PMDC Electrical Motors

1
PMDC Electrical Motors
( an introduction course )
Guest Lecture for Project Construction
Principles
Bachelor level education for full time and dual
program students ir. Wouter J. Hijink
(M.Sc.) 17 September 2010 (last update 30
october 2010)
30-1-2009
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2
Contents
Of this lecture
Core principles electric drives Different
types of electrical motors Permanent Magnet DC
motors Gearing systems Questions
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Magnetism, electric currents and Lorentz force
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Lorentz force F results when current I runs
perpendicular to magnetic field B
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Core principles of electrical drives
Homopolar motors the first electrical motors
ever !
A true Lorentz force motor No commutation
required ! Continuous electric current results
in continuous torque
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Michael Faradays demonstration at Royal
Institute 1821
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Bipolar motors typical explanation of DC motors
Is not really how most DC motors sold today work !
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Is still found in moving coil meters and some
core-less DC motors
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Bipolar motors replacing magnets by coils
First ever use of the main components of any DC
motor. - Stator - Rotor - Commutator Driven by
magnetic fields rather than by Lorentz forces !
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Ányos Jedliks class room demonstrations of 1827
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Modern DC motors use coils in the rotor
Uses a different right hand rule !
Compare to the field around a permanent magnet
A coil that conducts electrical current creates a
magnetic field around itself
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Core principles of electrical drives
Modern DC motors use coils and magnetic alignment
The magnetic fields try to align themselves to
each other
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Modern DC motors are typically tripolar of higher
30-1-2009
Allows for smoother running and self starting
from every position
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Introduction to PMDC electrical motors - guest
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Core principles of electrical drives
Example brushed DC motor (with soft iron core)
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Core principles of electrical drives
Brushless DC motors with and without iron cores
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BLDC requires electronic commutation !
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Three different types DC motor
Perm. magnet or coil / brushed or brush-less
Permanent Magnets in stator or rotor
Coils in both stator and rotor
Permanent Magnet Direct Current (PMDC) and
Core-Less DC (CLDC)
Brush-Less Direct Current (BLDC)
Series/Shunt/Compound Wound Direct Current (SWDC
/ SHWDC / CWDC)
Many different types of DC electrical motors are
possible
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Three different types DC motor
Motors curves of various DC motors
This course focuses on PMDC motors (red line)
only !
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Different types of electrical motors
There are also Alternating Current motors (AC)
The choice of electrical drives is much wider
then just DC motors !
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Permanent Magnet DC motors
PMDC characteristic curves definitions
Knowing the values for these 4 points completely
fixes the PMDC behavior
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Permanent Magnet DC motors
Various PMDC Motor constants
Torque constant
Back-EMF constant
Velocity constant, Motor constant
Where the units are different !
The values are related by
Suppliers use different ways to define the
standard plots understand these !
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Permanent Magnet DC motors
PMDC characteristic curves magnetic field
strength
Relative Magnetic Field Strengths
Stronger motor by using stronger magnets is paid
for by lower maximal speed !
Rule of thumb with PMDC motors somewhere else
you pay for your gains
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Permanent Magnet DC motors
PMDC characteristic curves Voltage dependence
A given torque is always related to the same
electrical current ! Hence the useful definition
of the torque constant
Higher voltages increase speed torque but dont
affect currents
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Introduction to PMDC electrical motors - guest
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Permanent Magnet DC motors
PMDC characteristic curves battery power
Voltage drops over long transmission lines or due
to other causes have similar effects. In real
life not always usage of regulated voltage
sources !
Battery packs are far from ideal power sources
this affects PMDC graphs
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Introduction to PMDC electrical motors - guest
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Permanent Magnet DC motors
Power created by PMDC motors
Supplied mechanical power area rectangles
Created power (rated power) is not constant over
working range
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Permanent Magnet DC motors
PMDC Motor characteristic curves
Max power and optimal efficiency typically do not
coincide !
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Permanent Magnet DC motors
Example brushless PMDC motor some real values
Notice the sharp drop-off of some key values
! Dont trust the brochures ! Make plots !
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PMDC and thermal limits
Motors can often not be used over full rpm range
30-1-2009
High torques are associated with high power
losses overheating motor
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PMDC and thermal limits
Motors can however be put overdrive momentarily
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Doing so has noticeable negative influence on its
life span however
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PMDC and thermal limits
Time constant of coils is not the only limiting
factor
Graph of earlier slide is only valid for relative
short bursts of intermittent operation !
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Coils need to off-load their heat to surroundings
that heat up as well !
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Gearing systems
Transforming speed and torque to desired values
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Gearing systems
Transforming speed and torque to desired values
Some standard relationships when dealing with
gear systems
For non-ideal systems
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When assuming ideal no-friction gear systems
(modeling friction separately)
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Gearing systems
Transforming speed and torque to desired values
Proving that transmitting enertia by gear systems
leads to a non-linear quadratic dependence.
and
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When assuming ideal no-friction gear systems
(modeling friction separately)
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Designing a PMDC drive system
Reduce the real setup to a simplied standard model
Replace non-ideal gearing by ideal version and
add friction torque
Remove gearing block by using the transformation
formulas
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Standard motor selection methods (calculations)
can now be used
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Thank you for your attention
Questions ?
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Subject header
Title of this particular sheet summary key point
This is the sheet template
Field for key points to be learned or reminder
for lecturer!
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Additional Comments
Delft University of Technology
Introduction to PMDC electrical motors - guest
lecture
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