chapter-2 principle-operation-applications of different...

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CHAPTER-2

Principle-Operation-Applications of Different Stepper Motors and

Significance of Permanent Magnet Hybrid Stepper Motor

2.1 Introduction

Stepper motor is defined [1] as a brushless dc motor whose rotor rotates in discrete

angular increments 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 stator windings. The rotor has no electrical windings, but has salient and / or

magnetized poles.

A stepper motor moves in increments, or steps, rather than turning smoothly as a conventional

motor. The size of the increment is measured in mechanical degrees and can vary depending on

the application. A stepper motor is a constant output power transducer and its torque is inversely

proportional to the motor speed. In this chapter different types of stepper motors and their

applications are discussed. Here the significance of PMH stepper motor is discussed among other

types of stepper motors. Constraints in the design of PMH stepper motor are also discussed.

Finally the reasons for the selection of PMH stepper motor for design and how to overcome the

constraints in its design are explained.

2.2 Stepper Motor Advantages, Disadvantages and Applications

2.2.1 Advantages and Disadvantages of Stepper Motors

The Advantages and disadvantages of stepper motor [2] are as mentioned below.

Advantages

1. The rotation angle of the motor is proportional to the input pulse.

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2. The motor has full torque at standstill (if the windings are energized).

3. It requires little or no maintenance.

4. It has excellent response to starting/stopping/reversing.

5. It is highly reliable since there are no contact brushes in the motor.

6. The life of the motor is simply dependant on the life of the bearing.

7. The motor’s response to digital input pulses provides open-loop control, making the

motor less costly and simpler to control.

8. It is possible to achieve very low speed synchronous rotation with a load that is

directly coupled to the shaft.

9. A wide range of rotational speeds can be realized as the speed is proportional to the

frequency of the input pulses.

Disadvantages

1. Resonances can occur in the motor if it is not properly controlled.

2. It is not easy to operate motor at very high speeds.

3. It has limited ability to handle large inertia load.

2.2.2 Applications of Stepper Motor

Stepper motors are used in a wide variety of applications in industry, including computer

peripherals, business machines, solar array tracking system, motion control and robotics [2]

which are included in process control and machine tool applications.

2.2.2 (A) Computer Peripherals

Table 2.1 shows applications of stepper motor for different computer peripherals like

floppy disk, printer, recorder and plotter etc.

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Table 2.1 Applications of stepper motor for different computer peripherals

Application Use

Floppy Disk Position Magnetic Pickup

Printer Carriage Drive

Printer Rotate Character Wheel

Printer Paper Feed

Printer Ribbon Wind/Rewind

Printer Position Matrix Print Head

Tape Reader Index Tape

Plotter X-Y-Z Positioning

Plotter Paper Feed

2.2.2 (B) Business Machines

Table 2.2 shows applications of stepper motor for different business machines like card

reader, copy machine, banking systems, automatic type writers and card sorter etc.

Table 2.2 Applications of stepper motor for different business machines

Application Use

Card Reader Position Cards

Copy Machine Paper Feed

Banking Systems Credit Card Positioning

Banking Systems Paper Feed

Typewriters (automatic) Head Positioning

Typewriters (automatic) Paper Feed

Copy Machine Lens Positioning

Card Sorter Route Card Flow

2.2.2 (C) Process Control

Table 2.3 shows applications of stepper motor for different process control applications

like adjustments for air-fuel mixture in carburetor, valve control, process gaze, I.C. Bonding,

laser trimming etc.

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Table 2.3 Applications of stepper motor for different process controls

Application Use

Carburetor Adjusting Air-fuel Mixture Adjust

Valve Control Fluid Gas Metering

Conveyor Main Drive

In-Process Gazing Parts Positioning

Assembly Lines Parts Positioning

Silicon Processing I. C. Wafer Slicing

I. C. Bonding Chip Positioning

Laser Trimming X-Y Positioning

Liquid Gasket Dispensing Valve Cover Positioning

Mail Handling Systems Feeding and Positioning Letters

2.2.2 (D) Machine Tool

Table 2.4 shows applications of stepper motor for different machine tool applications like

milling, drilling, grinding, laser cutting, sewing, mail handling etc.

Table 2.4 Applications of stepper motor for different machine tools

Application Use

Milling Machines X-Y-Z Table Positioning

Drilling Machines X-Y Table Positioning

Grinding Machines Down-feed Grinding Wheel

Grinding Machines automatic wheel dressing

Electron Beam Welder

X-Y-Z Positioning

Laser Cutting X-Y-Z Positioning

Lathes X-Y Positioning

Sewing Chip Positioning

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2.3 Types of Stepper Motors

Stepper motors are classified according to their rotor design and stator winding type

[3-10]. There are three basic types of stepping motors according to the rotor design, namely

variable reluctance Stepper motors (VR), permanent magnet stepper motors (PM) and permanent

magnet hybrid (PMH) stepper motors. Permanent magnet motors have a permanent magnet rotor,

while variable reluctance motors have salient pole soft-iron rotors. Hybrid stepping motors

combine features of both permanent magnet and variable reluctance motors technology. Both

permanent magnet and hybrid stepper motors are classified according to their stator winding as

unipolar winding motors, bipolar windings motors and bifilar winding motors.

2.3.1 Variable Reluctance Stepper Motor

Variable Reluctance Motors (also called variable switched reluctance motors) have three

to five windings connected to a common terminal. Fig. 2.1 shows the cross section of a three

winding, 300 per step variable reluctance motor. The rotor in this motor has four teeth and the

stator has six poles, with each winding wrapped around opposing poles.

Fig. 2.1 Cross section of a three winding 6/4 pole, 300 per step variable reluctance stepper motor

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The rotor teeth are attracted to winding 1 when it is energized. This attraction is caused

by the magnetic flux path generated around the coil and the rotor. The rotor experiences a torque

and moves the rotor in line with the energized coils, minimizing the flux path. The motor moves

clockwise when winding 1 is turned off and winding 2 in energized. The rotor teeth are attracted

to winding 2. Continuous clockwise motion is achieved by sequentially energizing and

de-energizing windings around the stator.

2.3.2 PM Stepper

The permanent-magnet stepper motor operates on the reaction between a permanent-

magnet rotor and an electromagnetic field. Fig. 2.2 shows a basic two-pole PM stepper motor.

The rotor shown in Fig. 2.2 (a) has a permanent magnet mounted at each end. The stator is

illustrated in Fig. 2.2 (b). Both the stator and rotor are shown as having teeth. The teeth on the

rotor surface and the stator pole faces are offset so that there will be only a limited number of

rotor teeth aligning themselves with an energized stator pole. 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. Greater the number of teeth, smaller the step angle for the motor.

Fig. 2.2 Two pole permanent magnet stepper motor; (a) Rotor; (b) Stator

When a PM stepper motor has a steady DC signal applied to one stator winding, the rotor

will overcome the residual torque and lineup with that stator field. The

defined as the amount of torque required to move the rotor one full

An important characteristic of the PM stepper motor is that it can maintain the holding torque

indefinitely when the rotor is stopped. When no power is applied to the windings, a small

magnetic force is developed between th

is called a residual, or detent torque. The detent torque can be noticed by turning a stepper motor

by hand and is generally about one

magnet stepper motor with four stator windings. By giving pulses the stator coils in a desired

sequence, it is possible to control the speed and direction of the motor.

diagram for the pulses required to rotate the PM stepper motor. This sequence

negative pulses causes the motor shaft to rotate counterclockwise in 90° steps.

Fig. 2.3 PM stepper motor excitation for 90

More recent development in PM stepper motor technology is the thin

type of stepper motor dissipates much less power in losses such as heat than the cylindrical rotor

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will overcome the residual torque and lineup with that stator field. The

defined as the amount of torque required to move the rotor one full step with the stator energized.



magnetic force is developed between the permanent magnet and the stator. This magnetic force


by hand and is generally about one-tenth of the holding torque. Fig. 2.3

pper motor with four stator windings. By giving pulses the stator coils in a desired

sequence, it is possible to control the speed and direction of the motor.

diagram for the pulses required to rotate the PM stepper motor. This sequence


motor excitation for 900 step

ore recent development in PM stepper motor technology is the thin



will overcome the residual torque and lineup with that stator field. The ‘holding torque’ is

step with the stator energized.



e permanent magnet and the stator. This magnetic force


Fig. 2.3 shows a permanent

pper motor with four stator windings. By giving pulses the stator coils in a desired

sequence, it is possible to control the speed and direction of the motor. It shows the timing

diagram for the pulses required to rotate the PM stepper motor. This sequence of positive and


ore recent development in PM stepper motor technology is the thin-disk rotor. This


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and as a result, it is considerably more efficient. Efficiency is a primary concern in industrial

circuits such as robotics, because a highly efficient motor will run cooler and produce more

torque or speed for its size. Thin-disk rotor PM stepper motors are also capable of producing

almost double the steps per second of a conventional PM stepper motor. Fig. 2.4 shows the basic

construction of a thin-disk rotor PM motor. The rotor is constructed of a special type of cobalt-

steel, and the stator poles are offset by one-half of a rotor segment.

Fig. 2.4 Thin-disk type rotor PM stepper motor

2.3.3 Unipolar Permanent Magnet Stepper Motor

Unipolar permanent magnet stepper motors are equipped of two windings, each with a

center tap. The center taps are either brought outside the motor as two separate wires or are

connected to each other internally and brought outside the motor as one wire. As a result,

unipolar motors have 5 or 6 wires. Regardless of the number of wires, unipolar motors are driven

in the same way. The center tap wire(s) is tied to a power supply and the ends of the coils are

alternately grounded. Unipolar stepping motors operate by attracting the north or south poles of

the permanently magnetized rotor to the stator poles. Thus, in these motors, the direction of the

current through the stator windings determines which rotor poles will be attracted to which stator

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poles. Current direction in unipolar motors is dependent on which half of a winding is energized.

Physically, the halves of the windings are wound parallel to one another. Therefore, one winding

acts as either a north or south pole depending on which half is powered. Fig. 2.5 shows the cross

section of a 300 per step unipolar permanent magnet stepper motor. Current flowing through the

center tap of winding 1 to terminal ‘a’ cause top stator pole as north pole and the bottom stator

pole as a south pole. This attracts the rotor into the position shown. If the power to winding 1 is

disconnected and winding 2 is energized, the rotor will turn 300, or one step. To rotate the motor

continuously, apply power to the two windings in sequence.

Fig. 2.5 Cross section of a 300 per step unipolar permanent magnet stepper motor

2.3.4 Bipolar Permanent Magnet Stepper Motor

Bipolar permanent magnet motors are constructed with exactly the same mechanism as is

used in unipolar motors, but the two windings are wired more simply, with no center taps. Thus,

the motor itself is simpler but the drive circuitry required to reverse the polarity of each pair of

motor poles is more complex. The schematic diagram in Fig. 2.6 illustrates cross section of a 300

per step bipolar permanent magnet stepper motor and shows how such a motor is wired, while

the motor cross section shown here is exactly the same as the cross section shown of a unipolar

motor. Current flow in the winding of a bipolar motor is bidirectional. This requires changing the

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polarity of each end of the windings. The current will flow from left to right in winding 1 when

1a is positive and 1b is negative. Current will flow in the opposite direction when the polarity on

each end is swapped.

Fig. 2.6 Cross section of a 300 per step bipolar permanent magnet stepper motor

2.3.5 Bifilar Permanent Magnet Stepper Motor

Bifilar windings on a stepping motor are applied to the same rotor and stator geometry as

a bipolar motor, but instead of winding each coil in the stator with a single wire, two wires are

wound in parallel with each other. As a result, the motor has 8 wires, not four. To use a bifilar

motor as a bipolar motor, the two wires of each winding are connected either in parallel or in

series. The schematic diagram in Fig. 2.7 illustrates cross section of a 300 per step bifilar

permanent magnet stepper motor and shows how such a motor is wired. Winding 2 in Fig. 2.7 is

shown with a parallel connection; this allows low voltage high-current operation. Winding 1 in

Fig. 2.7 is shown with a series connection; if the center tap is ignored, this allows operation at a

higher voltage and lower current than would be used with the windings in parallel.

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Fig. 2.7 Cross section of a 300 per step bifilar permanent magnet stepper motor

2.4 Permanent Magnet Hybrid (PMH) Stepper Motor

2.4.1 Significance of PMH Stepper Motor

The motor having the permanent magnet rotor and multiple teeth both on the stator and

rotor poles, with excitation in stator poles is called the permanent magnet hybrid stepper motor.

The term hybrid is derived from the fact that the motor is operated under the combined principles

of permanent magnet and variable reluctance motors. Hybrid stepper motors are widely used in

space applications, office and factory automation applications. Hybrid stepper motors are highly

preferred in space applications as they can provide accurate positioning in open loop system. The

positional accuracy of the stepper motors will be high only when its step angle is very small.

Hence for space applications hybrid stepper motor is the best choice as it can offer small step

angles in the ranges of 0.50 to 1.80. The other classes of stepper motors such as variable

reluctance stepper motor and permanent magnet stepper motor will be suitable only for

applications which require large step angles.

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2.4.2 Working Principle of PMH Stepper Motor

The simple model of hybrid stepper motor is shown in Fig. 2.8 (for only the winding of

phase A) [11]. A larger number of teeth on the stator and rotor give a smaller basic step size. The

stator has a two-phase winding. Each phase winding consists of two sections. The stator has 8

poles each with 5 teeth, making a total of 40 teeth. If a tooth is placed in each of the zones

between the stator poles, there would be a total of 48 stator teeth. The rotor consists of an axially

magnetized PM located between two ferro-magnetic disks with 50 teeth per disk, two more than

the number of uniformly distributed stator teeth. There is a half-tooth displacement between the

two sections of the rotor. If rotor and stator teeth are aligned at 12 o'clock, they will also be

aligned at 6 o'clock. At 3 o'clock and 9 o'clock the teeth will be misaligned. However, due to the

displacement between the sets of rotor teeth, alignment will occur at 3 o'clock and 9 o'clock at

the other end of the rotor.

The windings are arranged in sets of four, and wound such that diametrically opposite

poles are the same. Referring to Fig. 2.8, the North poles at 12 o'clock and 6 o'clock attract the

South-pole teeth at the front of the rotor; the South poles at 3 o'clock and 9 o'clock attract the

North-pole teeth at the back. By switching current to the second set of coils, the stator field

pattern rotates through 450 but to align with this new field the rotor only has to turn through a

step angle of 1.80 mechanical (θm) as shown in eqn (2.1)

θm = ��

�� . �� (2.1)

For 2-phase motor having 50 rotor teeth per disk, step angle is 1.80 mechanical. This is

equivalent to one quarter of a tooth pitch or 7.20 mechanical on the rotor, giving 200 full steps

per revolution. There are as many detent positions as there are full steps per revolution, normally

200. The detent positions correspond to rotor teeth being fully aligned with stator teeth. When

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the power is applied, there is usually current in both phases (zero phase state). The resulting rotor

position does not correspond with a natural detent position, so an unloaded motor will always

move by at least one half a step at power-on. If the system is turned off other than at the zero

phase state, or if the motor is moved in the meantime, a greater movement may be seen.

Fig. 2.8 Hybrid stepper motor performing 200 steps per revolution: (a) Cross Section

(b) Rotor.1- Stator core 2- Permanent magnet (PM) 3- Ferro magnetic rotor disk with teeth 4- Shaft

For a given current pattern in the windings, there are as many stable positions as there

are rotor teeth (50 for a 200-step motor). If a motor is desynchronized, the resulting positional

error will always be a whole number of rotor teeth or a multiple of 7.20 mechanical. Torque is

created in the hybrid motor by the interaction of the magnetic field of the permanent magnet and

the magnetic field produced by the stator.

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2.5 Constraints in the Design of PMH Stepper Motor

2.5.1 Double Slotting

The design of a hybrid stepper motor is not easy because of the complex air gap geometry

which results in complex air gap permeance variation unlike that of conventional ac motors such

as induction motor and synchronous motor using equivalent magnetic circuit analysis [12].

Hybrid stepper motor has many variations in the stator-rotor teeth configuration and that the

operating characteristics, especially the torque characteristics, are significantly affected by the

teeth and winding arrangements.

2.5.2 Presence of Permanent Magnet on Rotor and Saturation Effect

When permanent magnet is placed between rotor disks, investigation of operating point

for the permanent magnet flux before and after excitation of stator coil is very difficult by

equivalent magnetic circuit model [13, 14].When there are two magnetic circuits both in stator

and rotor, it is very difficult to estimate saturation limits with equivalent circuit model of PMH

stepper motor complex geometry.

2.6 Summary

Stepper motors are discrete torque motors used for different industrial applications in

different fields. They are categorized according to their rotor. PMH stepper motor is widely used

stepper motor because of its narrow stepping angle, good steady-state and dynamic

performances. Design and analysis of PMH stepper motor is difficult with equivalent circuit

model. Finite element method (FEM) is used to overcome the constraints in the design and

analysis of the PMH stepper motor.

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