3.12.3 Operations Operations of of and and Factors Factors Affecting Affecting Output Output and and Direction Direction ofof Current Flow
Current Flow
The commutator and brush gear of a dc machine have two distinct functions:
CollectionCollection - the transference of current between the moving armature and the fixed external circuit.
CommutationCommutation - the periodic reversal of current during transfer between the armature and the external circuit to produce dc.
These two operations are independent, but faulty collection or incorrect commutation produce similar results, i.e. the formation of a destructive spark or arc between the trailing edges of the brushes and the commutator surface.
Faulty Collection Faulty Collection
This is normally the result of poor brush fittings and maintenance. Sparking occurs between the brush trailing edge and the commutator surface and is very destructive.
Electromagnetic Problems Electromagnetic Problems
In addition to the problems associated with actual collection, two problems which are associated with the electromagnetic functions in the generator also exist. Though having similar effects, they are created by different things, may be compensated for by different design features and should therefore be understood as separate entities.
These are:
Armature Reaction.
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Armature Reaction/Reactive Sparking Armature Reaction/Reactive Sparking
Since an armature is wound with coils of wire, a magnetic field is set up in the armature whenever a current flows in the coils. This is called the armature flux and its field is right angles to the generator field, (also known as the field flux). This is called cross magnetisation of the armature. The effect of the armature flux is to distort the field flux and shift the magnetic neutral axis as illustrated. This effect is known as armature reaction and is proportional to the current flowing in the armature coils.
Resultant Magnetic Fields due to
Resultant Magnetic Fields due to Armature ReactionArmature Reaction
The magnetic neutral axis (MNA) is the resultant of the armature flux and the field flux interacting with one another.
The Geometric Neutral Axis (GNA) is the axis running through opposite poles.
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The brushes of a generator must be set in the MNA which means that they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the MNA, they would short-circuit ‘live’ coils and cause arcing and loss of power (reactive sparking).
In an ideal machine, the MNA will be equal to the GNA, which means there would be no distortion of the field flux and so no shifting of the MNA away from the brushes. This would result in no armature reaction or reactive sparking. However, the ideal machine has never been invented and armature reaction is something that has to be accepted and compensated for, and there are three principle methods with which it is overcome.
The first method is to shift the position of the brushes so that they are in the MNA when the generator is producing its normal load current.
The second method is by using special field poles, called INTERPOLES.INTERPOLES.
The third is by the use of COMPENSATING WINDINGSCOMPENSATING WINDINGS, both of which counteract the effect of armature reaction.
The brush-setting method is only satisfactory in installations in which the generator operates under a fairly constant load.
If the load varies to a marked degree, the MNA will shift proportionally, and the brushes will not be in the correct position at all times. This method is most commonly used in smaller generators (those producing 1kW or less) because it is less expensive. Larger generators require the use of interpoles.
Generator Circuit with Interpoles Generator Circuit with Interpoles
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effects of the interpoles are proportional to the load. The polarities of the interpoles are such that their effect is oppositeopposite to that of the armature field; i.e. the interpoles are of the same polarity as the next field pole in the direction of rotation. With this polarity, the interpoles are said to pull the generator field back into the correct position. A typical interpoles system is shown.
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In many generators, compensating windings are used to overcome the problem of armature reaction. These are windings placed in slots in the pole faces.
Use of Compensating Windings to overcome Armature Reaction Use of Compensating Windings to overcome Armature Reaction The current flowing in them travels in the opposite direction to that in the armature conductors, and by connecting them in seriesseries with the armature, the current in the windings is the samesame as that in the armature. With this method, the armature flux is cancelledcancelled out by the compensating flux under all
conditions of load resulting in the MNA and GNA being equal and commutation remains static.
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
In some machines, interpoles are used to minimize reactive sparking and armature reaction. However, for more efficient reduction of both, interpoles and compensation windings would be used as shown. The compensating windings are in seriesseries with the interpolesinterpoles and increase their effectiveness. The spark-less commutation obtained by the use of interpoles and
compensating winding.
Increases the life of the brushes and commutator.
Reduces radio interference.
Greatly improves the efficiency of the generator.
Generator with Interpoles and Compensating Winding Generator with Interpoles and Compensating Winding
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EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
Typical Generator Fault Chart Typical Generator Fault Chart
Defect
Defect Possible Possible Cause Cause Appropriate ActionAppropriate Action
1. Failure to excite
1. Failure to excite Loss of residual magnetism
Remagnetise. Disconnect shunt field winding and connect battery across the winding; positive of battery to positive end of winding. 2. Voltage fails to
2. Voltage fails to build up build up
a. Dirty commutator Clean as described b. Glazed contact
surface on brushes owing to prolonged ‘off load’ running.
Clean contact surface of brushes with Grade 00 glass paper.
c. Brushes not in contact with commutator
If result of sticking brushes, treat as described. d. Incorrect brush
position.
Check position and correct as necessary.
e. Disconnection in field circuit.
Check all connections, test field winding for continuity f. Reversed field connections Reconnect correctly g. Incorrect direction of rotation Reverse drive h. Machine run up on load (shunt machines only)
Disconnect load, run up ‘off load’ 3. Reversed 3. Reversed Polarity Polarity Residual magnetism reversed
Remagnetise. See 1 above. 4. Insufficient
4. Insufficient Voltage Voltage
a. Excessive load Reduce load
b. Weak field Reduce resistance of shunt field rheostat
c. Insufficient speed Reduce speed of prime mover 5. Excessive 5. Excessive Voltage Voltage a. Excessive field strength
Increase resistance of shunt field rheostat
b. Excessive speed Reduce speed of prime mover
6. Uniform sparking 6. Uniform sparking
at all brushes at all brushes
a. Dirty commutator Clean commutator b. Excessive load Reduce load
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Notes: Notes:
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EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
3.12.4:
3.12.4: DC Motor DC Motor Principle of Principle of OperationOperation Introduction
Introduction
An electric motor is a machine for converting electrical energy into mechanical energy. Its function is the reverse of a generator. There is little difference between the construction of a dc motor and a dc generator. Both have essentially the same parts and they look alike. In fact, in many cases, a dc machine can be used either as a motor or a generator.
Current Carrying Conductor in a Magnetic Field Current Carrying Conductor in a Magnetic Field
A current flowing through a wire placed in a magnetic field causes the wire to move; a motor works on this principle. It is the reaction of two magnetic fields that produces the motion that produces the torque that we see as the output of the motor.
The force with which the conductor moves is clearly dependent upon the strengths of the two interacting magnetic field. In turn this force relates to the speed at which a motor containing the current carrying conductor will turn.
Effects of a
Effects of a Current Carrying Conductor in a Magnetic FieldCurrent Carrying Conductor in a Magnetic Field The figures show the magnetic field between the poles of a magnet and the magnetic field round a wire carrying a current. If the wire is placed in the magnetic field the overlapping field pattern would seem to be as shown in (c). Of course, as we have seen earlier, lines of flux cannot cross and this pattern cannot exist. The resultant field is as shown in (d). The lines of flux reinforce each other in the space above the conductor and oppose each other below it. Lines of flux act as if they are pushing away from each other and also tend to straighten out. In this way they apply a force to the conductor tending to
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
It may therefore be appropriate to remember that the force is directly proportional to the Flux Density (B) of the major field, the current () in the Conductor (producing the minor field), and the length of the Conductor (L). This is expressed as:
Force =Flux Density
B Current
Length of conductor
L BILF
The direction in which the conductor moves depends on the direction of the current in the wire and also on the direction of the magnetic field. The direction of motion is given by Fleming’s LEFT HAND RULELEFT HAND RULE for motors: ‘The first finger, the second finger and the thumb of the left hand are held at right angles to each other’ . With the first finger pointing in the direction of the field (N to S) and the second finger in the direction of conventional current, the thumb shows the direction of motion of the wire.
To change the direction of rotation of a motor having an electro-magnetic field we need to reversereverse the direction of current in the armature OROR the direction of the current in the field.
Changing the supply connections to the motor will not have any effect; the current being reversed in direction in bothboth the armature and the field, the motor continues to run in the same direction.
Permanent magnetic motors are however, reversible by simply changing over the supply connections.
Fleming’s Left Hand Rule Fleming’s Left Hand Rule
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The simplest form of motor has a single loop of wire able to rotate freely between the poles of a permanent magnet. Connection is made from the dc supply voltage to the loop by BRUSHESBRUSHES bearing on a COMMUTATORCOMMUTATOR, the two segments of which are connected to the loop, as shown.
Fig 1 Simple DC Motor Fig 1 Simple DC Motor
The forces acting on the two sides of the loop combine to apply a force, known as a TORQUETORQUE, to turn the loop in an anticlockwise direction.
Action of DC Motor Action of DC Motor
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By applying Fleming’s Left Hand Rule it can be clearly seen that when:
The loop is in position (A), side ‘P’ of the loop tends to move downwards and side ‘Q’ upwards.
As the loop passes through the vertical position (B), the direction of the current flow must be reversed to keep the loop rotating in the same direction, and it is the action of the commutator that does this.
Because the commutator is two halves of a ring separated by insulation, the result of the loop rotating is such that as one half of the commutator leaves a brush, the other half comes into contact with it.
So now, at (C), when we apply Fleming’s Rule, side ‘Q’ will move in a downward direction and side ‘P’ upward, keeping the rotation of the loop in an anticlockwise direction.
At position (D), the loop passes through the vertical and the current reverses direction again until we get to (E) where the loop is back to where it was at the start (A) and the process goes on.
A single loop dc motor would not be able to turn heavy loads, so to obtain a large smooth mechanical output; some improvements have to be made. A laminated iron core carrying a number of armature coils is used together with a corresponding number of commutator segments. The magnetic field is produced by an electromagnet and its field coils, with the spacing between the armature and the pole pieces kept as small as possible.
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