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
Book 4 page 22
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
Book 4 page 23
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
Book 4 page 24
What is the unit of flux density?
Tesla
Book 4 page 24
How do capacitors and inductors compare?
What is the unit of inductance?
Henry (H)
Book 4 page 25
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
Book 4 page 26
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)
Book 4 page 27
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
Book 4 page 30
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
Book 4 page 32
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
