Short Intro to Stepper Motor

8/7/2019 Short Intro to Stepper Motor

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Conventionally position control was carried out usingDC servomotor , gearbox ,a position sensor , an errordetector and amplifier for activating the servomotor -An elaborate arrangement required.

With the advent of digital computers,electromechanical actuator was needed -directly drivenby digital pulses from a computer for the accuratepositioning of devices like printer heads , machinetools.

Stepper motor fulfills the criteria. Typical stepper motor- step angle-1.8 with 200 steps

per revolution. Stepper motors are usually open-loop devices- low

speed low power devices.


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Digital Electromechanical device whichtranslates the input digital information in theform of digital electric pulses into discrete steps

of shaft rotation. Input in the form of electric pulses.

Pulse input fed not directly to the motorwindings but through a controller.

Controller on receiving the pulse ,switches offone phase and energises a new phase of theseveral phase windings housed in the statorwith DC current.


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Instead of giving a 3 phaseac supply, energise phasewinding through a DC

source, one after theanother through switcharrangement.

If SA closed (keeping other

switches open),currentflows through phase Awinding

Stationary poles created inthe airgap.


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Polarised rotorinitially at some

arbitrary pointmoves andalligns itself.

N and S of therotor faces Nand S of thestator.


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Phase A de-energised andphase B energised. Statorpoles shift in space by 120.

N and S of the rotor faces Nand S of the stator.

Rotor experiences a torqueit alligns to the new statorpole axis after few damped

oscillations


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Phase B de-energised andphase C energised. Statorpoles shift in space by 120.

N and S of the rotor faces Nand S of the stator.

Rotor experiences a torque it alligns to the new statorpole axis.

In this way , motor is madeto step by 120 by each

switching. Step-for every input pulse

,the motor shaft turnsthrough a specified numberof degrees.


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Conventional motors Stepper motor

I t-c st t lt e s rce. I t electric lses.

Free r i sh ft Sh ft m es thr h l r ste s

Feedb ck req ired N feedb ck req ired

A l electr mech ic l de ice Di it l electr mech ic l de ice


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Permanent magnet(PM) stepper motors.

Hybrid stepper motors.

Variable Reluctance stepper motors


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Operates on the reactionbetween a permanentmagnet rotor and anelectromagnetic field.

the rotor magnetized withalternating north andsouth poles.

These magnetized rotorpoles provide anincreased magnetic flux

intensity. because of this the PM

motor exhibits improvedtorque characteristicswhen compared with theVR type.


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Residual or detent torque- when no powerapplied to the windings, a small magnetic forcedeveloped between the permanent magnet andthe stator.

When a PM stepper motor has a steady DC signalapplied to one stator winding, the rotor willovercome the residual torque and line up withthe stator field.

Holding torque- amount of torque required tomove the rotor one full step with the statorenergised.

Step angle-7.5 to 15. (48 24 steps/revolution)


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Reluctance motorused as steppermotor.

Stator-salient polewith concentricwindings.

Rotor-salientstructure withprominent teeth andslots but without any

winding.


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No of phases=4

No of statorpoles=8

No ofrotor poles=6

step angle=360/24=15


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Whenever a stator field energized with DCcurrent ,the nearest tooth axis (or maximumpermeance axis) aligns itself with that phase.

When a new phase energized by switching off

the old phase , the rotor moves in such a waythat its tooth axis generally different from theearlier tooth axis but closest to the new excitedphase , aligns itself with the new stator mmfaxis.

Remains in its position till yet another newphase energized. Angular displacement of the rotor generally

much smaller than the mechanicaldisplacement of the stator mmf.


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Wave Drive (1 phase on)

Full Step Drive (2 phases on)

Half Step Drive (1 & 2 phases on)

Microstepping (Continuously varying motorcurrents)


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(a) (b)


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A B C

ON

OFF

OFF 0

OFF ON OFF 30

OFF OFF ON 60

ON OFF OFF 90

OFF ON OFF 120


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A B C

ON

ON

OFF 15

OFF ON ON 45

ON OFF ON 75

ON ON OFF 105

OFF ON ON 135


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A B C

ON

OFF

OFF 0

ON ON OFF 15

OFF ON OFF 30

OFF ON ON 45

ON OFF OFF 60


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each phase wound onopposite stator poles.

6 stator poles and 4 rotorpoles.

Phase AA` supplied with DCthe toothed rotor structure,aligns itself

Such that the maximumpermeance axis (1-3) alongthe stator mmf axis.

Phase BB` energised,rotor

moves by 30 moves CCW. Nearest tooth axis 2-4 alligns

with the stator mmf axis. Rotor moves CCW by

another 30


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Step angle=360 /(NPh * Ph)=360/N

Following combinations,popular

Torque prop to sin (elec.)

Combination(1) better. Ns and Nr are even

Torque= (1/2)* i2* (dL/d

Ns Nr Stepangle(mech.)

Stepangle(elec.)

8 6 15 90

6 4 30 120

6 8 15 120

Short Intro to Stepper Motor

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