EE3092 -Laboratory Practice V

STEPPER MOTOR

Instructor: - Miss. W.G. Pavithra

Name: - V.I.P. Dasanayake

Index Number: - 090075M

Group: - 3

Submission date: -27/02/2012

Micro processor excitation sequence for different modes of stepper motor operation

One phase on operation

Speed of rotor =148.3 rpmPulse width = 8ms

Two phase on operation

Speed of rotor =149 rpmPulse width = 16ms

Three phase on operation

Speed of rotor =74.5 rpmPulse width = 25ms

Basic step angle

Basic step angle

Calculations

1. Calculate basic Step Angle for 6 / 4 pole 3 phase & 8 / 6 pole 4 phase stepper motor using

manual switching sequence?

Considering 6/4 pole 3 phase stepper motor;

Figure 1

Figure 1 shows a basic arrangement of a 6/4 pole 3 phase motor. Since this has 6 number of stator poles and 4 number of rotor poles, with 4 numbers of phases, which is connected to stator poles.

Basic Step Angle=¿ 3600

(Rotor poles x Phase)

∴Basic Step Angle= 3604 ×3

=300

Considering 8/6 pole 4 phase stepper motor;

Figure 2

Figure 1 shows a basic arrangement of a 6/4 pole 3 phase motor. Since this has 6 number of stator poles and 4 number of rotor poles, with 4 numbers of phases, which is connected to stator poles.

Basic Step Angle=¿ 3600

(Rotor poles x Phase)

∴Basic Step Angle= 3606× 4

=150

2. Calculate the number of pulses required for the 6/4 pole 3 phase stepper to revolve one

revolution and verify the answer with the observed value?

Number of pulses required to revolve 1 revolution= Number of rotor poles Number of phases = 4 x 3 = 12

Since the observed valued for one revolution is also 12, the answer is verified.

3. Calculate the basic steps angle for the 8/6 pole 4 phase stepper by having speed and pulse

rate (Hz) of the motor?

Take,

rpm of the motor= r

frequency of pulses=f

Basic step angle ¿ 3600

pulses need for onerevolution= 3600

time for rotate onecycle × f--------------- (1)

Time for rotate one cycle (in seconds) = 60/r------------------ (2)

From (1) to (2)

Basic step angle= 3600

pulses need for onerevolution

=3600

(60 /r)× f

=6 ×r

f degrees

4. Calculate the basic steps angle for each operation?

Basic step angle for single phase on operations= 6×148×8×10−3= 7 .1040

Basic step angle for two phase operation = 6×149×16×10−3= 14 . 3040

Basic step angle for half step operation= 6×74 . 5×25×10−3= 11. 1750

Discussion

1. Discuss the detent torque and holding torque involve with different types of stepper motor?

Mainly there are 3 types of stepper motors, such as

1. Permanent Magnet Stepper

2. Hybrid Synchronous Stepper

3. Variable Reluctance Stepper

Detent torque

This is also called residual torque. Normally this can be seen in Permanent magnet stepper

motors. Detent torque use to rotate the motor shaft. When there is no power applied to the windings, a

small magnetic force will be induced between the permanent magnet and the stator. And it causes for

occurring the detent torque. This torque can be notice by turning a stepper motor by hand. Usually it

is about one-tenth of the holding torque.

When this torque is occurred, the rotor returns to the normal rest position even the excitation

ends. Those positions are called detent positions.

Holding torque

The torque which is used to move the rotor one full step with the stator energized. This is called

static torque as well. Holding torque is the maximum torque produce by the standstill motor. When it

comes to permanent magnet stepper motor, it is able to maintain this holding torque indefinitely, even

the rotor is stopped. As the motor is energize, it has high holding torque rating, so torque available to

running motor is become less. It can be clearly seen in the following figure.

Figure 1-stepper speed characteristics

Static Characteristics of a stepper motor with the holding torque and detent torque:

Torque displacement characteristics curve Torque current characteristics curve

Torque-displacement characteristics: This curve shows the relationship electromagnetic torque and

displacement angle (0 from steady state position)

Torque-current characteristics: This curve shows the relationship between holding toque and the

current. Holding torque increases with the exiting current.

Relationship between holding torque and the detent torque:

Holding torque is not a constant value. It varies as the shaft moves a full step. The

reason for this would be the detent torque, which adds and subtracts from the electrically induced torque

as the motor shaft moves. Here the detent torque can be considered as sinusoidal changing torque,

which repeats with every full step of the motor

2. Discuss the main points of various modes of operation of the stepper motor?

There are 4 types of stepping modes for a stepper motor but only 3 of the excitation modes are very

common in stepper motors.

A. Full step

B. Half step

C. Micro step

Full step: - consider a two phase stepper motor. Here motor energized both phases constantly to have

the full rated torque at all positions of the motor. Since it has 4 rotor teeth, or 4 full step for every

revolution, dividing the 4 steps by 3600, we can have 900 full step angle. Full step mode can be

achieved by energizing both windings, while reversing the current in alternatively. For full step

Figure 2-Torque displacement characteristics curve Figure 3-Torque current characteristics curve

operation a unipolar motor driver, energizes a single phase. For bipolar stepper motor it energized both

coils to make a full step (an example of full step is shown in the following images)

 

Above explanation is for two phase motor. Since there are more types of stepper motors

(considering number of phases), such as three phase stepper motor, four phase stepper motor…etc,

same theory can be applied to those motor types as well. The only difference would be there will

be more coils.

Half step: - if we consider a two phase motor, there will be 8 steps for a one revolution. It is done by,

energizing one winding, while other 2 windings are energized alternatively. So it can rotate half a

distance or 450. Half step mode gives a smoother motion than a full step mode of operation. This

provides approximately 30% less torque than full step mode. It can be demonstrate with following

images. (And similar theory can be consider for other stepper motors as well such as, three phase

stepper motors, four phase stepper motors...etc)

Figure 4- single coil full step operation

Sequence of the energizing coils would be1, 2, 3, 4, 1, 2, 3…

Sequence of the energizing coils would be1&4, 4&3, 3&2, 2&1, 1&4, 4&3, 3&2…

Figure 5-Dual core full step mode

Micro step: - This is a new technology, which control the motor winding current to a degree such that,

it causes for subdivides the number of positions between poles. As an example Omegamation micro

stepping drives have a full step (1.8º) into 256 micro steps, resulting in 51,200 steps for every

revolution (.007º/step). This is normally used in application where accurate, smooth poisonings are

required highly. This also provides approximately 30% less torque than full step mode like half step

mode.

References

http://en.wikipedia.org/wiki/Stepper_motor

http://zone.ni.com/devzone/cda/ph/p/id/287

http://mediatoget.blogspot.com/2011/06/important-definitions-in-single-phase.html

http://www.sapiensman.com/ESDictionary/docs/d6.htm

http://turboblogsite.com/stepper-motors-guide.html

http://www.omega.com/prodinfo/stepper_motors.html

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