What is a Resistor? What are the Types of Resistors? What are the Applications of Resistor?

What is a Resistor?

A resistor is a passive two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law. The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor. 

Figure 1: A typical axial-lead resistor.

Figure 2: Two common schematic symbols of resistor.

In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.

Theory of Operation:

Ohm's law: The behavior of an ideal resistor is dictated by the relationship specified by Ohm's law:

             V = I/R

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.

Practical resistors also have some inductance and capacitance which affect the relation between voltage and current in alternating current circuits.

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10^-3 Ω), kilohm (1 kΩ = 10³ Ω), and megohm (1 MΩ = 10⁶ Ω) are also in common usage. 

Figure 3: A few types of resistors.

Types of Resistors:

1.     Linear resistors.

                                 i.         Fixed resistors

a)    Led arrangement

b)    Carbon composition

c)     Carbon Pile

d)    Carbon film

e)    Printed carbon resistor

f)      Thick and thin film

g)    Metal film

h)    Metal oxide film

i)      Wire wound

j)      Foil resistor

k)    Ammeter shunt

l)      Grid resistor

m) Special verities

                                    ii.         Variable resistor

a)       Adjustable resistor

b)       Potentiometers

c)        Resistance and decade boxes

d)       Special devices.

2.     Non-linear resistors.

Applications of Resistors:

• In general, a resistor is used to create a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to create a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to create a known current proportional to that difference.  
• Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.  
• A series resistor can be used for speed regulation of DC motors, such as used on locomotives and train sets.  
• An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.  
• A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize reflections of the signal.  
• All resistors dissipate heat. This is the principle behind electric heaters.  

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The Discovery of Electricity Timeline.

The Discovery of Electricity:

Schematic diagram of a copper–zinc voltaic pile.

v	Democritus (460?–370? BC) proposes an “atomic theory” wherein all matter is made up of indivisible particles, or atoms.
v	Charles de Coulomb (1736–1806) discovers that the force of attraction between electric charges is proportional to the product of the two charges and inversely proportional to the distance between them.
v	Luigi Galvani (1737–1798) discovers that two unlike metals immersed in blood cause the muscles of a frog’s legs to twitch.
v	Alessandro Volta (1745–1827) discovers that a current flows between two connected unlike metals in a salt solution and, thus, invents the battery.
v	John Dalton (1766–1844) proposes the first table of atomic weights of elements.
v	André Ampere (1775–1836) develops the theory of magnetic lines of force and quantifies electric current for the first time.
v	Hans Christian Ørsted (1777–1851) discovers a connection between electric current and magnetism and a way to measure electric current by the deflection of a magnet.
v	Georg Ohm (1787–1854) discovers the relationship (Ohm’s Law) between voltage, current, and resistance in a circuit.
v	Michael Faraday (1791–1867) analyzes the chemical reactions in batteries and defines the terms “electrode,” “anode,” “cathode,” and “electrolyte.”
v	James Clerk Maxwell (1831–1879) develops the mathematical equations relating electricity and magnetism.
v	Joseph Thomson (1856–1940) proves that electricity consists of electrons.

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State the main effects of electric current? What are the applications of it?

The three main effects of an electric current are:

1. Magnetic effect
2. Chemical effect and
3. Heating effect.

Some practical applications of the effects of an electric current include:

Magnetic effect: 

When electric current flows through a wire, it behaves like a magnet. This is called magnetic effect of electric current.

Figure 1: Electric bell - examples of magnetic effect.

Applications of magnetic effect:

• Bells
• Relays
• Motors
• Generators
• Transformers
• Telephones
• Car-ignition and 
• Lifting magnets.

Chemical effect: 

The passage of an electric current through a conducting liquid causes chemical reactions. The resulting effects are called chemical effects of electric current.

Figure 2: Battery - Example of chemecal effect.

Applications of chemical effect:

• Primary and secondary cells and 
• Electroplating.

Heating effect:  

When electric current flows through a wire, the wire gets heated. This is called the heating effect of electric current.

Figure 3: Irons - example of heating effects.

Applications of heating effect:

• Cookers
• Water heaters
• Electric fires
• Irons
• Furnaces
• Kettles and 
• Soldering irons

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What is the most defining drawback of DC motors in general?

The biggest drawback in any DC motor is the use of the commutator and brush arrangement and the relative motion between them. As a result of this arrangement, they:

•  Require additional maintenance.
•  Have a shorter life span.
•  Possess a higher chance of sparking at the commutator and brush assembly.    

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Suppose a man touches a metal toaster and is shocked. The toaster, having a polarized plug, is inspected, and nothing is found amiss. The inspector then looks at the outlet. He measures 120 V between the long slot and ground, and 0 V between the short slot and ground. What is the problem?

The long slot is the neutral connection. This slot should be connected to ground at the circuit breaker panel, thus having a voltage of 0 V with respect to ground. The short slot is the hot connection and should have a voltage of 120 V with respect to ground. The inspection has revealed that the measurements are the opposite of what they should be. Therefore, the outlet is wired backward, with the hot and neutral wires swapped.    

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37 Interview Questions and Answers on DC Generators - Part-3

Q. 1. How may the number of parallel paths in an armature be increased?

Ans. By increasing the number of magnetic poles.

Q. 2. How are brushes connected in a d.c. generator?

Ans. Usually, all positive brushes are connected together and all the negative brushes together (figure-a.).

Q. 3. What is meant by armature reaction?

Ans. It is the effect of armature magnetic field on the distribution of flux under main poles of a generator. The armature magnetic field has two effects:

1. It demagnetises or weakens the main flux and
2. It cross-magnetizes or distorts it.

Figure - (a)

Q. 4. What is the effect of this distortion on the operation of the machine?

Ans. It acts as a magnetic drag on the armature which consequently requires more power to turn it.

Q. 5. How can field distortion be remedied?

Ans. By using compensating windings which are embedded in the slots in the pole-shoe and are connected in series with the armature.

Q. 6. What is meant by normal neutral plane?

Ans. It is a plane which passes through the axis of the armature perpendicular to the magnetic field of the generator when there is no flow of current through the armature.

Q. 7. What is the importance of this plane in the working of the machine?

Ans. It gives the position where brushes would be placed to prevent sparking during the operation of the generator where the main pole field not distorted by armature field and were there no self-induction in the coils.

Q. 8. How do you differentiate between normal neutral plane?

Ans. The NNP is the position of zero induction and hence minimum sparking assuming no field distortion i.e. on no-load.  It is perpendicular to magnetic axis.  NA is the position of zero induction and hence minimum sparking with distorted field i.e. when generator is on load.

Q. 9. How do you define ‘commutating plane?

Ans. It is the plane which passes through the axis of the armature and through centre of contact of the brushes as shown in Figure (b).

Q. 10. What is the angle of lead?

Ans. It is the angle between the NNP and the commutating plane.

Q. 11. What affects this angle?

Ans. For sparkless commutation, the angle of lead varies directly with load. Its value can be kept small by making main pole field considerably more powerful than the armature field.

Q. 12. What is the best way of minimizing eddy currents in an armature?

Ans. Lamination.

Q. 13. How should the armature be laminated for the purpose?

Ans. It should be laminated at right angle to its axis.

Q. 14. How does field distortion affect communication?

Ans. The neutral plane no longer coincides with the normal neutral plane but is advanced by a certain angle in the direction of rotation of the armature.

Q. 15. Should the brushes of a loaded generator be placed in the neutral plane?

Ans. No.

Q. 16. Why not?

Ans. The brushes must be advanced by a certain angle (called brush lead) beyond the neutral plane to prevent sparking.

Q. 17. What causes sparking at the brushes?

Ans. It is due to the self-induction of the coil undergoing commutation.

Q. 18. What is the standard direction of rotation of the d.c. generators?

Ans. Clockwise when viewed from the end opposite to the driven end.

Q. 19. What is meant by build-up of a generator?

Ans. It means the gradual increase in the generator voltage to its maximum value after the generator is started from rest.

Q. 20. How should a generator be started?

Ans. It is usually brought up to speed with the help of the driving engine called prime-mover.

Q. 21. How should a shunt or compound generator be started?

Ans. Such machines excite best when all switches controlling the external circuit are open.

Q. 22. How about a series generator?

Ans. In this case, the external circuit must be closed otherwise the generator will not build-up.

Q. 23. What is the procedure for shunting down a generator?

Ans. First, the load should be gradually reduced, if possible, by easing down the driving engine, then when the generator is supplying little or no current, the main switch should be opened. When the voltmeter reads almost zero, then brushes should be raised from the commutator.

Q. 24. What are the indications and causes of an overloaded generator?

Ans. A generator is said to be overloaded if a greater output is taken from it that it can safely carry. Overloading is indicated by (i) excessive sparking at brushes and (ii) overheating of the armature and other parts of the generator. Most likely causes of overloading are :

1. Excessive voltage-as indicated by the voltmeter or the increased brilliancy of the pilot lamp. This could be due to over-excitation of field magnets or too high speed of the engine.
2. Excessive current–which could be due to bad feeding of the load.
3. Reversal of polarity–this happens occasionally when the series or compound-wound generators are running in parallel.  Polarity reversal occurs during stopping by the current from the machines at work.
4. Short-circuit or ground in the generator itself or in the external circuit.

Figure - (b)

Q. 25. Mention and explain the various causes for the failure of the generator to build up.

Ans. Principal causes due to which a generator may fail to excite are :

1. Brushes not properly adjusted–if brushes are not in their proper positions, then whole of the armature voltage will not be utilized and so would be insufficient to excite the machine.
2. Defective contacts-unclean contacts may interpose large resistance in the path of the exciting current and reduce it to such a small value that it fails to excite the machine.
3. Incorrect adjustment of regulators–in the case of shunt and compound generators, it is possibly that the resistance of field regulator may be too high to permit the passage of sufficient current through the field windings.
4. Speed too low–in the case of shunt–and compound-wound generators, there is certain critical armature speed below which they will not excite.
5. Open-circuit–in the case of series machines.
6. Short-circuit–in the generator of external circuit.
7. Reversed field polarity–usually caused by the reversed connections of the field coils.
8. Insufficient residual magnetism–The trouble normally occurs when the generator is new. It can be remedied by passing a strong direct current through the field coils.

Q. 26. How do we conclude that connections between field coils and armature are correct?

Ans. If the generator builds up when brought to full speed. If it does not, then connections are reversed.

Q. 27. When a generator loses its residual magnetism either due to lighting or short circuit, how can it be made to build up?

Ans. By temporarily magnetisng the main poles with the help of current from an external battery.

Q. 28. Can a generator be reversed by reversing the connections between the armature and field coils?

Ans. No, because if these connections are reversed, the generator will not build up at all.

Q. 29. Will a generator build up if it becomes reversed?

Ans. Yes.

Q. 30. Then, what is the objection to a reversed generator?

Ans. Since the current of such a reversed generator is also reversed, serious trouble can occur if attempt is made to connect it in parallel with other machines which are not reversed.

Q. 31. What are the two kinds of sparking produced in a generator?

Ans. One kind of sparking is due to bad adjustment of brushes and the other due to bad condition of the commutator. The sparking of the first are bluish whereas those of the other are reddish in colour.

Q. 32. What is the probable reason if sparking does not disappear in any position when brushes are rocked around the commutator?

Ans. 

1. The brushes may not be separated at correct distance.
2. The neutral plane may not be situated in the true theoretical position on the commutator due to faulty winding.

Q. 33. What is the permissible rise of temperature in a well-designed generator?

Ans. 27°C above the surrounding air.

Q. 34. What are the causes of hot bearings?

Ans. 

1. Lack of oil 
2. Belt too tight 
3. Armature not centered with respect of pole pieces
4. Bearing too tight or not in line.

Q. 35. What causes heating of armature?

Ans. 

1. Eddy currents.
2. Moisture which almost short-circuits the armature.
3. Unequal strength of magnetic poles.
4. Operation above rated voltage and below normal speed.

Q. 36. What is the commutator pitch of a 4-pole d.c. armature having 49 commutator bars?

Ans. Yc  = (49 ± 1)/2 = 24 or 25.

Q. 37. Will it make any difference if lower figure of 24 is selected in preference to other?

Ans. Yes.  Direction of armature rotation would be reversed.

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Source: A Textbook of Electrical Technology by BL Theraja, A K Theraja, Sedha R S_Volume_II

Read More:

1. 16 Common Interview Questions and Answers in Generators - Part-1
2. 21 Common Interview Question and Answers in AC Generator - Part-2

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