DE5304 electrical fundamentals Module 6 (level 6 Diploma in Mechanical engineering) PDF

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Summary

electrical machinne basics.
DC Theory and AC and Electrical safety
Basic electronics systems
DC motors and starters
Digital and analogue systems
Concepts of open and closed loop control, proportional, sequential...

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DE5304

Module 6: DC and AC machines and their characteristics Outcomes

Introduction Rotating electrical machinery is a common part of equipment used for industrial, domestic, or military purposes. From a simple machine such as a dryer, to a complicated electronic device – all of these items depend upon the proper functioning of rotating electrical equipment. There are two types of motors and generators commonly in use: direct current or DC motors and generators, and alternating current or AC motors and generators. The study of DC machines includes both DC generators and DC motors. While the construction of DC motors and generators is virtually the same, DC generators convert mechanical energy into electrical energy, whereas DC motors use electrical energy to get mechanical work done. This module will show you the principles governing the operation of DC and AC motors and generators. First we will discuss DC machines (generators and motors), followed by AC motors.

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Take note

You will find the online resources ‘DC motors and generators’ and ‘AC motors and generators’ to be useful supplements to the learning materials in this module. You can access these at: https://www.youtube.com/watch?v=OpL0joqJmqY (34:11) https://www.youtube.com/watch?v=07uXnc1C5CA (23:51)

DC generators In DC generators, direct current electrical energy is produced mechanically. The generated electrical energy is then used to power many devices, from normal household electrical appliances to industrial applications, including handling heavy material hoists, compressors and other machines.

Construction of a generator The energy conversion in DC generators is based on the principle of production of dynamically induced electromotive force (emf) (electricaleasy.com, 2014). The emf is induced by a moving conductor in a magnetic field. There are many turns of conductor wound within an iron core. This rotating part is called the armature, as shown in Fig.6.1.

Lifting eyes

Terminal box

Bearings

Cable outlets Commutator

Ventilating ducts Air vents

Interpoles Carbon brush holders

Armature Fan

End shield

Poles secured by screws Spring arms

Air vents

Main field coils Steel frame

End shield

Fig. 6.1 Main components of a DC machine

In Fig. 6.1, the permanent magnet of the simple generator is replaced by an electromagnet having the poles wound with insulated copper wire. These are the main field coils. They are referred to as the fixed field system.

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The field strength, or excitation, depends upon the number of turns of wire on each pole, and the magnitude of current flowing through the wire. The induced emf developed in any generator depends on the strength of magnetic flux in the field, the speed of the armature, and the number of conductors (turns) in series between the brushes. An induced emf is proportional to the speed at which a conductor cuts through a magnetic field flux. If ϕ is the flux, N is the speed, and ‘Z’ is the number of conductors in series, then emf ‘E’ is given by the following relation: E ∝ φNZ

The armature The armature of a DC generator consists of coils wound on the iron core (also called the rotor). This core provides an easy path for the magnetic field. When rotating, the iron core of the armature cuts the magnetic field flux. The core is laminated to reduce the eddy currents produced by the magnetic field. Conductors are wound into coils with the requisite number of turns before being fitted into the slots on the armature core. Each slot is fitted with an insulating liner. After the armature coils have been wound into the coil slots, the tail ends are soldered or brazed into the commutator bar risers. The slots are then closed with insulated wedges. In large machines, high-tensile steel wire bands are formed around the armature to keep the windings in place against centrifugal forces. The armature is shown in Fig. 6.2.

Laminations Shaft

Lead Insulating throw Riser spiders

Commutator

Mica Segme

Armature core

Shaft Windings

Slots

Commutator Core

Fig. 6.2 The armature

Commutator The induced emf across the conductor is AC in nature, so this AC output is converted into DC with the help of the commutator. The commutator has wedge-shaped segments made up of hard drawn copper. The armature coils are attached to the commutator segments. As each segment passes under the rectangular-shaped carbon brushes, the emf in the coil is mechanically reversed. The commutator also acts as a moving connection so that the brushes can pick up current generated in the armature coils. The brushes are positioned on the commutator so that each coil is short-circuited as it moves through its own electrical neutral plane. (The neutral plane is an imaginary line perpendicular to the lines of magnetic flux passing between the field poles of an electric motor or generator. If the brushes are mounted along this neutral plane, the segments of the commutator they short

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across have the same electrical potential, and there will be no sparking at the brushes.) The brushes are made of carbon, and these brushes collect current from the commutator. This current is then supplied to the connected load and/or field circuit. The bars of the commutator have the same hardness to reduce wear and tear between bars. Figure 6.3 shows the principal parts of the commutator. Tightening nut Iron ring Mica V-ring Front V-ring Commmutator bars Mica insulation between bars

Note taper from top to bottom

Commutator bar Mica Iron shell

Back V-ring with mica inner and outer rings for insulation

(a) Commutator

End view

Slots for leads Back end V-cut

Front end V-cut

(b) One segment

Fig. 6.3 Principal parts of the commutator

In small machines, the commutator is carried on a sleeve or brush and pressed on to the shaft. On large machines, the commutator is carried on a spider, which in turn is pressed on to a boss or hub. Commutators with a high surface speed must run true. Otherwise, brush bounce, with resultant sparking and loss of power, can occur.

Field system The prewound field coils are fitted on to the pole pieces, which are then bolted to the frame of the machine. The poles are usually laminated to reduce eddy current losses. They serve the same purpose as the iron core of an electromagnet in concentrating the lines of force produced by the field coils. Figure 6.4 shows how the field coils are wound alternately north and south. A variable resistance may be included in the field circuit to vary the excitation. The outer frame (also called the yoke) forms part of the magnetic circuit and provides mechanical strength for whole assembly. For this reason, it is made of a material of high permeability, such as cast steel.

Magnetic path

Figure 6.4 shows the magnetic path through the frame, the pole piece, and the armature for a fourpole machine. Fig. 6.4 Magnetic field paths

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Excitation of the field system Classification of DC generators is based on the types of connections used in field windings. The term ‘field excitation’ is used where the magnetic field is produced by the process of giving DC voltage to the field winding. For a generator to have an emf induced in its armature coils, the coils must cut a magnetic field. The coil is rotated in a clockwise direction in a uniform magnetic field. Emf is zero at the starting point, as the direction of the coil is perpendicular to the lines of flux. The rate of linked flux is at its maximum when the direction of the plane of the coil is parallel to the lines of flux. Permanent magnets can provide this magnetic field, or it can be produced by: ½

the supply or current from the armature itself

½

a separate DC supply.

DC generators can be classified by two types based on their excitation: self-excited DC generators and separately excited DC generators. A generator that supplies current to its own field is called a self-excited generator. There must be a small amount of residual magnetism in the pole pieces to induce the small emf required in the armature at the start. A generator that uses a separate DC supply is called a separately excited generator. A selfexcited DC generator is further divided into three categories: ½

series generator

½

shunt generator

½

compound generator (long shunt compound generator and short shunt compound generator).

Variation of voltage with speed Separately excited generators, which have a constant field flux, generate an emf that is proportional to speed: E ∝ N Figure 6.5 shows a graph of generated emf against armature speed. For the separately excited generator, this graph will be a straight line.

Armature e.m.f. in volts

240 Variation of e.m.f. with speed in generator with constant flux at no load 160 Rev/min

80

400

800

EMF

200 400 600 800

40 80 120 160

1000 1200

200 240

1200

Revs per min

Fig. 6.5 Armature speed vs armature emf graph

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This straight-line graph is used in certain types of speed indicator. The generator is coupled to the machine, whose speed must be known. The terminals are connected to a voltmeter that has its scale calibrated in revolutions per minute instead of volts. Example

If an emf of 150 V is generated at 1500 rpm by a separately excited generator, calculate the emf produced at 1000 rpm. Solution: Assuming N is the speed and E is the voltage. From above, we have: E2 = 150 V N1 = 1000 rpm N2 = 1500 rpm E1 = ? Now, in a separately excited generator, generated emf is proportional to speed: E ∝ N ∴ E1 ∝ N1 and E2 ∝ N2 Now

E1 E2

=

∴ E1 = =

N1 N2 E2 1

×

N1 N2

150 1000 × 1 1500

= 100 V Therefore, at 1000 rpm the generated emf is 100 V.

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Generator connections and characteristics The fundamental quantities voltage ‘V’ across terminals, the field or exciting current If, the armature current Ia, and the speed of rotation ‘N’ can be related together to analyse the properties of generators. The most important characteristic of a DC generator is the  E  internal characteristic   . The next important characteristic is the no-load saturation  Ia   E  characteristic   . Performance characteristics are categorised as external characteristics  If     V (Rajput, 2005, p. 90).  I 

Separately excited generators The simplest way to produce a magnetic field in a generator is to connect the field coils to a separate DC source, such as a battery. In practice, however, batteries are seldom used to supply the field current. A small self-excited generator, which may be driven from the main generator shaft or by a separate motor, is usually used. This additional generator is called an exciter. There is no electrical connection between the armature winding and the field.

Field Constantvoltage d.c. supply

A Arm

V Load

Terminal voltage

The load characteristic or regulation curve shows the relationship between the load current taken from the generator and the output voltage across its terminals when it is driven at constant speed. These load characteristics of a separately excited machine can be found by driving the armature at constant speed, while keeping the field current constant. The voltage and current is measured for progressively increasing load currents. The connections for such a test are shown in Fig. 6.6(a), and typical results are shown by the graph in Fig. 6.6(b).

Load current

(a) Connections for testing

(b) Typical results

Fig. 6.6 Voltage at various load s

The output voltage drops as the load increases, because the resistance of the armature winding acts in the same way as the internal resistance of a cell. Therefore, the terminal voltage is the difference between generated emf and the internal voltage drop: terminal voltage V = E – Iara

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where ‘ra’ is the internal resistance of the armature and Ia is the armature current. Therefore, terminal voltage is reduced with the increase in the armature current Ia: V = E – Iara – Vd Vd is the additional voltage drop across the brushes

Self-excited generators In a self-excited generator, the current for the field windings is provided from the emf produced by the generator. As we have already seen, there are three ways in which the armature and field windings can be connected within the generator: ½

shunt connection

½

series connection

½

compound connection (mixture of shunt and series).

Characteristics of shunt-wound generators In the most common form of self-excited generator, the field windings are connected across the armature, as shown in Fig. 6.7(a). It is thus called a shunt-wound generator, and its load characteristic is shown in the graph of Fig. 6.7(b).

Shunt field Load

(a) Field winding connetctions

Terminal voltage

Separate excitation

Shun t exc itatio n

Full load current

Load current

(b) Load characteristic

Fig. 6.7 Load characteristic for shunt-wound generator

The successful working of a shunt machine depends on the existence of residual magnetism in the frame and pole pieces. Without it, the machine will not generate an emf that is, emf ‘E’ is zero. When residual magnetism is present, the armature develops a small emf as soon as it begins to rotate, even with no current flowing in the field winding. This small emf will cause a current to flow in the field winding, thus increasing the field flux. The increase in field flux will increase the armature’s emf, and more current will flow in the field. This process will continue until rated field current is reached. If, however, the polarity of the initial emf is in the wrong direction, the current flowing in the field winding will destroy any residual magnetism and the generator will not function. Wrong field connections can cause this to happen.

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If no residual magnetism remains, connect a battery across the field terminals for 1second. If the generator still does not build up an emf, reverse the battery connections and try again. Like the separately excited generator, the output voltage of the shunt-wound generator drops with increasing load because of internal voltage drop. The fall in voltage will also reduce field current by a small amount, which will in turn reduce the generated emf, thus aggravating the fall in terminal voltage. The external load characteristic in Fig.6.7(b) shows a comparatively large drop of terminal voltage after normal full-load current is exceeded. The last part of the curve shows that when the load exceeds a certain critical point, the fall in terminal voltage is such that the field flux can no longer sustain the required load current. Any further reduction in the resistance of the load will see both voltage and current fall rapidly. Shunt-wound generators are used as exciters for alternators in hydro stations. They are also used for static battery charging sets and small electroplating plants. Shunt generators are very efficient when the load they supply remains almost constant.

Characteristics of series-wound DC generators The connections for a series-wound generator are shown in Fig. 6.8. The field coils are wound with a few turns of wire that has a large cross-sectional area. Take note

As this is a series connection, the load current is also the field current.

Series field Armature

e.m.f. output

Fig. 6.8 Series-wound DC generator

The external circuit must be closed before the series-wound DC generator will excite. The changes in terminal voltage as per changes in load at a constant speed are shown in Fig. 6.9. At no load, the generated emf is due to the residual magnetism. The flux starts increasing as the load is connected, causing more generated emf and thereby increasing the flow of current.

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This increases the field flux even further. A stable point is reached when the field poles are saturated, and so any further increase in load (around point X in Fig. 6.9) will cause a fall-off in terminal voltage.

Full load

Terminal voltage

Like the shunt-wound generator, the difference between the generated emf and the terminal voltage is due to internal resistance (IR) losses and armature reaction. The generated output is controlled by a field diverter resistor connected in parallel with the series winding. The control is small compared with the variations produced by load changes.

X

No-load

Load current

Fig. 6.9 Load characteristic of a series-wound DC generator

Uses of series-wound DC generators The series-wound generator was used in the past as a voltage booster for traction systems where a DC voltage had to be increased because of large current demands. The resistance in the overhead lines caused an increased voltage drop. The series-wound generator is little used today because its large changes of terminal voltage over a range of load currents means it is not a source of constant voltage.

Characteristics of compound-wound DC generators In a compound-wound generator, a series winding is connected to the shunt winding on the same field poles to improve the shunt-wound generator’s load characteristic. Figure 6.10 shows the two field connections. The shunt field is across the armature (a short shunt) or across the output terminals of the generator (a long shunt). The difference is the amount of current flowing through the series field winding. Shunt field Shunt field Series field Armature

Series field Armature

e.m.f. output

e.m.f. output

(a) Short shunt

(b) Long shunt

Fig. 6.10 Compound-wound DC generator

There are two types of compounded generators:

10

½

differentially compounded DC generators

½

cumulatively compounded DC generators.

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Differentially compounded DC generators When current is drawn from a differentially compounded generator, the flux will decrease because the series winding and shunt winding are opposing each other, and so the generated emf will drop. As the current load increases the voltage falls quickly, due to the decreasing flux. This is shown by its characteristic in Fig. 6.11.

Over compound Level compound

Full load

Terminal voltage

Under compound