ELECTRICAL MACHINE II EEE303

Stepper MotorsPresentation

OUT LINE

• Introduction• Working principle• Types of stepper motors• Applications• Conclusion

Introduction

A stepper motor (or step motor) is a brushless DC electric motor that divides a full rotation into a number of equal steps.

The name stepper is used because this motor rotates through a fixed angular step in response to each input current pulse received by its controller.

Their popularity is due to the fact that they can be controlled directly by computers, microprocessors and programmable controllers(PLC).

Introduction

To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step.

N.B torque ~ current

as the motor speeds up, less and less time is spent at full current.

As speeds further increase, the current will not reach the rated value, and eventually the motor will cease to produce torque.

Pictures of some stepper motors

Working principle

The unique feature of a stepper motor is that its output shaft rotates in a series of discrete angular intervals or steps,

Such motors develop torques ranging from 1 µN-m (in a tiny wrist watch motor of 3 mm diameter) up to 40 N-m in a motor of 15 cm diameter suitable for machine tool applications.

Their power output ranges from about 1 W to a maximum of 2500 W. the only moving part in a stepping motor is its rotor which has no windings, commutation or brushes.

This feature makes the motor quite robust and reliable

Stepper motor specifications

Steppers should be sized according to published torque curve, which is specified by the manufacturer at particular drive voltages or using their own drive circuitry.

Step motors adapted to harsh environments are often referred to as IP65 rated

Stepper motor system

A stepper motor system consists of three basic elements,

Indexers - The indexer (or controller) is a microprocessor capable of generating step pulses and direction signals for the driver.

Drivers - The driver (or amplifier) converts the indexer command signals into the power necessary to energize the motor windings.

.Stepper motors - The stepper motor is an electromagnetic device that

converts digital pulses into mechanical shaft rotation.

Merits and demerits of stepper motors

• Advantage

low cost, high reliability, high torque at low speeds

and a simple, rugged

construction that operates in almost any environment.

• Disadvantage

resonance effect often exhibited at low speeds

decreasing torque with increasing speed.

Current waveforms

• A stepper motor is a poly_phase AC synchronous motor and it is ideally driven by sinusoidal current.

• A full step waveform is a gross approximation of a sinusoid, and is the reason why the motor exhibits so much vibration..

Continue…Step Angle The angle through which the motor shaft rotates for each command pulse is called

the step angle β. The most common step sizes are 1.80 , 2.50 , 7.50 and 150 . The value of step angle can be expressed either in terms of the rotor and stator

poles (teeth) Nr and Ns respectively or in terms of the number of stator phases (m) and the number of rotor teeth.

The number of mechanical degrees corresponding to a given number of electrical degrees is

continue… Stepping motors are designed to operate for long periods with

the rotor held in a fixed position and with rated current flowing in the stator windings.

If f is the stepping frequency (or pulse rate) in pulses per second (pps) and β is the step angle, then motor shaft speed is given by

n = β × f/ 360 rps = pulse frequency resolution

Simple Stepper motor and its associated control unit

Two-pole three-phase stator with a permanent-magnet rotor. If a dc voltage is applied to phase a of the stator and no voltage is applied to phases b and c, then a torque will be induced in the rotor which causes it to line up with the stator magnetic field B.

(a) sketch of the output voltage from the control unit as a series of control pulses are input. (b) A 2-nd table showing the output voltage from the control unit as a function of pulse number.

In general, if a stator has N phases, it takes 2N pulses per electrical revolution in that motor. Therefore, the relationship between the speed of the motor in revolutions per minute and the number of pulses per minute becomes

Types of Stepper Motors

stepper motors can be divided into the following 3 basic categories :1. Permanent Magnet Stepper Motor2. Variable Reluctance Stepper Motor3. Hybrid Stepper Motor

Permanent magnet motors use a permanent magnet (PM) in the rotor and operate on the attraction or repulsion between the rotor PM and the stator electromagnets.

Variable reluctance (VR) motors have a plain iron rotor and operate based on the principle that minimum reluctance occurs with minimum gap, hence the rotor points are attracted toward the stator magnet poles.

Hybrid stepper motors are named because they use a combination of PM and VR techniques to achieve maximum power in a smallpackage size.

Applications

. Such motors are used for operation control in computer peripherals textile industry IC fabrications and robotics etc.

• Applications requiring incremental motion are typewriters, line printers, tape drives, floppy disk drives, numerically-controlled machine tools, process control systems

and X-Y plotters.

Conclusion

• Any Questions just Google it!!! thanks for your attention!!!