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Tuesday, 17 March 2020

Stepper Motor Working, Types, Applications

STEPPER MOTORS:


Certain terminologies which are involved in the operations of stepper motors are given below:

a. Step - Angular rotation created by the output shaft all time the motor receives a step pulse.

b. Step angle - The angle measured in degrees for each rotation of the output shaft.  The step angle can be as small as 0.72° to as large as 90° in steps of 1.8°, 2.5°, 7.5° and 15°. It can be expressed in terms of rotor and stator poles (teeth) i.e., Nf and Ni

i.e. β = (Ns—Nf )/ ( Ns x Nf ) x 360
= 360/ No: of stator phase x No: of rotor teeth

c . Steps per second - The number of steps the motor takes per second.

d. Resolution - The total number of steps required for the output shaft to make one complete revolution i.e.,
No. of step / Revolution = 360°/β 

e. Holding torque - The torque required to rotate the output shaft to full step with the motor energized and in a standstill condition.

f. Step response - The time required for the motor to make one complete step. Time usually will be in milli seconds.

g. Start without error rate - The stepping at which a motor with external load inertia can start and stop without loosing a step.

h. Slew rate - The maximum step rate at which a motor will run under a rated load once the motor has started rotating. This is usually substantially greater than the start without error rate.

i. Ramping - A control method used to vary the pulse rate to accelerate from zero step per second to the running rate or from any step rate to a different rate whether it is accelerating or decelerating.


BASIC PRINCIPLE OF WORKING OF STEPPER MOTOR

Consider a motor consisting of a four step with two pole permanent magnet rotor as shown in Fig.
The rotor of such a motor aligns with the axis of the stator field with torque being proportional to the sin θ, where θ is the angle or displacement between the rotor axis and the stator field axis. The torque angle characteristic is shown in .Fig. with 'a' and ‘b’ coils excited. The rotor has assumed a position with θ = 45o, when 'b' above is excited the rotor takes a position θ = 90°. From this it can be observed that the stable position of the rotor corresponds to that angle at which the torque is zero and is positive for smaller angles and negative for larger angles. The torque has a maximum value at θ = 90°, Thus with phase 'a' excited, the stable position is θ = 0°. But not θ = 180° (unstable).

Therefore each excitation position corresponds to a unique position of the rotor. Another unique feature of permanent magnet stepper motor is that when excited it seeks a preferred position which offers an added advantage in certain applications.
Torque Angle Characteristics Permanent Magnet Stepper Motor
TYPES OF STEPPER MOTORS

The different types of stepper motors are :

a. Variable reluctance stepper motor
b. Permanent magnet stepper motor  
c. Hybrid stepper motor.

VARIABLE RELUCTANCE MOTORS

The simplest of electrical machines is variable reluctance machines generally termed as VRM. It consists of a stator with excitation winding and a magnetic rotor with salient poles. It can be either singly salient or doubly salient. In either case there are no windings or permanent magnets on their rotors. Fig. shows both the types of VRM. In Fig. (a) the cross sectional view of singly salient VRM is shown to consists of non-salient stator and two pole rotor, both constructed of high permeability magnetic material. In the figure two phase windings are shown although any number of phases is possible. The inductance of each of the stator phase winding varies with rotor position such that the inductance is maximum when the rotor axis is aligned with the magnetic axis of that phase and minimum when the two axes are perpendicular.

In Fig. (b) cross section of doubly salient two phase VRM is shown. The salient stator has four poles each with a winding. The windings on opposite poles are of the same phase and can be considered to be connected in either series or parallel. Hereto the phase inductance varies from a maximum value when the rotor axis is aligned with the axis of that phase to minimum when they are perpendicular.

VARIABLE RELUCTANCE STEPPER MOTOR

Fig.1 shows a variable reluctance motor with salient poles constructed from ferromagnetic material. The stator is made from stack of steel laminations and six equally placed poles each wound with an exciting coil. The rotor may be solid or laminated and has four projecting teeth of the same width as that of the stator. There are three independent stator circuit as shown in Fig.2 each one can be energized by a d.c. pulse from the drive circuit. The coils are excited in proper sequence and each phase has its own independent switch. Diametrically opposite pairs of the stator coils are connected in series such that when one tooth becomes north, the other becomes south. In practice the switching of phase currents is done with the help of solid state control. When there is no current in the stator coils, the rotor is free to rotate but when one or more stator coils are energized the rotor takes a step forward position to form least reluctance path with magnetized stator teeth. The step angle of three phase, four rotor teeth motor is given by

P = 360/4x3 = 30°

PERMANENT MAGNET STEPPER MOTOR


Fig (a) & (b) shows a 2-phase 4 pole permanent magnet stepper motor. The rotor is made of ferrite or any magnetic material which can be magnetized permanently. The stator stack of phase B is staggered from that of phase A by an angle of 90°. When phase A is excited the rotor is aligned as shown in Fig (a). If now phase B is also the effective stator pole shifts counter clockwise by 22 ½o causing the rotor to move by the same angle. If at this time, phase A is de-energized with phase B energized the rotor will move by another 22 ½o. The switching arrangement is shown in Fig compared to VRM, permanent magnet stepper motors operate at larger steps upto 90o at a maximum response rates at 300 pps. Permanent magnet stepper motors has a low power requirement but possesses a high detente torque as compared to a variable reluctance stepper motor. The motor has high inertia and so slower acceleration. It produces more torque per ampere stator current than a variable reluctance motor.
HYBRID STEPPER MOTOR :

It is a combination of permanent magnet stepper motor and variable reluctance stepper motor. The rotor consists of a permanent magnet that is magnetized axially to create a pair of poles as shown in Fig. Two end caps are fitted at both ends of these axial magnets which consists of equal number of teeth which are magnetized by the respective polarities of the axial magnets. The rotor teeth of one end cap are offset by a half tooth pitch so that a tooth at one end cap coincides with the slot at the other. The end views of Hybrid stepper motor shown in Fig at XX' and YY' are shown in Fig. (a) & (b).
It can be seen from Fig. (a) and (b) the stator consists of two pair of poles and the rotor has five poles. When excited at one end the rotor poles are N and at the other end S. The step angle of such a motor is,

Step angle = (5 - 4) x 360o/ (5 x 4) = 18o

A
B
θ
+
0
0o
0
+
18o
-
0
36o
0
-
54o
+
0
72o

When phase A is excited, the north pole of the rotor attracts the south pole as shown in Fig (a). Taking this as initial position, phase A is de-energized and simultaneously phase B is energized. The rotor would turn counter clockwise by a full step of 18o. Next phases A and B are energized one after the other to produce a further rotation of the rotor by 18° in the same direction. The truth table for such operation is shown in Figure. To reverse the direction of rotation the phase sequence should be A+, B-, A-, B+, A+ etc. Even fractional step can be obtained by suitably proportioning the excitation of the two phases. Such stepping is called micro stepping. Typical step angles for stepper motors are 15°, 7.5°, 2° and 0.72°. The choice of the angle depends upon the angular resolution required for application.

APPLICATIONS OF STEPPER MOTORS

Stepper motors are a digital device, its output being determined by the number of in-pulses. The motors can position itself at any desired value accurately depending on the input command which is in the form of pulses provided it is used in the response range. Simplicity in construction and economy in operation are the main advantages of the stepper motor. It is a device which is compatible to many of the applications. Some of the applications are given below :

a. In machine tool operations when the job demands different operations by different tools one after the other, the tool port has to rotate by a specific angle and move towards the job a distance which is pre-determined for each operation. At such places these motors serve its purpose.

b. In the X-Y plotter, the pen has to move specific distance on the axis accurately. These movements of the pen are controlled by these motors.

c. In computer application, the disc drives, the printer head movement and the paper feed movements has to be precisely accurate which are controlled by these motors.

d. In the case of the line printer these motors are used for driving the paper feed mechanism.

e. In military applications these motors are used in guns to position the target precisely in relation to the radar tracking.

Besides the above there are innumerable applications such as remote indicators, as positioners, etc. which are even expanding day by day.

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