Basic principles:Basic principles of electrical machines

Basic principles of electrical machines

Electromechanical energy conversion

The electromechanical energy conversion device is a link between electrical and mechanical systems.

When the mechanical system delivers energy through the device to the electrical system, the device is called a generator.

The process is reversible; however, the part of energy converted to heat is lost and is irreversible. An electric machine can be made to work either as a generator or as a motor. The electromechanical conversion depends on the interrelation between:

• Electric and magnetic fields

• Mechanical forces and motion.

In rotating machines, power is generated by the relative motion of the coils.

In the case of a generator, the winding is rotated mechanically in the magnetic field. This causes the flux linkages with the windings to change causing induced voltages.

In the case of a motor, the current-carrying conductor is allowed inside a magnetic field. Mechanical force is exerted on a current-carrying conductor in a magnetic field and hence a resultant torque is produced to act on the rotor.

In both a generator as well as a motor, the current-carrying conductor is in the magnetic field. The conductors and flux travel with respect to each other at a definite speed. In rotating machines, both voltage and torque are produced. Only the direction of power flow determines whether the machine is working as a generator or a motor. For a generator, e and i are in the same direction.

In a generator, the power is supplied by the prime mover. Electrical power is produced by the action of the generator and the resultant power produced due to friction is lost. Whereas in the case of a motor, the power is supplied by the electrical power supply inputs, and there is a slight loss of the resultant mechanical power produced due to friction.

Basic principles of electromagnetism

Magnetic and electric fields

As you are aware, each electric charge has its own electric field; i.e., lines of force. Electric field lines point away from the positive charges and towards negative charges (Figure 1.7). Each charge exerts force on the other charge, which is always tangential to the lines of force created by the other charge.

Similarly, the magnetic field lines ‘flow’ away from the N-pole and towards the S-pole (Figure 1.8). A current moving the electric charges creates a magnetic field. Every orbiting electron forms a current loop that creates its own magnetic field. Magnetic field lines always form circles around the current creating them.

Magnetic field produced by a current-carrying conductor

If a conductor carries a current, it produces a magnetic field surrounding it. The direction of the current and the direction of the field so produced have a definite relation that is given by the following rules:

The right hand rule Hold the conductor in the right hand with the fingers closed around the conductor and the thumb pointing towards in the direction of the current. The fingers will point towards the direction of the magnetic lines of the flux produced around the conductor.

Flux produced by a current-carrying coil Flux can be produced by causing the current to flow through a coil instead of a conductor. Introduction of magnetic material in the core on which the coil is wound increases flux. The direction of the magnetic flux in the coil is given by the right-hand rule.

In the case of a motor, the direction of the emf induced is such as to oppose the flow of current. Whereas, in a generator the emf induced is in such a direction as to establish a current.

Fleming’s left hand rule This defines the relationship between the direction of the current, the direction of field, and the direction of the motion. If the forefinger of the left hand points in the direction of the field, the middle finger points in the direction of the current, and the thumb points in the direction of the motion.

The basic principle of motor

The basic working of a motor is based on the fact that when ‘a current carrying conductor is placed in a magnetic field, it experiences a force’.

If you take a simple DC motor, it has a current-carrying coil supported in between two permanent magnets (opposite pole facing) so that the coil can rotate freely inside. When the coil ends are connected to a DC source then the current will flow through it and it behaves like a bar magnet, as shown in Figure 1.9. As the current starts flowing, the magnetic flux lines of the coil will interact with the flux lines of the permanent magnet. This will cause a movement of the coil (Figures 1.9(a), (b), (c), (d)) due to the force of attraction and repulsion between two fields. The coil will rotate until it achieves the 180° position, because now the opposite poles will be in front of each other (Figure 1.9(e)) and the force of attraction or repulsion will not exist.

The role of the commutator: The commutator brushes just reverse the polarity of DC supply connected to the coil. This will cause a change in the direction of the current of the

magnetic field and start rotating the coil by another 180° (Figure 1.9(f ).

The brushes will move on like this to achieve continuous coil rotation of the motor.

Similarly, the AC motor also functions on the above principle; except here, the commutator contacts remain stationary, because AC current direction continually changes during each half-cycle (every 180°).

Basic principle of generator

We have discussed the basic working of a motor and through the diagrams we have seen a generator action as well.

In principle, an AC generator’s construction is similar to the construction of the motor. Instead of putting current in, current is taken out from the coil in an alternator.

A mechanical prime mover rotates the coil in between the poles of a permanent magnet and an AC potential is induced in the coil. To further define: if an AC current will make a coil turn, then turning the coil will create an AC current.

As per Faraday’s law, when a wire is moved in to cut across magnetic field lines, a force is exerted on the charge (electrons) in the wire by trying to move them along the wire. This is how current will start flowing if a complete circuit is provided to it. The magnetic field is provided not by magnets, but by field coils.

The coil in which the voltage is induced is called armature winding, while the coil that provides the magnetic field is called field winding.

In high-voltage generators, it is not good practice to have armatures rotating because current-collecting brushes of high ratings are required. Rather, the armature is kept stationary and the field is kept rotating.

Alternators of low capacity use a permanent magnet as a field, while in high-capacity alternators field winding supply is derived from the exciter assembly. An exciter assembly is a small alternator connected on the same shaft.

Idealized machines

There is a stationary member called a stator and a rotating member called a rotor. The rotating member is mounted on bearings fixed to the stationary member. The stator and the rotor have cylindrical iron cores, separated by an air gap. Windings are wound on the stator and the rotor core. A common magnetic flux passes across the air gap from one core to another forming a combined magnetic circuit. Two cylindrical iron surfaces with an air gap between them move relative to each other. The cylindrical surface may be divided by an even number of salient poles with spaces in between, or it may be continuous with slot openings uniformly spaced around the circle. This structure may be for either of the stator or the rotor.

The common features of an ideal electrical machine are shown in the Figure 1.10. For windings, conductors run parallel to the axis of the cylinders near the surface. The

conductors are connected into coils by the end connections outside the core and the coils are connected to form the windings of the machine.

The operation of the machine depends on the distribution of the currents around the core surfaces and the voltages applied to the windings.

In various types of electrical machines, the arrangement differs in the distribution of the conductors, windings, and in core constructions, depending on whether it is a continuous or a salient pole type. The magnetic flux permeates the iron cores in a complex manner. However, as the iron has a high permeability, the accurate working of a machine can be determined by considering the flux distribution in the air gap. The conductors are actually located in slots formed in the laminations of the core.

A typical cross section and the corresponding development diagram of an electrical machine with four poles, perpendicular to the axis of the cores is shown in Figure 1.11.

As shown in the diagram, the distribution of flux and current repeats itself at every pair of poles. On the poles, the windings are so wound that the current flows in the opposite direction and produces a field corresponding to the north and south polarities. Maximum flux is along the center of the pole and reduces to zero between the interpole gaps.

Basic principles of electrical machines

In an electrical machine, the currents in all the windings combine to produce the resultant flux. The field system produces flux. Voltages are induced in the windings such as those of an armature. When the armature carries current, the interaction between the flux and the current produces torque.

Types of electrical machine windings

(a) Coil winding

The winding consists of coils wound on all the poles of the machine and connected together to form a suitable series or parallel circuit. The direction of the current in the alternate pole will be opposite so that when one pole is the North Pole, the other adjacent pole will be a South Pole. This produces the flux in the proper direction, completing the magnetic circuit from the North Pole to the South Pole through the iron cores of both the stator and the rotor.

The coil may be wound on the stator or on the rotor, forming the salient or non-salient poles of the machine. The DC supply is given to these windings and they produce a field proportional to the magnitude of the current through the windings. If the poles are on the stator, a stationary field is produced in the air gap.

(b) Commutator winding

The commutator winding is on the rotor. The armature has open slots and the conductors are located in these slots and connected to the commutator segments in a continuous sequence.

Polyphase winding is a distributed winding. Individual conductors are distributed in slots in a suitable way and connected into a number of separate circuits, one for each phase. The group of conductors forming the phase bands is distributed in a regular sequence over the successive pole pitches so that there is balanced winding that produces an equal voltage per phase. This type of winding is mainly used for the stator. When supplied with three-phase currents it produces a rotating field in the air gap. This is of a constant magnitude but rotating at a constant synchronous speed.

Types of electrical machines

Depending on the type of combinations of windings used on the stator and the rotor, electrical machines are classified in different types as follows:

1. DC machines

The DC machines have an edge over AC machines when it comes to the speed control of a motor. It is easier and cheaper.

(a) Shunt motor

This machine has field winding mounted in yoke and the armature winding is mounted on rotor. The shunt motor is used where speed regulation is important.

Self excited Field winding is connected in parallel (shunt) with the armature winding on the same supply. Changing the field current can vary the speed. Torque is proportional to armature current.

This machine can also act as a generator. To limit the high starting current of the motor drive release the voltage in the ramp. For this motor, a variable resistor is connected in

series with the field circuit to change the flux value and the speed by a small amount.

Separately excited Field winding is connected in parallel (shunt) with the armature winding with separate excitation.

Torque is proportional to armature current. In a separately excited shunt motor, speed can be varied up to a certain limit by changing armature voltage. After that using field weakening (reducing field current), it is possible to increase the speed of motor above base speed. Other features remain same as that of the self-excited one.

(b) Series motor

As the name suggests in this type of motors, field winding is connected in series with the armature winding. Naturally, heavy current will pass through it; hence field winding of a thicker gage is used. A series motor is used where speed regulation is not important.

The main advantage of this motor is that a high torque can be obtained, which makes it useful for applications such as diesel locomotives, cranes, etc.

The relationship between Torque and current is as follows:

T α Ia2

It is important to start this motor in a loaded condition else it could lead to damage of the motor and its surroundings.

If we combine both series and shunt motors then we will have a compound motor. This combines the good features of both types such as high torque characteristics of a series motor and the speed regulation of a shunt motor.

2. AC machines

(a) Squirrel-cage induction motor

AC machines are simple and sturdy. The most common machine of this type is the Squirrel cage induction motor (the name was derived from its construction type). The basic working of this was dealt with in the previous sections. The following relation gives the speed of this motor:

For example, if the motor has two poles, then at 50 Hz frequency the motor rpm will be 3000 (rpm).

However, you will not find 3000 or 1500 rpm on the motor nameplate because the motor rpm will not be 3000 rpm at full load. This is because of a slip associated with an induction motor.

The RPM of the motor is controlled by controlling the frequency ( f ) – as frequency increases, motor speed will also increase. High starting current is limited using a star/delta starter or reduced-voltage starters.

(b) Wound rotor motor

This is similar in construction to the squirrel cage and works similarly too, except that slip rings are provided. The main feature of the slip ring motor is that resistors, which are connected in series with the rotor circuit, limit the starting current.

The motor starts with a full resistance bank, but as speed of the motor increases, the resistances are shorted, one by one. As the motor reaches full speed, the whole bank of resistance is shorted out and the motor now runs as a normal induction motor.

(c) Synchronous motors

Synchronous motor is a constant-speed motor, which can be used to correct the power factor of the three-phase system. Like the induction motor in terms of the stator, the synchronous machine has either a permanent magnet arrangement or an electromagnet (with current supplied via slip rings) rotor. In simple terms, the rotor will keep locking with the rotating magnetic field in the stator. So, a two-pole machine will run at exactly 3000 rpm. In many synchronous machines, a squirrel cage is incorporated into the rotor for starting. Therefore, the machine acts as an induction motor when starting and as it approaches synchronous speed, it will suddenly ‘lock in’ to the synchronous speed.

Basic characteristics of electrical machines

The following are the basic characteristics of electrical machines:

• The voltages induced in the windings, the load currents, and the terminal voltages depend on the following different loading conditions – the speed at which the machine works under different loading conditions and the frequency.

• The power input or the output received from the machine.

• The torque produced under different loading conditions.

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