Short Notes: Principle of Energy Conversion | Electrical Machines - Electrical Engineering (EE) PDF Download

Introduction

An electromechanical energy conversion device is a machine that converts electrical energy into mechanical energy or mechanical energy into electrical energy. The conversion between the two energy forms is achieved through a magnetic field, which acts as the coupling medium between the electrical and mechanical subsystems because of its high energy-storing capacity.

Such a converter may be divided into three main parts:

• Mechanical system - rotor, shaft and the driven or driving mechanical parts.
• Coupling medium - magnetic field and magnetic circuit that transfer energy between electrical and mechanical domains.
• Electrical system - windings, stator, supply or load connections and electrical circuits supplying or extracting power.

Electromechanical converters are commonly classified into two groups:

• Gross-motion devices - machines that produce appreciable motion of parts; examples include electrical motors and generators.
• Incremental-motion devices - devices producing small or discrete displacements; examples include microphones, loudspeakers, electromagnetic relays and many electrical measuring instruments.

When conversion proceeds from electrical to mechanical energy the device is called a motor. When conversion proceeds from mechanical to electrical energy the device is called a generator.

Basic electromagnetic effects involved

• Electromagnetic induction - when a conductor moves in a magnetic field (or the magnetic flux through a circuit changes), an electromotive force (EMF) is induced in the conductor (Faraday's law).
• Electromagnetic force (Lorentz force) - when a current-carrying conductor is placed in a magnetic field a mechanical force acts on it; for a straight conductor of length l carrying current I in a field B the force is F = B I l (direction given by Fleming's left-hand rule).

Both effects occur simultaneously in practical machines. In a motor, current in the rotor or armature conductors interacts with the magnetic field to produce a force on each conductor and hence an electromagnetic torque on the rotor. The rotor then turns and delivers mechanical power through the shaft. Because the conductors move in the magnetic field, an EMF is also induced in them during rotation.

In a generator, the rotor is driven by a prime mover. Motion of conductors in the magnetic field induces an EMF and causes current to flow to the electrical load. The current in the conductors interacts with the field and produces a reaction torque that opposes the prime mover (the machine resists the driving torque when generating).

Principle of Conservation of Energy in Electromechanical Conversion

The principle of conservation of energy states that energy can neither be created nor destroyed; it can only be converted from one form to another. In an electromechanical energy converter the total input energy equals the sum of energy dissipated, energy stored and useful output energy.

• Energy dissipated - losses such as copper (I2R) losses, core (hysteresis and eddy current) losses, mechanical losses (friction, windage).
• Energy stored - energy temporarily stored in the magnetic field (magnetic energy) of the machine.
• Useful output energy - mechanical output work (in a motor) or electrical output energy delivered to a load (in a generator).

General energy (power) balance - instantaneous form

For motoring action (electrical → mechanical), the instantaneous power balance may be written as

Electrical input power = Power dissipated in electrical and magnetic losses + Rate of change of stored magnetic energy + Mechanical output power.

For generating action (mechanical → electrical), the instantaneous power balance may be written as

Mechanical input power = Electrical output power + Rate of change of stored magnetic energy + Power dissipated.

Energy stored in the magnetic field

The magnetic field stores energy. For a continuous field, the magnetic energy stored in a volume V is

Wm = 1/2 ∫_V B · H dv

For lumped magnetic systems or inductances the stored energy may be written as

Wm = 1/2 L i²

or, using flux linkage λ = L i,

Wm = 1/2 λ i

Induced EMF and torque relations (key relations used in machine analysis)

• Induced EMF - the instantaneous induced EMF in a winding is related to the rate of change of flux linkage: e = -dλ/dt.
• Electromagnetic force on a conductor - F = B I l for a straight conductor of length l in a magnetic field B carrying current I.
• Torque in rotating machines - electromagnetic torque can be related to the derivative of magnetic energy or co-energy with respect to rotor position. For many analyses the torque T is obtained from co-energy W′ as

T = ∂W′/∂θ (with currents held constant)

Here W′ (co-energy) is used because it gives a convenient expression when currents are independent variables; using energy or co-energy depends on whether flux linkages or currents are treated as independent.

Energy conversion in a simple rotating conductor (conceptual)

Consider a conductor of length l moving with velocity v perpendicular to a magnetic field B. The induced EMF along the conductor is e = B l v. If the conductor carries current I in the field, the force on it is F = B I l and the mechanical power developed (or absorbed) is Pmech = F v = B I l v. Since e = B l v, electrical power e I equals mechanical power F v (sign depends on motoring or generating action), demonstrating power equivalence between electrical and mechanical domains apart from losses and stored energy.

Examples, Applications and Remarks

• Typical gross-motion devices include DC motors, induction motors, synchronous machines (used as either motors or generators).
• Typical incremental devices include moving-coil meters, relays, microphones and loudspeakers where small displacements or forces are produced.
• In machine design and analysis, the stored magnetic energy and the co-energy concepts are used to derive expressions for torque, starting behaviour and dynamic response.
• Energy balance equations are central in performance calculations: steady-state operation often assumes stored energy term averages to zero over a cycle, leaving input power = losses + output power.

Conclusion

The principle of electromechanical energy conversion rests on two fundamental electromagnetic effects - induction of EMF by changing flux and mechanical force on current-carrying conductors in a magnetic field. Magnetic fields act as the coupling medium and store energy; the conservation of energy requires that electrical input (or mechanical input) equals losses plus the rate of change of stored energy plus useful output. Understanding stored magnetic energy, induced EMF and torque relations (including co-energy methods) is essential for analysing and designing electrical machines.