Sean DeHart Smriti Chopra

Hannes Daepp

2

Overview

DC Motors (Brushed and Brushless)

Brief Introduction to AC Motors Stepper Motors Linear Motors

Sean DeHart

3

Electric Motor Basic Principles Interaction between magnetic field and

current carrying wire produces a force Opposite of a generator

Sean DeHart

4

Conventional (Brushed) DC Motors Permanent

magnets for outer stator

Rotating coils for inner rotor 

Commutation performed with metal contact brushes and contacts designed to reverse the polarity of the rotor as it reaches horizontalSean DeHart

5

2 pole brushed DC motor commutation

Sean DeHart

6

Conventional (Brushed) DC Motors

Common Applications:Small/cheap devices such as toys, electric

tooth brushes, small drillsLab 3

Pros:Cheap, simpleEasy to control - speed is governed by the

voltage and torque by the current through the armature

Cons:Mechanical brushes - electrical noise, arcing,

sparking, friction, wear, inefficient, shorting

Sean DeHart

7

DC Motor considerations Back EMF - every motor is also a generator More current = more torque; more voltage =

more speed Load, torque, speed characteristics

Shunt-wound, series-wound (aka universal motor), compound DC motors

Sean DeHart

8

Brushless DC Motors

Essential difference - commutation is performed electronically with controller rather than mechanically with brushes

Sean DeHart

9

Brushless DC Motor Commutation

Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit)Similarity with stepper motor, but with

less # polesNeeds rotor positional closed loop

feedback: hall effect sensors, back EMF, photo transistorsSean DeHart

10

Delta               Wye

BLDC (3-Pole) Motor Connections

Has 3 leads instead of 2 like brushed DC Delta (greater speed) and Wye (greater

torque) stator windings 

Sean DeHart

11

Brushless DC Motors

ApplicationsCPU cooling fansCD/DVD PlayersElectric automobiles

Pros (compared to brushed DC)Higher efficiencyLonger lifespan, low maintenanceClean, fast, no sparking/issues with brushed

contacts Cons

Higher costMore complex circuitry and requires a

controllerSean DeHart

AC Motors 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.

12Sean DeHart

13

AC Induction Motors (3 Phase) Use poly-phase (usually 3) AC current to create a

rotating magnetic field on the stator This induces a magnetic field on the rotor, which

tries to follow stator - slipping required to produce torque

Workhorses of the industry - high powered applications

Sean DeHart

AC induction MotorsInduction 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.

14Sean DeHart

Synchronous AC Motors 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.

15Sean DeHart

Stepper Motor

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

Main features

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

Stepper Motor Characteristics 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 vs. SpeedTorque 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

Disadvantages of stepper motors

There are two main disadvantages of stepper motors:

Resonance can occur if not properly controlled.

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. Not easy to operate at extremely high speeds.

20

Working principle

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

Understanding resolution

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

Two phase stepper motors

There are two basic winding arrangements for the electromagnetic coils in a two phase stepper motor: bipolar and unipolar.

unipolar bipolarSmriti 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.

Main difference

Smriti Chopra

Stepping modesWave 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 (Continuouslyvarying motor currents)A microstep driver may split a full step into as many as 256

microsteps. Smriti Chopra

Types of Stepper Motors

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 energizedwith DC Current, the poles become magnetized.

Rotation occurs when the rotor teethare attracted to the energized statorpoles.

Variable Reluctance motor

Smriti Chopra

Permanent Magnet motor

The rotor no longer has teeth as withthe VR motor.

Instead the rotor ismagnetized with alternating northand south poles situated in a straightline 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

Hybrid Synchronous motor

The rotor is multi-toothed like the VR motor andcontains an axially magnetized concentricmagnet around its shaft.

The teeth on the rotor provide an evenbetter path which helps guide themagnetic flux to preferred locations inthe air gap.

Smriti Chopra

Applications 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 printers plotters medical equipment fax machines automotive and scientific equipment etc.

Smriti Chopra

Linear MotorsHannes Daepp

Hannes Daepp

Basics of Linear Motors [1],[4]

IAnalogous to Unrolled DC Motor

• Force (F) is generated when the current (I) (along vector L) and the flux density (B) interact

• F = LI x B

Hannes Daepp

Linear Motors in Action

http://www.parkermotion.com/video/Braas_Trilogy_T3E_Video.MPG

Hannes Daepp

Analysis of Linear Motors [1],[5]

Analysis is similar to that of rotary machinesLinear dimension and displacements replace angular onesForces replace torquesCommutation cycle is distance between two consecutive pole pairs instead of 360 degrees

Hannes Daepp

Benefits of Linear Motors [2]High Maximum Speed

Limited primarily by bus voltage, control electronicsHigh 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)

StiffnessNo mechanical linkage, stiffness depends mostly on gain &

currentDurable

Modern linear motors have few/no contacting parts no wear

Hannes Daepp

Downsides of Linear Motors [2]

CostLow production volume (relative to demand)High price of magnetsLinear encoders (feedback) are much more expensive than

rotary encoders, cost increases with length

Higher Bandwidth Drives and ControlsLower force per package sizeHeating issues

Forcer is usually attached to load I2R losses are directly coupled to load

No (minimal) FrictionNo automatic brake

Hannes Daepp

Components of Linear Motors [2],[3]

Forcer (Motor Coil)Windings (coils) provide current (I)Windings are encapsulated within

core materialMounting Plate on topUsually contains sensors (hall

effect and thermal)

Magnet RailIron Plate / Base PlateRare Earth Magnets of alternating

polarity provide flux (B)Single or double rail

F = lI x B

Hannes Daepp

Types of Linear Motors [1],[2],[3]

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

Hannes Daepp

Linear Motor Types: Iron Core [1],[2]

Distinguishing Feature Copper windings around forcer laminations over a single

magnet railAdvantages: 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)

Hannes Daepp

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

Top View

ForcerMountingPlateRare EarthMagnetsHorseshoeShapedbackiron

Winding, heldby epoxy

Hall Effect and ThermalSensors in coil

Front View

Linear Motor Types: Ironless [1],[2]

Disadvantages: Low force per package size Lower Stiffness; limited max load without improved structure Poor heat dissipation Higher cost (2x Magnets!)

Hannes Daepp

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

Side View

Front View

Backiron

Mountingplate

CoilassemblyThermal

sensor

Rare EarthMagnets

Ironplate

Linear Motor Types: Slotless [1],[2]

Hannes Daepp

Disadvantages Some attractive force and

cogging Less efficient than iron core and

ironless - more heat to do the same job

Side View

Front View

Backiron

Mountingplate

CoilassemblyThermal

sensor

Rare EarthMagnets

Ironplate

Linear Motor Types: Slotless [2],[3]

Hannes Daepp

Linear Brushless DC Motor Type

Feature Iron Core Ironless Slotless

Attraction Force Most None Moderate

Cost Medium High Lowest

Force Cogging Highest None Medium

Power Density Highest Medium Medium

Forcer Weight Heaviest Lightest Moderate

Linear Motor Type Comparison [2]

Hannes Daepp

Components of a “Complete” Linear Motor System [3]

1. Motor components2. Base/Bearings3. Servo

controller/feedback elements

• Typical sensors include Hall Effect (for position) and thermal sensors

4. Cable management

Hannes Daepp

Sample Pricing$3529 Trilogy T1S Ironless

linear motor 110V, 1 pole motor Single bearing rail ~12’’ travel magnetic encoder Peak Velocity = 7 m/s Resolution = 5μm

Hannes Daepp

Applications [3],[5],[6]

Small Linear Motors Packaging and Material Handling Automated Assembly Reciprocating compressors and

alternators Large Linear Induction Machines

(3 phase) Transportation Materials handling Extrusion presses

References[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.

References (continued)

49

http://www.physclips.unsw.edu.au/jw/electricmotors.html

http://www.speedace.info/solar_car_motor_and_drivetrain.htm

http://www.allaboutcircuits.com/vol_2/chpt_13/1.html http://www.tpub.com/neets/book5/18d.htm single

phase induction motor http://www.stefanv.com/rcstuff/qf200212.html

Brushless DC motors https://www.geckodrive.com/upload/

Step_motor_basics.pdf http://www.solarbotics.net/library/pdflib/pdf/

motorbas.pdf