Special motors pdf

Such DC motors have the field winding also known as shunt field winding connected in parallel with the armature winding. It allows a full terminal voltage across the field winding while both of windings have the same amount of voltage across it. Such electric motors are used for its constant speed application since it maintains its speed over a range of loads connected with it.

The shunt winding refers to the windings connected in parallel. Cumulative Compound. The total flux generated in such case in always greater than the original flux.

Differentially Compound. They total flux generated is always less than the original flux. Short Shunt DC Motor. It is also known as compound wound motor. Long Shunt DC Motor. A DC motor is said to be long shunt motor if the said shunt field windings is in parallel to the both armature winding as well as the field winding. It has the usual armature like the rest of brushed DC electric motors explained above.

However, there is no stator or field winding, the magnetic field is generated using a permanent magnet placed in the stator. However, the strength of the magnet reduces with time. There is an extra excitation field found in some designs that helps increase its magnetic strength when it reduces. PMDC does not need field excitation to generate the field flux as it is produced by the permanent magnet.

This increases its efficiency as no extra power is consumed for excitation. The absence of the field windings greatly reduces the size of overall motor. Therefore PMDC motors have compact designs. It has multiple stator windings each situated at a different angle to generate flux in different directions. Since the DC input to the stator needs to be switched, such electric motors used electronic commutations instead of mechanical commutation using switching devices such as thyristors.

These switches are controlled using microcontroller to precisely switch the input between the stators windings. It essentially switches the DC input into a 3-phase supply which generates a smooth rotating magnetic field.

The brushless motor speed depends on the frequency of the AC power supplied by the controller. Which is why it is also called synchronous motor. But the cost of the controller is far greater than the motor itself. It helps in increasing the life span of the motor as well as the efficiency of the motor.

The energy dissipated in brushes is converted into mechanical output. And they are also maintenance free. As the name suggests, such DC motors have no laminated iron core. The rotor winding is wound in skewed or honeycomb shape to form a self-supporting hollow cage often made with using epoxy. The rotor made of permanent magnet sets in the hollow rotor.

There are several types of special electric motors that are the modified versions of other motor designed for special purposes. Some of these electric motors are given below. Servo motor can be designed to run on AC as well as DC power supply. It specifies much weight the servo can lift at a specific distance. The weight lifting capacity decreases with an increase in the distance. The servo motor has a gear assembly, controller, a sensor and a feedback system.

Servo motors has three wires. It is controlled by providing a pulsating signal thorough a microcontroller using PWM pulse width modulation. Direct drive motor or also known as torque motor is another type of motor that produces high torque at low speed even when it is stalling. The payload is directly connected to the rotor thus eliminating the use of gearbox, belts, speed reducers etc. The fact that it has less mechanical parts means it require less maintenance and low cost.

The armature windings are designed in a linear fashion which carries 3 phase current to generate a magnetic field. The magnetic field interacts with the magnetic field generated by the flat permanent magnet lying below it.

The interaction between them generates a linear force upon each other thus the armature moves forward or backward. It is an AC powered motor with a controller such as in servo motor. The power is supplied to the primary part of the motor that contains windings.

It generates its own magnetic field whose polarity depends on the phase of the AC supply. The amount of current determines the force while the rate of change of current determines the speed of the primary part. A stepper motor or a stepping motor is a brushless DC motor whose full rotation is divided into a number of equal steps.

Such motor rotates in steps fixed degrees instead of rotating continuously. Such stepping movement offers great precision which is utilized is robotics. The stepper motor operates on pulses. Each pulse moves the motor by one step. The precision of the motor depends on the number of steps per revolution. The steps size is determined during its design. However, the speed of the motor can be controlled by applying the pulse train of variable frequency.

Start on. Show related SlideShares at end. WordPress Shortcode. Share Email. Top clipped slide. Download Now Download Download to read offline. Principles of Special Motors Feb. Naila Syed Follow. Cat os. Infrared thermography. The human brain. Facts we need to know. Wide area measurements synchrophasor measurements in Power Systems. Variable frequency drives for industrial applications.

Related Books Free with a 30 day trial from Scribd. Related Audiobooks Free with a 30 day trial from Scribd. Elizabeth Howell. Principles of Special Motors 1. It is a varying-speed machine. ECE 7 7. How do you communicate the angle at which the servo should turn? The control wire is used to communicate the angle.

The angle is determined by the duration of a pulse that is applied to the control wire. This is called Pulse Coded Modulation. The servo expects to see a pulse every 20 milliseconds. The length of the pulse will determine how far the motor turns. If the pulse is shorter than 1. If the pulse is longer than 1. Torque is generated through the phenomenon of magnetic reluctance.

Disadvantages are high torque ripple when operated at low speed, and noise caused by torque ripple. Until recently, their use has been limited by the complexity inherent in both designing the motors and controlling them. These challenges are being overcome by advances in the theory, by the use of sophisticated computer design tools, and by the use of low-cost embedded systems for motor control. These control systems are typically based on microcontrollers using control algorithms and real-time computing to tailor drive waveforms according to rotor position and current or voltage feedback.

Design and operating fundamentals The stator consists of multiple salient ie. The rotor consists of soft magnetic material, such as laminated silicon steel, which has multiple projections acting as salient magnetic poles through magnetic reluctance.

The number of rotor poles is typically less than the number of stator poles, which minimizes torque ripple and prevents the poles from all aligning simultaneously—a position which can not generate torque.

When a rotor pole is equidistant from the two adjacent stator poles, the rotor pole is said to be in the "fully unaligned position". This is the position of maximum magnetic reluctance for the rotor pole. In the "aligned position", two or more rotor poles are fully aligned with two or more stator poles, which means the rotor poles completely face the stator poles and is a position of minimum reluctance.

When a stator pole is energized, the rotor torque is in the direction that will reduce reluctance. Thus the nearest rotor pole is pulled from the unaligned position into alignment with the stator field a position of less reluctance. This is the same effect used by a solenoid, or when picking up ferromagnetic metal with a magnet. In order to sustain rotation, the stator field must rotate in advance of the rotor poles, thus constantly "pulling" the rotor along.

Some motor variants will run on 3-phase AC power see the synchronous reluctance variant below. Most modern designs are of the switched reluctance type, because electronic commutation gives significant control advantages for motor starting, speed control, and smooth operation low torque ripple. The main difference is that a capacitor is placed in series with the start winding. The capacitor start motor produces considerably more starting torque than the split phase motor.

After 6. Logically, capacitor start motors should be used where the load acceleration requirements exceed the capacity of a split phase motor. A round dual run capacitor described below is used in some air conditioner compressor units, to boost both the fan and compressor motors. Run capacitors Run capacitors are designed for continuous duty, and they are energized the entire time the motor is running.

Single phase electric motors need a capacitor to energize a second-phase winding. If the wrong run capacitor is installed, the motor will not have an even magnetic field, and this will cause the rotor to hesitate at those spots that are uneven.

This hesitation can cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat. However, a motor will not be ruined just because a run capacitor is faulty. Start capacitors Start capacitors briefly increase motor starting torque and allow a motor to be cycled on and off rapidly.

As in other induction motors the rotating part is a squirrel-cage rotor. All single-phase motors require a means of producing a rotating magnetic field for starting.

In the shaded-pole type, a part of the face of each field pole carries a copper ring called a shading coil. Currents in this coil delay the phase of magnetic flux in that part of the pole enough to provide a rotating field. The effect produces only a low starting torque compared to other classes of single- phase motors.

These motors have only one winding, no capacitor nor starting switch, making them economical and reliable. Because their starting torque is low they are best suited to driving fans or other loads that are easily started. Moreover, they are compatible with TRIAC-based variable-speed controls, which often are used with fans. For larger motors, other designs offer better characteristics.

The first photo is of a common C-frame motor. With the shading coils positioned as shown, this motor will start in a clockwise direction as viewed from the long shaft end. The second photo shows detail of the shading coils Construction Of Shaded-Pole Motors The three types of shaded-pole motor are salient-pole, skeleton and distributed-winding.

Salient-pole construction has many main-winding coils. The number of poles is the same as the number of coils. In most cases there are four coils. Some of the older refrigerator fans are silent-pole, shaded-pole motors. They used a felt soaked in oil for lubricating the rotary part. The skeleton type is used for horsepower from 0. This type of motor uses triple shading. Three shading coils of different throw for each pole are used.

In this type bearings are self-aligning iolite. Wick-type oilers are used to spread the lubrication so that it covers the whole rotating shaft. A squirrel-cage rotor to be slightly offset from the pole, and it easily compressed as the coil is energized. The sucking effect of the coil causes the rotor to be pulled back inside the hole in laminated core. This type of motor is used on can openers, small knife sharpeners, clocks and timers. It has an advantage over the synchronous type originally used in clocks, as it is self-starting and will start again if the power fails.

The old clock-type synchronous motors had to be started by hand. The third type of construction of shaded-pole motors is the distributed-winding. The stator laminations are similar to those used for single-phase or polyphase induction motors. The main winding is also similar to the start winding except that it is short- circuited upon itself.

Performance Of Shaded-Pole Motors If this type of motor of motor is properly lubricated in most cases the skeleton type is sealed it will last continuously operated for over 25 years. Most clock motors and fan motors are limited only by the physical abuse they receive. If they are kept plugged in in the case of a clock and operating continuously as timers or similar devices, without overloading, there is no reason why they cannot last indefinitely.