SIM_U4_P2.pptx ELECTRICAL MACHINES UNIT 4

Synchronous machines are AC electromechanical devices that operate at a constant speed (synchronous speed), directly proportional to the frequency of the power supply and the number of magnetic poles. They function as either a synchronous generator (alternator), converting mechanical energy into AC electrical energy for power generation, or as a synchronous motor, converting electrical energy into mechanical energy for constant-speed applications.

1. Electrical Machine (ETME-209) MAE & ME -3 rd semester Unit IV ,Part 2 Govind Gupta ( Assistant Professor) Electrical & Electronics Engineering Department Maharaja Agrasen Institute of Technology, Delhi

2. Stepper Motor Hysteresis Motor Servomotor Universal Motor AC Series Motor Index

3. Stepper Motor The name Stepper Motor itself shows that the rotor movement is in the form of various steps or discrete steps. It is also known as Stepping Motor. The number of pulses fed into the controller circuit determines the angular rotation of the motor. Each input pulse produces one step of the angular movement . The drive is considered as an analog to digital converter. It has an inbuilt logic, which causes appropriate windings to be energised and de-energized by the solid state switches in the required sequence.

4. Types of Stepper Motor

5. Step Angle in Stepper Motor Step angle is defined as the angle which the rotor of a stepper motor moves when one pulse is applied to the input of the stator. The positioning of a motor is decided by the step angle and is expressed in degrees. The resolution or the step number of a motor is the number of steps it makes in one revolution of the rotor. Smaller the step angle higher the resolution of the positioning of the stepper motor . The accuracy of positioning of the objects by the motor depends on the resolution. Higher the resolution greater will be the accuracy. Some precision motors can make 1000 steps in one revolution with a step angle of 0.36 degrees. A standard motor will have a step angle of 1.8 degrees with 200 steps per revolution. The various step angles like 90, 45 and 15 degrees are common in simple motors. The number of phases can vary from two to six. Small steps angle can be obtained by using slotted pole pieces.

6. Characteristics of a Stepper Motor The Torque pulse rate Characteristics of a Stepper Motor gives the variation of an electromagnetic torque as a function of stepping rate in pulse per second (PPS ). Curve one is denoted by a blue colour line is known as the Pull-in torque. It shows the maximum stepping rate for the various values of the load torque at which the motor can start, synchronise , stop or reverse . Similarly, the curve 2 represented by Red colour line is known as pullout torque characteristics. It shows the maximum stepping rate of the motor where it can run for the various values of load torque. But it cannot start, stop or reverse at this rate. The motor can start, synchronise and stop or reverse for the load torque Ʈ L  if the pulse rate is less than S 1 . The stepping rate can be increased for the same load as the rotor started the rotation and synchronised . Now, for the load Ʈ L1,  after starting and synchronising , the stepping rate can be increased up to S 2  without losing the synchronism. If the stepping rate is increased beyond S 2 , the motor will lose synchronism. Thus, the area between curves 1 and 2 represents the various torque values, the range of stepping rate, which the motors follow without losing the synchronism when it has already been started and synchronised . This is known as Slew Range. The motor is said to operate in slewing mode.

7. Stepper Motor Applications The Stepper motor is manufactured in various sizes ranging from milliwatts to hundreds of watts. Its maximum torque value ranges up to 15 Newton Meter and the step angle ranges from 1.8 to 90 degrees . The  Stepper Motor Applications have a wide range. Some of the applications are given below. As the stepper motor are digitally controlled using an input pulse, they are suitable for use with computer controlled systems. They are used in numeric control of machine tools. Used in tape drives, floppy disc drives, printers and electric watches. The stepper motor also use in X-Y plotter and robotics. It has wide application in textile industries and integrated circuit fabrications. The other applications of the Stepper Motor are in spacecrafts launched for scientific explorations of the planets etc. These motors also find a variety of commercial, medical and military applications and also used in the production of science fiction movies. Stepper motors of microwatts are used in the wrist watches. In the machine tool, the stepper motors with ratings of several tens of kilowatts is used

8. Hysteresis Motor Hysteresis Motor is a synchronous motor with a cylindrical rotor and does not require any dc excitation to the rotor and it uses non-projected poles . It is a single phase motor with rotor made up of ferromagnetic material. Initially, when hysteresis motor is started it behaves as a single phase induction motor and while running it behaves as a synchronous motor . When stator is energised with single phase AC supply it produces rotating magnetic field . The main and auxiliary, both the windings must be supplied continuously at the start as well as in running condition so as to maintain the rotating magnetic field . This field induces poles in the rotor.  The  hysteresis phenomenon is dominant for the rotor material chosen and due to which rotor pole axis lag behind the axis of rotating magnetic field. Due to this, rotor poles get attracted towards the moving stator field poles . Thus rotor gets subjected to torque called hysteresis torque . This torque is constant at all speeds . When the stator field axis moves forward, due to high retentivity the rotor pole strength remains maintained . So higher the retentivity , higher is the hysteresis torque.       Initially, rotor starts rotating due to the combined effect of hysteresis torque as well as torque due to eddy currents induced in the rotor. Once the speed is near about the synchronous, the stator pulls rotor into synchronism. In such case, as relative motion between stator field and rotor vanishes, so the torque due to eddy currents vanishes . Only  hysteresis torque is present which keeps rotor running at synchronous speed. The high retentivity ensures the continuous magnetic locking between stator and rotor . Due to the principle of magnetic locking, the Hysteresis Motor rotates at synchronous speed

9. Construction of Hysteresis Motor Hysteresis Motor consists of ( i )Stator (ii)Rotor (iii)Main windings (iv)Auxiliary windings (v)Shaft The stator is wounded with main and auxiliary windings so as to produce rotating magnetic field . In some hysteresis motor designs, the stator can also be shaded pole type . To develop synchronising revolving field from single phase supply, the stator of hysteresis motor is designed accordingly . The rotor is smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity . This requires selecting a material with high hysteresis loop area . The rotor does not carry any winding or teeth . The rotor of hysteresis motor has high resistance to reduce eddy current loss . This cylindrical rotor is mounted on the shaft through arbour made up of aluminium .

10. Torque Speed characteristics of Hysteresis Motor The starting torque and running torque is almost equal in this Hysteresis Motor.As stator carries mainly the two windings its direction can be reversed by interchanging the terminals of either main winding or auxiliary winding. The torque speed characteristics of hysteresis motor are as shown in the figure as seen from the characteristics torque at the start is almost same throughout the operation of the Hysteresis Motor.

11. Hysteresis Torque Equation in the Hysteresis Motor: The eddy current loss in the machines is given by Pe = Kef²2B² Ke = eddy current constant f2 = frequency of eddy currents B = flux density We know the relation between rotor frequency f2 and supply frequency f1 f2 = sf1 where Pe = KeS²f²1B² The torque due to eddy currents is given by So when the rotor rotates at synchronous speed, the slip becomes zero and torque due to eddy current component vanishes . It only helps at the start . The hysteresis power loss is given by , Thus the hysteresis torque component at all the rotor speeds. Hysteresis Power Loss ( Ph ) in Hysteresis Motor: Hysteresis power loss in rotor is given by where f1 = supply frequency f2 = frequency of eddy currents B = flux density Kh = hysteresis constant S = slip

12. Advantages and Limitation of using Hysteresis Motors The  advantages of Hysteresis Motor are   As rotor has no teeth, no winding, there are no mechanical vibrations Due to the absence of vibrations, the operation is quiet and noiseless Suitability to accelerate high inertia loads  Possibility of multispeed operation by employing gear train.  Disadvantages of using Hysteresis Motors: The disadvantages of Hysteresis Motor are Low efficiency Low torque Low power factor Available in very small size  Poor output

13. Applications of Hysteresis Motor: Due to the noiseless operation, Hysteresis motor is used in  Sound recording instruments Sound producing equipment High-quality record players Tape recorders Electric Clocks Teleprinters Timing devices

14. Servomotor A servomotor (or servo motor) is a simple electric motor, controlled with the help of servomechanism. If the motor as a controlled device, associated with servomechanism is DC motor, then it is commonly known as a DC Servo Motor. If AC operates the controlled motor, it is known as a AC Servo Motor . A servomotor is a linear actuator or rotary actuator that allows for precise control of linear or angular position, acceleration, and velocity. It consists of a motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.

15. Servomotor Basics and Application There are some special types of applications of an electric motor where the rotation of the motor is required for just a certain angle. For these applications, we require some special types of motor with some special arrangement which makes the motor rotate a certain angle for a given electrical input (signal). For this purpose, servo motor comes into the picture . The servo motor is usually a simple DC motor controlled for specific angular rotation with the help of additional servomechanism (a typical closed-loop feedback control system). Nowadays, servo systems are used widely in industrial applications. Servo motor applications are also commonly seen in remote-controlled toy cars for controlling the direction of motion, and it is also very widely used as the motor which moves the tray of a CD or DVD player. Besides these, there are hundreds of servo motor applications we see in our daily life. The main reason behind using a servo is that it provides angular precision, i.e. it will only rotate as much we want and then stop and wait for the next signal to take further action. The servo motor is unlike a standard electric motor which starts turning as when we apply power to it, and the rotation continues until we switch off the power. We cannot control the rotational progress of electrical motor, but we can only control the speed of rotation and can turn it ON and OFF. Small servo motors are included many beginner Arduino starter kits, as they are easy to operate as part of a small electronics projects

16. Torque speed characteristic The Torque speed characteristic of a normal induction motor is highly nonlinear and has a positive slope for some portion of the curve. This is not desirable for control applications. as the positive slope makes the systems unstable . The torque speed characteristic of an ac servo motor is fairly linear and has negative slope throughout. The rotor construction is usually squirrel cage or drag cup type for an ac servo motor. The diameter is small compared to the length of the rotor which reduces inertia of the moving parts. Thus it has good accelerating characteristic and good dynamic response. The supplies to the two windings of ac servo motor are not balanced as in the case of a normal induction motor. The control voltage varies both in magnitude and phase with respect to the constant reference is applied to the reference winding.The direction of rotation of the motor depends on the phase (± 90°) of the control voltage with respect to the reference voltage. For different rms values of control voltage the torque speed characteristics are shown in Fig. The torque varies approximately linearly with respect to speed and also controls voltage . The torque speed characteristics can be linearised at the operating point and the transfer function of the motor can be obtained.

17. The Torque S peed characteristic of the two phase AC servomotor The negative slope represents a high rotor resistance and provides the motor with positive damping for better stability. The curve is linear for almost various control voltages. The response of the motor to a light control signal is improved by reducing the weight and inertia of the motor in a design known as the Drag Cup Servo motor .

18. Universal Motor The motors which can be used with a single phase AC source as well as a DC source of supply and voltages are called as Universal Motor. It is also known as Single Phase Series Motor. A universal motor is a commutation type motor. If the polarity of the line terminals of a DC Series Motor is reversed, the motor will continue to run in the same direction.

19. C ertain modifications are made in a DC series motor so that it can work even on the AC current. They are as follows:- The field core is made up of the material having a low hysteresis loss. It is laminated to reduce the eddy current loss. The area of the field poles is increased to reduce the flux density. As a result, the iron loss and the reactive voltage drop are reduced. To get the required torque the number of conductors in the armature is increased. A compensating winding is used for reducing the effect of the armature reaction and improving the commutation process. The winding is placed in the stator slots as shown. The winding is put in the stator slot. The axis of compensating winding is 90 degrees with the main field axis. The compensating winding is connected in series with both the armature and the field, hence, it is called Conductively compensated . If the compensating winding is short circuited, the motor is said to be inductively compensated.

20. The direction is determined by both field polarity and the direction of current through the armature. As torque is proportional to the flux and the armature current. Let the DC series motor be connected across a single phase AC supply. Since the same current flows through the field winding and the armature winding . The AC reversal from positive to negative or vice versa will affect the field flux polarity and the current direction through the armature. The direction of the developed torque will remain positive, and direction of the rotation will be as it was before. The nature of the torque will be pulsating, and the frequency will be twice that of line frequency as shown in the waveform.

21. Applications of Universal Motor The Universal motor is used for the purposes where speed control and high values of the speed are necessary. The various applications of the Universal Motor are as follows :- Portable drill machine. Used in hair dryers, grinders and table fans. A universal motor is also used in blowers, polishers and kitchen appliances.

22. Characteristic of Universal motor The characteristic of Universal motor is similar to that of the DC series motor. When operating from an AC supply, the series motor develops less torque . By interchanging connections of the fields with respect to the armature, the direction of rotation can be altered. Speed control of the universal motors is obtained by solid state devices. This motor is most suitable for applications requiring high speeds. Since the speed of these motors is not limited by the supply frequency and is as high as 20000 rpm.

23. AC Series Motor

24. Doubt clearing Time

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