IAM - 22523 Notes On Unit 5 Fractional Horsepower Motors Industrial Ac Machines

 

IAM - 22523 Notes On Unit 5  Fractional Horsepower Motors  Industrial Ac Machines Download Link ⬇️

Unit5 Fractional Horsepower Motors (FHP) 5.1 Construction and working: Synchronous Reluctance Motor, Switched Reluctance Motor, BLDC Permanent Magnet Synchronous Motors, stepper motors, AC and DC servomotors. 5.2Torque speed characteristics of above motors. 5.3 Applications of above motors. 1 Construction and working of Synchronous Reluctance Motor Torque speed characteristics Construction of Reluctance Motor: We know about different types of synchronous motors, apart from all these motor works based on reluctance. So it is called Reluctance Motor. Here we will discuss construction and working principle of Reluctance Motor. The reluctance motor has basically two main parts called stator and rotor. the stator has a laminated construction, made up of stampings. The stampings are slotted on its periphery to carry the winding called stator winding. The stator carries only one winding. This is excited by single-phase a.c. supply. The laminated construction keeps iron losses to a minimum. The stampings are made up of material from silicon steel which minimises the hysteresis loss. The stator winding is wound for certain definite number of poles. The rotor has a particular shape. Due to its shape, the air gap between stator and rotor is not uniform.No d.c supply is given to the rotor. The rotor is free to rotate. The reluctance i.e., the resistance of the magnetic circuit depends on the air gap. More the air gap, more is the reluctance and vice-versa. Due to the variable air gap between stator and rotor, when the rotor rotates, reluctance between stator and rotor also changes. The stator and rotor are designed in such a manner that the variation of the inductance of the windings is sinusoidal with respect to the rotor position. The construction of Reluctance Motor is shown in figure(a) while the practical rotor of Reluctance Motor is shown in figure(b) below. Construction of reluctance motor Working Principle of Reluctance Motor: The stator consists of a Single Winding called main winding. But single winding cannot produce rotating magnetic field. So for production of rotating magnetic field, there must be at least two windings separated by the certain phase angle. Hence stator consists of an additional winding called auxiliary winding which consists of a capacitor in series with it. Thus there exists a phase difference between the currents carried by the two windings and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field. The technique is called split phase technique of production of the rotating magnetic field. The speed of this field is the synchronous speed which is decided by the number of poles for which stator winding is wound. The rotor carries the short-circuited copper or aluminium bars and it acts as a squirrel-cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically. Similarly, in the reluctance motor, rotor tries to align itself with the axis of rotating magnetic field in a minimum reluctance position. But due to rotor inertia, it is not possible when the rotor is standstill. So rotor starts rotating near synchronous speed as a squirrel cage induction motor. When the rotor speed is about synchronous, stator magnetic field pulls rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked. Then rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very shall and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor. Torque - speed characteristics of Reluctance Motor: The torque speed characteristics are shown in below figure. The starting torque is highly dependent on the position of the rotor. Torque speed characteristics of reluctance motor Reluctance Motor Advantages: The reluctance motor has the following advantages, 1) No d.c. supply is necessary for the rotor. 2) Constant speed characteristics. 3) Robust construction. 4) Less maintenance. Reluctance Motor Disadvantages: The reluctance motor has following limitations, 1) Less efficiency 2) Poor power factor 3) Need of very low inertia rotor 4) Less capacity to drive the loads Applications of Reluctance Motor: Reluctance motor is used in  Signalling Devices  Control Apparatus  Automatic regulators  Recording Instruments  Clocks  All timing devices  Teleprinters  Gramophones 2 Construction and working of Switched Reluctance Motor Torque speed characteristics Construction of SRM In switched reluctance motor, the stator and rotor have projected pole made up of soft iron and silicon stampings. Silicon stamping is used to reduce hysteresis losses. Stator => Inward projection Rotor => Outward projection. The rotor does not have winding and stator only carries main field winding. Each winding in the stator is connected in series with the opposite poles to increase the MMF of the circuit. It is called phase winding. Refer to fig 1.1 AA’, BB’ and CC’. Learn More: How to Run Three Phase Motor on Single Phase Power Supply Linear SRM – Fig 1.1 Pole concern, the number of poles in the stator will be around 6 to 8 numbers. But the rotor carries less number of poles with respect to the stator. The rotor poles will be 4 to 8 numbers. By increasing the number of poles we can get a low angle of rotation from the motor. The rotor’s shaft is mounted with a position sensor. The position sensor is used to determine the position of the rotor by a control circuit. The control circuit always collects the information of the rotor position and based on that the controller gives the input to the motor. Block diagram of SRM Block Diagram of SRM Fig 1.2 The DC input is connected to the driver/converter circuit and the output is connected to the motor. The rotor sensor’s feedback wire is connected to the controller circuit and it provides the position of the rotor with reference to the reference axis. Finally, the controller collects all information and based on that, reference will be given to the stator. Also, the controller monitors the motor current to protect the motor from internal and external faults. Learn More: What is Synchronous Machine? The converter circuit: Also, note that the output of the controller is DC. And the output will be as shown in the figure 1.3. Switching Pulse of SRM Fig 1.3 Working Principle The working principle of switched reluctance motor is simple, let we take an iron piece. If we keep it in a magnetic field means, the iron piece will align with the minimum reluctance position and get locked magnetically. The same principle is followed in the switched reluctance motor. The minimum reluctance portion of the rotor tries to align itself with the stator magnetic field. Hence the reluctance torque is developed in the rotor. Switched Reluctance motor Working, types In our motor, let us consider the following notation for better understanding. Stator Poles: AA’ poles axis for A phase BB’ poles axis for B phase CC’ poles axis for C Phase Rotor poles: aa’ rotor poles axis for Position 1 bb’ rotor poles axis for position 2 Now the input is given to the A-phase, other B and C phase neither maximum nor minimum, then stator pole axis AA‘ and rotor pole axis aa‘ are in alignment. Ref picture Fig 1.4 Figure 1.4 indicates that the A-phase reached the minimum reluctance position. Advantage of SRM 1. It does not require an external ventilation system as the stator and rotor slots projected. The airflow maintained between the slots. 2. The rotor does not have winding since therefore no need keeps the carbon brush and slip ring assembly. 3. Since the absence of permanent magnet, such motors are available at a cheaper price. 4. Simple three or two-phase pulse generator is enough to drive the motor 5. The direction of the motor can be reversed by changing the phase sequence. 6. Self-starting and does not require external arrangements. 7. Starting torque can be very high without excessive inrush currents. 8. High Fault Tolerance 9. Phase losses do not affect motor operations. 10.High torque/inertia ratio 11.High starting torque can be achieved. The disadvantage of Switched reluctance motor Creates Torque ripple at high-speed operation The external rotor position sensor is required. Noise level is high At a higher speed, the motor generates harmonics, to reduce this, we need to install larger size capacitors. Since the absence of Permanent Magnet, the motor has to designed to carry high input current. It increases the converter KVA requirement. Application of SRM Domestic appliances such as washing machines, vacuum cleaners, fans etc. 3 Construction and working of BLDC Motor Torque speed characteristics Working Principle and Operation of BLDC Motor BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves. When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate. Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate. Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction. Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit. Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft. Brushless DC Motor Drive As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor. Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit. In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives. Advantages of BLDC Motor BLDC motor has several advantages over conventional DC motors and some of these are  It has no mechanical commutator and associated problems  High efficiency due to the use of permanent magnet rotor  High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed  Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors  Long life as no inspection and maintenance is required for commutator system  Higher dynamic response due to low inertia and carrying windings in the stator  Less electromagnetic interference  Quite operation (or low noise) due to absence of brushes Disadvantages of Brushless Motor  These motors are costly  Electronic controller required control this motor is expensive  Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors  Requires complex drive circuitry  Need of additional sensors Applications of Brushless DC Motors (BLDC) Brushless DC Motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments, etc. Some specific applications of BLDC motors are  Computer hard drives and DVD/CD players  Electric vehicles, hybrid vehicles, and electric bicycles  Industrial robots, CNC machine tools, and simple belt driven systems  Washing machines, compressors and dryers  Fans, pumps and blowers 4. Construction and working of Permanent Magnet Synchronous Motors and Torque speed characteristics Construction of Permanent Magnet Synchronous Motor (PMSM): The basic construction of PMSM is same as that of synchronous motor. The only difference lies with the rotor. Unlike synchronous motor, there is no filed winding on the rotor of PMSM. Field poles are created by using permanent magnet. These Permanent magnets are made up of high permeability and high coercivity materials like Samarium-Cobalt and Neodium-Iron-Boron. Neodium-Iron-Boron is mostly used due to its ease of availability and cost effectiveness. Theses permanent magnets are mounted on the rotor core. Based on the mounting arrangement of magnet on rotor core, Permanent Magnet Synchronous Motor (PMSM) can be categorized into two types: Surface Mounted PMSMs and Buried or interior PMSMs In Surface Mounted PMSM, permanent magnet is mounted on the rotor surface as shown in figure below. This type of PMSM is not robust and therefore not suited for high speed application. Since the permeability of magnet and air gap is almost same, therefore this type of construction provides a uniform air gap. Therefore, there is no reluctance torque present. Thus the dynamic performance of this motor is superior and hence used in high performance machine tool drives and robotics. In Interior or Buried PMSM, the permanent magnets are embedded into the rotor instead of mounting on the surface. This provides robustness and hence can be used in high speed applications. Due to presence of saliency, reluctance torque is present in this type of PMSM. Working Principle of Permanent Magnet Synchronous Motor (PMSM): The working principle of permanent magnet synchronous motor is same as that of synchronous motor. When three phase winding of stator is energized from 3 phase supply, rotating magnetic field is set up in the air gap. At synchronous speed, the rotor field poles locks with the rotating magnetic field to produce torque and hence rotor continues to rotate. As we know that synchronous motors are not self starting, PMSM needs to be started somehow. Since there is no winding on the rotor, induction windings for starting is not applicable for such motors and therefore variable frequency power supply for this purpose. Applications: Permanent Magnet Synchronous Motor can be used as an alternative for servo drives. It is widely used in various industrial application viz. robotics, traction, aerospace etc. 5. Construction and working of Stepper Motors and Torque speed characteristics: Construction: Stepper motor is made up of the stator and rotor. The rotor is the movable part which has no winding, brushes and a commutator. The stator is made up of multipole and multiphase winding, usually of three or four phases winding wound for a required number of poles decided by desired angular displacement per input pulses. Working: Stepper motor works on the principle of electromagnetism. The magnetic rotor shaft is surrounded by the electromagnetism stators. Rotor and stator have poles which may or may not be teethed depending upon the types of the stator. Whenever the stators have energized the rotor, it moves to align itself along with the stator. In this fashion, the stators are energized in the sequence at different poles to rotate the stepper motor. Due to the very good control of the speed, rotation, direction and angular position, these are of particular interest in industrial process control system, CNC machine, robotics, manufacturing automation system and instrumentation. Types of stepper motor: 1. Variable reluctance stepper motor. 2. Permanent magnet stepper motor. 3. Hybrid stepper motor. 1. Variable reluctance stepper motor: Variable reluctance stepper motor has the simple design with soft iron, nonmagnetic toothed rotor and wound electromagnetic stators. No attraction between the rotor and stator winding when the winding being energized since the rotor is not magnetised. When an opposite pair of winding has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of poles on the rotor. It gives the steps angle of 7.5 or 15 degrees. 2. Permanent magnet stepper motor: Permanent magnet stepper motor has a permanent magnet rotor that is axially magnetised. It means that it has alternating north and south poles parallel to the rotor shaft. Each pole is wound with a field winding, the coils on opposite pair of poles in series. Current is supplied from D.C source to the winding through switches. The rotor is a permanent magnet and thus when a pair of the stator poles has a current switched to it the rotor will move to line up with it. steps angle of this motor are 1.8, 7.5,15,30,34 and 90 degrees. 3. Hybrid stepper motor: The hybrid stepper motor is the combination of both permanent and variable reluctance motor. It has a permanent magnet, toothed rotor made up two sections or cup, which are opposite in polarity and whose teeth are offset to each other. The rotor set itself in the minimum reluctance position in response to a pair of stator coil energised .Step angle of this motor are 0.9, or 1.8 degrees. You can also watch this video to understand the different types and working of stepper motor. Advantages: 1. The rotation angle is proportional to the input pulses. 2. Full torque at standstill. 3. Very low-speed synchronous rotation is possible to achieve. 4. There are no brushes so it is reliable. 5. Speed is directly proportional to the frequency of the input as pulses; hence a wide range of rotational speed can be realized. 6. Low speed with high precision. Disadvantages: 1. No feedback system. 2. Low effitiency. 3. May produce more noise. 4. Difficult to operate at very high speed. 5. For the smooth move, micro stepping is required. Applications: 1. Factory automation. 2. Packaging. 3. Material handling. 4. Aerospace industry especially in avionics. 5. 3D pictures acquisition system. 6. Laser measurements. 7. Robotics. 6. Construction and working of Servomotor and Torque speed characteristics AC Servomotor Construction of AC Servomotor We have already said in the beginning that an ac servomotor is regarded as a twophase induction motor. However, ac servomotors have some special design features which are not present in normal induction motor, thus it is said that two somewhat differs in construction. It is mainly composed of two major units, stator and rotor.  Stator: First have a look at the figure shown below, representing stator of ac servomotor: The stator of ac servo motor consists of two separate windings uniformly distributed and separated at 90°, in space. Out of the two windings, one is referred as main or fixed winding while the other one is called control winding. A constant ac signal as input is provided to the main winding of the stator. However, as the name suggests, the control winding is provided with the variable control voltage. This variable control voltage is obtained from the servo amplifier. It is to be noted here that to have a rotating magnetic field, the voltage applied to the control winding must be 90° out of phase w.r.t the input ac voltage.  Rotor: The rotor is generally of two types; one is squirrel cage type while the other is drag cup type. The squirrel cage type of rotor is shown below: In this type of rotor, the length is large while the diameter is small and is constructed with aluminium conductors thus weighs less. It is to be noted here that the torque-speed characteristics of a normal induction motor have both positive as well as negative slope regions that represent unstable and stable regions, respectively. However, ac servo motors are designed to possess high stability thus, its torqueslip characteristics must not have a positive slip region. Along with this the torque developed in the motor must reduce in a linear manner with speed. To achieve this the rotor circuit resistance should have a high value, with low inertia. Due to this reason, while constructing the rotor, the diameter to length ratio is kept smaller. The reduced air gaps between the aluminium bars in the squirrel cage motor facilitate a reduction in magnetizing current. Let us now see the representation of the drag cup type rotor: This type of rotor is different in construction from that of squirrel cage one. It consists of a laminated core of aluminium around which drag cup is present with certain air gaps on both the side. These drag cups are attached with a driving shaft that facilitates its operation. The two air gaps in both sides of the core lead to reducing the inertia thus is used in applications where there is a low power requirement. Working Principle of AC Servomotor The figure below represents the AC two-phase induction motor that uses the principle of servomechanism: Initially, a constant ac voltage is provided at the main winding of the stator of the ac servomotor. The other stator terminal of the servomotor is connected to the control transformer through the control winding. Due to the provided reference voltage, the shaft of the synchro generator rotates with a particular speed and attains a certain angular position. Also, the shaft of the control transformer has a certain specific angular position which is compared with the angular position of the shaft of the synchro generator. Further, the comparison of two angular positions provides the error signal. More specifically, the voltage levels of the corresponding shaft positions are compared which generates the error signal. This error signal corresponds to the voltage level present at the control transformer. This signal is then provided to the servo amplifier which generates variable control voltage. With this applied voltage, the rotor again attains a specific speed and starts rotation and sustains until the value of the error signal reaches 0, thereby attaining the desired position of the motor in the AC servomotors. Torque-Speed Characteristics The figure below represents the torque-speed characteristics of the two-phase induction motor We have already discussed that the motor must be designed in a way to provide linear torque-speed characteristics, in which the torque changes in a linear manner with the speed. However, as we have seen in the above figure that the torque-speed characteristics here are not actually linear. This is so because it depends on the ratio of reactance to resistance. The low value of the ratio of reactance to resistance implies that motor possesses high resistance and low reactance, in such case, the characteristics is more linear that high value of the ratio for reactance to resistance. Applications of AC Servomotors Due to the various advantages offered by the AC servomotors, these majorly finds applications in the instruments that operate on servomechanism, in position controlling devices, computers. Along with this these also find applications in tracking systems, machine tools and robotics machinery. DC Servomotors: In the case of DC servomotor, the applied dc input causes the motor to rotate and acquire the desired position at the specified angle. It is a closed-loop operation and uses position feedback to precisely adjust the position at the desired angle. The DC servomotor is further classified on the basis of control provided to it. Basically, controlling to the DC servomotor is either provided from the field side or from the armature side. And this further classifies the DC servomotor. Here in this article, we will discuss each type separately. Field Controlled DC Servomotor In this type of DC servomotor, the controlling is provided to the field winding. More specifically, we can say, the controlled signal received from the amplifier is fed to the field winding. Thus, it is named so. While the armature current is maintained at a constant value using a constant current source. The figure given below represents the field controlled DC servomotor: It is to be noted here that the field in this type of dc servomotor can be either electromagnetic type where a salient pole is present with a field winding wound around it and excitation to it is provided with dc current or a permanent magnet type. Basically, according to the general torque equation of DC motor, the torque is directly proportional to the product of field flux and the armature current. However, in this specific type, the armature current is kept constant thus, T α Φ We have already discussed the amplified error signal from the servoamplifier excites the field. And in this way, the excitation provided controls the torque i.e., the rotation of the motor. In case the value of the current source applied at the armature is quite large, then for a small change in field current, there will be a proportional change in the torque of the motor. It is to be noted here that the direction in which the shaft rotates can be changed according to the polarity of the field or the by using split field dc motor. When the direction of rotation is polarity dependent then this signifies that with the change in polarity of the field, the rotational direction also changes. However, sometimes by the use of split field dc motor, the rotational direction of the motor changes. This is so because as the name suggests in this type of motor, the winding is split into two parts, where one part is wounded in a clockwise direction while the other in an anticlockwise direction. In this case, the error signal is provided at the junction of the winding. As the two windings possess a magnetic field in the opposite direction. Thus, when the error signal is provided then one side of the magnetic field will dominate the other. So, the motor rotates in the direction of the winding of the dominating side according to the error signal. Here, the ratio of reactance to resistance is quite large thereby exhibiting the high value of the time constant. This implies that for quick changes in the control signals the response will be slower. Thus, is majorly used in small rated motors. However, here the power requirement is less as the motor is controlled by the field. Armature Controlled DC Servomotor In armature controlled dc servomotor, the controlling is provided at the armature. This means, here the signal from the servoamplifier is provided at the armature and constant current is provided at the field winding. The voltage from the servoamplifier, Va(t) with resistance Ra and inductance La is provided at the armature. And this input voltage at the armature controls the shaft. The figure below represents the arrangement of armature coupled dc servomotor: Here the constant field is provided using the permanent magnets and hence no field coils are required. In the field controlled dc servomotor, we have already discussed that torque is in direct proportion the field flux and the armature current. Thus, its operating principle is such that if the field flux is quite large then even with the small change in the armature current there will be a large change in torque. Thereby making the servomotor sensitive to armature current. It is to be noted here that in armature controlled dc servomotor, the sensitivity towards the field current should be low. As the armature controlled motor must not respond to the field current. It offers a small value of the time constant so there is a rapid change in the armature current of the motor with the change in the voltage applied at the armature. Thus, it provides a faster dynamic response where the direction of rotation changes with the change in polarity of the error signal. Characteristics of DC Servomotor The figure below shows the torque-speed characteristics of armature controlled dc servomotor: It is noteworthy here that, the above-shown characteristics are similar to the torque-speed characteristics of ac servomotor.

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