Module 2 â Motors & Generators
Physics – Module 2 – Motors & Generators 1. Motors use the effect of forces on current-carrying conductors in magnetic fields Discuss the effect on the magnitude of the force on a current carrying conductor of variations in:
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Magnetic Field: An area in space which exerts a magnetic force on charged particles and magnetic dipoles. Magnetic fields can be produced by magnets or moving charges • When a conductor carrying electricity is placed in a magnetic field, it will move The Motor Effect: The force on a current carrying conductor in a magnetic field (right hand palm rule) Force is proportional to: • B – The Magnetic Field Strength • I – The size of the current • L – The length of the conductor inside the magnetic field • ʘ - The angle between the conductor and external magnetic field. In order to understand the sin term in the equation, we must understand that the current carrying conductor only experiences a force when it is at right angle to the magnetic field. Hence, where the wire is at an angle of ʘ to the field, it is only the component of the wire perpendicular to the field that produces the motor effect
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Describe qualitatively and quantitatively the force between long parallel current-carrying conductors
HOW IS FORCE PRODUCED
A conductor carrying a current into the page will produce its own magnetic field given by the right hand grip rule. When wires carry current in the opposite direction, the force due to the motor effect will cause the other wire to move apart For current carrying conductors, ‘like attracts’ – that is, two wires carrying current in the same direction will produce an attractive force. Wires carrying current in the opposite direction repel.
FIELD LINES AROUND WIRES
MAGNITUDE OF FORCE
The force between parallel current carrying conductors depends on the current flow direction. If current flow is in the same direction, the wires will attract. The formula shows that as length and current increases, force increases. As distance increases, force decreases in a linear relationship
• Define torque as the turning moment of a force Torque: Is the turning moment or turning effect of a force. It occurs when a force is applied to an object tangentially rather than straight at it. Torque depends of force and perpendicular distance from pivot. ʘ à Angle between perpendiculars ɸ à Angle between force and pivotal radius Ø T = F.d (when perp.) Ø T = F.r.Cosʘ Ø T = F.r.Sinɸ Describe the forces experiences by a current-carrying loop in a magnetic field and describe the net result of the forces A current carrying loop will experience force due to the motor effect. Perpendicular sides of the loop have current moving in opposite directions, so they experience opposite forces. If the coil is able to pivot around its center, one of the sides will experience an upward force, and the other will experience a downwards force. Because of the structure of the loop, each of the sides produces torque, as they experience a force acting tangentially to the pivot point where the loop is horizontal. This causes the loop to rotate (refer to talent) •
T = nBIAcosʘ
Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field The motor effect is caused by the moving of electrons in a magnetic field. When placed in a magnetic field, moving charges experience a force. This force is dependent on the direction of the field. When charge is moved through the wire, the wire experiences a force. This force is what constitutes the motor effect – the conversion of moving electric charge in a magnetic field into kinetic energy •
Describe the main features of a DC electric motor and the role of each feature We can use the motor effect to create torque; this is the basis of a motor o Current is fed through coils which experience a force due to the motor effect o Electrical Energy is converted into Mechanical Energy •
PART Armature Stator
Conducting Brushes
Split Ring Commutator
EXPLANATION OF FUNCTION Turns of wire, through which current is passed, and rotate in the presence of the magnetic field The stationary part of the motor and provides the magnetic field. It can be either permanent or electromagnet. To provide electrical contact between the armature and external electrical circuit to allow current to flow through the armature Reverses the current in the armature every half cycle to ensure continuous torque in one direction. If the motor were allowed to spin freely without the SRC, the direction of torque on the rotor would be reversed every half revolution and the rotor would spin back and stop.
Identify that the required magnetic fields in DC motors can be produced by current-carrying coils or permanent magnets In order for a DC motor to operate, the armature must be immersed in a magnetic field. This allows moving charge in the armature to generate force (motor effect). How this magnetic field is generated however, is immaterial. Permanents magnets as well as current-carrying coils can provide this field. Moving charge produces a field, and so current carrying coils can produce magnetic fields similar to those produced by bar magnets. This principle can be used to generate the field in a DC motor, just like permanent magnets. If electromagnets are used, the wire is usually wrapped around an iron core to increase the strength of the field. •
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Magnetic Field: An area in space which exerts a magnetic force on charged particles and magnetic dipoles. Magnetic fields can be produced by magnets or moving charges • When a conductor carrying electricity is placed in a magnetic field, it will move The Motor Effect: The force on a current carrying conductor in a magnetic field (right hand palm rule) Force is proportional to: • B – The Magnetic Field Strength • I – The size of the current • L – The length of the conductor inside the magnetic field • ʘ - The angle between the conductor and external magnetic field. In order to understand the sin term in the equation, we must understand that the current carrying conductor only experiences a force when it is at right angle to the magnetic field. Hence, where the wire is at an angle of ʘ to the field, it is only the component of the wire perpendicular to the field that produces the motor effect
•
Describe qualitatively and quantitatively the force between long parallel current-carrying conductors
HOW IS FORCE PRODUCED
A conductor carrying a current into the page will produce its own magnetic field given by the right hand grip rule. When wires carry current in the opposite direction, the force due to the motor effect will cause the other wire to move apart For current carrying conductors, ‘like attracts’ – that is, two wires carrying current in the same direction will produce an attractive force. Wires carrying current in the opposite direction repel.
FIELD LINES AROUND WIRES
MAGNITUDE OF FORCE
The force between parallel current carrying conductors depends on the current flow direction. If current flow is in the same direction, the wires will attract. The formula shows that as length and current increases, force increases. As distance increases, force decreases in a linear relationship
• Define torque as the turning moment of a force Torque: Is the turning moment or turning effect of a force. It occurs when a force is applied to an object tangentially rather than straight at it. Torque depends of force and perpendicular distance from pivot. ʘ à Angle between perpendiculars ɸ à Angle between force and pivotal radius Ø T = F.d (when perp.) Ø T = F.r.Cosʘ Ø T = F.r.Sinɸ Describe the forces experiences by a current-carrying loop in a magnetic field and describe the net result of the forces A current carrying loop will experience force due to the motor effect. Perpendicular sides of the loop have current moving in opposite directions, so they experience opposite forces. If the coil is able to pivot around its center, one of the sides will experience an upward force, and the other will experience a downwards force. Because of the structure of the loop, each of the sides produces torque, as they experience a force acting tangentially to the pivot point where the loop is horizontal. This causes the loop to rotate (refer to talent) •
T = nBIAcosʘ
Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field The motor effect is caused by the moving of electrons in a magnetic field. When placed in a magnetic field, moving charges experience a force. This force is dependent on the direction of the field. When charge is moved through the wire, the wire experiences a force. This force is what constitutes the motor effect – the conversion of moving electric charge in a magnetic field into kinetic energy •
Describe the main features of a DC electric motor and the role of each feature We can use the motor effect to create torque; this is the basis of a motor o Current is fed through coils which experience a force due to the motor effect o Electrical Energy is converted into Mechanical Energy •
PART Armature Stator
Conducting Brushes
Split Ring Commutator
EXPLANATION OF FUNCTION Turns of wire, through which current is passed, and rotate in the presence of the magnetic field The stationary part of the motor and provides the magnetic field. It can be either permanent or electromagnet. To provide electrical contact between the armature and external electrical circuit to allow current to flow through the armature Reverses the current in the armature every half cycle to ensure continuous torque in one direction. If the motor were allowed to spin freely without the SRC, the direction of torque on the rotor would be reversed every half revolution and the rotor would spin back and stop.
Identify that the required magnetic fields in DC motors can be produced by current-carrying coils or permanent magnets In order for a DC motor to operate, the armature must be immersed in a magnetic field. This allows moving charge in the armature to generate force (motor effect). How this magnetic field is generated however, is immaterial. Permanents magnets as well as current-carrying coils can provide this field. Moving charge produces a field, and so current carrying coils can produce magnetic fields similar to those produced by bar magnets. This principle can be used to generate the field in a DC motor, just like permanent magnets. If electromagnets are used, the wire is usually wrapped around an iron core to increase the strength of the field. •
