2. Electromagnetism and Actuation Flashcards
What is a solenoid?
A type of linear actuator in which a coil of wire carrying an electric current attracts a moving ferromagnetic core
Glossary
How does an electromagnet differ from a solenoid?
They are similar in that they both consist of an iron core inside a coil, except that with the electromagnet, the core is fixed
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What advantages do semiconductor devices have over relays?
- A relay can only switch a motor on or off, not control its speed
- Sparking at the relay contacts when they switch can gradually erode them, limiting their life
- The current in the relay coil, while smaller than the switched current, may still be too high for a digital logic component to supply
- Relays take a few milliseconds to operate, so they are unsuitable for high-speed switching applications
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What are ferromagnetic materials?
Materials which are strongly attracted to magnets or are easily magnetised
Book 4 page 23
What is the definition of permeability?
Magnetic permeability is a measure of how easily the material is magnetised by a magnetic field
Glossary
What is the definition of flux density?
The strength of a magnetic field
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What is the unit of flux density?
Tesla
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How do capacitors and inductors compare?
What is the unit of inductance?
Henry (H)
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What are the implications for actuators and the electronics that drive them owing to the presence of inductors?
- Takes a finite time for the magnetic field to become established
- When switched off, the magnetic field collapses and the energy has to go somewhere, which can result in a large voltage spike across the coil terminals
- Impedance may be quite large at high frequencies, therefore putting an upper limit on the frequency at which a motor controlled by pulse-width modulation can be switched efficiently
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What is the equation to express the force experienced by a current carrying conductor passing through a magnetic field at right angles?
F = BIl
F is force (N)
B is flux density (T)
l is length (m)
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What is the major advantage of ironless rotors?
They have low mass, unlike iron rotors, and so can accelerate from a standstill to full speed very quickly
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What does a motor which has a detent or ‘cogging’ torque mean?
It has a permanent magnet because when it is unpowered and you try to rotate the shaft, it resists movement in some position more than in others
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Why does a laminated structure efficiently deal with eddy currents?
Reduces the incidence of eddy currents by inhibiting eddies from circulating
Book 4 page 33
What is the equation which expresses the relationship between a straight conductor being moved at right angles in a magnetic field?
V = Blv
V is the potential difference (V)
B is the flux density (T)
l is the length (m)
v is the velocity (m s-1)
Book 4 page 34
How does back emf affect the speed of a motor?
When the motor is spinning, the generator effect is to create a voltage that opposes the supply voltage to the motor’s terminals (back emf)
Subtracts from the supply voltage and therefore reduces the current through the motor
When back emf equals the supply voltage, the current drops to zero and so the motor spins no faster
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What is motor braking?
If there is a short circuit or low resistance between the motor terminals of a rotating motor which has been switched off, then current will flow due to the generated emf and power will be dissipated
This removes kinetic energy from the spinning motor
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What is regenerative braking?
The same as motor braking, except the energy released is not just dissipated as heat, but used to charge a battery, capacitor or other storage device
Book 4 page 34
