Transcript Document

MECH1200
Electric Motors
Fundamentals of DC Electric
Machinery
History
MECH1200
•
• Introduction
• Principle of Operation of Electric Machinery
• Principle of Operation of AC Synchronous
Machines
• Principle of Operation of DC Machines
• Types of Electric Machines
• Construction of DC Machines
• DC Machine Torque Characteristics
• DC Machine Speed Control
• Exercise Questions
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History
- Very advanced mechanisms are developed in the middle ages.
- Development of the steam engine leads to the industrial
revolution in the mid 1700’s.
- Michael Faraday runs the first electric motor experiment in
1821.
A
mechanical
clock made
in 1510
James Watt
Faraday
and
Daniell in
the lab
Images are in the public domain. Source: Wikimedia Commons, Wikimedia Foundation
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Introduction
What is an Electric Motor?
• Electromechanical device that converts
electrical energy to mechanical energy
• Mechanical energy used to:
• Rotate pump impeller, fan, blower
• Drive compressors
• Lift materials
• Motors in industry: 70% of electrical
load
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• Electric Machinery:
- Electric Motors: convert electrical
energy into mechanical energy.
- Electric Generators: convert mechanical
energy to electric energy.
• The Electric machine is composed of:
- Stator
- Rotor
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The Stator is the stationary part of a motor.
The Rotor is a moving component of an
electromagnetic system in the electric
motor
How Does an Electric
Motor Work?
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Principle of Operation of AC Synchronous Machines
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N
S
N
Alternators such as
the one shown above
are
three
phase
synchronous machines.
In synchronous motors, the
magnetic field in the stator
rotates, and the rotor tries to
follow the stator’s magnetic
field, i.e. it tries to synchronize
itself with it, and thence the
name synchronous motor.
NS
N
S
S
S
N
When
Once
A the
Stationary
the
rotor
rotor
isthe
rotates
energized,
field
itaflowing
rotates
cycle,
intechnique
the
itin
to
stator
switches
the
ABut
Every
circuit
how
half
that
can
cycle
we
usesmake
amagnetic
commutation
current
thehalf
magnetic
field
the
alternate
rotor
ismost
used
Principle
of
Operation
of
DC
Machines
polarity,
An alternating
so
stable
the
magnets
position:
field
are again
in
thethe
repelling
rotor
switches
to
supply
direction
anthat
alternating
inmagnetic
due
the
to
rotor?
current
commutation,
into
watch:
rotor.
Commutators
Brushes
(To maintain
electrical contact)
N
S
Commutation Circuit
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Types of Electric Motors
Electric Motors
DC Motors
AC Motors
Permanent Magnet
DC (PMDC)
Series Motor
Special Purpose
Motors
Stepper motor
Induction
Motor
Brushless DC
motor
Hysteresis motor
Shunt Motor
Compounded
Motor
Synchronous
Motor
Reluctance motor
Universal motor
Separately Excited
motor
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Classification of Motors
Electric Motors
Alternating Current (AC)
Motors
Synchronous
Induction
Single-Phase
Three-Phase
Direct Current (DC)
Motors
Separately
Excited
Series
Self Excited
Compound
Shunt
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Three types of Motor Load
Motor loads
Description
Examples
Constant torque Output power varies but
loads
torque is constant
Conveyors, rotary kilns,
constant-displacement
pumps
Variable torque Torque varies with square
loads
of operation speed
Centrifugal pumps, fans
Constant power Torque changes inversely
loads
with speed
Machine tools
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DC Motors
• Parts of a DC motor:
–
–
–
–
Armature with windings
Brush assembly
Field windings
Commutator
Construction of DC Machines
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• Field Windings: In DC machines, this is the winding in
•
•
•
•
the stator. Note: Permanent magnet dc motors do not have
field winding but rather a permanent magnet.
Armature Windings: The windings in the rotor .
Commutator segments: the method to convert the dc
current to an alternating current in the rotor of the dc motor
is called commutation. They are connected to the rotor
windings, and stay in contact with the brushes.
Brushes: pieces made of carbon or graphite and pushed
against a spring to maintain electrical contact with the
commutator segments.
Interpole windings: A third set of windings are
mounted on the stator, and connected in series with the
rotor to reduce the sparking between brushes and the
commutator.
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• If the armature and field windings are connected in
series, then the machine that results is called the DC
series machine.
• If the armature and field windings are connected in
parallel, then the machine that results is called the
DC shunt machine.
• If the armature is connected with a field winding in
series and another field winding in parallel, then the
resulting machine is called the
compounded
machine.
• Note that the interpole windings are always
connected in series with the rotor.
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• Name the type of each of the motors A, B, and
C shown in the figure below:
A: Shunt motor
B: Series motor
C: Compound motor
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DC Motor Construction
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Interpole Windings
• Due to shifting of the
magnetic flux lines, arcing
and short-circuiting can
result
• Interpole windings
counteract this shift and
reduces the arcing
• Are in series with field coils
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Counter (or Back) EMF
N
S
Likewise, as the armature cuts through
the magnetic field of the field coils
(stator), a voltage is induced. (Think
how a generator works.)
S
Recall from 1100: an induced voltage in
a wire (or any conductor) results from
relative motion of either the wire or the
magnetic field.
N
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Counter (or Back) EMF
Combined Armature and Field
Lines of Force
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Counter (or Back) EMF
This induced voltage causes current to flow in the
opposite direction of the applied DC current, thus
“countering” it, reducing the overall current and
voltage.
Counter EMF current
is “out of page” (+z direction)
Armature current
is “into page” (- z direction)
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Permanent Magnet DC Motors
• Excellent starting torque
• Good speed regulation
• Limited to low HP loads
• Torque is limited to 150%
of rated torque.
• Reverse direction of
rotation by interchanging
armature connections.
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Series Wound - DC Motors
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Series Wound - DC Motors
• Highest starting torque
• Not used with loads that are belt or chain
coupled
• Loads should always be directly coupled.
• Reverse direction of rotation by interchanging
armature connections.
• Cannot be used without a load attached
Examples: Locomotives, Cranes, Hoists..
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Series Wound – DC Motors
Runaway motor at
low/no loads
High starting
torque
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Shunt Wound DC Motors
• Best speed regulation
• Open shunt field winding
will cause motor speed to
increase to dangerous
levels.
• Field loss relay should always
be used.
• Allows simplified control
for reversing the motor.
• http://www.youtube.com/watch
?v=VGmoKK5Iqrs
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Shunt Wound DC Motors
• Field winding are connected in parallel to the
armature windings.
• Field and armature windings can be hooked to
same or different sources.
• “Separately excited” – independent control of
armature and field.
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Shunt Wound DC Motors
Self excited –
When constant speed is desired.
Field is independent of armature
current.
Doesn’t provide high starting torque
Ex: Elevator, Centrifugal Pumps
Separately excited –
Can operate above its rated speed.
Speed can be increased by weakening the field in the field
windings. Results in lower torque
Requires two power supplies
Ex: Woodworking Machinery, Printing Press
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• Properties:
Shunt Wound
DC Motors
Speed constant
independent of load
up to certain torque
• Field winding parallel
with armature winding
• Current = field current
+ armature current
Speed control:
insert resistance
in armature or
field current
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Shunt Wound DC Motors
• Reverse direction by switching either
armature or field winding polarities
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Shunt DC Motor Terminal Characteristics
Question: At which speed should we operate the motor to achieve
maximum power?
Hint: The power corresponds to the area of the rectangle under the
operating point on the speed-torque line. Which of the three rectangles
has the largest area?
Speed (RPM)
No load speed
3000
Maximum power is at 50% of no load speed
2400
1800
1200
Stall point
600
Torque
(ft-lb)
5
1
0
Shunt DC Motor Speed Control
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• There are three methods to control the speed of a DC motor:
1. Changing the armature voltage.
2. Changing armature resistance (by adding a resistor in series
with
armature)
3. Change the field (stator) magnetic field
Speed (ω)
Speed (ω)
As Va increases
As Ra increases
Torque
Torque
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Compound Wound DC Motors
• Starting torque :
– Better than shunt-wound DC
motor.
– Not as good as series-wound DC
motor.
• Speed regulation
– Better than series-wound DC
motor
– Not as good as with shunt-wound
DC motor.
• Reverse direction of rotation
by interchanging armature
connections.
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DC Motors
DC compound motor
Suited for high
starting torque if high
% compounding:
cranes, hoists
Field winding in
series and
parallel with
armature winding
Good torque and
stable speed
Higher %
compound in
series = high
starting torque
Series DC Motor Terminal Characteristics
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Speed (RPM)
If the series motor is unloaded, speed increases
significantly (this condition is called Runaway,
and it must be avoided)
Shunt motor
Series motor
Very high starting
torque
Torque
(ft-lb)
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DC Motor Rating Plates
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DC STEPPER MOTORS
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DC STEPPER MOTORS
• Voltage Rating
– provides desired torque
• Resistance-per-winding determines
– the current draw of the motor
– Maximum operating speed
• Degrees per Step
– Sets the number of degrees the shaft
will rotate for each full step
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STEPPER MOTOR CONTROL
• 4 signal wires fired in the correct sequence will
turn the motor
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Stepper motor location
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How does the stepper motor fit in
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Stepper motor
STEPS
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What are Poles in a Motor?
•Winding(s) which produce the magnetic field(s)
necessary to cause the rotor to turn.
3 Phase; 2 Pole Motor
Brushless DC Motors
• Rotor is permanent
magnet
• Stator is wired to
provide rotating
magnetic field
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Types of Motor Enclosures
• ODP – Open Drip Proof
– Air flows through motor (fan blades help flow)
– Used in environments free from contaminants
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Types of Motor Enclosures
• TENV – Totally Enclosed Non-Ventilating
– Protect motor from corrosive and harmful
elements
– Frame fins help to dissipate heat
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Types of Motor Enclosures
• TEFC – Totally enclosed Fan Cooled
– Similar to TENV except has external fan for cooling
Types of Motor Enclosures
• XP – Explosion Proof
– Similar to TEFC but enclosures are cast iron
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Hazardous Locations
Division I – Hazardous material present in the air
as a norm
Division II - Hazardous material present in the air
as an abnormal event
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Hazardous Locations/ Classes
Summary
• DC motors are:
–
–
–
–
permanent magnet
series-wound
shunt-wound
compound-wound
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Exercise Questions
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1. How does the rotor of a dc motor maintain
electrical contact with its commutation circuit?
By using brushes that are pushed against a
spring.
2. Name two motors that do not need brushes for
their rotor windings.
Squirrel cage induction motor and the
brushless DC motor, and some types of
stepper motors
3. Which motor supplies the highest output
torque to weight ratio?
The series DC motor.
Exercise Questions
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4. Why is it not recommended to use brushed dc motors
in spacecraft and some aircraft applications?
Because in the space environment there is no air to
carry away the heat generated by the brushes of the dc
motor, causing them to wear very quickly. The same
thing applies in the low pressure environment of
aircraft. Thus a brushless dc motor is used.
5. In an environment that contains explosive gases, such
as in mines, which motor do you recommend using:
a) series dc motor
b) shunt dc motor
c) induction motor
d) universal motor
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6. Which motor is recommended for artificial heart
applications
a) Series dc motor
b) Shunt dc motor
c) Induction motor
e) Brushless dc motor
7. What type of machines is used as an electric
generator in vehicles to recharge the battery?
The synchronous generator
8. What is added to the dc motor to reduce sparking
in dc motor brushes?
Interpole windings are added.
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9. Which machines have a more complex construction, DC or
AC machines?
DC machines are much more complex, due to existence of
interpoles and commutation circuits.
10. Which motor is potentially dangerous to operate with no
load on it?
The series dc motor
11. Why series dc motors should not be used in belt drives?
Because if the belt breaks, the motor will be unloaded.
Series dc motors are dangerous to operate if unloaded, as
their speed will increase significantly until the motor is
damaged.
12. Which type of motors can operate on both ac and dc power
supplies?
The universal motor