Chpt 22 Subfractional Horsepower Permanent Magnet Brushed DC Motor Flashcards
What is the least expensive and most popular DC Motor
Sub-fractional horsepower permanent magnet brushed DC Motor
What does “subfractional horsepower” mean
Limited power output
What does “brushed” mean
The method of communication (the way in which coils are energised to establish the motors other magnetic field)
What phenomenon do motors exploit( one of maxwells equations)
Amperes law
What equation describes the resulting force when a current carrying wire is placed in an external magnetic field
F=I(LxBext)
I current ; L length of wire ; Bext external magnetic field density
What does the stator do in a motor
The stationary element which is made up of a powerful permanent magnets that generates a static nonrotating magnetic field
What is the armature in a motor
The moving element , in the case of permanent magnet brushed DC motor the element that incorporates the windings or coils
What does the rotor carry in a motor
The armature and commutator
What shape is the is the stator commonly shaped as
A thick walled tube, the rotor and armature fit in the hollow space
Where is the current introduced into the coils
The commutator –to- brush interface
What shape is the commutator
Shaped like a smooth ring with longitudinally arranged strips of conductive material with alternating strips of insulating material on its outer surface
What material are brushes commonly made from
Low friction material such as graphite or precious metal (eg platinum)
What happens if one of the coils periodically has both leads connected to the same brush
No current flows in that single coil
What is the weakest component in a motor
The brushes they are a sacrificial component
What causes brush drag and what is it
The sliding contact of the commutator or frictional forces, which is the unavoidable result of dragging one material over another. Overcoming these forces requires torque that is no longer available to do work.
How are the brushes pressed onto the commutator and what does this create
Pressed on by springs which creates a dynamic system with resonances
What causes brush bounce and whats the effects
When the brushes get worn down they are not able to maintain good electrical contact because of brush bounce. This limits max speed. It is a consequence of the brushes and springs making a dynamic system
What is a some solution to brush bounce and its downfall
Increasing the spring force on the brushes to the commutator but increases wear
What pros and cons of brushes made of graphite
Relatively high electrical resistance (1-10 ohms) , physically dirty , casts off particles as wear creating dust
Why is the brush-commutator interface electrically noisy
Connections to individual coils in the armature are continuously made and broken. These can be a significant source of electromagnetic interference (EMI)
What three basic electrical components can a permanent magnet brushed DC motor be modelled as
A resistor as a result of the finite resistance per unit length of the wire used to construct the coils in the
, an inductor as the result of coils of wire that make up the armature windings,
a source of EMF
How is torque generated by a permanent magnet DC motor
Result of the current flowing in the armature coils in the presence of the stators magnetic field
What is the effect of torque generated by a permanent magnet DC motor known as
Lorentz force law
What is faradays law
Describes the result of moving a coil of wire through an external magnetic field and a voltage is generated
