Motors - PowerPoint PPT Presentation

Motors

Sean DeHart

Smriti Chopra

Hannes Daepp

Overview

• DC Motors (Brushed and Brushless)
• Brief Introduction to AC Motors
• Stepper Motors
• Linear Motors

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Sean DeHart

Electric Motor Basic Principles

• Interaction between magnetic field and current carrying wire produces a force
• Opposite of a generator

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Sean DeHart

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 horizontal

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Sean DeHart

2 pole brushed DC motor commutation

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Sean DeHart

Conventional (Brushed) DC Motors

• Common Applications:
• Small/cheap devices such as toys, electric tooth brushes, small drills
• Lab 3
• Pros:
• Cheap, simple
• Easy 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

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Sean DeHart

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

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Sean DeHart

Brushless DC Motors

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

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Sean DeHart

Brushless DC Motor Commutation

• Commutation is performed electronically using a controller (e.g. HCS12 or logic circuit)
• Similarity with stepper motor, but with less # poles
• Needs rotor positional closed loop feedback: hall effect sensors, back EMF, photo transistors

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Sean DeHart

BLDC (3-Pole) Motor Connections

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

Delta               Wye

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Sean DeHart

Brushless DC Motors

• Applications
• CPU cooling fans
• CD/DVD Players
• Electric automobiles
• Pros (compared to brushed DC)
• Higher efficiency
• Longer lifespan, low maintenance
• Clean, fast, no sparking/issues with brushed contacts
• Cons
• Higher cost
• More complex circuitry and requires a controller

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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

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

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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:

• 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.

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

Main difference

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

Variable Reluctance 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

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

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

Smriti Chopra

Linear MotorsHannes Daepp

Analogous 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

I

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)

Front View

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

Top View

Forcer

Mounting

Plate

Winding, held

by epoxy

Rare

Earth

Magnets

Disadvantages:

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

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

Coil

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

Coil

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

Linear Motor Type Comparison [2]

• Motor components
• Base/Bearings
• Servo controller/feedback elements
• Typical sensors include Hall Effect (for position) and thermal sensors
• Cable management

$3529

Trilogy T1S Ironless linear motor

110V, 1 pole motor

Single bearing rail

~12’’ travel

magnetic encoder

Peak Velocity = 7 m/s

Resolution = 5μm

• Small Linear Motors
• Packaging and Material Handling
• Automated Assembly
• Reciprocating compressors and alternators
• Large Linear Induction Machines (3 phase)
• Transportation
• Materials handling
• Extrusion presses

[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.

• 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

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