AEE Electrical Engineering Core · Chapter 12

Utilization of Electrical Energy: Heating, Welding and Traction

What to remember

Electric heating types: resistance (oven, element), induction (eddy current in metal), dielectric (loss in insulators), arc (furnace). Heat needed = m × c × ΔT; 1 kWh = 3600 kJ.
Welding: arc welding uses an arc between electrode and work; resistance welding uses I²Rt heat and pressure. The welding transformer has a drooping characteristic.
Traction: total tractive effort = Fa + Fg + Fr. Indian main-line railways use 25 kV, single-phase, 50 Hz AC overhead supply. Specific energy is in Wh per tonne-km.

Electric heating

Advantages: clean, easy to control, high efficiency at point of use, no flue gas, uniform temperature, automatic control.

Heat required and efficiency

Heat = mass × specific heat × temperature rise. For water, c is about 4.2 kJ/kg·K.
1 kWh = 3600 kJ = 860 kcal (approx.).
Efficiency = useful heat ÷ electrical energy input.

Worked example. Heat 10 kg of water by 60 °C: 10 × 4.2 × 60 = 2520 kJ = 0.7 kWh. At 70% efficiency, the energy used is 1 kWh.

Resistance heating

Direct: current passes through the material (salt bath, electrode boiler).
Indirect: current passes through a heating element and the heat transfers by conduction, convection and radiation (room heater, oven).
Power P = V²/R = I²R. A 220 V heater of 48.4 ohm takes 1000 W.
Heating element materials: nichrome (nickel-chromium) is common because of high resistivity, high melting point, and good oxidation resistance. Others: kanthal, silicon carbide, molybdenum disilicide for higher temperature.
Temperature control by switching, changing the number of elements, or a thermostat; star-delta change of elements.
Used in domestic heaters, ovens, soldering irons, salt bath furnaces, and for tempering and annealing.

Induction heating (principle: eddy current and hysteresis loss in the work)

A coil carries high-frequency current; the job (conductive material) is heated by eddy currents.
Skin effect: current flows near the surface. Depth of penetration varies inversely with the square root of frequency and permeability. High frequency heats a thin layer (surface hardening); low frequency heats deeper (through heating).
Core-type (low-frequency) furnace: like a transformer with the molten metal as a single-turn secondary; it needs a closed channel of metal; used for melting brass and copper.
Coreless furnace: crucible within a coil; high frequency; stirring action of molten metal; used for melting steel and alloys.
Applications: surface hardening, brazing, melting, forging.

Dielectric heating (for insulating materials)

Material is placed between two plates connected to a high-frequency supply. Heat arises from dielectric loss (molecular friction). Heat ∝ V² × f × tan δ (loss tangent).
Frequencies range from several MHz to tens of MHz.
Heats uniformly throughout the material (not only at the surface).
Uses: wood gluing and drying, plastic welding, drying of textiles, food processing.

Arc furnace

Direct arc: arc strikes between electrodes and the charge. Used for steel making, with three graphite electrodes in a three-phase supply.
Indirect arc: arc between electrodes above the charge; heat radiates to the charge. Used for non-ferrous metals.
Furnace transformer has a low secondary voltage and heavy current with tap changing, and large reactance to limit short-circuits during melting.

Comparison

Method	Material	Frequency	Typical use
Resistance	Any conductor	50 Hz	Ovens, heaters
Induction	Conducting metal	50 Hz to many kHz	Melting, hardening
Dielectric	Insulators	MHz	Plywood gluing, plastics
Arc	Metals, ores	50 Hz	Steel making

Electric welding

Arc welding. An arc between the electrode and the work melts both; the filler metal from the electrode fuses the joint.

Electrodes: bare, coated (flux coated). The coating gives a gas shield, forms slag, and stabilises the arc.
Striking voltage (OCV): about 50 to 100 V. Operating arc voltage is much lower, about 20 to 40 V.
Drooping characteristic: voltage falls steeply as current rises. This keeps arc current almost constant when arc length changes.
DC arc welding: straight polarity (electrode negative, work positive) gives more heat at the work. Reverse polarity gives more heat at the electrode.
AC arc welding uses a welding transformer with high leakage reactance or a series reactor. It is cheaper and has no arc blow, but the arc is less stable than DC.
Welding generators (DC), rectifier sets and inverter based machines are used.
TIG (tungsten inert gas) uses a non-consumable tungsten electrode with argon or helium shielding; MIG (metal inert gas) uses a consumable wire fed continuously.
Atomic hydrogen welding: arc between two tungsten electrodes in hydrogen gas.

Resistance welding. Heat H = I²Rt, with pressure applied to make the joint. The contact resistance of the joint is the main source. No filler metal is needed.

Spot welding: overlapping sheets joined at points; Seam welding: continuous series of spots with roller electrodes; Projection welding: raised points on one sheet; Butt welding: ends of rods or tubes joined; Flash welding: ends arced, then forced together.
Transformer has a low-voltage, high-current secondary; current is controlled by a timer and tap changing.
If the current is doubled and time is the same, heat becomes four times.

Comparison

Feature	Arc welding	Resistance welding
Heat source	Arc	I²R at contact
Filler	Electrode	Usually none
Pressure	Not essential	Needed
Current	A few hundred A	Several thousand A at low voltage
Use	Structural steel	Sheet metal, mass production

Electric traction

Systems of track electrification

DC system: 600 V to 1500 V for tramways and suburban lines; 750 V DC third rail is used in some metros. DC series motors directly.
Single-phase AC at 25 kV, 50 Hz: used on main-line Indian Railways. Overhead catenary and pantograph pick up power. Fewer substations, light overhead, lower cost. Disadvantage: interference with nearby telecommunication lines.
Three-phase induction motors with converter-inverter drive in modern locomotives; AC supply is rectified and then inverted at variable voltage and frequency. They are strong, need little maintenance and allow regenerative braking.
Composite systems convert on the locomotive.

Parts. Overhead catenary, contact wire, pantograph, traction substation, feeder, return through the rails.

Traction motor requirements: high starting torque, speed control, ability to withstand overload and vibration, compact size. The DC series motor suits as torque rises with the square of current at low speed.

Speed-time curve. A plot of speed against time for a run between two stops.

Phases: acceleration (constant acceleration), free running (constant speed), coasting (power off, train slows by resistance), braking.
Trapezoidal curve (no coasting, long free run; approximates main-line services) and quadrilateral curve (with coasting, no free run; approximates urban and suburban services).
Area under the curve = distance travelled.
Average speed = distance ÷ running time; schedule speed = distance ÷ (running time + stop time). Schedule speed is lower than average speed.
Example: acceleration 2 km/h/s up to 60 km/h takes 30 s; distance in this period = ½ × (60/3.6) × 30 = 250 m.

Tractive effort. Total Ft = Fa + Fg + Fr.

Acceleration: Fa = 277.8 × We × α (newton), where We is effective (accelerating) weight in tonne (dead weight plus rotational inertia allowance, about 5 to 10% more), and α in km/h/s.
Gradient: Fg = 98.1 × W × G (newton) for G percent slope and W in tonne.
Resistance: Fr = W × r (newton), where r is the train resistance in N/tonne (from friction, flange, and air resistance).
Power output of motor = Ft × v / 3600 kW, with Ft in N and v in km/h. If Ft = 36,000 N and v = 100 km/h, power = 1000 kW.
Worked example: We = W = 200 t; α = 2 km/h/s: Fa = 277.8 × 200 × 2 = 111,120 N. For a 1% gradient, Fg = 98.1 × 200 × 1 = 19,620 N.

Adhesion. Maximum tractive effort without wheel slip = μ × adhesive weight × g. For a locomotive with 100 t on driven wheels and μ = 0.2: 0.2 × 100 × 1000 × 9.81 = 196,200 N. The coefficient of adhesion falls with wet or greasy rails. Sanding improves it.

Specific energy consumption is energy per tonne-km at the axle (Wh/tonne-km). It is lower when the schedule has coasting, a higher acceleration is used only to the needed speed, and stops are far apart.

Braking

Regenerative: motors act as generators and return energy to the supply; saves energy and wear.
Rheostatic (dynamic): energy is dissipated in resistors.
Plugging (counter-current): reverses connection; not used for regular braking.
Mechanical: friction and air brakes are always used for final stopping.

Motor control. Series-parallel control of DC motors, with resistance start; AC drives use variable-voltage, variable-frequency converters.

Illumination basics (brief)

Luminous flux unit: lumen; luminous intensity: candela; illuminance: lux = lumen per square metre.
Inverse square law: E = I / d². Illuminance falls to one-quarter when distance doubles (100 lux at 2 m gives 25 lux at 4 m).
Cosine law: E = (I / d²) cos θ for an inclined surface.
Luminous efficacy: lumen per watt; LED lamps have far higher efficacy than incandescent lamps.

Exam traps

Induction heating is for conductors; dielectric heating is for insulators.
High frequency in induction heating gives shallow heating (skin effect).
Core-type induction furnace runs at low frequency; coreless furnace at high frequency.
Welding transformer needs drooping characteristics and high leakage reactance, unlike a power transformer.
Resistance welding uses low voltage and very high current.
Indian main-line electrification is 25 kV single-phase 50 Hz AC, not 1500 V DC.
Schedule speed is less than average (running) speed.
Fa uses effective weight (We), but Fg and Fr use dead weight (W).
Regenerative braking returns energy; rheostatic dissipates it.
DC series motor: torque is high at low speed.
Straight polarity in DC welding means electrode negative.
Coasting saves energy but increases journey time.

One-liners

1. 1 kWh = 3600 kJ.
2. Nichrome is common for heating elements.
3. Dielectric heating heats the whole volume of an insulator uniformly.
4. Skin depth reduces as frequency increases.
5. Direct arc furnaces use three-phase supply for steel making.
6. Arc welding needs a drooping characteristic for stable arc current.
7. Heat in resistance welding is I²Rt.
8. Spot welding joins overlapping sheets at points.
9. 25 kV AC, 50 Hz is used for main-line electric traction in India.
10. Fa = 277.8 We α, in newton.
11. Fg = 98.1 W G, in newton, for G percent gradient.
12. Regenerative braking returns power to the supply.

Practice questions

The heat needed to raise 10 kg of water by 60 °C (specific heat 4.2 kJ/kg·K) is:
1. 2.52 kWh
2. 7 kWh
3. 0.7 kWh
4. 0.42 kWh
Answer
C. 0.7 kWh
Heat = 10 × 4.2 × 60 = 2520 kJ; 2520/3600 = 0.7 kWh.
The heating above is done in a heater of 70% efficiency. The energy drawn from the supply is:
1. 0.49 kWh
2. 1.4 kWh
3. 0.7 kWh
4. 1 kWh
Answer
D. 1 kWh
Input = 0.7/0.7 = 1 kWh.
A heater of resistance 48.4 ohm is connected to a 220 V supply. Its power is:
1. 1000 W
2. 4.5 W
3. 2000 W
4. 500 W
Answer
A. 1000 W
P = V²/R = 48400/48.4 = 1000 W.
One kilowatt-hour equals:
1. 36 kJ
2. 860 kJ
3. 3600 kJ
4. 1000 kJ
Answer
C. 3600 kJ
1 kW × 3600 s = 3600 kJ.
In resistance welding, if the current is doubled and the time is unchanged, the heat produced becomes:
1. Two times
2. Four times
3. Half
4. The same
Answer
B. Four times
H = I²Rt, so doubling I gives four times the heat.
A train has effective weight 200 t and an acceleration of 2 km/h/s. The tractive effort for acceleration is (Fa = 277.8 We α):
1. 55,560 N
2. 222,240 N
3. 111,120 N
4. 11,112 N
Answer
C. 111,120 N
Fa = 277.8 × 200 × 2 = 111,120 N.
The tractive effort to overcome a 1% gradient for a 200 t train (Fg = 98.1 W G) is:
1. 9,810 N
2. 19,620 N
3. 981 N
4. 196,200 N
Answer
B. 19,620 N
Fg = 98.1 × 200 × 1 = 19,620 N.
A locomotive exerts 36,000 N at 100 km/h. The power at the wheel rim is:
1. 1000 kW
2. 3600 kW
3. 360 kW
4. 100 kW
Answer
A. 1000 kW
P = F × v/3600 = 36000 × 100/3600 = 1000 kW.
A locomotive has 100 t on its driving wheels and a coefficient of adhesion of 0.2. The maximum tractive effort without slip is about:
1. 20 kN
2. 981 kN
3. 98 kN
4. 196 kN
Answer
D. 196 kN
F = 0.2 × 100 × 1000 × 9.81 = 196,200 N.
A train accelerates from rest to 60 km/h at 2 km/h/s. The distance travelled during acceleration is:
1. 250 m
2. 125 m
3. 500 m
4. 900 m
Answer
A. 250 m
Time = 30 s; distance = ½ × (60/3.6) × 30 = 250 m.
A train covers 3 km in a running time of 4 minutes. Its average speed is:
1. 36 km/h
2. 75 km/h
3. 45 km/h
4. 12 km/h
Answer
C. 45 km/h
Speed = 3/(4/60) = 45 km/h.
For the same run with a 1-minute stop, the schedule speed is:
1. 45 km/h
2. 36 km/h
3. 60 km/h
4. 30 km/h
Answer
B. 36 km/h
Schedule speed = 3 km / (5/60 h) = 36 km/h.
A lamp gives 100 lux at 2 m on a surface facing it. At 4 m the illuminance is:
1. 200 lux
2. 12.5 lux
3. 50 lux
4. 25 lux
Answer
D. 25 lux
By the inverse square law, doubling the distance gives one-quarter.
A lamp of 10 W gives 1000 lumen. Its luminous efficacy is:
1. 10 lm/W
2. 0.01 lm/W
3. 100 lm/W
4. 1000 lm/W
Answer
C. 100 lm/W
Efficacy = lumen ÷ watt = 1000/10.
The tractive effort required to overcome train resistance of 5 N/tonne for a 200 t train is:
1. 1000 N
2. 200 N
3. 40 N
4. 5000 N
Answer
A. 1000 N
Fr = W × r = 200 × 5 = 1000 N.
A train accelerates uniformly at 2 km/h/s. The time taken to reach 60 km/h is:
1. 3 s
2. 120 s
3. 60 s
4. 30 s
Answer
D. 30 s
t = 60/2 = 30 s.
In direct resistance heating:
1. Current passes through the material to be heated
2. Heat is transferred from an element by radiation only
3. Eddy currents heat the material
4. Dielectric loss heats the material
Answer
A. Current passes through the material to be heated
In indirect heating the current goes through a separate element.
Nichrome is widely used as a heating element because it has:
1. Very low resistivity
2. Low melting point
3. High thermal conductivity only
4. High resistivity and good resistance to oxidation at high temperature
Answer
D. High resistivity and good resistance to oxidation at high temperature
It combines a high melting point with high resistivity.
Induction heating can be applied directly to:
1. Glass
2. Dry wood
3. Electrically conducting materials
4. Plastic only
Answer
C. Electrically conducting materials
Eddy currents are induced in conductors.
In induction heating, increasing the frequency causes:
1. Deeper heating
2. Shallower heating because of the skin effect
3. No change in the depth
4. Lower heating of the surface
Answer
B. Shallower heating because of the skin effect
Skin depth decreases as frequency increases.
A core-type induction furnace operates at:
1. Low frequency (mains frequency)
2. Very high radio frequency
3. DC only
4. Ultrasonic frequency
Answer
A. Low frequency (mains frequency)
It acts like a transformer with the molten metal as a single-turn secondary.
Dielectric heating is used for:
1. Steel billets
2. Insulating materials such as wood and plastics
3. Copper wires
4. Aluminium ingots
Answer
B. Insulating materials such as wood and plastics
Heat is produced by dielectric loss in the material.
Dielectric heating generally uses frequencies in the range of:
1. 50 Hz
2. 10 kHz only
3. Several MHz to tens of MHz
4. DC
Answer
C. Several MHz to tens of MHz
The loss increases with frequency.
Which furnace is generally used for making steel from scrap?
1. Core-type induction furnace
2. Dielectric heater
3. Resistance oven
4. Direct arc furnace
Answer
D. Direct arc furnace
An arc strikes between graphite electrodes and the charge.
A welding transformer needs a drooping characteristic so that:
1. It can supply constant voltage
2. The arc current stays nearly constant as the arc length changes
3. Welding is done at low current
4. The no-load voltage is zero
Answer
B. The arc current stays nearly constant as the arc length changes
A steep fall in voltage with current limits short-circuit current.
The coating (flux) on a welding electrode is used to:
1. Reduce the current
2. Raise resistance of the work
3. Melt only the electrode
4. Shield the weld, form slag and stabilise the arc
Answer
D. Shield the weld, form slag and stabilise the arc
The flux produces a gas shield and slag that protects the molten metal.
In arc welding, the open-circuit voltage compared with the arc voltage is:
1. Higher
2. The same
3. Lower
4. Zero
Answer
A. Higher
A higher striking voltage is needed to start the arc; the arc voltage during welding is lower.
In DC arc welding with straight polarity:
1. The electrode is positive and the work is negative
2. The electrode is negative and the work is positive
3. Both are earthed
4. The arc is AC
Answer
B. The electrode is negative and the work is positive
More heat is generated at the work in this case.
A non-consumable tungsten electrode with an inert gas shield is used in:
1. Seam welding
2. MIG welding
3. TIG welding
4. Spot welding
Answer
C. TIG welding
MIG uses a consumable wire electrode.
Spot welding is a type of:
1. Resistance welding
2. Atomic hydrogen welding
3. Gas welding
4. Arc welding
Answer
A. Resistance welding
Heat is produced by I²Rt at the contact between overlapping sheets.
In resistance welding, the main source of heat is:
1. A gas flame
2. An arc between electrodes
3. I²R loss at the contact resistance of the joint
4. Induced eddy currents in a coil
Answer
C. I²R loss at the contact resistance of the joint
Heat is I²Rt, with pressure applied.
The standard main-line railway electrification system in India is:
1. 1500 V DC
2. 11 kV three-phase
3. 440 V DC
4. 25 kV, single-phase, 50 Hz AC
Answer
D. 25 kV, single-phase, 50 Hz AC
Overhead catenary at 25 kV AC is used on main lines.
A drawback of 25 kV single-phase AC traction is:
1. A large number of substations
2. Interference with nearby communication lines
3. Inability to use regenerative braking
4. Very heavy overhead structure
Answer
B. Interference with nearby communication lines
The single-phase current induces noise in telecom circuits.
The area under the speed-time curve of a train represents:
1. The tractive effort
2. The energy used
3. The distance travelled
4. The time taken
Answer
C. The distance travelled
Distance is the integral of speed over time.
Compared with average (running) speed, the schedule speed is:
1. Lower, since it includes the stop time
2. Unrelated
3. Higher
4. The same
Answer
A. Lower, since it includes the stop time
Schedule speed = distance ÷ (running time + stop time).
Regenerative braking in a traction system:
1. Dissipates energy in resistors only
2. Returns energy to the supply
3. Uses friction only
4. Reverses the supply
Answer
B. Returns energy to the supply
Motors act as generators during braking.
A DC series motor suits electric traction because it has:
1. Very low starting torque
2. Poor overload capacity
3. Constant speed at all loads
4. High starting torque
Answer
D. High starting torque
Torque is high at low speed.
Coasting in a speed-time curve is a period during which:
1. The brakes are applied
2. The train is at rest
3. Power is switched off and the train runs on its momentum
4. The train accelerates at full power
Answer
C. Power is switched off and the train runs on its momentum
Coasting saves energy but lengthens the run.
The adhesion of locomotive wheels to the rail is improved by:
1. Reducing the weight on driving wheels
2. Wet rail
3. Oiling the rail
4. Sanding the rail
Answer
D. Sanding the rail
Sand increases the coefficient of adhesion.
Statements: 1. Induction heating is for conducting materials. 2. Dielectric heating is used for conductors only.
1. 2 only
2. 1 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 1 only
Dielectric heating is used for insulating materials.
Statements on arc welding: 1. The welding transformer has a drooping characteristic. 2. It has high leakage reactance.
1. Both 1 and 2
2. 1 only
3. 2 only
4. Neither 1 nor 2
Answer
A. Both 1 and 2
Both are true; the leakage reactance gives the drooping characteristic.
Statements on tractive effort: 1. Fa uses the effective (accelerating) weight. 2. Fg uses the effective weight, not the dead weight.
1. 2 only
2. Both 1 and 2
3. Neither 1 nor 2
4. 1 only
Answer
D. 1 only
Gradient and resistance forces use the dead weight W.
Which pair is correctly matched?
1. Coreless furnace – mains frequency only
2. Dielectric heating – steel billets
3. Spot welding – overlapping sheets joined at points
4. Regenerative braking – dissipates energy in resistors
Answer
C. Spot welding – overlapping sheets joined at points
A coreless furnace uses high frequency; dielectric heating is for insulators; rheostatic braking dissipates energy.
Statements on traction: 1. Schedule speed is greater than average running speed. 2. Regenerative braking is possible only with a DC series motor and never with AC drives.
1. 1 only
2. Neither 1 nor 2
3. 2 only
4. Both 1 and 2
Answer
B. Neither 1 nor 2
Schedule speed is lower since it includes stops; AC converter drives also give regenerative braking.
Statements on induction heating: 1. Depth of heating decreases as frequency increases. 2. A coreless furnace uses high frequency.
1. Both 1 and 2
2. 1 only
3. 2 only
4. Neither 1 nor 2
Answer
A. Both 1 and 2
Both follow from the skin effect and the design of coreless furnaces.

Page 1 of 1

Chapter quiz · 10 questions

0/10 answered

On what you just read, from the live question bank. 🔒 Answers and solutions are shown to registered students (free).
अध्याय पढ़ने के बाद अभ्यास करें।

1.The device which converts mechanical energy into electrical energy is ______.
2.The rate at which electrical work is done or the rate at which electrical energy is consumed is called-
3.The rate at which electrical energy is dissipated in an electric circuit is called ___________.
4.Which of the following materials is used for making the heating element of an electrical iron?
5.Which of the following materials is preferably used in making heating elements of electrical heating devices?
6.Which of the following components store energy in the form of electrical charges?
7.Which of the following correctly describes the SI unit of electric power and the commercial unit of electrical energy, respectively?
8.Which of the following statements is/are correct? Statements: I) Electromagnet is a device used to convert magnetic field into electrical energy. II) Electromagnet is a device used to convert mechanical energy into magnetic energy.
9.What is the commercial unit of electrical energy consumed?
10.Which energy of fossil fuel is transformed into electrical energy at thermal power plants?

10 left. You can check any time.