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
How is back emf induced
The armature is spinning inside the field created by the stator, this induces a voltage (back EMF) across the coil as it spins. This voltage is opposed to the voltage placed across the coil that made the motor spin.
The magnitude of the back EMF is related to the rotational speed through a constant Ke called the speed constant, what is the units for Ke
E=Ke x w
What factors affect the value of Ke
Construction, geometry, and material properties of the motor
What is kirchoffs law for steady state current flow
V = IR + Ke . w
when is current and torque a maximum
when w is zero
this is the stall torque
when does the motor stall
when the motor is attempting to spin or move against a force that exceeds the amount of torque it can generally internally
stall torque occurs any time the motor starts from a stop
what is mechanical power defined as
P = Tw
when is max power output for a permanent magnet brushed DC motor
when T = 1/2Tstall
what is efficiency of the motor
as the ratio of mechanical power produced by the motor to electrical power consumed by the motor
what is the efficiency equation
n = P out/P in = Tw/VI
where is the desirable region to operate a DC motor in
between the point of maximum efficiency and the point of maximum power
why is high speed/low torque operation inefficient
dominated by friction therefore not efficient
why is low speed/high current operation inefficient
dominated by I^2R losses through heating of the coils
what is the gear ratio
Gear Ratio = N = win/wout
why is a gearhead (gearbox) used in DC Motors
the most efficient operating currents lie above no-load current and below condition for maximum power, 1/2Tstall
this region is especially desirable if a motor is to be in a high duty cycle or continuous use, as most motors aren’t rated for long periods of high torque output the coils in the armature will increase temp and eventually fail.
Given most motors have output shaft speeds of several thousand rpm and little torque near the point of max efficiency, many applications require substantially lower speeds and higher torques which can be provided by the gearhead.
what is gearhead effciency
n=Pout/Pin = Tout . Wout / Tin.Win
what is the avaidable torque at the output shaft of a gearbox
Tout = n.Tin.N
