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Presented by
Dr.P.SUNDARARAMAN ,
Assistant professor -EECE,
GITAM University ,
Bangalore.
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CONTENTS
● Introduction
● How Stepper Motors Work
● Types of Stepper Motors
● Construction
● Design Considerations
● Torque Prediction
● Characteristics
● Drive Circuits
● Comparison
● Applications
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INTRODUCTION
Basic Stepper Motor System
• Controller, also known as a pulse generator or indexer. The
controller is a microprocessor capable of generating step pulses
and direct signals for the driver.
• Driver (or Amplifier) converts the controller command signals
into the power necessary to energize the motor windings.
• Step (or stepping or stepper) Motor is an electromagnetic device
that converts digital pulses into mechanical shaft rotation.
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ABOUT STEP MOTOR…
• 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.
• The sequence of the applied pulses is directly related to the
direction of motor shafts rotation.
• The speed of the motor shaft rotation is directly related to
the frequency of the input pulses applied.
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SIMPLE APPLICATION
Each time the controller
receives an input signal, the
paper is driven a certain
incremental distance.
Paper drive mechanism
using stepper machine
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HOW STEPPER MOTORS WORK
Function:
• To move a rotor through a
precise angular step when the
current in one or more of the
stator windings is switched on.
• The rotor may be driven at a
high speed by switching the
currents rapidly;
• However, the controlled quantity
is angular rotor position rather
than its velocity
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FULL STEPPING
(One Phase ON)
00 900
1800
2700
on
off
off
off
a.4
off
on
off
off
a.3
off
off
on
off
a.2
off
off
off
on
a.1
Coil 1
Coil 2
Coil 3
Coil 4
Step
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FULL STEPPING
(Two Phase ON)
450 1350
2250 3150
on
off
off
on
b.4
on
on
off
off
b.3
off
on
on
off
b.2
off
off
on
on
b.1
Coil 1
Coil 2
Coil 3
Coil 4
Step
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HALF STEPPING
on
off
off
on
b.4
on
off
off
off
a.4
on
on
off
off
b.3
off
on
off
off
a.3
off
on
on
off
b.2
off
off
on
off
a.2
off
off
on
on
b.1
off
off
off
on
a.1
Coil 1
Coil 2
Coil 3
Coil 4
Step
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MULTIPOLE MOTOR
• In position 1, the north
pole of the rotor's
permanent magnet is
aligned with the south
pole of the stator's
electromagnet.
• In position 2, the upper electromagnet is deactivated and the
next one to its immediate left is activated, causing the rotor to
rotate a precise amount of degrees. After eight steps the
sequence repeats.
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TYPES OF STEPPER MOTORS
STEPPER MOTORS
With Permanent Magnet Without Permanent Magnet
Claw Pole
(PM)
Hybrid
(PMH)
Enhanced
Hybrid
(EHYB)
Disc Magnet
(DM)
Variable Reluctance
(VR)
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VARIABLE RELUCTANCE
(VR) STEPPER MOTOR
● The variable-reluctance (VR) stepper motor differs from the
PM stepper in that it has no permanent-magnet rotor and no
residual torque to hold the rotor at one position when turned
off.
● When the stator coils are energized, the rotor teeth will align
with the energized stator poles.
● This type of motor operates on the principle of minimizing
the reluctance along the path of the applied magnetic field.
● By alternating the windings that are energized in the stator,
the stator field changes, and the rotor is moved to a new
position.
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VR STEPPER MOTOR (Cont..)
Construction
• This type of construction is good in non industrial applications
that do not require a high degree of motor torque.
• The stator of a VR stepper
motor has a magnetic core
constructed with a stack of
steel laminations.
• The rotor is made of
unmagnetized soft steel
with teeth and slots.
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VR STEPPER MOTOR (Cont..)
Principle of Operation
• Current applied to pole 1 through the motor winding causes a
magnetic attraction that aligns the rotor (tooth) to pole 1.
• Energizing stator pole 2 causes the rotor to rotate 30 degrees in
alignment with pole 2.
• This process will continue with pole 3 and back to 1 in a clockwise
direction. Reversing the procedure (3 to 1) would result in a
counterclockwise rotation.
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The relationship among step angle, rotor teeth,
and stator teeth is expressed using the following
equation:
Where,
ψ – Step angle
Ns – Number of stator teeth
Nr – Number of rotor teeth
Example
Determine the step angle of a Variable Reluctance stepper motor with 12
teeth in the stator and 8 rotor teeth.
Here, Ns = 12; Nr = 8;
Step angle = = 150
17. MULTISTACK STEPPER MOTOR
●The VR stepper motors mentioned up to this point are all single-
stack motors. That is, all the phases are arranged in a single stack,
or plane.
●The disadvantage of this design for a stepper motor is that the
steps are generally quite large (above 15°).
●Multi-stack stepper motors can produce smaller step sizes because
the motor is divided along its axial length into magnetically
isolated sections, or stacks.
●Each of these sections is excited by a separate winding, or phase.
●In this type of motor, each stack corresponds to a phase, and the
stator and rotor have the same tooth pitch.
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PERMANENT MAGNET (PM)
STEPPER MOTOR
● The permanent magnet motor,
also referred to as a "canstack"
motor.
● It's simple construction and low
cost make it an ideal choice for
non industrial applications such
as a line printer print wheel
positioner.
+12v dc, four-phase, unipolar, permanent magnet, 3.6° per step
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PM STEPPER MOTOR (Cont..)
Construction
• The rotor has a permanent magnet mounted at each end.
• The number of teeth on the rotor and stator determine the step
angle that will occur each time the polarity of the winding is
reversed.
• The greater is the number of teeth, the smaller is the step angle.
20. 20
PM STEPPER MOTOR (Cont..)
Full Step Mode
• This animation demonstrates the
principle for a stepper motor using
full step commutation.
• The rotor of a permanent magnet
stepper motor consists of
permanent magnets and the stator
has two pairs of windings.
• Just as the rotor aligns with one of
the stator poles, the second phase is
energized.
• The two phases alternate on and off
and also reverse polarity. There are
four steps.
21. 21
PM STEPPER MOTOR (Cont..)
Half Step Mode
• This animation shows the stepping
pattern for a half-step stepper
motor. The commutation sequence
for a half-step stepper motor has
eight steps instead of four.
• The main difference is that the
second phase is turned on before the
first phase is turned off.
• Thus, sometimes both phases are
energized at the same time.
• During the half-steps the rotor is held in between the two full-step
positions.
• A half-step motor has twice the resolution of a full step motor.
• It is very popular for this reason.
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DISC PM MOTOR
• This stepper motor dissipates much
less power in losses such as heat
than the cylindrical rotor and as a
result, it is considerably more
efficient.
• Thin-disk rotor PM stepper motors
are also capable of producing
almost double the steps per second
of a conventional PM stepper
motor.
• The rotor is constructed of a special
type of cobalt-steel.
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HYBRID STEPPER MOTOR
• Hybrid motors combine the best
characteristics of the variable
reluctance and permanent magnet
motors.
• They are constructed with multi-
toothed stator poles and a
permanent magnet rotor.
• Standard hybrid motors have 200 rotor teeth and rotate at 1.80
step angles. Other hybrid motors are available in 0.9ºand 3.6º
step angle configurations.
• Because they exhibit high static and dynamic torque and run at
very high step rates, hybrid motors are used in a wide variety of
industrial applications.
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MOTOR WINDINGS
Unipolar Winding
Figure 2.12
Step Q1 Q2 Q3 Q4
1 ON OFF ON OFF
2 OFF ON ON OFF
3 OFF ON OFF ON
4 ON OFF OFF ON
1 ON OFF ON OFF
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MOTOR WINDINGS (Cont..)
Bipolar Winding
Step Q2-Q3 Q1-Q4 Q6-Q7 Q5-Q8
1 ON OFF ON OFF
2 OFF ON ON OFF
3 OFF ON OFF ON
4 ON OFF OFF ON
1 ON OFF ON OFF
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Prepared by M.Srinivasan
BIPOLAR MOTOR
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MOTOR WINDINGS (Cont..)
MULTIPHASE WINDINGS
• In the context of 3-phase motors, these configurations would be
described as Delta and Y configurations, but they are also used
with 5-phase motors, as illustrated in Figure 1.5.
• Some multiphase motors expose all ends of all motor windings,
leaving it to the user to decide between the Delta and Y
configurations, or alternatively, allowing each winding to be
driven independently.
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Design Considerations
● Resistance -determines the current drawn by the motor, as
well as affects the motor's torque curve and maximum operating
speed.
● Inductance - A high inductance motor will provide a
greater amount of torque at low speeds and similarly the reverse
is true.
● Series, Parallel Connection
❖A series connection provides a
high inductance and therefore
greater performance at low speeds.
❖ A parallel connection will lower the
inductance but increase
the torque at faster speeds.
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Design Considerations (Cont..)
•Driver Voltage - The higher the output voltage from the
driver, the higher the level of torque vs. speed. Generally, the
driver output voltage should be rated higher than the motor
voltage rating.
•Motor Stiffness - By design, stepping motors tend to run
stiff. Reducing the current flow to the motor by a small percentage
will smooth the rotation. Likewise, increasing the motor current
will increase the stiffness but will also provide more torque.
•Motor Heat -Step motors are designed to run hot (50º-90º
C). However, too much current may cause excessive heating and
damage to the motor insulation and windings.
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TORQUE PREDICTIONS
Assumed that the magnetic circuit is linear (unsaturated).
Let
e(t) = voltage applied per stack
R = winding resistance per stack
L(θ) = winding inductance per stack (a function of rotor position
only and independent of coil current because of linear
magnetic circuit assumption)
i(t) = current per stack
θ(t) = angular position of rotor
Kirchoff’s mesh equation for stator winding is
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TORQUE PREDICTIONS(Cont..)
where λ = flux linkages of stator winding = i L(θ).
Therefore,
…………....(1)
Transformer emf speed emf
Energy stored in air gap is
W = ½ L(θ) i2(t)………………(2)
Mechanical torque developed is given by
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TORQUE PREDICTIONS(Cont..)
…………………....(3)
……………….(4)
Rotor dynamics is governed by
In a toothed structure, reluctance and therefore winding inductance
varies cosinusoidally (even function) as function of θ over and above
an average value, i.e.,
………………………….(5)
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TORQUE PREDICTIONS(Cont..)
Substituting in equation 3,
……………………………….(6)
This indeed is the reluctance torque and has sinusoidal form
compared to the torque-angle curve.
Equations 1, 6 and 4 govern the dynamic behavior of one stack of
a stepper motor under application of e(t), a pulse wave shape.
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STEPPER MOTOR
CHARACTERISTICS
Static Characteristics
Torque-Angle curve:
It is seen that the torque
increases, almost sinusoidally,
with angle θ from equilibrium
position.
T
TH
θ
θM
θs
Holding Torque (TH):
It is the maximum load
torque which the energized
stepper motor can withstand
without slipping from
equilibrium position.
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STATIC CHARACTERISTICS
(Cont..)
Détente Torque (TD):
• It is the maximum load torque
which is unenergized stepper motor
can withstand without slipping.
• Détente torque is due to
residual magnetism, and is ,
therefore, available only in PM
and Hybrid stepper motor.
• It is about 5 to 10 percentage of holding torque.
• It is typically a fourth harmonic torque.
• It is also known as cogging torque.
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STATIC CHARACTERISTICS
(Cont..)
Torque – Current Curve T
TH
TD
IRated I
Torque Constant (Kt):
It 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
unit is N-m/A.
The curve is linear but, later
on, its slope progressively decreases as
the magnetic circuit of the motor
saturates.
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DYNAMIC CHARACTERISTICS
Current - Time Curve
• An important consideration
in designing high-speed
stepping motor controllers is
the effect of the inductance of
the motor windings.
•The inductance of the motor winding determines the rise and fall
time of the current through the windings.
•The rise time is determined by the drive voltage and drive
circuitry, while the fall time depends on the circuitry used to
dissipate the stored energy in the motor winding.
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DYNAMIC CHARS. (Cont..)
Torque – Speed Characteristics
Torque
Speed
Slewing
mode
Normal
mode
Two distinct modes of operation:
• Locked-step (normal) mode
• Slewing mode
•In the first, the rotor comes to rest
between steps and the rotor can be
started, stopped, reversed
•Slewing mode does not allow stopping or reversal of the rotor,
although it advances in synchronism with the stepping sequence
(e.g. rewinding a tape drive)
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CHARACTERISTIC PARAMETERS
Curve A: Pull-out torque
Curve B: Pull-in torque
Torque
Speed
Slewing
mode
Normal
mode
Curve A
Curve B
Pull-in rate
Pull-out rate
Max pull-out rate
Max pull-in rate
• Pull-out torque: is the
maximum torque which can
be applied to a motor, running
at a given stepping rate,
without losing synchronism
• Pull-in torque: is the
maximum torque against
which a motor will start, at a
given pulse rate, and reach
synchronism without losing a
step.
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CHARACTERISTIC PARAMETERS
(Cont..)
• Pull-out rate: is the maximum switching rate at which a motor
will remain in synchronism while the switching rate is gradually
increased.
• Pull-in rate: is the maximum switching rate at which a loaded
motor can start without losing steps.
• Slew range: is the range of stepping ( or switching rates) between
pull-in and pull-out in which a motor will run in synchronism but
cannot start or reverse.
• Response Range: is the range of stepping rates at which the
stepper motor can start or stop, without losing synchronism at a
given torque, T.
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DRIVE CIRCUITS
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STEPPER MOTOR ADVANTAGES
• Low cost
• Ruggedness
• Simplicity in construction
• High reliability since there is no contact brushes in the motor
• No maintenance
• Wide acceptance
• No feedback components are needed
• They work in just about any environment
• Inherently more failsafe than servo motors.
• Excellent response to starting/ stopping/reversing.
• It is possible to achieve very low speed synchronous rotation
with a load that is directly coupled to the shaft.
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STEPPER MOTOR DISADVANTAGES
• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.
• Rough performance at low speed unless a micro-step drive is
used
• Liability to undetected position loss as a result of operating
open-loop
• They consume current regardless of load conditions and
therefore tend to run hot
• Losses at speed are relatively high and can cause excessive
heating, and they are frequently noisy (especially at high
speeds).
Many of these drawbacks can be overcome by the use of
a closed-loop control scheme.
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Stepper Motors Applications
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When to find the stepper motors ?
●A stepper motor 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.
Where to find the stepper motors ?
• Printers
• Plotters
• High end office equipment
• Hard disk drives
• Medical equipment
• Fax machines
• Automotive
• Machine tools
• Process control systems
• programmable controllers
and many more.
