7/27/2019 Guide to Stepper Motor Selection

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Astrosyn

Astrosyn International Technology Ltd,

The Old Courthouse, New Rd Ave,

Chatham, Kent, ME4 6BE, England

Tel +44 (0) 1634 815175Fax +44(0) 1634 826552

www.astrosyn.com

Guide to Stepper Motor Selection

7/27/2019 Guide to Stepper Motor Selection

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Astrosyn GUIDE TO STEPPER MOTOR SELECTION

Astrosyn International Technology LtdThe Old Courthouse, New Rd Ave, Chatham, Kent, ME4 6BE, EnglandTel +44 (0) 1634 815175 Fax +44 (0) 1634 826552 www.astrosyn.com

Introduction

Choosing the right stepper motor for a particular

application can be confusing for anyone without

significant experience in the field. Simon Hunt ofAstrosyn International Technology outlines some stepper

motor fundamentals and offers advice on their selection

for industrial applications.

Stepper motors have a number of features that make themthe motor of choice for a wide range of applications,particularly in measurement and control. They are low cost,highly reliable, produce high torque at low speeds andbenefit from a simple, rugged construction.

They also have the advantage of operating in open loopmode. As long as the motor runs within its specified torque,the position of the shaft is known at all times without theneed for a feedback loop.

Stepper motors are available in three main types:

Permanent magnet motors.These are widely used in non-industrial applications, such as

computer peripherals, and arelow-cost, low-torque and low-

speed devices. Their designmeans that their step angles arerelatively large, but theirsimplicity lends itself to high-volume production at low cost.

Variable reluctance motors.These do not contain permanent magnets, giving them agood torque to inertia ratio. They are frequently used insmall sizes for applications such as micro-positioning tables.However, they require a different driving arrangement fromthe other two types and are seldom used in industrial

applications.

Hybrid motors. As their name suggests, these combine theoperating principles of the previous two types, whichenables them to provide higher torques. Experience atAstrosyn and elsewhere shows that these are by far the mostwidely used stepper motors in industrial applications and forthat reason we will consider only this type here.

Making the right choiceSeveral factors affect the choice of stepper motors forparticular applications, such as the type of motor, the torque

requirement of thesystem, complexity of thecontroller, and thephysical characteristics ofthe motor.

Choosing the right

stepper motor dependsupon the application. Themotors specification willinclude the followingparameters:-

the number of fullsteps perrevolution

whether tooperate in fullstep, half step or

microstep mode the rotational

inertia of the rotorand shaft

holding torque -the maximumrestoring torquedeveloped by therotor when one ormore phases ofthe motor areenergised

detent torque the torquerequired to rotatethe motors shaftwhen thewindings are notenergised

pull-out torque the maximumtorque that can beapplied to a

stepper motor

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GUIDE TO STEPPER MOTOR SELECTION Astrosyn

Astrosyn International Technology LtdThe Old Courthouse, New Rd Ave, Chatham, Kent, ME4 6BE, England

Tel +44 (0) 1634 815175 Fax +44 (0) 1634 826552 www.astrosyn.com

running at constant speed without causing a loss ofsynchronism

expected ambient temperature rangeA standard 1.8 stepper motor has 200 steps per revolution.

Full-step mode in single phase iswhen the motor is operated withonly one phase energised at atime. This requires the least powerfrom the driver power supply, andcan be used where the motor isoperated at a fixed speed. Dualphase provides increased torque,but uses more power.

Half-step operation involves alternating single and dualphase operation and provides twice the resolution, whichresults in increased smoothness at low speeds.

Microstep operation divides each full step into much smallerangles by using sine and cosine functions to drive thewindings. Astrosyn supplies motors with resolutions of up to12 800 microsteps per revolution, and this mode is usedwhen smoother motion or increased resolution is required.

To design stepper motors with higher torque output,manufacturers need to increase the strength of both thepermanent magnet in the rotor and the field produced bythe stator. Rare earth magnets offer increased magnetic fieldstrength, and further torque increases can be achievedthrough mechanical changes.

A stronger rotor magnet can beobtained by increasing itsdiameter, producing a larger cross-sectional area, but this decreasesthe acceleration of the motorbecause the torque-to-inertia ratiobecomes worse.However, torque output can be increased without degradingacceleration by adding further magnet sections or stacks tothe same shaft. A second stack will enable twice the torqueto be produced with double the inertia, so the torque-to-

inertia ratio remains the same. Hence stepper motors areoften available in single, double and three stack versions ineach frame size.

Practical tipsAs a rule of thumb, the torque-to-inertia ratio reduces by afactor of two with each increase in frame size. An unloaded34-size motor can accelerate twice as fast as a 42-size motor,regardless of the number of stacks.

In the manufacture of stepper motors, the rotor ismagnetised after assembly to produce the highest possible

flux density. This meansthat stepper motorsshould not be dismantled,since this flux will belargely destroyed if therotor is removed. Unlikewith DC servo motors, it isnot generally possible todemagnetise steppermotors by applying excesscurrent.

However, too high acurrent can damage themotor in other ways:excess heating may meltthe insulation or the

winding transformers, andmay soften the bondingmaterial holding the rotorlaminations. If thelaminations are displaced,the effects can be thesame as if the rotor wasdemagnetised.

Note that continuousoperation at high speedscan lead to overheating of

the rotor, but high speedscan be used successfullyfor positioningapplications.

Because the shaft of ahybrid stepper motorpasses through the centreof the permanent magnet,it must be made of nonmagnetic material toavoid a magnetic short

circuit. Stepper motorshafts are therefore madeof stainless steel, and thisshould be borne in mindwhen handling the motor.

Small diameter motors areparticularly vulnerable ifthey are dropped on theirshaft end, as this is likelyto bend the shaft.