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Stepper motor


Introduction:

In solar tracking application we must select the motor that can carry photo voltic panel towards the sun any type of motor can used.

Using induction motor for example is a choice but we will use gear box and complex control circuit to control induction motor.

For that we will use a stepper dc motor since the application discrete mechanical movements so we prefer using stepper 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 motors rotation has several direct relationships to these applied input pulses. The sequence of the applied pulses is directly related to the direction of motor shafts rotation. The speed of the motor shafts rotation is directly related to the frequency of the input pulses and the length of rotation is directly related to the number of input pulses applied.

Advantages

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

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

3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to the next.

4. Excellent response to starting/ stopping/reversing.

5. Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependant on the life of the bearing.

6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.

7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.

8. 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 if not properly controlled.

2. Not easy to operate at extremely high speeds.


Open Loop Operation

One of the most significant advantages of a stepper motor is its ability to be accurately controlled in an open loop system. Open loop control means no feedback information about position is needed. This type of control eliminates the need for expensive sensing and feedback devices such as optical encoders. Your position is known simply by keeping track of the input step pulses.


Motor Characteristics

There are a few characteristics common to all motors that you should keep in mind when looking for motors:

Voltage

The rated voltage of a motor is the voltage at which it operates at peak efficiency. Most DC motors can be operated somewhat above or below their range, but it’s best to plan to operate them at their rated voltage. Dropping below rated voltage reduces the motor’s power, and operating above the rated voltage may burn the motor out. Plan on the motor’s top speed being at rated voltage, and slowest speed at no more than 50% less than the rated voltage.

Current

Motors draw current depending on the load they’re pulling. Usually more load means more current. Every motor has a stall current, which is the current it draws when it’s stopped by an opposing force. This stall current is much greater than the running current, or current that it draws under no load. Your power supply for a motor should be able to handle the stall current with extra amperage to spare. Motors may draw near the stall current for a brief period of time when starting up, to overcome their inertia.

Speed

Motor speed is given in revolutions per minute (RPM’s).

Torque

Torque is the measure of a motor’s pulling force. It’s measured by the force a motor can pull when the opposing force is attached to a shaft attached to its center rod. If the shaft sticks out a foot from the motor’s center, and the motor can pull one pound on that shaft, the motor’s torque is one foot-pound. Motor manufacturers haven’t standardized this measurement, so sometimes you will see it as ft.-lb., lb-ft., oz.-in, in.-oz., g-cm (gram-centimeter), and any other weight to length variation you can think of.

Resistance

Often you’ll see a motor rated in ohms. This just gives you the resistance that the motor’s coil offers. Using Ohm’s Law (voltage = current x resistance), you can calculate the motor’s current draw if you know the rated voltage and the coil resistance.



Stepper Motor Types

There are three basic stepper motor types. They are :

• Variable-reluctance

• Permanent-magnet

• Hybrid


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