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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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.
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.
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.
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.
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.
The motors response to digital input pulses provides open-loop
control, making the motor simpler and less costly to control.
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.
The rotation angle of the motor is proportional to the input pulse.
Speed increases -> torque decreases
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.
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
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.
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.
There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.
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.
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
Half Step Drive (1 & 2 phases on)
A1B2 – B2 – B2A2 – A2 ----
varying motor currents)
A microstep driver may split a full step into as many as 256 microsteps.
There are three main types of stepper motors:
Variable Reluctance stepper motor
Permanent Magnet stepper motor
Hybrid Synchronous stepper motor
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
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.
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.
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.
Analogous to Unrolled DC Motor
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
Accuracy, resolution, repeatability limited by feedback device, budget
Zero backlash: No mechanical transmission components.
Response rate can be over 100 times that of a mechanical transmission faster accelerations, settling time (more throughput)
No mechanical linkage, stiffness depends mostly on gain & current
Modern linear motors have few/no contacting parts no wear
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
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)
Iron Plate / Base Plate
Rare Earth Magnets of alternating polarity provide flux (B)
Single or double rail
F = lI x B
Coils wound around teeth of laminations on forcer
Dual back iron separated by spacer
Coils held together with epoxy
Coil and back iron held together with epoxy
Laminated forcer assembly and mounting plate
Coil wound Around
Rare earth magnets
Copper windings around forcer laminations over a single magnet rail
Highest force available per unit volume
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 ,
Hall Effect and
Sensors in coil
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
No attractive forces in forcer
Low weight forcer - No iron means higher accel/decel rates
Linear Motor Types: Slotless ,
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
Linear Motor Types: Slotless ,
Some attractive force and cogging
Less efficient than iron core and ironless - more heat to do the same job
Trilogy T1S Ironless linear motor
110V, 1 pole motor
Single bearing rail
Peak Velocity = 7 m/s
Resolution = 5μm
 S. Cetinkunt, Mechatronics, John Wiley & Sons, Inc., Hoboken 2007.
 J. Barrett, T. Harned, J. Monnich, Linear Motor Basics, Parker Hannifin Corporation, http://www.parkermotion.com/whitepages/linearmotorarticle.pdf
 Trilogy Linear Motor & Linear Motor Positioners, Parker Hannifin Corporation, 2008, http://www.parkermotion.com/pdfs/Trilogy_Catalog.pdf
 Rockwell Automation, http://www.rockwellautomation.com/anorad/products/linearmotors/questions.html
 J. Marsh, Motor Parameters Application Note, Parker-Trilogy Linear Motors, 2003. http://www.parkermotion.com/whitepages/Linear_Motor_Parameter_Application_Note.pdf
 Greg Paula, Linear motors take center stage, The American Society of Mechanical Engineers, 1998.