AC Motor,Servo Motor and Stepper Motor

AC Motor Servo Motor Stepper Motor

By: Ahmed Sayed Zaghloul

AC Motors AC motors are used worldwide in many applications to transform electrical energy into mechanical energy. There are many types of AC motors, but this course focuses on threephase AC induction motors, the most common type of motor used in industrial applications. An AC motor of this type may be part of a pump or fan or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer. Siemens manufactures a wide variety of AC motors. In addition to providing basic information about AC motors in general, this course also includes an overview of Siemens AC motors.

AC Motor Construction Three-phase AC induction motors are commonly used in industrial applications. This type of motor has three main parts, rotor, stator, and enclosure. The stator and rotor do the work, and the enclosure protects the stator and rotor.

Stator Core The stator is the stationary part of the motor’s electromagnetic circuit. The stator core is made up of many thin metal sheets,called laminations. Laminations are used to reduce energy loses that would result if a solid core were used.

Stator Windings Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.

When the assembled motor is in operation, the stator windings are connected directly to the power source. Each grouping of coils, together with the steel core it surrounds, becomes an electromagnet when current is applied. Electromagnetism is the basic principle behind motor operation.

Rotor Construction The rotor is the rotating part of the motor’s electromagnetic circuit. The most common type of rotor used in a three-phase induction motor is a squirrel cage rotor. Other types of rotor construction is discussed later in the course. The squirrel cage rotor is so called because its construction is reminiscent of the rotating exercise wheels found in some pet cages.

A squirrel cage rotor core is made by stacking thin steel laminations to form a cylinder.

Rather than using coils of wire as conductors, conductor bars are die cast into the slots evenly spaced around the cylinder. Most squirrel cage rotors are made by die casting aluminum to form the conductor bars. Siemens also makes motors with die cast copper rotor conductors.

Advantages of AC Motor  Low cost  Speed variation  High power factor  Reliable operation  Disadvantages of AC Motor  Inability to operate at low speeds  Poor positioning control

Servo Motors What is a servo motor? Servo motors (or servos) are self-contained electric devices that rotate or push parts of a machine with great precision. Servos are found in many places: from toys to home electronics to cars and airplanes. If you have a radio-controlled model car, airplane, or helicopter, you are using at least a few servos. In a model car or aircraft, servos move levers back and forth to control steering or adjust wing surfaces. By rotating a shaft connected to the engine throttle, a servo regulates the speed of a fuel-powered car or aircraft. Servos also appear behind the scenes in devices we use every day. Electronic devices such as DVD and Blu-ray DiscTM players use servos to extend or retract the disc trays. In 21st-century automobiles, servos manage the car's speed: The gas pedal, similar to the volume control on a radio, sends an electrical signal that tells the car's computer how far down it is pressed. The car's computer calculates that information and other data from other sensors and sends a signal to the servo attached to the throttle to adjust the engine speed. Commercial aircraft use servos and a related hydraulic technology to push and pull just about everything in the plane.

Design of Motor The magnetic force is be made by permanent magnet and the current is used to generate torque. Then high torque and efficiency are available at low current and small size. It has no brush so there is little noise/vibration and no dirt. And high precision control is available with high resolution encoder.

How does a servo motor work? The simplicity of a servo is among the features that make them so reliable. The heart of a servo is a small direct current (DC) motor, similar to what you might find in an inexpensive toy. These motors run on electricity from a battery and spin at high RPM (rotations per minute) but put out very low torque (a twisting force used to do work— you apply torque when you open a jar). An arrangement of gears takes the high speed of the motor and slows it down while at the

same time increasing the torque. (Basic law of physics: work = force x distance.) A tiny electric motor does not have much torque, but it can spin really fast (small force, big distance). The gear design inside the servo case converts the output to a much slower rotation speed but with more torque (big force, little distance). The amount of actual work is the same, just more useful. Gears in an inexpensive servo motor are generally made of plastic to keep it lighter and less costly .On a servo designed to provide more torque for heavier work, the gears are made of metal and are harder to damage.

Advantages 

high performance, higher speeds, accurate positioning (because of feedback)

Disadvantages 

higher cost (less of an issue today), requires setup/tuning

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.

How Stepper Motors Work Stepper motors consist of a permanent magnetic rotating shaft, called the rotor, and electromagnets on the stationary portion that surrounds the motor, called the stator. Figure 1 illustrates one complete rotation of a stepper motor. At position 1, we can see that the rotor is beginning at the upper electromagnet, which is currently active (has voltage applied to it). To move the rotor clockwise (CW), the upper electromagnet is deactivated and the right electromagnet is activated, causing the rotor to move 90 degrees CW, aligning itself with the active magnet. This process is repeated in the same manner at the south and west electromagnets until we once again reach the starting position.

Types of Stepper Motor By construction the step motors come into three broad classes: 1.

Permanent Magnet Stepper

2.

Variable Reluctance Stepper

3.

Hybrid Step Motor

These three types have been explained in detail in the following sections.

Stepper Motor Advantages and Disadvantages 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. 3. Once motor is not controled well, it can easily cause resonance vibration.