Voltage and Current Relationship

Question:

                How Voltage and current are related to each other?

Answer:

               

A charge is carried by electrons. The amount of charge flowing is called Current.

Voltage is potential difference. i.e. in this picture water will flow from top to bottom. Similarly, current flows from high concentration of current to low concentration of current . That is potential difference. So, the work done in moving a charge between two points is called voltage.

Power = Voltage * Current

Read More

USB DESIGNS

USB stands for: Universal Serial Bus

It is a portable device which is used to store data. it is available in different data storage capacity. The development of USB device began by a group of seven companies includes IBM, Intel, COMPAC, DEC, NEC and Microsoft. USB flash drive includes a flash drive with an integrated USB interface. it isused for same purpose such as data storage and data transfer from one computer to another lieother storage devices such as floppy disks and compact disks.
this device were invented by Amir Ban, Dov Moran and Oron Ogdan, all of these are of Israeli company M-Systems. it were invented in April 1999.
It has different uses like: Personal data transport, Application carriers, Computer forensics and law enforcement.

Unique designs of USB flash drive:

Fast food USB:

Jogger USB:

Sleepers USB:

Stethoscope USB for Doctors:

Glasses USB:

Heart USB:

Ghost USB:

Insect USB:

Robotic USB:

Cartoon  USB:

Mouse USB:

:

Tie USB for managers:

Syringe USB for Doctors:

Football USB for football lovers:

Building block USB : 

IPOd USB for Singers:

Car USB:

Wine USB:

Strawberry USB:

Shopping bag USB:

Camera USB for Photographer:

Lock shaped USB:

Bulb shaped USB for Electrical Engineers:

USB with Battery:

Nail cutter shaped USB:

Cartoon USB:

Read More

LED Dot Matrix Dislplay

Parts List:

• IC CD4017
• LED
• PIC16F877A
• Crystal 20 MHz
• Crystal socket
• R1 = 4.7k

Read More

Questions. . . ! Single phase induction motor

Q1: Where do we require single-phase induction motors?

Ans: Single-phase induction motors are required where

(i) 3 phase supply is not available 

(ii) efficiency is of lesser importance

(iii) Rating is less than one H.P. 

(iv) Equipment is portable

Q2: Why is the power factor of a single-phase induction motor low?

Ans: It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature.

Q3: What is the function of centrifugal starting switch in a single-phase induction motor?

Ans: The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

Q4: What happens when the centrifugal starting switch fails to open?

Ans: If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.

Q5: What happens when the centrifugal switch fails to close when needed?

Ans: If the centrifugal starting switch fails to close, the motor will overheat the main winding without any failure of the main winding.

Q6: Why are resistance split-phase inductions motors most popular?

Ans: These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent. They drive fans, pumps, washing machines, small machine tools etc. They have power ranting between 60 watts and 250 watts.

Q7: What is the draw back of the resistance split-phase induction motor?

Ans: The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding. Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.

Q8: Why is the starting torque of a resistance split-phase induction motor not high?

Ans: The starting torque is given as, Ts = K Im Is Sin Ф

Where

            K = constant whose magnitude depends upon the design of the motor

(i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small.

(ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.

Q9: Why is the starting torque of a capacitor start induction motor high?

Ans: The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.

Q10: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Ans: Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q11: What is the principle of operation of shaded-pole induction motor?

Ans: A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole. The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.

Q12: Which type of torque is developed in single phase motors?

Ans: Pulsating torque is produced.

Q13: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?

Ans: Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q14: What is a fractional H.P. motor?

Ans: A small motor having H.P. less than unit is called fractional H.P. motor.

Q15: Which type of rotor is used in single phase motors?

Ans: Squirrel cage type

Q16: How the starting winding produce rotation in a single phase resistance start induction motor?

Ans: The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.    

    

Q17: How the starting winding is made resistive?

Ans: It consists of only few turns of smaller diameter.

Q18: How the speed of rotation of a split phase induction motor is reversed?

Ans: The terminal connections of the starting windings are reversed with respect to main running windings.

Q19: What will happen if the centrifugal switch fails to open the starting winding?

Ans: Excessive heat will be produced due to high resistance of the starting winding due to which stator temperature will rise and eventually both windings will burn.

Q20: How speed control is made in single phase motors?

Ans: It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.

Q21: Is there any relation between the capacitances of two capacitors used in two value capacitor motor?

Ans: Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.

Q22:  What is size of shaded-pole motor?

Ans: These are usually built in small fractional H.P, not exceed 1/4 H.P.

Q23: Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc.

Ans: Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.

Q24: How can a universal motor be reversed?

Ans: By reversing either the field leads or armature leads but not both.

Q25: What are applications of Stepper motors?

Ans: (i) Paper feed motors in typewriters and printers 
(ii) Positioning of print heads 
(iii) Pens in XY-plotters 
(iv) Recording heads in computer disc drives etc.

Q26: Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?

Ans: Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

Q27: If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?

Ans: Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

Q28: What is a fractional H.P. motor?

Ans: A small motor having H.P. less than unit is called fractional H.P. motor.

Q29: Which type of rotor is used in single phase motors?

Ans: Squirrel cage type

Q30: How the starting winding produce rotation in a single phase resistance start induction motor?

Ans: The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.  

      

Q31: How the starting winding is made resistive?

Ans: It consists of only few turns of smaller diameter.

Q32: How the speed of rotation of a split phase induction motor is reversed?

Ans: The terminal connections of the starting windings are reversed with respect to main running windings.

Read More

Split – Phase Induction Motor

• Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90^o electrical from the main winding and operates for a short time when the motor starts up.
• The two windings are so designed that the starting winding has a high resistance and relatively small reactance while the main winding has relatively low resistance and large reactance. This can be done by having main winding of large diameter and number of turns while starting winding of a fine wire of a small number of turns. The currents taken by these two windings are out of phase (25^o to 30^o). Schematic diagram and phasor diagram of split phase induction motor are shown in figure
• Operation: When the stator windings are energised from a single phase supply, the main winding carries current Im and the starting winding carries current Is. These two currents are out of phase (25^o to 30^o) and produce a weak rotating field which starts the motor. The starting torque is given as,

                                    Ts = K Im Is Sin Ф

Where

            K = constant whose magnitude depends upon the design of the motor

When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed is below the synchronous speed and depends upon the load on the motor.

Characteristics of Split-Phase I / M:

1. The starting torque is 1.5 to 2 times the full load torque and the starting current is 6 to 8 times the full-load current.
2. The power ratings of such motors are from 60 Watts to 250 Watts.
3. They have efficiency of 50 – 70 % and overload of 50%.
4. As they have moderate starting torque and low starting current, so they are commonly used to drive fans, blowers, centrifugal pumps, domestic refrigerators, washing machines, oil burners, small machine tools, etc.
5. These motors are essentially constant-speed motors. The speed variation is 2 to 5 % from no-load to full-load.
6. The speed range of such motors is from 2875 to 700 r.p.m.
7. Such a motor can be operated with a power factor of 0.55 – 0.65.
8. The percent slip for such motors is about 4 – 6 percent.

Read More

A.C. Series Motor OR Universal Motor

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.

§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.

1.    The entire magnetic circuit is laminated in order to reduce the eddy current loss. Hence an a.c. series motor requires a more expensive construction than a d.c. series motor.

2.    The series field winding uses a few turns to reduce the reactance of the field winding. This reduces the voltage drop across the field winding.

3.    A high field flux is obtained by using a low – reluctance magnetic circuit.

4.    When the motor is operated on a.c. supply, there will be possible sparking between the brushes and the commutator. This can be eliminated by using high-resistance leads to connect the coils to the commutator segments.

• Operation: When the motor is connected to a.c. supply, the same alternating current flows in the field and armature windings. The field winding produces an alternating flux that reacts with the current flowing in the armature to produce a torque. The flux produced is only alternating not the rotating.

Characteristics of Universal Motor:

1. It is self starting motor, starting torque is twice or thrice, therefore no starting device is required.
2. The speed increases to a high value with a decrease in load. In small motors, the losses are enough at no-load to limit the speed to a definite value. However, in a d.c. series motor, the speed at no-load may rise to a dangerously high value.
3. The motor torque is high for large armature currents, thus giving a high starting torque.
4. At full load, the power factor is about 90%.
5. They operate at high speeds (1500 – 15000 r.p.m)

Application:

These motors are used to drive

(1) high speed vacuum cleaners   

(2) Sewing machines 

(3) Electric shavers 

(4) Drills 

(5) Machine tools etc.

Read More

Single Phase Motors

"Single phase motors are also used to convert Electrical energy to mechanical energy but the difference is that they are operated on single phase supply".

Types of single phase motors:

Single phase motors are generally built in the fractional –horsepower range and are classified into following four basic types:

1. Single-Phase Induction Motors

      (i) Split-phase type

     (ii) Capacitor type 

     (iii) Shaded-pole type

2. AC Series Motor or Universal Motor
3. Repulsion Motors

      (i) Repulsion-start induction-run motor 

      (ii) Repulsion-induction motor

4. Synchronous Motors

            (i) Reluctance motor 
           (ii) Hysteresis motor

Single phase Induction motors:

Split – Phase Induction Motor

• Split phase induction motor has two windings its stator, a main winding and an auxiliary winding or starting winding. The starting winding is located 90^o electrical from the main winding and operates for a short time when the motor starts up.

• When the motor reaches about 75% of synchronous speed, then the centrifugal switch or relay opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed is below the synchronous speed and depends upon the load on the motor.

Capacitor-Start Motor

• The capacitor motor is similar to a split-phase induction motor but with two differences: (i) the starting winding of capacitor start motor have as many turns as the main winding and (ii) a capacitor is connected in series with the starting winding.
• Capacitors are used to improve
• The starting-performance
• Running performance
• Both starting and running performance of a split-phase induction motor.

Capacitor-Start Capacitor-Run Motor

• This motor is similar to capacitor-start motor except that starting winding is not opened after starting so that both the windings of the stator remain connected to the supply when running as well as at starting.

Shaded-Pole Motor

• A shaded-pole motor has salient pole stator similar to the stator of dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in figure F6. A short-circuited turn of copper ring called shading coil is placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-third section of the pole arc is called unshaded portion. The main single phase winding is wound on the entire pole section.  The rotor used is similar to the rotor of any other single phase induction motor.

Single phase Synchronous  Motor:

Synchronous Reluctance Motor:

This motor has variable air-gap which decides the reluctance of the magnetic path. More the air gap, more is the reluctance and vice versa. The reluctance is greatest when the magnetic field of the stator aligns with the cut out portion of the rotor whereas reluctance is minimum when it aligns with the center position of the rotor teeth. Therefore the reluctance of the magnetic circuit is a function of the air-gap.  The salient poles created on the rotor must be equal to the poles on the stator. The rotor salient poles offer low reluctance to the stator flux and therefore, become strongly magnetized.

A.C. Series Motor OR Universal Motor:

§  A universal motor is built like a series d.c. motor with the difference that both its stator and armature are laminated. A universal motor is defined as a motor which can be operated on both a.c. supply and d.c. supply at approximately the same speed and output.

§  A d.c. series motor (both armature and field windings are in series) can be operated on a single phase a.c. supply. It is then called an a.c. series motor. Following changes are made in d.c. series motor to operate satisfactory on a.c. supply.

Read More