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UNIT - II

STEPPING MOTOR

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CONTENTS
2. Technical Terms
2.1 INTRODUCTION

2.2 CLASSIFICATION OF STEPPER MOTORS

2.3 SINGLE STACK VARIABLE RELUCTANCE STEPPER MOTOR

2.3.1 Construction

2.3.2 Electrical Connection

2.3.3 Principle of Operation

2.4 MICRO STEPPING CONTROL OF STEPPING MOTOR

2.4.1 Principle of micro stepping

2.5. MULTISTACK VARIABLE RELUCTANCE STEPPER MOTOR

2.6. HYBRID STEPPER MOTOR

2.7 CLAW TOOTH PM MOTOR

2.8. SINGLE PHASE STEPPING MOTOR

2.8.1 Construction

2.8.2. Principle of Operation

2.9. THEORY OF TORQUE PREDICTION

2.10 TERMINOLOGIES USED IN STEPPER MOTOR

2.11. CHARACTERISTICS OF STEPPER MOTOR

2.11.1. Static characteristics

2.11.2. Dynamic characteristics

2.12. TORQUE-SPEED CHARACTERISTICS

2.13 FIGURES OF MERIT FM'S

2.14 DRIVE SYSTEM AND CONTROL CIRCUITRY FOR STEPPER MOTOR

2.14.1 Drive System

2.14.2 Logic Sequencer

2.14.3. Power Driver Circuit

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2.14.4 Improvement of current buildup/special driver circuit

2.14.5. Suppressor circuits

2.15. LINEAR AND NON LINEAR ANALYSIS

2.16 APPLICATION OF STEPPER MOTOR

Question Bank

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2. Technical Terms
1. Stepper motor -- Stepper motor is a brushless DC motor whose rotor rotates in
discrete displacements when its stator windings are energized
in a programmer manner. The rotor has no winding, magnets
or case winding.

2. Full step operation -- Single phase on mode is the one in which at time only one
phase winding is energized.

3. Step Angle -- It is the angular displacement of the rotor of the stepper

motor for every pulse of excitation given to the stator
windings of the motor

4. Resolution -- It is number of steps per revolution.

5. Stepping -- The number of steps per second

rate/frequency
6. Hold Position -- It is corresponds to the rest position when the stepper motor
is excited or energized.

7. Stepping Error -- Actual step angle is slightly different from the theoretical
step angle.

8. Positional Error -- The maximum range of cumulative percentage of error taken

over a complete rotation of steeper motor.

9. Holding torque -- It is the maximum load torque which the energized stepper
motor can withstand slipping from equilibrium position.

10. Detent torque -- It is the maximum load torque which the un-energized
stepper motor can withstand without slipping.

11. Static stiffness -- The ability of the actuator to resist disturbing torques and
forces and thereby to maintain position.

12. Band width -- It is a measure of the frequencies up to which the actuator or

servo system can respond.

13. Synchronism -- It is the one to one correspondence between the numbers of

pulses applied to the stepper motor and the number of steps
through which the motor has actually moved.

14 Half step operation -- It is alternate one phase on and two phase on mode operation.
Here the rotor rotates through half of the full step angle.

15. Slewing -- Steeper may operate at high steeping rates, 25,000 steps per
second.

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2.1 INTRODUCTION

It is an electrodynamics and electromagnetic equipment.

These motors are also referred to as step motors or stepping motors.

On account of its unusual construction, operation and characteristics it is difficult to

define a stepper motor. Definition given in British Standard specification (BSS) is

A stepper motor is brushless dc motor whose rotor rotates in discrete angular

displacements when its stator windings are energized in a programmed manner. Rotation
occurs because of magnetic interaction between rotor poles and poles of the sequentially
energized winding. The rotor has no electrical windings, but has salient and magnetic/or
magnetized poles.

The stepper motor is a digital actuator whose input is in the form of digital signals and
whose output is in the form of discrete angular rotation. The angular rotation is dependent on
the number of input pulses the motor is suitable for controlling the position by controlling the
number of input pulses. Thus they are identically suited for open position and speed control.

Applications:

 Printers
 Graph plotters
 Tape driver
 Disk Drives
 Machine Tools
 X-Y Recorders
 Robotics space Vehicle
 IC Fabrication and Electric Watches

2.2 CLASSIFICATION OF STEPPER MOTORS

As construction is concerned stepper motors may be divided into two major groups.

1. Without Permanent Magnet (PM)

(a) Single Stack
(b) Multi Stack
2. With Permanent Magnet
(a) Claw Pole Motor
(b) Hybrid Motors

2.3 SINGLE STACK VARIABLE RELUCTANCE STEPPER MOTOR

2.3.1 Construction:

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The VR stepper motor characterized by the fact there is no permanent magnet either
on the rotor or the stator. The construction of a 3-phase VR stepper motor with 6 poles on the
stator and 4-pole on the rotor as shown.

Fig 2.1 Single Stack Variable Reluctance Stepper Motor

The Stator is made up of silicon steel stampings with inward projected even or odd
number of poles or teeth. Each and every stator poles carries a field coil an exciting coil. In
case of even number of poles the exciting coils of opposite poles are connected in series. The
two coils are connected such that their MMF gets added .the combination of two coils is
known as phase winding.

The rotor is also made up of silicon steel stampings with outward projected poles and
it does not have any electrical windings. The number of rotor poles should be different from
that of stators in order to have self-starting capability and bi direction. The width of rotor
teeth should be same as stator teeth. Solid silicon steel rotors are extensively employed. Both
the stator and rotor materials must have lowering a high magnetic flux to pass through them
even if a low magneto motive force is applied.

2.3.2 Electrical Connection

Electrical connection of VR stepper as shown fig. Coil A and A’ are connected in

series to form a phase winding. This phase winding is connected to a DC source with the help
of semiconductor switch S1.Similary B and B’ and C and C’ are connected to the same
source through semiconductor switches S2 and S3 respectively. The motor has 3 –phases a, b
and c.

 a phase consist of A and A’ Coils

 b phase consist of B and B’ Coils
 c phase consist of C and C’ Coils

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Fig 2.2 Electrical Connection of VR stepper motor

2.3.3 Principle of Operation

It works on the principle of variable reluctance. The principle of operation of VR

stepper motor explained by referring fig.

(a).Mode 1 : One phase ON or full step operation

In this mode of operation of stepper motor only one phase is energized at any time. If
current is applied to the coils of phase ‘a’ (or) phase ‘a’ is excited, the reluctance torque
causes the rotor to run until aligns with the axis of phase a. The axis of rotor poles 1 and 3 are
in alignment with the axis of stator poles ‘A’ and ‘A’’. Then angle θ = 0° the magnetic
reluctance is minimized and this state provides a rest or equilibrium position to the rotor and
rotor cannot move until phase ‘a’ is energized.

Next phase b is energized by turning on the semiconductor switch S2 and phase ‘a’ is
de –energized by turning off S1.Then the rotor poles 1 and 3 and 2 and 4 experience torques
in opposite direction. When the rotor and stator teeth are out of alignment in the excited
phase the magnetic reluctance is large. The torque experienced by 1 and 3 are in clockwise
direction and that of 2 and 4 is in counter clockwise direction. The latter is more than the
former. As a result the rotor makes an angular displacement of 30° in counterclockwise
direction so that B and B’ and 2 and 4 in alignment. The phases are excited in sequence a, b
and c the rotor turns with a step of 30° in counter clockwise direction. The direction of
rotation can be reversed by reversing the switching sequence in which are energized and is
independent of the direction of currents through the phase winding.

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Fig 2.3 step motions as switching sequence process in a three phase VR motor

The truth table for mode I operation in counter and clockwise directions are given in
the table

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Table 2.1: Counter Clockwise Rotation (CCW) Table 2Clockwise Rotation (CW)

S1 S2 S3 θ S1 S2 S3 θ
* - - 0 * - - 0
- * - 30 - - * 30
- - * 60 - * - 60
* - - 90 * - - 90
- * - 120 - - * 120
- - * 150 - * - 150
* - - 180 * - - 180
- * - 210 - - * 210
- - * 240 - * - 240
* - - 270 * - - 270
- * - 300 - - * 300
- - * 330 - * - 330
* - - 360 * - - 360

In this mode two stator phases are excited simultaneously. When phases a and b are
energized together, the rotor experiences torque from both phases and comes to rest in a point
mid-way between the two adjacent full step position. If the phases b and c are excited, the
rotor occupies a position such that angle between AA’ axis of stator and 1-3 axis of rotor is
equal to 45°.To reverse the direction of rotation switching sequence is changed a and b,a and
c etc. The main advantage of this type of operation is that torque developed by the stepper
motor is more than that due to single phase ON mode of operation.

The truth table for mode II operation in counter clockwise and clockwise directions is given
in a tableTable

2.3: Counter Clockwise Rotation (CCW) Table 2.4: Clockwise Rotation (CW) (C)

S1 S2 S3 θ° S1 S2 S3 θ
* * - 15° AB AC - * - 15°
- * * 45° BC CB - * * 45°
- * - 75° CA BA * * - 75°
* * - 105° AB AC - * - 105°
- * * 135° BC CB - * * 135°
- * - 165° CA BA * * - 165°
* * - 195° AB AC - * - 195°
- * * 225° BC CB - * * 225°
- * - 255° CA BA * * - 255°
* * - 285° AB AC 285°

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Mode III: Half step Mode

In this type of mode of operation on phase is ON for some duration and two phases
are ON during some other duration. The step angle can be reduced from 30° to 15° by
exciting phase sequence a, a+b, b,b+c, c etc. The technique of shifting excitation from one
phase to another from a to b with an intermediate step of a+b is known as half step and is
used to realize smaller steps continuous half stepping produces smoother shaft rotation.

The truth table for mode III operation in counter and clockwise directions are given in
the table

Table 2.5: Counter ClockwiseRotation (CCW) Table 2.6: Clockwise Rotation (CW)
S1 S2 S3 θ
S1 S2 S3 θ
* - - 0° A°
* - - 0° A°
* - * 15° AB°
* * - 15° AB°
- - * 30° B°
- * - 30° B°
- * * 45° BC°
- * * 45° BC°
- - * 60° C°
- - * 60° C°
- * - 75° CA°
* - * 75° CA°
* * - 90° A°
* - - 90° A°
* - - 105° AB°
* * - 105° AB°
* - * 120° B°
- * - 120° B°
- - - 135° BC°
- * * 135° BC°
- * * 150° C°
- * - 150° C°
- * - 165° CA°
* - * 165° CA°
2.4 MICRO STEPPING CONTROL OF STEPPING MOTOR

Stepping motor is a digital actuator which moves in steps of θs in response to input pulses.
such incremental motion results in the following limitations of the stepper motor

Limited resolution

As θs is the smallest angle through which the stepper motor can move, this has an effect on
position accuracy of incremental servo system employing stepper motors because the stepper
motor cannot position the load to an accuracy finer than θs.

Mid frequency Resonance

A phenomenon in which the motor torque suddenly drops to a low value at certain pulse
frequencies as in fig

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Fig 2.4 Mid frequency Resonance

A new principal known as micro stepping control has been developed with a view of
overcoming the above limitation .It enables the stepping motor to move through a tiny micro
step of size ∆ θs << θs full step angle is response to input pulses.

2.4.1 Principle of micro stepping

Assume a two phase stepper motor operating in ‘one phase ON’ sequence. Assume
also that only B2 winding is On and carrying current IB2 = IR, the rated phase current. All
the other winding are OFF. In this state the stator magnetic field is along the positive real axis
as show in fig (a). Naturally the rotor will also as be in θ = 0° position.

When the next input pulse comes, B2 is switched OFF while A1 is switched ON.In
this condition IA1= IR while all the phase current are zero. As a result the stator magnetic
field rotates through 90® in counter clockwise direction as show in fig (a).

The rotor follows suit by rotating through 90° in the process of aligning itself with
stator magnetic field. Thus with a conventional controller the stator magnetic field rotates
through 90° when a new input pulse is received causing the rotor to rotate full step.

However in micro stepping we want the stator magnetic field to rote through a small
angle θs << 90° in respect to input pulse. This is achieved by modulating the current through
B2 and A1 winding as show in fig (b) such that

IA1= IR sin θ

IB1= IR cos θ

Then the resulting stator magnetic field will be at an angle θ ° with respect to the
positive real axis. consequently the rotor will rotate through an angle θs << 90° .

This method of modulating current through stator winding so as to obtain rotation of

stator magnetic field through a small angle θ °

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Fig 2.5 Principle of micro stepping

2.5. MULTISTACK VARIABLE RELUCTANCE STEPPER MOTOR

These are used to obtain smaller step sizes, typically in the range of 2° to 15°. Although three
stacks are common a multistack motor may employ as many as seven stacks. This type is also
known as the cascade type. A cutaway view of a three stack motor is shown in fig. 2.6.

Fig. 2.6: Construction of multi-stack VR motor.

A multistack (or m-stack) variable reluctance stepper motor can be considered to be made up
of ’m’ identical single stack variable reluctance motors with their rotors mounted on a single
shaft. The stators and rotors have the same number of poles (or teeth) and therefore same pole
(tooth) pitch. For a m0stack motor, the stator poles (or teeth) in all m stacks are aligned, but
the rotor poles (teeth) are displaced by 1/m of the pole pitch angle from one another. All the
stator pole windings in a given stack are exited simultaneously and, therefore the stator
winding of each stack forms one phase. Thus the motor has the same number of phases as
number of stacks.

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Fig. 2.7: Cross-section of a 3-stack, VR stepper motor parallel to the shaft.

Figure 2.7 shows the cross section of a three stack (3-phase) motor parallel to the shaft. In
each stack, stator and rotors have 12 poles (teeth). For a 12 pole rotor, pole pitch is 30° and
therefore, the rotor poles (teeth) are displaced from each other by 1/3rd of the pole pitch or
10°. The stator teeth in each stack are aligned. When the phase winding A is excited rotor
teeth of stack A are aligned with the stator teeth as shown in fig. 2.8.

When phase A is de-energized and phase B is excited the rotor teeth of stack B are aligned
with stator teeth. The new alignment is made by the rotor movement of 10° in the
anticlockwise direction. Thus the motor moves one step (equal to ½ pole pitch) due to change
of excitation from stack A to stack B

Next phase B is de-energized and phase C is excited. The rotor moves by another step 1/3rd
of pole pitch in the anticlockwise direction. Another change of excitation from stack C to
stack A will once more align the stator and rotor teeth in stack A. however during this process
(A → B → C → A) the rotor has moved one rotor tooth pitch.

Fig. 2.8: Position of stator & rotor teeth of 3 stack VR motor

Let Nr be the number of rotor teeth and ‘m’ the number of stacks or phases, then

Tooth pitch Tp= 360/Nr ……………… (2.1)

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Step Angle α= 360°/mNr ………………. (2.2)

2.6. Hybrid stepper motor

Principle of operation

Most widely used hybrid motor is the two phase type as shown in fig2.11. This
model has four poles and operates on one phase on excitation.

Fig2.9cross-section of a two phase hybrid motor

The coil in pole 1 and that in pole 3 are connected in series consisting of phase A, and
pole 2 and 4 are for phase B. Fig 2.12 shows the proce3ss of rotor journey as the winding
currents are switched in one phase ON excitation.

Fig2.10 one-phase on operation of a two-phase hybrid motor.

The poles of phase A are excited the teeth of pole 1 attract some of the rotors north
poles, while the teeth of pole 3 align with rotor’s south poles. Current is then switched to
phase B, The rotor will travel a quarter tooth pitch so that tooth alignment takes place in 2
and 4.

Next current is switched back to phase A but in opposite polarity to before, the rotor
will make another quarter tooth journey. The tooth alignment occurs in opposite magnetic
polarity to state 1. When current is switched to phase B in opposite polarity (4) Occurs as a
result of quarter tooth pitch journey.

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The structures of two phase motor considered in fig.2.11 will not produce force in a
symmetrical manner with respect to the axis. The motor having 8 poles in the stator shown in
fig2.13 considered as the structure in which torque is generated at a symmetrical position on
the surface.

Fig2.11 Two-phase hybrid motor with 8 stator poles.

2.7 CLAW TOOTH PM MOTOR

This is another type of stepping motor. This is also known as can-stack Stepping
motor, as the stator of this motor consists of a sort of metal can. Teeth are punched out of a
circular metal sheet and the circle is then drawn into a bell shape. The teeth are then drawn
inside to form claw teeth. A Stack of the stator is formed by joining two bell shaped casings
so that the teeth of both of them are intermeshed and the toroid coil is contained within them

This type of motor shown in fig 2.14 is usually of two stacks. Since the rotor has
magnetic poles that are axially aligned and is common for both stator stacks, the stator tooth
pitches are misaligned by a quarter pitches between the two stacks.

Fig. 2.12 Cutaway diagram of a claw-tooth PM motor

The sequence of excitation is shown in fig. when phase A is excited, the rotor moves by
the tension of magnetic lines (state 1).state 2 is the equilibrium position with phase A excited.
Next if current is switched to phase B , the rotor will be driven further in the same direction,
because the stator teeth in stack B are misaligned by a quarter tooth pitch to the left

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Fig. 2.13 Current waveform supplied to a claw-tooth PM motor

with respect to the teeth in stack A. State 3 shows the result due to this excitation. To advance
the rotor further to the left and place in the next state (4), phase B is de-energized and phase
A is excited. Next, current will be switched to phase B.

The claw tooth motor has low manufacturing cost through it cannot realize a very small
step angle.

2.8. SINGLE PHASE STEPPING MOTOR

These are motors which are designed to be operated from single phase supply. They
are widely use in watches and clocks, timers and counters. Present single phase stepping
motors use one or more (two) permanent magnets, because permanent magnets are quite
necessary to raise the ratio of torque to input power in a miniature motor.

The two requirements of single phase stepping motor are

To detent the motor at a particular position when the coil is not excited.

To rotate the motor at desired direction by switching the magnetic polarity of only one coil.

2.8.1 CONSTRUCTION

It is a permanent magnet type stepper motor with two poles. Rotor is a circular type of
permanent magnet as shown in figure 2.27.ststor is made of silicon steel stampings with two
salient poles. Stator carries a coil which is connected to a pulsed supply. The air gap is
specially designed so that specific reluctance at different radial axes are different. Minimum
values occur at one tip of the poles. Under normal conditions the rotor occupies any one of
the decent position shown in fig 2.28(a0 or as in (b) to minimum reluctance position. two
positions shown in figures 2.28(a) & (b) are the detent positions of the rotor of the stepper
motor.

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Fig. 2.14 A single-phase stepping motor

Fig. 2.15 Detent positions and coil polarity to rotate motor.

2.8.2. PRINCIPLE OF OPERATION

When the coil is given an electric positive pulse, pole A in position 1 as shown in
figure. 2.28(a) it experiences a torque in clockwise direction and finally attains a steady state
as in fig 2.28(b).then pulse given to the coil is zero. After a lapse of a second, from the start
of the pulse, a negative pulse is given to the coil which makes the pole A as south and pole B
as north. Rotor experiences another torque in figure 2.28(a).by repeating the cycle the rotor
rotates continuously in step .it is not possible to develop torque in counter clockwise direction
by altering pulses.

2.9. THEORY OF TORQUE PREDICTION

According to Faradays laws of electromagnetic induction

Flux linkages λ=Nφ ……….. (2.3)

λ=Li ……..… (2.4)

Flux linkages can be varied by

Varying flux φ

Varying the current ‘i’ of an electromagnet (i.e) equivalent of varying the mmf

N²
Varying the reluctance L =
S

By varying reluctance

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mmf = Nφ …………….. (2.5)

1
Reluctance = ………….(2.6)
Aμ
¿
Flux = S ……….… (2.7)

¿ N²
Flux linkages λ = N .∋ S ¿ =
S
………. (2.8)

flux linkages
Inductance L = ………. (2.9)
Ampere

N ²i
L= ………….. (2.10)
Si
2
N
L= ………….(2.11)
S

If the reluctance of magnetic circuit can be varied, inductance L and the flux linkages λ can
also be varied.

Consider a magnetic circuit as shown in fig. 2.29.

Fig. 2.16 Magnetic circuit

The stator consists magnetic core with two pole arrangement. Stator core carries a
coil. Rotor is also made up of ferrous material. The motor core is similar to a salient pole
machine. Let the angle between the axis of stator pole and rotor pole be θ. let the angular
displacement be illustrated using fig. 2.29 (a, b and c).

Case 1: θ = 0

As shown in fig. 2.29 (a) the air gap between the stator and rotor is very very small.
Thereby the reluctance of the magnetic path is least. Due to minimum reluctance, the
inductance of the circuit is minimum. Let it be Lmax

Case 2 : θ = 45 0

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As shown in fig. 2.29(b) in this only a portion of rotor poles cover the stator poles.
Therefore reluctance of the magnetic path is more than that of case 1.due to which the
inductance becomes less than Lmax .

Case 3: θ = 90 0

As shown in fig. 2.29(c) the air gap between the stator poles has maximum value.
Thereby reluctance has a value yielding minimum inductance. Let it be Lmin.

Variation in inductance with respect to the angle between the stator and rotor poles is
shown in fig. 2.30.

Fig. 2.17 Variation in induction w.r to θ.

Derivation for reluctance torque

As per faradays law of electromagnetic induction an emf induced in an electric circuit

when there exists a change in flux linkages.

∂y
emf induced e = -
∂t

Where λ = NΦ or λ = Li …….… (2.12)

d
Therefore e = - [Li] ……….…
dt
(2.13)

∂i ∂L
=-L -i …..….. (2.14(a))
∂t ∂t

∂i ∂ L ∂θ
=-L -i × ……... (2.14(b))
∂t ∂θ ∂ t

∂i ∂L
=-L -iω ……....
∂t ∂θ
(2.14(c))

di ∂L
Magnitude of e = L +ω i ………..…..
dt ∂θ
(2.15)

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If the direction of current I is opposite to that of e, then the electric power is

transferred from the source to the inductor. On the other hand, if the direction of current I is
same as that of e, then the source gets the electrical power from the inductor.

On the basis of magnetic circuit/field theory it is known that the stored energy in a
magnetic field.

1
We = Li2 ………. (2.16)
2

The rate of change of energy transfer due to variation in stored energy or power due to
variation in stored energy.

dWe 1 ∂i 1 ∂L
= L. 2i + = i2
dt 2 ∂t 2 ∂t
……… (2.17)

Mechanical power developed/consumed = power received from the electrical source –

power due to change in stored energy in the inductor

Power received from the electrical source = ei

di ∂L
∴ ei=i L + ω i² ….… (2.18)
dt ∂θ

Power due to change in stored energy

di 1 ∂L
= Li + ωi ² …… (2.19)
dt 2 ∂θ

Mechanical power developed

di ∂L di 1 ∂L
= iL +ω i ² + Li + ω i ² …… (2.20)
dt ∂θ dt 2 ∂θ

Mechanical power developed

1 ∂L
Pm = ωi2
2 ∂θ
…… (2.21)

2 πNT
Pm = …… (2.22)
60

Pm = ωT …… (2.23)

2 πN
Where ω =
60

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Pm
Therefore reluctance torque T = …… (2.24)
ω

1 ∂L
Reluctance torque T = i² …… (2.25)
2 ∂θ

Note:

∂L
* Torque corresponds to monitoring when is +ve.
∂θ

∂L
* Torque corresponds to generating when is -ve.
∂θ

* Torque is proportional to i2 : Therefore it does not depend upon the direction of the
current.

2.10 TERMINOLOGIES USED IN STEPPER MOTOR

1. Step motor

2. Resolution

3. Stepping rate

4. Hold position

5. Detent position

6. Stepping error

7. Position Error

1. Step angle (θs or β)

It is the angular displacement of rotor of a stepper motor for every pulse of excitation
given to the stator winding of the motor. it is determined by the number of teeth on the rotor
and stator, as well as the number of steps in the energisation sequence. It is given by

360
Θs = β =
mNr

Where

m = Number of phases (m and q)

Nr- number of teeth on rotor.

Also, Θs=((Ns~Nr)/(Ns.Nr))*360

2. Resolution

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It is the number of steps per revolution. It is denoted as S or Z. it is given by

Z=360/(Θs)

For variable reluctance motor Z=(q Nr) or (m Nr)

For PM motor and hybrid motor Z=2q Nr

Also , Z=(Ns.Nr)/(Ns~Nr)

Where Ns-number of teeth/poles on stator.

3. Stepping Rate

The number of steps per second is known as stepping rate or stepping frequency.

4. Hold Position

It corresponds to the rest position when the stepper motor is excited or energized(this
corresponds to align position of VR motor)

5. Detent Position

It corresponds to rest position of the motor when it is not excited.

6. Stepping Error

Actual step angle is slightly different from the theoretical step angle. This is mainly
due to tolerances in the manufacture of stepper motor and the properties of the magnetic and
other materials used.

The error in the step angle is expressed as a percentage of the theoretical step angle.

%error= ((step angle – theoretical step angle)/theoretical step angle)*100

Percentage error is restricted to ± 5%.In some cases it is restricted to ±2%. The

cumulative error between the actual angular displacement and theoretical angular
displacement is expressed as a percentage of theoretical angular displacement. It is usually
considered for one complete cycle.

7. Positional Error

The maximum range of cumulative percentage of error taken over a complete rotation
of stepper motor is referred to as positional accuracy as shown in fig below.

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Fig. 2.18 Positional Accuracy

2.11. CHARACTERISTICS OF STEPPER MOTOR

Stepper motor characteristics are divided into two groups

 Static characteristics
 Dynamic characteristics

2.11.1. Static characteristics

It is divided into two charteristics.

(i)Torque Angle curve

(ii)Torque current curve

(i)Torque-Angle curve

Torque angle curve of a step motor is shown in fig.2.32. it is seen that the Torque
increases almost sinusoid ally, with angle Θ from equilibrium.

Fig. 2.19 Torque Angle

Holding Torque (TH)

It is the maximum load torque which the energized stepper motor can withstand without
slipping from equilibrium position. If the holding torque is exceeded, the motor suddenly
slips from the present equilibrium position and goes to the static equilibrium position.

Detent torque (TD):

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It is the maximum load torque which the un-energized stepper motor can withstand slipping.
Detent torque is due to magnetism, and is therefore available only in permanent magnet and
hybrid stepper motor. It is about 5-10 % of holding torque.

Torque current curve

A typical torque curve for a stepper motor is shown in fig.2.34. It is seen the curve is
initially linear but later on its slope progressively decreases as the magnetic circuit of the
motor saturates.

Fig.2.20 Torque-current Curve

Torque constant (Kt)

Torque constant of the stepper is defined as the initial slope of the torque-current (T-I)
curve of the stepper motor. It is also known as torque sensitivity. Its units N-mA, kg-cm/A or
OZ-in/A

2.11.2. Dynamic characteristics

A stepper motor is said to be operated in synchronism when there exist strictly one to
one correspondence between number of pulses applied and the number of steps through
which the motor has actually moved. There are two modes of operation.

Start-Stop mode

Also called as pull in curve or single stepping mode.

Slewing mode

In start –stop mode the stepper motor always operate in synchronism and the
motor can be started and stopped without using synchronism. In slewing mode the motor will
be in synchronism, but it cannot be started or stopped without losing synchronism. To operate
the motor in slewing mode first the motor is to be started in start stop mode and then to

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slewing mode. Similarly to stop the motor operating in slewing mode, first the motor is to be
brought to the start stop mode and then stop.

Start Stop mode

Start stop mode of operation of stepper motor is shown in fig.2.35 (a).In this second
pulse is given to the stepper motor only after the rotor attained a steady or rest position due to
first pulse. The region of start-stop mode of operation depends on the operation depends on
the torque developed and the stepping rate or stepping frequency of stepper motor.

Fig. 2.21 Modes of operation

2.12. TORQUE-SPEED CHARACTERISTICS

Torque developed by the stepper motor and stepping rate characteristics for both
modes of operation are shown in fig.2.36.the curve ABC represents the "pull in"
characteristics and the curve ADE represents the "pull-out" characteristics.

Fig. 2.22 Torque-Speed Characteristics

The area OABCO represents the region for start stop mode of operation. At any
operating point in the region the motor can start and stop without losing synchronism. The

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area ABCEDA refers to the region for slewing mode of operation. At any operating point
without losing synchronism to attain an operating point in the slewing mode at first the motor
is to operate at a point in the start-stop mode and then stepping rate is increased to operate in
slewing mode, similarly while switching off it is essential to operate the motor from slewing
mode to start-stop mode before it is stopped.

Pull in torque

It is the maximum torque developed by the stepper motor for a given stepping rate in the
start-stop mode of operation without losing synchronism. In the fig.2.36 LM represents the
pull in torque (i.e)TPI corresponding to the stepping rate F (i.e.) OL.

Pull out torque

It is the maximum torque developed by the stepper motor for a given stepping rate in
the slewing mode without losing synchronism. In fig.2.36 LN represents the pull in torque
(i.e.) TPO corresponding to F (i.e.) OL.

Pull in range

It is the maximum stepping rate at which the stepper motor can operate in start-stop
mode developing a specific torque (without losing synchronism).In fig. 2.36 PIT represents
pull in range for a torque of T (i.e.) OP. This range is also known as response range of
stepping rate for the given torque T.

Pull out range

It is the maximum stepping rate at which the stepper motor can operate in slewing
mode developing a specified torque without losing synchronism. In fig.2.36 PIPO represents
the pull out range for a torque of T. The range PIPO is known slewing range.

Pull in rate (FPI)

It is the maximum stepping rate at which the stepper motor will start or stop without
losing synchronism against a given load torque T.

Pull out rate (FPO)

It is the maximum stepping rate at which the stepper motor will slew, without missing
steps, against load torque T.

Synchronism

This term means one to one correspondence between the number of pulses applied to
the stepper motor and the number of steps through which the motor has actually moved.

Mid frequency resonance

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The phenomenon at which the motor torque drops to a low value at certain input pulse
frequencies.

2.13 FIGURES OF MERIT (FM'S)

Figures of merit (FM'S) are performance indices which give quantitative information
on certain aspects of performance and design of actuators such as stepper motors. DC or AC
servomotors etc.

1. Electrical Time constant (Te)

Te=Lm/Rm ……….. (2.26)

where Lm-Inductance of motor winding

Rm-resistance of motor.

Te governs the rate at which current rises when the motor winding is turned on.It also
determines how quickly the current decays when the winding is turned off.

In motion control, the speed of response is of importance. Hence electrical time

constant Te must be minimized.

Te dependent upon inductance and resistance of the motor winding. Inductance is

determined by magnetic circuit. (i.e.) magnet iron volume as well as volume of copper used
in the motor design. Once these have been designed, neither reducing conductor size nor
increasing the number of turns will reduce Te. Otherwise magnetic circuit itself has to be
redesigned.

2. Motor time constant (Tm)

Tm=J/(Ke.KtRm)=JRm/Ke ………… (2.27)

J-moment of inertia of motor (kg-m2)

Rm-resistance of the motor winding (Ω)

Ke-back emf constant(volt s/ rad)

Kt- torque constant (Nm/A)

Motor back emf and torque constants are determined by magnetic circuit and phase
winding design. Winding resistance also from winding design. Moment of inertia is
determined by mechanical design.

In this way motor time constant Tm combines all the three aspects of motor design
viz, magnetic circuit, electrical circuit and mechanical design. Achieving a low Tm requires
excellence in motor design. As a thumb rule the ratio of Te/Tm 0.1

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Initial Acceleration (a0):

A0=T/J(rad/S2)

Where T-rated torque (N-M)

J-moment of inertia(kg-m2)

a0 gives a quantitative idea of how fast the motor accelerates to its final velocity or
position. Maximization of a0 calls for good magnetic circuit design to produce high torque in
conjunction with good mechanical design to minimize rotor inertia. The moment of inertia of
the load coupled to motor also determines a0.

Motor Constant (km)

km=T/√ ω

where T- rated motor torque

ω -rated power(w) of the motor

km=√Kt Ke/Rm

This shows that maximizing km causes minimizing R, maximizing Ke and Kt. Maximizing
Ke and Kt. Call for optimization of magnetic circuit design, decreasing electrical time
constant Te which is undesirable. A trade off between electrical and magnetic circuit design
is necessary to achieve a good km.

Power rate (dP/dt):

Power rate is (dP/dt)=(d/dt)(T.(dϴ/dt))= T.(d2ϴ/dt2)=T.(T/J)=(T2/J) …..(2.28)

Now T=Kt I so

2.14 DRIVE SYSTEM AND CONTROL CIRCUITRY FOR STEPPER MOTOR

2.14.1 DRIVE SYSTEM

The stepper motor is a digital device that needs binary (digital) signals for its
operation .Depending on the stator construction two or more phases have to be sequentially
switched using a master clock pulse input. The clock frequency determines the stepping rate,
and hence the speed of the motor. The control circuit generating the sequence is called a
translator or logic sequencer.

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Fig. 2.23 Block Diagram of the drive system of a stepping motor.

The fig 2.38 shows the block diagram of a typical control circuit for a stepper
motor. It consists of a logic sequencer, power driver and essential protective circuits for
current and voltage limiting. This control circuit enables the stepper motor to be run at a
desired speed in either direction. The power driver is essentially a current amplifier, since the
sequence generator can supply only logic but not any power. The controller structure for VR
or hybrid types of stepper motor

Fig. 2.24 Block diagram of a typical step motor control

2.14.2 LOGIC SEQUENCER

The logic sequencer is a logic circuit which control the excitation of the
winding sequentially, responding to step command pulses. A logic sequencer is usually
composed of a shifter register and logic gates such as NANDs, NORs etc. But one can
assemble a logic sequencer for a particular purpose by a proper combination of JK flip flop,
IC chips and logic gate chips.

Two simple types of sequencer build with only two JK-FFs are shown in fig
2.39 for unidirectional case. Truth tables for logic sequencer also given for both the
directions.

TABLE 2.7 Logic Sequencer

R 1 2 3 4 5 6 ….

Ph A,Q1 0 1 1 0 0 1 1 ….

Ph B,Q2 0 0 1 1 0 0 1 ….

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R 1 2 3 4 5 6 ….
Ph 1 0 0 1 1 0 0 … Ph A,Q1 0 0 1 1 0 0 1 ….
A, .
Q1 Ph B,Q2 0 1 1 0 0 1 1 ….

Ph 1 1 0 0 1 1 0 … Ph A,Q1 1 1 0 0 1 1 0 ….
B,
Ph B,Q2 1 0 0 1 1 0 0 ….
Q2

Fig.2.25 A unidirectional logic sequencer for two phase on operation of a

two phase hybrid motor

The corresponding between the output terminals of the sequencer and the phase windings to
be controlled is as follows.

Q1-----Ph A

Q1-----Ph A

Q2-----Ph B

Q2-----Ph B

If Q1 is on the H level the winding Ph A is excited and if Q1is on L level, Ph

A is not excited.

To reserve the rotational direction, the connection of the sequencer must be

interchanged. The direction switching circuits shown in fig 2.40 may be used for this
purpose. The essential functions being in the combination of three NAND gates or two AND
gates and a NOR gate.

2.14.3. Power Driver Circuit

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The number of logic signals discussed above is equal to the number of phases and the
power circuitry is identical for all phases. Fig. 2.44(a) shows the simplest possible circuit of
one phase consisting of a Darlington pair current amplifier and associated protection circuits.
The switching waveform shown in fig. 2.44(c) is the typical R-L response with an
exponential rise followed by decay at the end of the pulses.

In view of the inductive switching operation, certain protective elements are introduced
in the driver circuit. These are the inverter gate 7408, the forward biased diode D1 and the
freewheeling diode D. The inverter IC provides some sort of isolation between the logic
circuit and the power driver.

There are some problems with this simple power circuit. They can be understood by
considering each phase winding as a R-L circuit shown in fig. 2.44(b) subject to repetitive
switching. On application of a positive step voltage, the current rises exponentially as

i(t)=I(1-e−t / Ԏ) … …(2.29)

Where I=V/R – rated current and

Ԏ=L/R winding time constant.

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Fig. 2.26 Power Driver Stage of Stepper Motor Controller

In practice, the time constant Ԏ limits the rise and fall of current in the winding. At
low stepping rate the current rises to the rated value in each ON interval and falls to zero
value in each OFF interval. However as the switching rate increases, the current is not able to
rise to the steady state, nor fall down to zero value with in the on/off time intervals set by the
pulse waveform. This in effect, smoothens the winding current reducing the swing as shown
in fig. 2.45. As a result the torque developed by the motor gets reduced considerably and for
higher frequencies, the motor just ‘vibrates’ or oscillates within one step of the current
mechanical position.

Fig. 2.27 Effect of increasing Stepping Rate on Current Swing

In order to overcome these problems and to make improvement of current build up several
methods of drive circuits have been developed.

For example when a transistor is turned on to9 excite a phase, the power supply must
overcome effect of winding inductances has tendency to oppose the current built up. As
switching frequency increases the position that the buildup time takes up within the switching
cycle becomes large and it results in decreased torque and slow response.

2.14.4 Improvement of current buildup/special driver circuit

(a) Resistance drive (L/R drive)

Here the initial slope of the current waveform is made higher by adding external resistance in
each winding and applying a higher voltage proportionally. While this increases the rate of
rise of the current, the maximum value remains unchanged as shown in fig. 2.46.

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Fig. 2.28 L/R drive

The circuit time constant is now reduced and the motor is able to develop normal torque even
at high frequencies. The disadvantage of this method is

Flow of current through external resistance causes I 2R losses and heating. This denotes
wastage of power as far as the motor is concerned.

In order to reach the same steady state current I R as before, the voltage required

To be applied is much higher than before. Hence this approach is suitable for small
instrument stepper motor with current ratings around 100 mA, and heating is not a major
problem.

(b) Dual voltage driver (or) Bi-level driver

To reduce the power dissipation in the driver and increase the performance of a
stepping motor, a dual-voltage driver is used. The scheme for one phase is shown in fig. 2.47.

When a step command pulse is given to the sequencer, a high level signal will be put
out from one of the output terminal to excite a phase winding. On this signal both T r 1 and T r
2 are turned on, and the higher voltage E H will be applied to the winding. The diode D1 is
now reverse biased to isolate the lower voltage supply. The current build up quickly due to
the higher voltage E H . The time constant of the monostable multivibrator is selected so that
transistor T r 1 is turned off when the winding current exceeds the rated current by a little.
After the higher

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Fig. 2.29 Improvement of current buildup by dual voltage drive

Voltage source is cut off the diode is forward biased and the winding current is supplied from
the lower voltage supply. A typical current wave form is shown in fig. 2.48.

Fig. 2.30 Voltage and current wave form in dual voltage driver

When the dual voltage method is employed for the two phase on drive of a two phase
hybrid motor, the circuit scheme will de such as that shown in fig.2.49. Two transistor T r 1
& T r2 and two diodes D1and D2 are used for switching the higher voltage. In dual voltage
scheme as the stepping rate is increased, the high voltage is turned on for a greater percentage
of time.

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Fig. 2.31 A dual-voltage driver for the two-phase-on drive of a two phase hybrid motor

This drive is good and energy efficient. However it is more complex as it requires
two regulated power supplies EH& EL end two power transistor switches Tr1 & Tr2 and
complex switching logic. Hence it is not very popular.

(c) Chopper drive

Here a higher voltage 5 to 10 times the related value is applied to the phase winding
as shown in fig.2.50(a) and the current is allowed to raise very fast. As soon as the current
reaches about 2 to 5% above the rated current, the voltage is cut off ,allowing the current to
decrease exponentially. Again as the current reaches some 2 to 5% below the rated value, the
voltage is applied again. The process is repeated some 5-6 times within the ON period before
the phase is switched off. During this period the current oscillates about the rated value as
shown in fig. A minor modification is to chop the applied dc voltage at a high frequency of
around 1khz, with the desired duty cycle so as to obtain the average on-state current equal to
the rated value.

Fig. 2.32 Chopper drive

The chopper drive is particularly suitable for high torque stepper motors. It is
ener4gy efficient like the bi-level drive but the control circuit is simpler.

(d) Problems with driver circuits

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A winding on a stepping motor is inductive and appears as a combination of inductance

and resistance in series. In addition, as a motor revolves a counter emf is produced in the
winding. The equivalent circuit to a winding is hence, such as that shown for designing a
power driver one must take into account necessary factors and behavior of this kind of circuit.
Firstly the worst case3 conditions of the stepping motor, power transistors, and supply
voltage must be considered. The motor parameters vary due to manufacturing tolerance and
operating conditions. Since stepping motors are designed to deliver the highest power from
the smallest size, the case temperature can be as high as about 100°c and the winding
resistance therefore increases by 20 to 25 per cent.

2.14.5. Suppressor circuits

These circuits are needed to ensure fast decay of current through the winding when it
is turned off. When the transistor in the above fig is turned off a high voltage builds up to
Ldi/dt and this voltage may damage the transistor. There are several methods of suppressing
this spike voltage and protecting the transistor as shown in the following.

(a) Diode suppressor

If a diode is put in parallel with the winding in the polarity as shown in fig. a circulating
current will flow after the transistor is turned off, and the current will decay with time. In this
scheme, no big change in current appears at turn off, and the collector potential is the supply
potential E plus the forward potential of the diode. This method is very simple but a
drawback is that the circulating current lasts for a considerable length of time and it produces
a braking torque.

Fig. 2.33 Diode suppressor

(b)Diode-Resistor suppressor

A resistor is connected in series with the diode as shown in fig to damp quickly the
circulating current. The voltage VCE applied to the collector at turn-off in this scheme is

VCE=E+IRS+VD

Where E= supply potential

I= Current before turning off

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Rs-resistance of suppressor resistor

VD-forward potential of diode

Fig. 2.34 Diode-resistor suppressor

A high resistance RS is required to achieve a quick current decay, but this also results in
a higher collector potential VCE, thus a transistor with a high maximum voltage rating is
necessary.

(d) Zener diode suppressor

In this zener diode are often used to connect in series with the ordinary diode as
shown in fig. Compared with preceding two cases zener diode which provides negative bias
causes the current to decay more quickly after turn off. In addition to this, it is a merit of this
method that the potential applied to the collector is the supply potential plus the zener
potential, independent of the current. This makes the determination of the rating of the
maximum collector potential easy. However zeners are signal diodes, rather than power
diodes. Their power dissipation is limited to 5w. Consequently, this suppressor can be used
for very small instrument stepper motors of typical size 8 to 11.

Comparison of effects of various suppressor schemes of various suppressor schemes

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Fig. 2.35 Zener diode Fig. 2.36 Comparison of effects

(d)Condenser suppressor

This scheme is often employed for bifilar-wound hybrid motor. An explanation is

given for the given for the circuit shown in fig:

Fig. 2.37 Condenser suppressor

A condenser is put between ph A and ph A͞ and between ph B and ph B͞ . These condensers

serve two fold purposes.

1. When a transistor is turned off, the condenser connected to it via a diode absorbs the
decaying current from the winding to protect the transistor.

Let us see the situation just after the Tr 1 is turned off in the one phase on mode. Either
Tr 2 or Tr 4 will turn on, but Tr 3 will still be in the turned off state. Since the winding of ph
A & ph A͞ are wound in the bifilar fashion, a transient current will circulate in loop. If Tr 3 is
turned on when the transient current becomes zero and the charge stored in the condenser
becomes maximum, a positive current can easily flow through phase A winding. By this
resonance mechanism, currents are used efficiently in this scheme. This feature remains in
the two phase on mode too. The condenser suppressor is suited to drives in which stepping
rate is limited in a narrow range.

2 .Another utility of condensers is as an electrical damper, a method of damping rotor

oscillations is to provide a mechanism to convert kinetic energy to joule heating. If a rotor
having a permanent magnet oscillates, an alternating emf is generated in the winding.
However if a current path is not provided or a high resistance is connected, no current will be
caused by this emf. When the condenser is connected between phases an oscillatory current
will flow in the closed loop and joule heat is generated in the windings, which means that the
condenser works as an electrical damper.

2.15. LINEAR AND NON LINEAR ANALYSIS

The linear and nonlinear analysis of the motor performance with respect to the torque
produced by the rotor of the motor is explained.

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Let

Tm be the motor torque produced by the rotor in Nm

J be the inertia of the rotor and load combination in kgm2

ω be the angular velocity of the rotor

D be the damping coefficient or viscous frictional coefficient

Tf be the frictional load torque independent of the speed

Θs be the step angle in radians

F be the stepping rate in steps/sec or pps

Frictional load torque Tf = K θ

According to rotor dynamics

Tm=―J*dω/dt+Dω+Tf ………….. (2.30)

Also θs=θ=ωt=step angle

ω=θs/t=f θs …………..(2.31)

where f=1/t ………….(2.32)

By putting ω=f θs

Tm=J *d/dt(f θs )+D(f θs )+Tf …………..(2.33)

θs=360/mNr is fixed for a particular type of motor

S o θs can be considered as constant

Therefore Tm=J θs* d/dt(f)+D θs(f)+Tf ………….(2.34)

In equation 2.47 if viscous friction constant is neglected the equation will be a linear
equation, the corresponding acceleration will be nonlinear and the equation will be nonlinear
which given rise to nonlinear analysis.

Linear acceleration on linear analysis

If the damping coefficient is neglected D=0

The expression for motor torque becomes

Tm=―J*dω/dt+Tf ………………(2.35)

Tm-Tf= J*dω/dt

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(Tm-Tf)/J= dω/dt

dω=((Tm-Tf)/J)dt ……………….. (2.36)

Integrating

ω=((Tm-Tf)/J)dt+ω1 ……………….. (2.37)

Where

ω1=Integration constant

Mathematically ω1 is the constant of integration but it indicates the initial angular velocity
of the motor before the occurrence of acceleration.

Therefore ω=θs f and ω1= θs f1

Substituting ω and ω1 in equation 2.50

((Tm-Tf)/J)t+ θs f1= θsf ………………..(2.38)

Dividing throughout by θs we get

((Tm-Tf)/J θs)t+ f1=f

Therefore stepping rate f=((Tm-Tf)/J θs)t+ f1 ……………(2.39)

And Tf = K θ

Figure 2.38 shows the linear acceleration from ω1 to ω2

Nonlinear (exponential) acceleration on Nonlinear analysis

Considering the torque produced by the motor

Tm=jθs df/dt +Dθsf+Tf …….(2.40)

(Tm-Tf)= jθs df/dt +Dθsf

Dividing throughout by jθs We get

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(df/dt)+(D/J)f-(Tm-Tf /j θs )=0

(or) (df/dt)+(D/J)f=(Tm-Tf /j θs ) …. (2.41)

The above eqn. 2.54 is of the form

(dy/dx)+py=Q Which have the solution of

ye∫pdx =∫Q e∫pdx+C …..………….(2.42 )

Here y=f; x=t; p=(D/j) and Q=(Tm-Tf)/jθs =constant

fe∫D/J dt=∫(Tm-Tf)/jθse∫D/J dt+C …………………(2.43)

fe∫D/J t=∫(Tm-Tf)/jθse∫D/J t+C ………….…….(2.44)

fe∫D/J t=(Tm-Tf)/jθs(e∫D/J t/(D/J))+C ………………..(2.45)

where C is the integration constant

To find C substituting initial condition at t=0; f=f(0)=f1f1e0==(Tm-Tf)/jθs(1/(D/J))

+C …………..…..(2.46)

f1===(Tm-Tf)/Jθs(J/D))+C …………..…...
(2.47)

f1=(Tm-Tf)/Dθs+C ………….…..
(2.48)

C= f1-(Tm-Tf)/Dθs . ……………...
(2.49)

Substituting eqn. (2.62) in eqn. (2.58)

f e(D/J)t=(Tm-Tf)/Jθs(J/D)e(D/J)t+( f1-(Tm-Tf)/Dθs) …………….….(2.50)

f e(D/J)t=(Tm-Tf)/Dθs e(D/J)t+( f1-(Tm-Tf)/Dθs) ………..……( 2.51)

Dividing throughout by e(D/J)t we get

F=Tm-Tf/Dθs +(f1-Tm-Tf/Dθs)e-D/j t ……………(2.52)

Stepping frequency f= Tm-Tf/Dθs +(f1-Tm-Tf/Dθs)e-D/j t

The above equation is a nonlinear exponential equation which gives rise to nonlinear
acceleration of the rotor of the motor.

2.16 APPLICATION OF STEPPER MOTOR:

The main application of stepper motor may be divided into the following
groups.

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1. Instrumentation applications.

2. Computer peripherals & Office equipment’s.

3. Numerical control of machine tools and robotics.

4. Applications in semiconductor technology.

5. Space vehicles and satellites.

6. Electro medical and

7. Miscellaneous applications.

1. Instrumentation application:

This involve low torque applications such as

Quartz watches.

Synchronized clocks.

Camera shutter operations.

2. Stepper motor application in computer peripherals:

This involve medium torque, high performance and high volume application such
as

Dot matrix and line printers.

Graph plotters.

Floppy disk drives

Digital X-Y plotters.

Magnetic tape drives.

Paper tape drives.

3. Application is office equipment:

Electronic typewriters.

Copiers

Facsimile machines.

4. Machine tool applications:

This involve high torque application such as

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Numerical control system for milling machine

X-Y tables and index table.

Home use and industrial sewing machines.

5. Application in semiconductor technology:

Stepper motors used in high vacuum.

Goniometer-An instrument used to determine crystalline structure.

Electron beam micro fabricator.

6. Stepper motor used in space vehicles and satellites.

7. Robotics.

8. Electro medical applications:

This involve high torque applications such as

X-ray machines.

Radiation therapy units.

Ultra sound scanner.

9. Miscellaneous applications:

Nuclear reactors.

Heavy industry applications.

Automatic focusing mechanism in camera.

Related Links

1. http://www.en.nanotec.com/steppermotor_animation.html
2. http://www.freescale.com/webapp/sps/site/overview.jsp?code=WBT_MOTORSTEPTUT_WP
3. http://seriss.com/opcs/halfstep-diagram.html
4. http://educypedia.karadimov.info/library/pas.swf
5. http://educypedia.karadimov.info/library/howsteppermotorswork_1273256291.gif
6. http://educypedia.karadimov.info/library/StepperMotorConstructionAllPhases1.gif

Question Bank

PART – A
1. What is stepper motor?

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2. Define step angle.

3. Define slewing.
4. Sketch the diagram of a VR Stepper motor.
5. What are the different modes of excitation used in variable reluctance stepper motor?
6. Mention some applications of stepper motor?
7. Define resolution.
8. What are the advantages and disadvantages of stepper motor?
9. What is meant by micro stepping in stepper motor?
10. Differential between VR, PM and hybrid stepper motor?
11. Define holding torque.
12. Define detent torque.
13. Define pull-in torque and pull-out torque.
14. Draw the typical dynamic characteristics of a stepper motor
15. What is slew range?
16. What is synchronism in stepper motor?
17. Draw the block diagram of the drive system of a stepper motor.
18. What is the step angle of a four phase stepper motor with 12 stator teeth and 3 rotor teeth?
19. A single stack, 3-phase VR motor has a step angle of 15°. Find the number of its rotor
and stator poles.
PART – B
1. Explain the construction and various modes of excitation of VR stepper motor.
2. Explain the construction and various modes of excitation of PM stepper motor.
3. Explain the construction and working principle of Hybrid Stepper motor.
4. State and explain the static and dynamic characteristics of a stepper motor.
5. Explain in detail about different types of power drive circuits for stepper motor.
6. Explain the mechanism of torque production in VR stepper motor.
7. Draw any two drive circuits for stepper motor.

UNIT-II 2. 44