Laboratory

Electromechanical Energy ConversionDIRECT CURRENT GENERATORS Prepared by: ADEL ELGAMMAL

Adel_elgammal2000@yahoo.com

Adel.elgammal@utt.edu.tt

Group #First NameLast Name

NOTE: All the students must strictly follow all of the safety precautions. In case of any question or

concern, please contact LAB INSTRUCTOR or TA.

8adel.elgammal@utt.edu.tt

Lab 1: Identify and Study Main Parts of DC

Generator

Objective:

To study main parts of a DC generator.

Apparatus:

DC generator Yoke, Poles, Armature, Commutator, Brushes.

Theory:

A DC generator is comprised of following main parts

1. Field system

2. Armature Core

3. Armature Winding

4. Commutator

5. Carbon Brushes

1. Field System:

The function of the field system is to produce uniform magnetic field within which the armature rotates. It consists of a number of salient poles (of course, even number) bolted to the inside of circular frame (generally called yoke). The yoke is usually made of solid cast steel whereas the pole pieces are composed of stacked laminations. Field coils are mounted on the poles and carry the d.c exciting current. The field coils are connected in such a way that adjacent pole shave opposite polarity.

The m.m.f. developed by the field coils produces a magnetic flux that passes through the pole pieces, the air gap, the armature and the frame Practical d.c. machines have air gaps ranging from 0.5 mm to 1.5 mm. Since armature and field systems are composed of materials that have high permeability, most of the m.m.f. of field coils is required to set up flux in the air gap. By reducing the length of air gap, we can reduce the size of field coils (i.e. number of turns).

2. Armature Core:

The armature core is keyed to the machine shaft and rotates between the field poles. It consists of slotted soft-iron laminations (about 0.4 to 0.6 mm thick) that are stacked to form a cylindrical core as shown in Fig. The laminations are individually coated with a thin insulating film so that they do not come in electrical contact with each other. The purpose of laminating the core is to reduce the eddy current loss. The laminations are slotted to accommodate and provide mechanical security to the armature winding and to give shorter air gap for the flux to crossbetween the pole face and the armature teeth.

3. Armature Winding:

The slots of the armature core hold insulated conductors that are connected in a suitable manner. This is known as armature winding. This is the winding in which working e.m.f. is induced. The armature conductors are connected in series-parallel; the conductors being connected in series so as to increase the voltage and in parallel paths so as to increase the current. The armature winding of a d.c. machine is a closed-circuit winding; the conductors being connected in a symmetrical manner forming a closed loop or series of closed loops.

4. Commutator:

A commutator is a mechanical rectifier which converts the alternating voltage generated in the armature winding into direct voltage across the brushes. The commutator is made of copper segments insulated from each other by mica sheets and mounted on the shaft of the machine (See Fig). The armature conductors are soldered to the commutator segments in a suitable manner to give rise to the armature winding. Great care is taken in building the commutator because any eccentricity will cause the brushes to bounce, producing unacceptable sparking. The sparks may bum the brushes and overheat and carbonize the commutator.

5. Carbon Brushes:

The purpose of brushes is to ensure electrical connections between the rotating commutator and stationary external load circuit. The brushes are made of carbon and rest on the commutator. The brush pressure is adjusted by means of adjustable springs (See Fig). If the brush pressure is very large, the friction produces heating of the commutator and the brushes. On the other hand, if it is too weak, the imperfect contact with the commutator may produce sparking. Multipole machines have as many brushes as they have poles. For example, a 4-pole machine has 4 brushes. As we go round the commutator, the successive brushes have positive and negative polarities. Brushes having the same polarity are connected together so that we have two terminals viz., the +ve terminal and the -ve terminal.

Lab 2: Different Types of Connections in Dc

Generators

Objectives:

To understand different types of DC Machines

1. Separately Excited DC Generator

2. Shunt Excited DC Generator

3. Series Excited DC Generator

Apparatus:

1. DC Generator SM 2641

2. DC Power Supply

3. Connecting Leads

4. Voltmeter

Circuit Diagram:

(Separately excited dc generator)

(Shunt dc-generator)

(Series dc generator)

Theory:

DC Machines are classified according to manner in which armature circuit is connected to the field circuit. So there are following main types1. Separate Excited DC Generator

2. Shunt Excited DC Generator

3. Series Excited DC Generator

In a separate excited DC Generator the armature and field circuits are supplied by separate voltage

sources.

In a shunt excited DC Generator both circuits are connected in parallel to each other.

In a series excited DC Machine both the field and armature circuits are connected in series to each

other. These connections are shown in the circuit diagram.

In this lab exercise our aim is to achieve above stated connections.

Procedure:

Make connections according to the given circuit diagram for each type of machine separately. After that you will see that we got different values for different connections.

Lab 3: Separately Excited DC Generator

(Open Circuit characteristics)

OBJECTIVE:

To determine open circuit characteristics of a Separate Excited DC Generator.

PRELAB: Answers: 1) There are three types of DC machines i) Series excited DC generator: this is where the field and armature circuits are connected in series.ii) Shunt excited DC generator: this is the same as the series expect that the field and armature circuits are connected parallel. This comes in two variable sizes long and short shunt generators.iii) Separate excited DC generator: in this case the field and armature are supplied by their separate voltages sources. This type of generator comes in two types series and compound.2) Critical resistance: this is the maximum field circuit resistance given at a certain speed in which the shunt generator will excite.Critical speed: this is the speed at which the critical resistance is excited at.3) The two things that mostly give magnetic resistance are magnetic saturation of the soft iron material that the poles/rotor body are made of and the air gaps between the poles and rotor. The flux in the air gaps and so also through the rotor are very strongly determined by the air gap - the ratio is an inverse one -more gap gives less flux.

4) OCC starts a little higher than zero because the residual magnetism in the field poles from the revious use f the generators. This may not be true if the generator has never been used previously. 5) OCC is also known as Magnetization characteristics because with respect to the curve which shows the relationships between the generated EMF at no load (Eo) and the field current (If) at a constant speed. The curve will be considered linear if the values for Eo and If are low, once these values increase the curve will no longer be considered a linear but will change due to the magnetic saturation.6) The external characteristics drop due to two reasons: i) the armature reaction weakens the main flux so that the actual EMF generated (E) on the load is less than that of the generated (Eo) on no load. ii) Across the armature resistance there is no voltage drop. (Il*Ra = Ia*Ra)

7)

8)

9)

10) The reasons DC separately excited generator fails to excite is because: i) A shunt generator usually dont develop its rated operating voltage due to its lost in residual magnetism. ii) Critical field resistance: shunt generator may fail to reach its operating voltage even though its residual magnetic field is satisfactory. This failure may be due to excessive resistance in the field circuit. Any generator has critical field resistance. The presence of resistance in the field circuit in excess of this critical value causes the generator to fail to build up to its rated operating voltage. Since field rheostats are used to control the voltage output at rated speed, it's important to reduce the resistance of the field rheostats to a minimum value before investigating other possible faults in the event of failure to develop rated voltage.iii) It also uses a Brush contact resistance. A brush contact resistance is a contact resistance at the brushes is another reason for the failure of the generator to develop its operating voltage.

PRE-CONNECTION:

[1] After the presentation, go to the designated lab setup and identify all the components shown in the diagram located in the lab set-up. Also identify the scales and multiplication factors of all instruments, i.e., find what range the meter is set for and what scale to read from properly. Improper reading may result in measurement and calculation errors.[2]Recorded the nameplate data of the DC machine in Table #1. Study and understand the name plate ratings.

You must familiarize yourself with the switches and push buttons and make sure all connections are correctly done before switching any supply to the unit.

Circuit Diagram:

Theory: Open circuit characteristics curve also sometimes called no-load characteristic, is a graph showing the relation between induced e.m.f of a generator on no-load and the field current. The e.m.f of thegenerator at no-load is given by:

Eo NIf the speed be kept constant while this characteristic is being drawn in that case Eo becomes proportional to flux , but flux is proportional to field current If. The curve between E0 and If is known as open circuit characteristic.The induced emf in the armature winding of a dc machine is directly proportional to flux and speed of rotation. Let us assume that the field winding is connected to a variable dc source that is capable of supplying a desired field current. If the armature terminals are left open and the armature is rotated at constant speed, then the induced emf in the armature is E = K , where K is a constant. In other words the

induced emf is directly proportional to the airgap flux. Flux depends on the magneto-motive force ( MMF) provided by the current in the field winding. That is, = Kf IF , where Kf depends on the operating flux density. Therefore induced emf can now be written as E = K Kf If . The magnetic circuit of a dc machine consists of both linear (air gap) and non-linear (magnetic material of the stator and rotor) parts. Hence, Kf changes (it decreases as the magnetic circuit gets saturated) with the change in flux density in the machine. The relationship between E and If can be determined by measuring the open circuit voltage (voltage across armature terminals) at different values of If at a constant speed. This curve is known as open circuit characteristics (O.C.C). It should be noted that E does not start at zero when the field current is zero but at some value (of the order of 1-5 V). This is due to residual magnetism.PRECAUTION:

1.The field rheostat on the motor side must be kept at minimum resistance position at the time of starting.2.The field potentiometer on the generator side must be kept at minimum potential position at the time of starting.3. All switches must be kept open at the time of power on.

Procedure:

1. The connections as shown in circuit diagram was achieved.

2.After checking minimum position of motor field rheostat, maximum position of generator field rheostat, Supply switch was closed and starting resistance was gradually removed.3. The motor is brought to rated speed by adjusting the field rheostat.

4.By adjusting the potentiometer on the generator field side suitably for various increasing field currents, note down the terminal voltages till around 125% of the rated voltage. The speed was maintained constant throughout this process.5. The values of output voltages against each value of field current was recorded.

6. After accomplishing the task the power was turned off on the machine.

7. The graph between E0 and If was drawn.

10adel.elgammal@utt.edu.ttObservations:

Speed = rpm

If (amps)Eg (volts)

Graph between E0 and If:

EgEg

If

If

Lab 4: External Characteristics of Separately

Excited DC Generator

Objective:

To determine external characteristics of a Separately Excited DC Generator.

Circuit Diagram:

Theory:

In External characteristics curve showing the relation between terminal voltage of a generator and load current. The terminal voltage will be less then E due to voltage drop in the armature circuit .Therefore, this curve will lie below the internal characteristic. The formula of terminal voltage for externalcharacteristic is

V = E- IaRa armature reaction drop

18adel.elgammal@utt.edu.tt

When the field current is held constant and the armature is rotating at a constant speed, the induced emf in an ideal generator is independent of the armature current, as shown by the dotted line in Fig. As the load current IL increases, the terminal voltage decreases as indicated by solid line. If the armature reaction is neglected, decrease in Va should be linear and equal to the voltage drop across Ra and carbon brushes. However, if the generator is operated at the knee point in the magnetization curve, the armature reaction causes a further drop in terminal voltage.

Armature Reaction

When there is no current in the armature winding (no-load condition), the flux produced by the field winding is uniformly distributed over the pole faces as shown in Fig. (A). Let us assume that this two pole machine is driven by a prime mover in the clockwise direction (generator operation). The direction of the currents in the armature conductors under load is shown in Fig. (B). The armature flux distribution due to armature mmf is also shown in this figure. Since both fluxes exist at the same time when the armature is loaded, the resultant flux gets distorted. It can be seen that the armature flux opposes the flux in one half of the pole and aids in the other half. If the magnetic circuit is unsaturated the decrease in flux in one-half of the pole is accompanied by an equal increase in the flux in the other half. The net flux per pole, therefore, is the same under load as at no load. On the other hand, if the magnetic circuit is very close to saturation point under no-load, the increase in flux is smaller than the decrease in ux. In this case there is a net reduction in the total flux.

Flux distribution due to field MMF only Flux distribution due to armature MMF only

Armature reaction in DC Generator

Procedure:

1. The connections as shown in circuit diagram was achieved.

2.DC Power Supply was turned on and the excitation voltage was gradually increased from zero to full value while keeping speed of prime mover to be constant.3. The value of output voltage against each value of load current was recorded.

4. After accomplishing the task the machine power was then turned off.

5. The graph between V and IL was drawn.

Observations:

IL (amps)VT (volts)

MODEL GRAPH:

V

IL

Conclusion: The results and specifications have been met connecting each type of generator that were a separately excited DC generator with no load characteristics and a separately excited DC generator with a variable load characteristics. The data have been plotted and verified successful in both cases.