have their own advantages and disadvantages from the application point of view. The advantage of a uni-polar motor is that we do not have to use a complex H bridge circuitry to control the stepper motor. Only a simple driver like ULN2003A will do the task satisfactorily. But, there is one disadvantage of uni-polar motors. The torque generated by them is quite less. This is because the current is flowing only through the half the winding. Hence they are used in low torque applications.

On the other hand, bipolar stepper motors are a little complex to wire as we have to use a current reversing H bridge driver IC like an L293D. But the advantage is that the current will flow through the full coil. The resulting torque generated by the motor is larger as compared to a uni-polar motor.

Author : Unknown Comments : 0

Posted :

Electric Lamp | Types of Electric Lamp

An electric lamp is a conventional light emitting component used in different circuits, mainly for lighting and indicating purposes. The construction of lamp is quite simple, it has one filament surrounding which, a transparent glass made spherical cover is provided. The filament of the lamp is mainly made of tungsten as it has high melting point temperature. A lamp emits light energy as the thin small tungsten filament of lamp glows without being melted, whileelectric current flows through it.

Uses of Electric Lamps

These lamps mainly get used for lighting and indicating purpose. Although, now days use of light emitting diodes for these two purposes dominates uses of conventional lamps. But still they are in use although at maximum places they are replaced by LEDs.

Rating of Electric Lamps

There are mainly three things that must be known before selecting a lamp for a specific purpose.

Voltage across which the lamp to be connected for getting normal brightness of light. This voltage rating of electric lamp must be marked on the lamps. If the supply of voltage across the lamp is lower than rated value, the lamp will not glow properly aselectric current flowing through it will be insufficient. Again if the voltage across the lamp crosses the rated value, the filament may not be able to sustain the over currentand it will blow out.
Power rating or current rating of electric lamp: When an electric lamp is connected across its rated voltage, its filament will carry an electric current depending upon the electrical resistance offered by the lamp. The filament of the lamp is so designed, that it must give most optimized brightness for that current. This electric current rating of electric lamp is very important parameter, as it determines the power consumption of the lamp. As a filament of lamp is considered as 100% resistive, the power consumption is nothing but the product of voltage and current rating of the lamp. Consumption of power is nothing but power rating of the lamp. Hence, if voltage rating of a lamp is known to us, then it is enough to know either current rating or power rating of that lamp, as power rating can be calculated from current rating and current rating can be calculated from power rating since electric power is product of voltage and electric current. It is normal practice that small rated lamps are rated by currentand higher rated electric lamps are rated by power.

Types of Electric Lamps

Another thing must be considered during selecting a lamp for a circuit. What types of electric lamp is suitable for specific circuits? Types of lamp depend upon designing pattern of the lamps. According to variation of deigns there are verities types of lamps are listed below,

Edison Screw type Lamps

The main feature of this type of lamp is that it has one contact in the base and other contact on the metallic body of the lamp. This metallic body of the lamp is of screw shaped. Commonly two types of Edison Screw lamps are available in market –

Miniature Edison Screw Lamp (MES) its bulb diameter is 10 mm (approx).
Lilliput Edison Screw Lamp (LES) its bulb diameter is 5 mm (approx).

Miniature Center Contact type Lamps

Like MES and LES, it has one contact in the base and other contact on the metallic body of the lamp. But main difference is that this metallic body of the lamp is not screw shaped-it has bayonet style fittings. Its bulb diameter is 10 mm (approx).

Small Bayonet Cap Lamps

It has also bayonet style fitting but has both contacts on the base of the lamp, hence there is no electrical connection with the body of the lamp. The standard bulb diameter is about 40 mm. It may have horizontal as well as vertical filament arrangements as shown. The power rating of the bulb can be up to 24 watt.

Wire Ended Lamps

These are very small size lamps of about 6 mm long and has 3 mm diameter. The design is very simple; here contact wires directly come out from the glass cashing. Wire ended lamps are mainly designed for very low power rating and are also available in the market in very cheap rate. This lamp does not need any lamp holder; it can be soldered directly to the circuit board with the help of the contact wires come out from the glass cashing.

Author : Unknown Comments : 0

Posted :

Wheatstone Bridge Circuit Theory and Principle

For measuring accurately any electrical resistance Wheatstone bridge is widely used. There are two known resistors, one variable resistor and one unknown resistor connected in bridge form as shown below. By adjusting the variable resistor the current through the Galvanometer is made zero. When the electric current through the galvanometer becomes zero, the ratio of two known resistors is exactly equal to the ratio of adjusted value of variable resistance and the value of unknown resistance. In this way the value of unknown electrical resistance can easily be measured by using a Wheatstone Bridge.

Wheatstone Bridge Theory

The general arrangement ofWheatstone bridge circuit is shown in the figure below. It is a four arms bridge circuit where arm AB, BC, CD and AD are consisting of electrical resistances P, Q, S and R respectively. Among these resistances P and Q are known fixed electrical resistances and these two arms are referred as ratio arms. An accurate and sensitive Galvanometer is connected between the terminals B and D through a switch S2. The voltage source of this Wheatstone bridge is connected to the terminals A and C via a switch S1 as shown. A variable resistor S is connected between point C and D. The potential at point D can be varied by adjusting the value of variable resistor. Suppose current I1 and current I2are flowing through the paths ABC and ADC respectively. If we vary the electrical resistance value of arm CD the value of current I2 will also be varied as the voltage across A and C is fixed. If we continue to adjust the variable resistance one situation may comes when voltage drop across the resistor S that is I2.S is becomes exactly equal to voltage drop across resistor Q that is I1.Q. Thus the potential at point B becomes equal to the potential at point D hence potential difference between these two points is zero hence current through galvanometer is nil. Then the deflection in the galvanometer is nil when the switch S2 is closed.

and

Now potential of point B in respect of point C is nothing but the voltage drop across the resistor Q and this is

Again potential of point D in respect of point C is nothing but the voltage drop across the resistor S and this is

Equating, equations (i) and (ii) we get,

Here in the above equation, the value of S and P ⁄ Q are known, so value of R can easily be determined.

The electrical resistances P and Q of the Wheatstone bridge are made of definite ratio such as 1:1; 10:1 or 100:1 known as ratio arms and S the rheostat arm is made continuously variable from 1 to 1,000 Ω or from 1 to 10,000 Ω

The above explanation is most basic Wheatstone bridge theory.

Author : Unknown Comments : 0

Posted :

Low Power Factor Wattmeter

As the name suggests the low power factor meter are the instruments that measures lower values of power factor accurately. Before we study more about the low power factor meter, it is very essential to know why there is a requirement of low power factor meter though we use ordinary electrodynamometer to measure power factor?

Answer to this question is very simple as it gives inaccurate results.

Now there are two main reasons that would suggests us that we should not use ordinary wattmeter in measuring the low value of power factor.

(1) The value of deflecting torque is very low even though we fully excite the electric current and pressure coils.
(2) Errors due pressure coil inductance.

Above two reasons gives very inaccurate results thus we should not use normal or ordinary wattmeters in measuring the
low value of power factor. However by doing some modification or adding some new features we can use modified electrodynamic wattmeter or low power factor to measure the low power factor accurately. Here we are going to discuss, where we need to do modification. These are discussed one by one below:

(1) Modification in pressure coil:
The electrical resistance of the ordinary wattmeter's pressure coil is reduced to low value such that current in the pressure coil circuit is increased, thus it leads to. In this category two cases diagrams arises and these are shown below:

In the first category both the ends of the pressure coil is connected to supply side (i.e. current coil is in series with the load). The supply voltage is equal to the voltage across the pressure coil. Thus in this case we have power shown by the first wattmeter is equal to the power loss in the load plus power loss in the current coil. Mathematically, P1 = power consumed by load + I2R1.
In the second category, the current coil is not in series with the load and the voltage across the pressure coil is not equal to the applied voltage. The voltage across pressure coil is equal to the voltage across the load. In this power shown by the second wattmeter is equal to the power loss in the load plus the power loss in the pressure coil. Mathematically P2 = power consumed by load + I2R2.
From the above discussion we conclude that in both cases we have some amount of errors hence there is need to do some modification in above circuits to have minimum error. The modified circuit is shown below:
We have used here a special coil called compensating coil, it carries current equal to the sum of two currents i.e load current plus pressure coil current. The pressure coil is placed such that the field produced by the compensating coil is opposed by the field produced by pressure coil as shown in the above circuit diagram.
wattmeter with compensating coil

Thus the net field is due to the current I only . Hence by this way error caused by pressure coil can be neutralised.

(2) We require compensating coil in the circuit in order to make the low power factor meter. It is the second modification that we have discussed in detail above.

(3) Now the third point deals with the compensation of the inductance of pressure coil, which can be achieved by doing modification in above circuit.

Now let us derive an expression for the correction factor for pressure coil inductance. And from this correction factor we are going to derive an an expression for error due to inductance of pressure coil. If we consider the inductance of pressure coil we dont have voltage across pressure in phase with the applied voltage. Hence it that case it lag by an angle
$b\; =\;tan^{-1} \frac{W_l}{R+r_p}$
where R is electrical resistance in series with pressure coil, rp is pressure coil resistance, here we also conclude that the electric current in the current coil is also lagging by some angle with the electric current in pressure coil. And this angle is given by C = A - b. At this time reading of the voltmeter is given by
$\frac{VIcos(C)\frac{dM}{dx}}{K+R_p}\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot(1)$
Where Rp is (rp+R) and x is angle. If we ignore the effect of inductance of pressure i.e putting b = 0 we have expression for true power as
$\frac{VI cos(A)\frac{dM}{dx}}{K\cdot R_p}\cdot\cdot\cdot\cdot\cdot\cdot\cdot\cdot(2)$
On taking ratio of equations (2) and (1) we have expression for correction factor as written below:
$\frac{cos(A)}{cos(b)cos(A-b)}$
And from this correction factor error can be calculated as
Error = (1 - (correction factor))*(actual reading of the voltmeter). On substituting the value of correction factor and taking suitable approximation we have expression for error as VIsin(A)*tan(b).
wattmeter

Now we know that the error caused by pressure coil inductance is given by the expression e=VIsin(A) tan(b), if power factor is low (i.e in our case the value of φ is large hence we have large error). Thus in order to avoid this situation we have connect the variable series resistance with a capacitor as shown in the above figure. This final modified circuit so obtained is called low power factor meter. A modern low power factor meter is designed such that it gives high accuracy while measuring power factors even lower than 0.1.

Author : Unknown Comments : 0

Posted :

MEGGER WORKING PRINCIPLE

MEGGER WORKING PRINCIPLE

Insulation resistance quality of an electrical system degrades with time, environment condition i.e. temperature, humidity, moisture & dust particles. It also get impacted negatively due to the presence of electrical & mechanical stress, so it’s become very necessary to check the IR (Insulation resistance) of equipment at a constant regular interval to avoid any measure fatal or electrical shock.

Electronics Megger

Uses of Megger

The device enable us to measure electrical leakage in wire, results are very reliable as we shall be passing electric currentthrough device while we are testing. The equipment basically use for verifying the electrical insulation level of any device such as motor, cable, generator winding, etc. This is a very poplar test being carried out since very long back. Not necessary it shows us exact area of electrical puncture but shows the amount of leakage current & level of moisture within electrical equipment/winding/system.

Types of Megger

This can be separated into mainly two categories:-
1. Electronic Type (Battery Operated)
2. Manual Type (Hand Operated)
But there are another types of megger which is motor operated type which does not usebattery to produce voltage it requires external source to rotate a electrical motor which in turn rotates the generator of the megger

Electronic Megger

Electronic Type Megger

Important parts:-
1. Digital Display :- A digital display to show IR value in digital form.

2. Wire leads :- Two nos of wire leads for connecting megger with electrical external system to be tested.

3. Selection switches :- Switches use to select electrical parameters ranges.

4. Indicators :- To indicates various parameters status i.e. On-Off. For Example Power, hold, Warning, etc.

Note: - Above construction is not similar for every megger, it difference appears manufacture to manufacture but basic construction & operation are same for all.

Advantages of Electronic Type Megger

• Level of accuracy is very high.
• IR value is digital type, easy to read.
• One person can operate very easily.
• Works perfectly even at very congested space.
• Very handy & safe to use.

Disadvantages of Electronic Type Megger

• Require an external source of energy to energies i.e. Dry cell.
• Costlier in market.

Hand Operated Megger

Important parts:-
Analog display: - Analog display provided on front face of tester for IR value recording.
Hand Crank:- Hand crank used to rotate helps to achieve desired RPM required generate voltagewhich runs through electrical system.
Wire Leads :- Used same as in electronic tester i.e. For connecting tester with electrical system.

Advantages of Hand Operated Megger

1. Still keeps important in such high-tech world as it’s an oldest method for IR value determination.
2. No external source required to operate.
3. Cheaper available in market.

Disadvantages of Hand Operated Megger

1. At least 2 person required to operate i.e. one for rotation of crank other to connect megger with electrical system to be tested.
2. Accuracy is not up to the level as it’s varies with rotation of crank.
3. Require very stable placement for operation which is a little hard to find at working sites.
4. Unstable placement of tester may impact the result of tester.
5. Provides an analog display result.
6. Require very high care & safety during use of the same.

Construction of Megger

Circuit Construction features :-

Megger Construction

1) Deflecting & Control coil : Connected parallel to the generator, mounted at right angle to each other and maintain polarities in such a way to produced torque in opposite direction.

2) Permanent Magnets: Produce magnetic field to deflect pointer with North-South pole magnet.

3) Pointer : One end of the pointer connected with coil another end deflects on scale from infinity to zero.

4) Scale : A scale is provided in front-top of the megger from range ‘zero’ to ‘infinity’, enable us to read the value.

5) D.C generator or Battery connection : Testing voltage is produced by hand operated D.C generator for manual operated Megger. Battery / electronic voltage charger is provided for automatic type Megger for same purpose.

6) Pressure coil resistance and Current coil resistance : Protect instrument from any damage because of low external electrical resistance under test.

Working Principle of Megger

• Voltage for testing produced by hand operated megger by rotation of crank in case of hand operated type, a battery is used for electronic tester.

• 500 Volt DC is sufficient for performing test on equipment range up to 440 Volts.

• 1000V to 5000V is used for testing for high voltage electrical systems.

• Deflecting coil or current coil connected in series and allows flowing the electric currenttaken by the circuit being tested.

• The control coil also known as pressure coil is connected across the circuit.

• Current limiting resistor (CCR & PCR ) connected in series with control & deflecting coil to protect damage in case of very low resistance in external circuit.

• In hand operated megger electromagnetic induction effect is used to produce the testvoltage i.e. armature arranges to move in permanent magnetic field or vice versa.

• Where as in electronic type megger batteries are used to produce the testing voltage.

• As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with a increases of current.

• Hence, resultant torque is directly proportional to voltage & inversely proportional to current.

• When electrical circuit being tested is open, torque due to voltage coil will be maximum & pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance within the circuit under test.

• If there is short circuit pointer shows ‘zero’, which means ‘NO’ resistance within circuit being tested.

Work philosophy based on ohm-meter or ratio-meter. The deflection torque is produced with megger tester due to the magnetic field produced by voltage & current, similarly like ‘Ohm's Law’

Torque of the megger varies in ration with V/I, (Ohm's Law :- V=IR or R=V/I). Electrical resistance to be measured is connected across the generator & in series with deflecting coil.

Produced torque shall be in opposite direction if current supplied to the coil.

1. High resistance = No current :- No current shall flow through deflecting coil, if resistance is very high i.e. infinity position of pointer

2. Small resistance = High current :- If circuit measures small resistance allows a highelectric current to pass through deflecting coil, i.e. produced torque make the pointer to set at ‘ZERO’.

3. Intermediate resistance = varied current :- If measured resistance is intermediate, produced torque align or set the pointer between the range of ‘ZERO to INIFINITY’

Connection Diagram of Megger for Testing

Author : Unknown Comments : 7

Posted :

Contributors

Blog Archive

Follow us on FaceBook

About

Popular Posts

Archive for June 2014

Bipolar Stepper Motor

Bipolar Stepper Basics

Interfacing to Micro-Controller

Interfacing Diagram

Bipolar Stepper Drives

Unipolar Stepper v/s Bipolar Stepper

Electric Lamp | Types of Electric Lamp

Uses of Electric Lamps

Rating of Electric Lamps

Types of Electric Lamps

Edison Screw type Lamps

Miniature Center Contact type Lamps

Small Bayonet Cap Lamps

Wire Ended Lamps

Wheatstone Bridge Circuit Theory and Principle

Wheatstone Bridge Theory

Low Power Factor Wattmeter

Uses of Megger

Types of Megger

Electronic Type Megger

Advantages of Electronic Type Megger

Disadvantages of Electronic Type Megger

Hand Operated Megger

Advantages of Hand Operated Megger

Disadvantages of Hand Operated Megger

Construction of Megger

Working Principle of Megger

Connection Diagram of Megger for Testing

Blogger templates

Blogger news

Blogroll

Popular Posts