stepper miotor

8/8/2019 stepper miotor

http://slidepdf.com/reader/full/stepper-miotor 1/40

STEPPER MOTORy A stepper motor is a brushless, synchronous electric motor

that converts digital pulses into mechanical shaft rotation.

y Every revolution of the stepper motor is divided into adiscrete number of steps, and the motor must be sent a

separate pulse for each step.y The stepper motor can only take one step at a time and each

step is the same size.

y Typical step sizes are 2°, 2.5° ,3°, 7.5° , 15°, 30°



TYPES OF STEPPER MOTORy V ARIABLE RELUCTANCE

yPERMANENT MAGNET

y

HYBRID



VARIABLE RELUCTANCE STEPPER

y The rotor is a soft iron cylinder with salient (protruding)poles.

y This is the least complex, most inexpensive stepper motor.

y The only type stepper with no detent torque in hand rotationof a de-energized motor shaft.

y Large step angle

y A lead screw is often mounted to the shaft for linear stepping

motion.



y In looking at this figure, note the following:

1. The field appears to rotate counter clockwise while therotor rotates clockwise. This arrangement is common with variable reluctance motors.

y 2. The field rotates in steps of 60 degrees while the rotormoves in 30 degree steps (in the opposite direction). This isreminiscent of the motion of the scale of a vernier caliper,and for this reason, variable reluctance motors aresometimes known as vernier motors.

y 3. It takes four complete cycles of the control system to rotatethe motor rotor through one cycle. This is because the rotorhas 4 poles!



Permanent Magnet Stepper motors

y Rotor is made of permanent magnetic material

y No teeth on rotor

y No freewheeling

y more torque during rotation

y Less acceleration

y Difficult to make small step pm rotors with a large number

of poles. i.e. step sizes are limited to 30° - 90°.



PERMANENT MAGNET STEPPER

MOTOR

y Components of a PM stepper motor: (a) Rotor; (b) stator

Figure 6-18 Components of a PM stepper motor: (a) Rotor; (b) stato



y

In looking at this figure, note the following:y This 6-pole rotor turns in a direction opposite the

rotation of the stator field; a two-pole rotor inside theexact same stator would rotate with the field.

y This illustration is based on half-step control, wherealternate half steps involve one and two motor windings.

y It takes three complete cycles of the control system toturn this 6-pole rotor one revolution. A two-pole rotor

would turn a full revolution per control system cycle.



HYBRID MOTORS

y The hybrid stepper motor combines features of both the variable reluctance stepper and the permanent magnetstepper to produce a smaller step angle.

y The rotor is a cylindrical permanent magnet, magnetizedalong the axis with radial soft iron teeth

y The stator coils are wound on alternating poles withcorresponding teeth.



HYBRID



CHARACTERSTI

CS

PERMANENT

MAGNET

VARIABLE

RELUCTANC

E

HYBRID

COST CHEAPEST MODERATE MOSTEXPENSI V E(due tomanufacturingprocess)

DESIGN MODERATELY

COMPLEX

SIMPLE COMPLEX

R ESOLUTION 30 deg 3 deg / step 1.8 deg /step andsmaller

1.8 deg /step andsmaller

TORQUE VS SPEED

LESSPRONOUNCEDTORQUE DROP ATHIGHER SPEEDS

NOISE QUIET NOISY QUIET



STEPPER MOTOR SERVO MOTOR

1. Steppers are good for small

load application

Servo dc motors good up to 1 hp,

and brushless can be up to 50,000

hp

2. Steppers are usually open loop,

count steps and may be has stop

sensor.

Servos are closed loop controlled

with feedback sensors.

3. They are commonly used in

printers , clocks etc .,

automation and CNC

applications.

4. The main difference is Steppermotor can operate with or

without feedback(Encoders ,

Resolvers).But the exact position

will not be known by thecontroller.

In case of Servo motor feedback must be used and also accuracy is

obtained.Most of the big

machines operate on Servo's.



CHARACTERSTICS

SERVO MOTOR STEPPER MOTOR

COST The cost for a servo motorand servo motor system ishigher than that of astepper motor system withequal power rating.

Steppers are generally cheaperthan servo motors that have thesame power rating.

VERSATILITY Servo motors are very versatile in their use for

automation and CNCapplications

Stepper motors are also very versatile in their use for automation

and CNC applications. Because of their simplicity stepper motors may be found on anything from printersto clocks.

LOW SPEED AND

HIGH TORQUE

Servo motors will do fine with low speed

applications given lowfriction and the correctgear ratio

Stepper motors provide mosttorque at low speed (RPM)

HIGH SPEED AND

HIGH TORQUE

Servo motors maintaintheir rated torque to about90% of their no load RPM

Stepper motors lose up to 80% of their maximum torque at 90% of their maximum RPM

OVER LOAD Servo motors may Stepper motors are unlikely to be



CHARACTERSTICS

SERVOMOTOR

STEPPER MOTOR

POWER TO

WEIGHT/SIZERATIO

Servo motors have anexcellent power to weightratio given theirefficiency

Stepper motors are less efficientthan servo motors which usually means a smaller power to weight/size ratio

EFFICIENY Servo motors are very efficient. Yielding 80-90% efficiency given light

loads

Stepper motors consume a lot of power given their output, much of which is converted to heat. Stepper

motors are usually about 70%efficient but this has some to do with the stepper drive

FLEXIBILITY IN

MOTOR

RESOLUTION

Since the encoder on aservo motor determinesthe motor resolution

servos have a wide rangeof resolutions available.

Stepper motors usually have 1.8 or0.9 degree resolution. Howeverthanks to micro-stepping steppers

can obtain higher resolutions. Thisis up to the driver and not themotor.

TORQUE TO

INERTIA RATIO

Servo motors are very capable of acceleratingloads

Stepper motors are also capable of accelerating loads but not as wellas servo motors. Stepper motors

may stall and skip steps if themotor is not powerful enough.



CHARACTERSTICS

SERVO MOTOR STEPPER MOTOR

NOISE Servo motors produce very little noise.

Stepper motors produce a slighthum due to the control process.However a high quality driver willdecrease the noise level.

MOTOR

IMPLI ITY

Servo motors are moremechanically complex due totheir internal parts and the

external encoders

Stepper motors are very simple indesign with no designed

consumable part

DIR E TDR IVE

APABILITY

Servo motors usually requiremore gearing ratios due totheir high RPM. It is very rare

to see a direct drive servomotor setup.

Stepper motors will work fine indirect drive mode. Many peoplesimple use a motor couple and

attach the motor shaft directly tothe lead screw or ball screw

POWER R ANGE Because servo motors areavailable in DC and AC servomotors have a very widepower availability range

The power availability range forstepper motors is not that of servomotors



APPLICATIONS OF STEPPER MOTORy Figure 6-17 illustrates a simple application for a stepper motor.

Each time the controller receives an input signal, the paper isdriven a certain incremental distance. In addition to the paperdrive mechanism in a printer, stepper motors are also popularin machine tools, process control systems, tape and disk drivesystems, and programmable controllers.



OTHER APPLICATIONS

-COMPUTER PHERIHERALS

APPLICATION USE

FLOPPY DISC POSITION MAGNETIC

PICKUP

PRINTER ROTATE CHAR ACTER

WHEEL , CARRIAGE

DRIVE, PAPER FEED

PLOTTER X-Y-Z POSTIONING ,

PAPER FEED



BUSINESS MACHINES

APPLICATION USE

Card Reader position cards

Banking Systems credit card positioning , paper feed

Card Sorter route card flow



PROCESS CONTROL

APPLICATION USE

Carburettor Adjusting air-fuel mixture adjust

V alve Control fluid gas metering

Laser Trimming X-Y positioning



STEPPER MOTOR SPECIFICATIONS

y PHASE :

This term refer s to the number of independent windings on the

stator . E.g. a four phase motor

y STEP A NGLE:

This is the angle through which the rotor for one switching

change for the stator coils .

y HOLDING TORQUE :

This is the maximum torque that can be applied to a powered

motor without moving it from its rest position and causing s pindlerotation

y PULL-IN TORQUE :

This is the maximum torque against which a motor will start ,

for a given pulse rate , and reach synchronism without losing a step .



Contd

y PULL- UT TORQUE :

This is the a imum t r ue that ca e a lie t a motor ,r unning at a given ste ing rate , ithout losing s nchr onism.

y PULL-I RATE :

This is the ma imum switching rate at which a loa e

motor can start without losing a ste .

y PULL-OUT RATE :

This is the switching rate at which a loa e motor will

remain in s nchr onism as the switching rate is re uce .y SLEW RANGE :

This is the range of switching rates etween ull-in and ull-out within which the motor r uns in s nchr onism ut cannot start

up or rever se .



TORQUE

SPEED

PULLIN

SPEED

PULLOUTSPEED

MAX

PULLIN

SPEED

MAX

PULLOUT

SPEED

SLEW

RANGE

CHARACTERSTICS OF

STEPPER MOTOR



MOTORSy DC MOTORS

y AC MOTORS



MOTOR PRINCIPLEy The Motor Principle: when a current-carrying conductor is

located in an external magnetic field perpendicular to theconductor, the conductor experiences a force perpendicularto itself and to the external magnetic field.

y The right-hand rule for force on a conductor can be used todetermine the direction of the force experienced on theconductor: if the right thumb points in the direction of thecurrent in the conductor and the fingers of the right hand

point in the direction of the external magnetic field, then theforce on the conductor is directed outward from the palm of the right hand.



PRINCIPLE



DC MOTOR WORKING



AC MOTOR



TYPES OF A.C MOTORS

y Single Phase Squirrel-cage induction motor

y Three Phase Induction motor

y Synchronous motor



SINGLE PHASE SQ UIRREL CAGE

INDUCTION MOTORy This motor consists of a squirrel-cage rotor, this being

copper or aluminium bars that fit into slots in end

rings to form complete electrical connections.y The basic motor consists of this rotor with a stator

having a set of windings.

y WORKING: When an a.c passes through the stator

windings an alternating magnetic field is produced. Asa result e.m.f is induced in the conductors of the rotorand current flow in the rotor.



y The motor is not self starting.

y A number of methods are used to make the motor self starting and give this initial impetus to start it; one is

to use an auxiliary starting winding to give rotor aninitial push.

y The motor runs at a speed determined by thefrequency of the a.c applied to the stator.

y For a constant frequency supply to a two-pole single-phase motor the magnetic field will alternate at thisfrequency. This speed of rotation of the magnetic fieldis termed the synchronous speed.



THREE PHASE INDUCTION MOTORy It is similar to single-phase induction motor but has a

stator with three windings located 120 degrees apart,each winding being connected to one of the three linesof the supply.

y Because the three phases reach their maximum

currents at different times, the magnetic field can beconsidered to rotate round the stator poles,completing one rotation in one full cycle of thecurrent.



y The three phase motor has a great advantage over thesingle phase motor of being self-starting.

y The direction of rotation is reversed by interchangingany two of the line connections, this changing thedirection of rotation of the magnetic field.



SYNCHRONOUS MOTORSy Synchronous motors have stators similar to those described

above for induction motors but a rotor which is apermanent magnet.

y The magnetic field produced by the stator rotates and sothe magnet rotates with it.

y

With one pair of poles per phase of the supply, themagnetic field rotates through 360 degrees in one cycle of the supply and so the frequency of rotation with thisarrangement is the same as the frequency of the supply.



y Synchronous motors are used when a precise speed isrequired.

y They are not self starting and some system has to beemployed to start them.

stepper miotor

Documents