STEPPER MOTOR

STEPPER MOTOR

A Stepper Motor or a step motor is a brushless, synchronous motor which divides a full rotation into a number of steps. Unlike a brushless DC motor which rotates continuously when a fixed DC voltage is applied to it, a step motor rotates in discrete step angles. The Stepper Motors therefore are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step. The stepper motor can be controlled with or without feedback.

WORKING PRINCIPLE:

• The unique feature of a stepper motor is that the shaft rotates in definite steps, one step being taken each time a command pulse is received. When a definite number of pulses are supplied, the shaft rotates through a definite known angle. The rotor of a stepper motor is gear shaped and it can be of ferromagnetic material or permanent magnet. Multiple toothed poles on which field coils are wound are arranged around the gear shaped rotor. The stator poles are magnetized in the appropriate manner by using a micro-controller or microprocessor or by other means.
• First, one pole is magnetized by supplying the corresponding field coil. This toothed pole then aligns the rotor teeth due to magnetic attraction. Rotor teeth are slightly offset from the next pole.
• At the next step, first pole is demagnetized and the second is magnetized. This causes the rotor to rotate in a fixed angle to align with the second pole and offset with the previous pole.
• This was the basic working principle of a stepper motor. The rotor can be made multiple stacked to achieve more steps. Also, different types of stepping (like full step, half step or micro step) can be used for achieving more steps.

 

Classification of Stepper Motors:

Based on the type of construction stepper motors can be classified as,

• Variable Reluctance (VR) stepper motor
• Permanent Magnet (PM) stepper motor
• Hybrid stepper motor

Variable Reluctance (VR) stepper motor:

• The variable reluctance stepper motors are those which have a rotor made of ferromagnetic substances. Hence when the stator is excited it becomes an electromagnet and the rotor feels a pull in that direction. The ferromagnetic substance always tries to align itself in the minimum reluctance path.
• By exciting the coils, a magnetic field is procured and air gap reluctance is varied. Hence it is called a variable reluctance stepper motor. In this motor, the direction of the motor is independent of the direction of the current flow in the windings.

Permanent Magnet (PM) stepper motor:

• Here the rotor is permanently magnetized. Hence, the movement of the motor is due to the attraction and repulsion between the stator and rotor magnetic poles.
• In this motor, the direction of the motor is directly dependent of the direction of the current flow in the windings as the magnetic poles are reversed my changing the direction of the current flowing through the rotor.

Hybrid stepper motor:

• The hybrid stepper motor, as the name suggest is a motor designed to provide better efficiency by combining the pros of both the permanent magnet stepper motor and variable reluctance stepper motor.
• The VR and PM stepper motors are the most common type of stepper motors. The only difference is that, in the variable reluctance stepper motor, the rotor is made of a ferromagnetic substance and in the case of permanent magnet stepper motor, the rotor is permanently magnetized.

ADVANTAGES:

• Positioning – Since steppers move in precise repeatable steps, they excel in applications requiring precise positioning such as 3D printers, CNC, Camera platforms and X ,Y Plotters. Some disk drives also use stepper motors to position the read/write head.
• Speed Control – Precise increments of movement also allow for excellent control of rotational speed for process automation and robotics.
• Low Speed Torque - Normal DC motors don't have very much torque at low speeds. A Stepper motor has maximum torque at low speeds, so they are a good choice for applications requiring low speed with high precision.

DISADVANTAGES:

• Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.
• Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.
• No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Applications:

•      Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated production equipments.
•        Security – new surveillance products for the security industry.
• .      Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery.
•     Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom functions.