Electrical Machine - II -English-QB Compose By Lalit Chaudhary

Special Purpose Machines 151 (i) Synchronous Speed: Synchronous speed of a linear induction motor is given by the relation, vl = 2wf Where, vl = linear synchronous speed (m/s), w = width of one pole pitch (m), f = supply frequency (Hz). (ii) Slip: The slip of a linear induction motor is given by the following relation, s = (vl – v) vl Where, v = actual speed of the motor (m/s). (iii) Thrust or Force: It is given by the following relation, F = P2 vl Where, P2 = active power supplied to the rotor. (iv) Active Power Flow: It is similar to that in a rotating motor and can be given by, Pcr = sP2 and Pm = (1 – s) P2 Q. 3. What are the advantages and disadvantages of a linear induction motor? Ans. Advantages: The advantages of a linear induction motor are as follows: (i) This motor has low initial cost. (ii) It has low maintenance cost because the absence of rotating parts. (iii) This motor has no limitations of tractive effort due to adhesion between the wheel and the rail. (iv) It has no limitation of maximum speed due to centrifugal forces. (v) It has no over heating of rotor because the motor moves continuously over cool rotor plate leaving behind heated rotor portion. (vi) This motor has power to weight ratio is better. (vii) It is simplicity. Disadvantages: The disadvantages of a linear induction motor are as follows: (i) Poor utilisation of motor due to transverse edge effect and end effect. (ii) In this larger air-gap and non-magnetic reaction rail (rotor plate) need more magnetizing current resulting in poor efficiency and low power factor. (iii) It has very high capital cost of reaction rail fixed along the centre line of the track.

152 Electrical Machines–II (iv) This motor has complication and high cost involved for providing three phase collector system along the track. (v) This motor has difficulties encountered in maintaining adequate clearances at points and crossings. Q.4. Write short note on stepper motor and its applications. (2007) Write short note on stepper motor. (S/2010, 11, S/13, 15, 17) Ans. These motors are also called stepping motors or step motors. 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. In recent year, there has been wide spread demand of stepping motor because of the explosive growth of the computer industry. Their popularity is due to the fact that they can be controlled directly by computers, microprocessors and programmable controllers. Stepping motor are ideally suited for situations where either precise positioning or precise speed control or both are required in automation system. The unique feature of stepper motor is that its output shaft rotates in a series of discrete angular intervals or steps, one step being taken each time a command pulse is received. When a definite number of pulses are supplied, the shaft turns through a definite known angle. This fact makes the motor well suited for open loop position control because no feedback need to be taken from the output shaft. Such motors develop torque ranging from 1 µ N-m (in any tiny wrist watch motor of 3 mm diameter) upto 40 N-m in a motor of 15 cm diameter suitable for machine tool applications. Their output power 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, commutator, or brushes. This feature makes the motor quite robust and reliable. Applications: The applications of stepper motor are as follows: (i) These motors are used for operation control in computer peripherals, textile industry for fabrication and rotarties etc. (ii) Application requiring incremental motion are typewriter, line printers, tape drivers, floppy disc drives, numerically controlled machine tools, process control system and X-Y plotters. (iii) Stepper motor also performs countless takes outside the computer industry. It includes commercial, military and medical applications where they perform mixing, cutting, striking, metering, blending and purging. Q.5. What is step angle in a stepper motor? Ans. The angle through which the motor shaft rotates for each command pulse is called the step angle β. Smaller the step angle, greater the number of step per revolution and higher the resolution or accuracy of positioning obtained. The step angle can be as small as 0.720 or as large as 900. But the most common step sizes are 1080, 2.50, 7.50 and 150.

Special Purpose Machines 153 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. β = (Ns – Nr) × 3600 NsNr or β = 3600 mNr = 3600 No. of stator phases × No. of rotor teeth Q.6. What is resolution is stepper motor? What do you understand by slewing? Ans. Resolution is given by the number of steps needed to complete one revolution of the rotor shaft. Higher the resolution, greater the accuracy of positioning of objects by the motor. ∴ Resolution = No. of steps Revolution = 3600 β Where, β is step angle. A stepping motor has the extraordinary ability to operate at very high stepping rates (upto 20,000 steps/second in some motor) and yet to remain fully in synchronism with the command pulses. When the pulse rate is high, the shaft rotation seems continuous. Operation at high speeds is called slewing. When in the slewing range, the motor generally emits an audible while having a fundamental frequency equal to the stepping rate. If f is the stepping frequency (or pulse rate) in pulses/second (pps) and β is the step angle, then motor shaft speed is given by, n =β × f rps = Pulse frequency resolution 360 If the stepping rate is increased too quickly, the motor loses and synchronism stops. Same thing happens if when the motor is slewing, command pulse are suddenly stopped instead of being progressively slowed. Q.7. What are the different types of stepper motors? State each motor in brief. Ans. There is large variety of stepper motors which can be divided into the three basic categories are as follows: (i) Variable Reluctance Stepper Motor: It has would stator poles but the rotor poles are made of a ferromagnetic material as shown in fig. 6.2 (a). It can be of single stack type or multitask type which gives smaller step angle. Direction of motor rotation independent of

154 Electrical Machines–II the polarity of the stator current. It is called variable reluctance motor because the reluctance of the magnetic circuit formed by the rotor and stator with varies with the angular position of the rotor. Fig. 6.2 (ii) Permanent Magnet Stepper Motor: It also has wound stator poles but its rotor poles are permanently magnetized. It has a cylindrical rotor as shown in fig. 6.2 (b). Its direction of rotation depends on the polarity of the stator current. PM stepper motor is also called variable speed brushless DC motor. (iii) Hybrid Stepper Motor: It has wound stator poles and permanently magnetized rotor poles as shown in fig. 6.2 (c). It is best suited when small step angles of 1.80, 2.50 etc. are required. The hybrid motor combines the features of VR stepper motor and PM stepper motor. Its stator constitution is similar to the single stack VR motor but the rotor is cylindrical and is composed of radically magnetized auxirally to give small stepping angle and low inertia. Q.8. Explain working of PM motor. (2010) Write a short note on permanent magnet motor. (S/2011, S/12) Ans. Permanent magnet (PM) AC motors are nothing but polyphase synchronous motors consisting of permanent magnet rotors. Unlike synchronous machines, these machines do not have the field windings. In these machines slip rings and brushes are not required. Construction: The constructional features of permanent magnet AC motors are shown in fig. 6.3. These motors having cage rotor with rate earth permanent magnet. When fixed frequency supply is given to such motors, then it is started as an induction motor initially and then runs synchronously. No, DC excitation is required for the rotor which makes the motor reliable and robust. These motors are similar to permanent magnet stepping motors having non salient stator. These motors having rating

Special Purpose Machines 155 upto 100 kW with maximum synchronous torque designed to about 150% of full load torque. The speed and position sensors are mounted on the rotor shaft for controlling the motor. For shaft positioning number of techniques such as hall effect devices, high emitting diodes and phototransistors in combination with pulsed wheel and inductance pickup can be employed. Fig. 6.3 Cross Section of Permanent Magnet AC Motor Working Principle and Operation: The operation of permanent magnet AC motor is similar to that of permanent magnet stepping motor. The shaft position sensor gives the information of the rotor position and accordingly the phase windings are excited sequentially in such a way that the required speed and torque is produced. The motor speed is decided by the frequency of the supply which excites the phase windings. The angular position between the rotor magnetic axis and that of a phase winding produces the necessary torque for operation. All these motors are capable of measuring rotor speed and position and their phase windings may be excited with variable frequency and amplitude, these motors are highly controllable and exhibit wide range of operating characteristing whenever coupled with appropriate electronics and drive system. Hence, in recent times these motors are widely used in many applications. These motors are also referred as brushless motors or brushless DC motors as these motors have the characteristics similar to that of DC motors and no brushes are required, since the armature current is electronically commutated by shaft position sensor and by switches like transistors and thyristors connected to armature winding. Q.9. Write the advantages, disadvantages and applications of permanent magnet motor.

156 Electrical Machines–II Ans. Advantages: The advantages of permanent magnet motor are as follows: (i) As the slip rings or brushes are absent so there is no sparking. (ii) These motor are flexible as they are highly controllable. (iii) No DC field excitation is required so field losses are reduced. (iv) It requires less space as field windings are absent. (v) They have low inertia and friction. (vi) These motors are reliable and having long life. Disadvantages: The disadvantages of permanent magnet motor are as follows: (i) The electronic circuitry driving the motor is complex and complicated. (ii) There is possibility of demagnetization due to excessive currents. Applications: The applications of permanent magnet motor are as follows: (i) These motors can be used where constant speed is required such as synthetic fiber drawing. (ii) They are also used in turn table drives in recording players, spindle drives in hard disk drives in computers and also in computer peripheral equipments. (iii) These motors are also used for tape drive for video recorders. (iv) They have more importance in the field of aerospace, gyroscope motors and biomedical instruments like artificial heat pumps. Q.10. Write note on synchros. Ans. Synchros are a set of two or more motors whose stators are wound in slots like the stator of a three phase induction motor and single phase salient pole (two poles) rotors. Their rotor windings are connected across a common single phase AC supply source, and their stators are connected together, phase to phase, through three pilot wires, as shown in fig. 6.4. When the rotors of the synchros are in the same position relative to each other, the electro motive forces induced in their stator windings oppose each other and hence, an electric current flows in them originally (the two rotors are in the same space position relative to their stator windings). But when a relative displacement error in the rotor position takes place, the emf balance is disturbed and then circulating currents flow in the stator circuits. These currents are such that they produce such torque tends to restore the rotor positions to a state of mutual correspondance. This restoring torque is approximately proportional to the error (expressed in degrees) itself. But with an error of 1800 the restoring torque drops down to zero and under this condition the synchro system becomes unstable, as then the lightest angular deviation of

Special Purpose Machines 157 Fig. 6.4 either of the machines sets up a torque which tends to increase the deviation. The synchros have the same general constructional features as other small motors. Both the stator and stator cores are laminated. The friction of rotor hearings must be very small, hence, low friction ball bearings are used. Dampers are employed to force the synchros to settle quickly to their angular position. Q.11. Enumerate the application of small synchros. Ans. The application of small synchros as follows: (i) Control system for indicating misalignment of a shaft in terms of an error emf. (ii) Instrument and indicator system are uses in position synchros. NUMERICAL PROBLEMS AND SOLUTIONS Prob.1. A single sided linear induction motor has 98 poles, and its pole pitch is 50 cm. The motor is used for propelling an electric vehicle. Determine the linear synchronous velocity and the vehicle speed in km/hour if the frequency is 50 Hz and slip is 0.25. Sol. Given, Supply frequency, f = 50 Hz Pole pitch, w = 50 cm = 0.5 m Slip, s = 0.25 Linear synchronous velocity, vs = 2wf = 2 × 0.5 × 50 = 50 m/s

158 Electrical Machines–II = 50 × 60 × 60 1000 = 180 km/h Ans. Vehicle speed, u = Vs (1 – s) = 180 (1 – 0.25) = 135 km/h Ans. Prob.2. A stepper motor has a step angle of 2.50. Determine: (i) resolution on, (ii) number of steps required for the shaft to make 25 revolutions, (iii) shaft speed, if the stepping frequency is 3600 rps. Sol. (i) Resolution on, = 3600 β = 3600 2.50 = 144 steps/revolution Ans. (ii) Now, steps/revolution = 144 Ans. Hence, steps required for making 25 resolutions = 144 × 25 = 3600 (iii) Shaftspeed, η = β × f 3600 = 2.50 × 3600 3600 = 25 rps Ans. FF