
Motors. Sean DeHart Smriti Chopra Hannes Daepp . Overview. DC Motors (Brushed and Brushless) Brief Introduction to AC Motors Stepper Motors Linear Motors. 2. Sean DeHart. Electric Motor Basic Principles. Interaction between magnetic field and current carrying wire produces a force
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Sean DeHart
Electric Motor Basic Principles
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Sean DeHart
Conventional (Brushed) DC Motors
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Sean DeHart
Conventional (Brushed) DC Motors
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Sean DeHart
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Sean DeHart
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Sean DeHart
Brushless DC Motor Commutation
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Sean DeHart
BLDC (3-Pole) Motor Connections
Delta Wye
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Sean DeHart
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Sean DeHart
Two main types of AC motor, Synchronous and Induction.
Synchronous motors supply power to both the rotor and the stator, where induction motors only supply power to the stator coils, and rely on induction to generate torque.
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Sean DeHart
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Sean DeHart
Induction motors only supply current to the stator, and rely on a second induced current in the rotor coils.
This requires a relative speed between the rotating magnetic field and the rotor. If the rotor somehow matches or exceeds the magnetic field speed, there is condition called slip.
Slip is required to produce torque, if there is no slip, there is no difference between the induced pole and the powered pole, and therefore no torque on the shaft.
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Sean DeHart
Current is applied to both the Rotor and the Stator.
This allows for precise control (stepper motors), but requires mechanical brushes or slip rings to supply DC current to the rotor.
There is no slip since the rotor does not rely on induction to produce torque.
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Sean DeHart
A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.
Smriti Chopra
The sequence of the applied pulses is directly related to the direction of motor shafts rotation.
The speed of the motor shafts rotation is directly related to the frequency of the input pulses.
The length of rotation is directly related to the number of input pulses applied.
Smriti Chopra
Open loop
The motors response to digital input pulses provides open-loop
control, making the motor simpler and less costly to control.
Brushless
Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.
Incremental steps/changes
The rotation angle of the motor is proportional to the input pulse.
Speed increases -> torque decreases
Smriti Chopra
Torque varies inversely with speed.
Current is proportional to torque.
Torque → ∞ means Current → ∞, which leads to motor damage.
Torque thus needs to be limited to rated value of motor.
Smriti Chopra
There are two main disadvantages of stepper motors:
This can be seen as a sudden loss or drop in torque at certain speeds which can result in missed steps or loss of synchronism. It occurs when the input step pulse rate coincides with the natural oscillation frequency of the rotor. Resonance can be minimised by using half stepping or microstepping.
Stepper motors consist of a permanent magnet rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator.
When a phase winding of a stepper
motor is energized with current, a
magnetic flux is developed in the
stator. The direction of this flux is
determined by the “Right Hand
Rule”.
Smriti Chopra
At position 1, the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it).
To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet.
This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position.
Smriti Chopra
Resolution is the number of degrees rotated per step.
Step angle = 360/(NPh * Ph) = 360/N
NPh = Number of equivalent poles per phase = number of rotor poles.
Ph = Number of phases.
N = Total number of poles for all phases together.
Example: for a three winding motor with a rotor having 4 teeth, the resolution is 30 degrees.
Smriti Chopra
There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.
bipolar
unipolar
Smriti Chopra
A unipolar stepper motor has two windings per phase, one for each direction of magnetic field. In this arrangement a magnetic pole can be reversed without switching the direction of current.
Bipolar motors have a single winding per phase. The current in a winding needs to be reversed in order to reverse a magnetic pole.
Bipolar motors have higher torque but need more complex driver circuits.
Smriti Chopra
Wave Drive (1 phase on)
A1 – B2 – A2 – B1
(25% of unipolar windings , 50% of bipolar)
Full Step Drive (2 phases on)
A1B2 – B2A2 – A2B1 – B1A1
(50% of unipolar windings , full bipolar
windings utilization)
Half Step Drive (1 & 2 phases on)
A1B2 – B2 – B2A2 – A2 ----
(increases resolution)
Microstepping (Continuously
varying motor currents)
A microstep driver may split a full step into as many as 256 microsteps.
Smriti Chopra
There are three main types of stepper motors:
Variable Reluctance stepper motor
Permanent Magnet stepper motor
Hybrid Synchronous stepper motor
Smriti Chopra
This type of motor consists of a soft iron multi-toothed
rotor and a wound stator.
When the stator windings are energized
with DC Current, the poles become magnetized.
Rotation occurs when the rotor teeth
are attracted to the energized stator
poles.
Smriti Chopra
The rotor no longer has teeth as with
the VR motor.
Instead the rotor is
magnetized with alternating north
and south poles situated in a straight
line parallel to the rotor shaft.
These magnetized rotor poles provide an increased
magnetic flux intensity and because of this
the PM motor exhibits improved torque characteristics
when compared with the VR type.
Smriti Chopra
The rotor is multi-toothed like the VR motor and
contains an axially magnetized concentric
magnet around its shaft.
The teeth on the rotor provide an even
better path which helps guide the
magnetic flux to preferred locations in
the air gap.
Smriti Chopra
Stepper motors can be a good choice whenever controlled movement is required.
They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism.
These include
Smriti Chopra
Analogous to Unrolled DC Motor
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http://www.parkermotion.com/video/Braas_Trilogy_T3E_Video.MPG
Analysis is similar to that of rotary machines
Linear dimension and displacements replace angular ones
Forces replace torques
Commutation cycle is distance between two consecutive pole pairs instead of 360 degrees
High Maximum Speed
Limited primarily by bus voltage, control electronics
High Precision
Accuracy, resolution, repeatability limited by feedback device, budget
Zero backlash: No mechanical transmission components.
Fast Response
Response rate can be over 100 times that of a mechanical transmission faster accelerations, settling time (more throughput)
Stiffness
No mechanical linkage, stiffness depends mostly on gain & current
Durable
Modern linear motors have few/no contacting parts no wear
Cost
Low production volume (relative to demand)
High price of magnets
Linear encoders (feedback) are much more expensive than rotary encoders, cost increases with length
Higher Bandwidth Drives and Controls
Lower force per package size
Heating issues
Forcer is usually attached to load I2R losses are directly coupled to load
No (minimal) Friction
No automatic brake
Forcer (Motor Coil)
Windings (coils) provide current (I)
Windings are encapsulated within core material
Mounting Plate on top
Usually contains sensors (hall effect and thermal)
Magnet Rail
Iron Plate / Base Plate
Rare Earth Magnets of alternating polarity provide flux (B)
Single or double rail
F = lI x B
Iron Core
Coils wound around teeth of laminations on forcer
Ironless Core
Dual back iron separated by spacer
Coils held together with epoxy
Slotless
Coil and back iron held together with epoxy
Linear Motor Types: Iron Core [1],[2]
Laminated forcer assembly and mounting plate
Hall effect
and thermal
sensors
Coil wound Around
Forcer lamination
Iron Plate
Rare earth magnets
Distinguishing Feature
Copper windings around forcer laminations over a single magnet rail
Advantages:
Highest force available per unit volume
Efficient Cooling
Lower cost
Disadvantages:
High attractive force between forcer & magnet track
Cogging: iron forcer affects thrust force as it passes over each magnet (aka velocity ripple)
Linear Motor Types: Ironless [1],[2]
Top View
Forcer
Mounting
Plate
Winding, held
by epoxy
Rare
Earth
Magnets
Disadvantages:
Hall Effect and
Thermal
Sensors in coil
Horseshoe
Shaped
backiron
Distinguishing Feature
Forcer constructed of wound coils held together with epoxy and running between two rails (North and South)
Also known as “Aircore” or “U-channel” motors
Advantages:
No attractive forces in forcer
No Cogging
Low weight forcer - No iron means higher accel/decel rates
assembly
Back
iron
Mounting
plate
Thermal
sensor
Rare
Earth
Magnets
Iron
plate
Linear Motor Types: Slotless [1],[2]
Side View
Front View
Distinguishing Feature
Mix of ironless and iron core: coils with back iron contained within aluminum housing over a single magnet rail
Advantages over ironless:
Lower cost (1x magnets)
Better heat dissipation
Structurally stronger forcer
More force per package size
Advantages over iron core:
Lighter weight and lower inertia forcer
Lower attractive forces
Less cogging
assembly
Back
iron
Mounting
plate
Thermal
sensor
Rare
Earth
Magnets
Iron
plate
Linear Motor Types: Slotless [2],[3]
Side View
Front View
Disadvantages
Some attractive force and cogging
Less efficient than iron core and ironless - more heat to do the same job
$3529
Trilogy T1S Ironless linear motor
110V, 1 pole motor
Single bearing rail
~12’’ travel
magnetic encoder
Peak Velocity = 7 m/s
Resolution = 5μm
[1] S. Cetinkunt, Mechatronics, John Wiley & Sons, Inc., Hoboken 2007.
[2] J. Barrett, T. Harned, J. Monnich, Linear Motor Basics, Parker Hannifin Corporation, http://www.parkermotion.com/whitepages/linearmotorarticle.pdf
[3] Trilogy Linear Motor & Linear Motor Positioners, Parker Hannifin Corporation, 2008, http://www.parkermotion.com/pdfs/Trilogy_Catalog.pdf
[4] Rockwell Automation, http://www.rockwellautomation.com/anorad/products/linearmotors/questions.html
[5] J. Marsh, Motor Parameters Application Note, Parker-Trilogy Linear Motors, 2003. http://www.parkermotion.com/whitepages/Linear_Motor_Parameter_Application_Note.pdf
[6] Greg Paula, Linear motors take center stage, The American Society of Mechanical Engineers, 1998.
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