Technical Means of Automation
Electric Motors
Institute of Information Engineering, Automation and Mathematics
November 3, 2015
Contents
DC motors (last week)
Brushed DC motors
Brushless DC motors
Servomotor
Stepper motor
AC motors
Induction motors
Synchronous motors
Variable frequency drives
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Classification of Electric Motors
An electric motor is an electric machine that converts electrical energy into
mechanical energy.
Electric Motors
DC Motors
AC Motors
Induction Motors
Synchronous Motors
1-Phase Ind. Motor 3-Phase Ind. Motor
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Alternating Current (AC)
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Alternating Current
AC power supply uses the principle of time-based voltage/current alternation in
the form of sinusoidal waves changing their amplitude across the zero voltage.
Number of energized wires:
1-P applicability:
single-phase (1P)
energy supply for homes (lighting,
three-phase (3P)
heat, ...)
Rated voltage:
line-to-neutral voltage (L-N)
line-lo-line (L-L – only for 3P)
non-industrial businesses
to run motors up to 4kW
3-P applicability:
industry, manufacturing and large
businesses
highly efficient for equipment
designed to run on 3-phase
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Single-phase AC
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Three-phase AC
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Generation of AC
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Generation of AC
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Rating of AC Voltage
L-N voltage rating (between line
and neutral – 230V)
L-L voltage rating (between two
lines ≈ 400V)
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AC Motors
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Induction (Asynchronous) Motors
Three-phase Induction Motor
Stator: 3 sets of induction coils in
120◦ shift
Rotor: laminated core + cage
3P current creates rotating
electromagnetic field (EMF)
current is inducted in rotor cage
bars
interaction of rotor’s and stator’s
EMFs causes the rotor to rotate in
the same direction as rotation of
stator’s EMF
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Induction (Asynchronous) Motors
Three-phase Induction Motor
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Induction (Asynchronous) Motors
Advantages and Drawbacks of Induction Motors
Pros:
relatively simple
no mechanical commutator
robust & reliable
economical
Cons:
fixed speed if powered
directly to 3P source
requires variable frequency
drive for speed control
very long lifetime
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Induction (Asynchronous) Motors
Single-phase Induction Motor
Stator: 2 set of perpendicular
induction coils (main and secondary)
Rotor: laminated core + cage
capacitor between main and
secondary coils
main coil – fluctuating EMF (same
phase as source)
secondary coil – fluctuating EMF
(90◦ phase shift)
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Synchronous Motors
Synchronous Motor
Stator: 3 sets of induction coils in
120◦ shift
Rotor: induction coils (DC power)
3P current creates rotating
electromagnetic field (EMF)
rotor creates stationary EMF
if rotor is moving, both EMFs are
fixed through polarities and rotor
moves at synchronous speed
for self start, cage must be used in
rotor
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Synchronous Motors
Synchronous Motor
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AC Motors Speed
Speed of synchronous motor (synchronous speed) is the same as the speed of
EMF rotation.
120f
P
where NS is synchronous speed in RPM, f is a frequency of AC and P is number
NS =
of poles.
Speed of induction motor NI is slightly smaller than synchronous speed. The
difference between synchronous speed and actual speed of motor is called a slip.
NI = NS (1 − s)
where NI is an asynchronous speed and s is a slip.
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AC Motors Control
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AC Motors Control
AC (induction and synchronous) motors exhibit the following relationships
between mechanical and electrical quantities:
Torque:
directly proportional to rotor magnetic field strength (current)
can be controlled by AC voltage
Speed:
directly proportional to speed of stator’s EMF rotation
can be controlled by AC frequency
Direction:
same as rotation of stator’s EMF
can be controlled by phase-shift timing
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AC Motors Control
Example: Why we need to control AC motors?
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AC Motors Control
Variable Frequency Drive (VFD)
Main reasons:
direction control
torque control
speed control
smooth run
reliability
variability
efficiency
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Variable Frequency Drives
What is VFD?
Adjustable-speed drive used in electro-mechanical drive systems to control AC
motor speed and torque by varying motor input frequency and voltage.
Around 25% of the world’s
electrical energy is consumed by
electric motors in industrial
applications. Up to 45% of
overall end-use electrical energy
is consumed by electric motors.
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Variable Frequency Drives
Energy Savings – Example
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Variable Frequency Drives
Energy Savings – Example
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Variable Frequency Drives
Energy Savings – Example
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Variable Frequency Drives
Energy Savings – Example
365 days in year, 1/2 - 90% flow, 1/2 - 60% flow
Payback in less than half of a year
Big saving every year
Safety for motor too
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Variable Frequency Drives
Circuit of VFD
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Variable Frequency Drives
Output of VFD Circuit
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Variable Frequency Drives
VFD Configuration
Setting the motor power, current, voltage, speed, maximum frequency in the
variable frequency drive, these parameters can be obtained from the motor’s
nameplate directly.
Rated motor voltage [V]
Rated motor current [A]
Rated motor power [kW,HP]
Rated motor power factor [cos(φ)]
Rated motor frequency [Hz]
Rated motor speed [RPM]
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Variable Frequency Drives
VFD Configuration
voltage (P0304)
current (P0305)
power (P0307)
power factor (P0308)
frequency (P0310)
speed (P0311)
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Variable Frequency Drives – Control Modes
V/F control
V/F control is basically open-loop control where desired frequency is
produced at VFD’s output
V/F control does not consider correction of slip or load
Suitable for non-precise applications, like fans and pumps.
Vector control
In the vector control mode, the VFD senses the motor’s speed and adjusts
the output to match the commanded speed.
Vector mode is basically a closed-loop control system that allows to
compensate the motor’s slip and load.
Apply to high performance general-purpose applications (centrifuge
machines, wire drawing machines, etc.)
Torque control
Torque can be changed very fast without overshoot.
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Variable Frequency Drives
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