CHAPTER 3 INDUCTION MOTOR AND DIFFERENT SPEED CONTROL METHODS

Transcription

1 CHAPTER 3 INDUCTION MOTOR AND DIFFERENT SPEED CONTROL METHODS Page No 3.1 Introduction Three Phase Induction Motor Stator Rotor Working principle of three phase induction motor Equivalent Circuit Slip Different Speed Control Methods Stator voltage control method Frequency control method Volts Hertz (V/F) control method Conclusion 71 57

2 CHAPTER 3 INDUCTION MOTOR AND DIFFERENT SPEED CONTROL METHODS 3.1 Introduction This chapter describes the construction, principal of operation, start up consideration and the basic speed control methods for induction motor. 3.2 Three Phase Induction Motor (IM) The three-phase induction motors are also called as asynchronous motors, which are most commonly used type of motor in industrial applications. In particular, the squirrel-cage induction motors are widely used electric motor in home and industrial applications, because these machines are very economical, rugged and reliable. They are available in the ranges of Fractional Horse Power (FHP) to multi-megawatt capacity. Fractional Horse Power motors are available in single-phase as well as poly-phase (three-phase).the three phase machines are used most often in variable-speed drives where the torque requirement is more. An induction motor or asynchronous motor is a type of alternating current motor where power is supplied to rotor by means of electromagnetic induction. An electric motor rotates because of magnetic force exerted between a stationary electromagnet called the stator and rotating electromagnet called the rotor. The current in stator side creates an electromagnetic field which interacts with the secondary to produce a resultant torque, transforming electrical energy into mechanical energy. The cross sectional view of induction motor and its various parts [4] are as shown in Fig 3.1 Construction: In this section, the construction details of induction motor are discussed. A three phase induction motor mainly consists of two parts, stator and rotor. Stator is the stationary 58

3 part while the rotor is the rotating part of the motor and they are separated by a small air gap depending on the rating of the motor. Fig. 3.1: Squirrel-cage three-phase Induction motor Fig. 3.2 Stator 59

4 3.2.1 Stator The stator is as shown in Fig 3.2, which consists of a steel frame which encloses a hollow cylindrical core made up of thin laminations of silicon steel to reduce eddy current and hysteresis loss. A large number of uniform slots are cut on the inner periphery of the core. The stator conductors are placed in these slots which are insulated from one another and also from the slots. These conductors are connected as a balanced three phase star or delta winding. The windings are wound for a definite number of poles depending on the requirement of speed. It is wound for more number of poles, if speed required is less and vice versa. According to the relation Ns = 120f / P (3.1) Where Ns is the synchronous speed in RPM f is the supply frequency P is the number of poles When a three phase supply is given to stator winding a magnetic field of constant magnitude and rotating at synchronous speed is produced. This rotating magnetic field is mainly responsible for producing the torque in the rotor, so that it can rotate at the rated speed Rotor The rotor is the rotating part of the induction motor and is mounted on the shaft of the motor to which any mechanical load can be connected. Based on the construction of the rotor, induction motors are broadly classified in two categories; squirrel cage motors and slip ring motors. The stator construction is the same in both motors. 60

5 Squirrel cage motors: Fig. 3.3: Squirrel cage type rotor Almost 90% of induction motors are squirrel cage motors. This is because the squirrel cage motor has a simple and rugged construction. Fig 3.3 shows the squirrel cage type rotor, it consists of cylindrical laminated core with axially placed parallel slots for carrying rotor conductors. The rotor conductors are heavy bars of copper or aluminium. Each slot carries a copper, aluminium or alloy bar. If the slots are semi closed, then these bars are inserted from the ends. These rotor bars are permanently short-circuited at both ends by means of the end rings, thus short circuiting themselves at both the ends. As rotor bars are short circuited on themselves, it is not possible to add any external resistance in series with rotor circuit during starting. The slots are slightly skewed, which helps in two ways i.e. It reduces noise due to magnetic hum and makes motor run quietly. It reduces locking tendency between rotor and stator. Advantages of squirrel cage rotor: 1. It is simple in construction, rugged and can withstand rough handling. 61

6 2. Maintenance cost is low. 3. It has better efficiency and power factor. 4. A simple star delta starter is sufficient to start the rotor. 5. It is explosion proof as there are no slip rings and brushes. Slip ring motors: In the slip ring motor, the windings on the rotor are terminated to three insulated slip rings mounted on the shaft with brushes resting on them. This allows an introduction of an external resistor to the rotor winding. The external resistor can be used to boost the starting torque of the motor and change the speed-torque characteristic. When running under normal conditions, the slip rings are short-circuited, using an external metal collar, which is pushed along the shaft to connect the rings. In normal conditions, the slip ring motor functions like a squirrel cage motor Working principle of three phase induction motor When a three phase supply is given to the three phase stator winding, a magnetic field of constant magnitude and rotating at synchronous speed N s is produced. This rotating magnetic field sweeps across the rotor conductors and hence an electromagnetic force (EMF) is induced in rotor conductors. As the rotor conductors are short circuited on themselves the induced EMF sets up a current in the rotor conductors in such a direction as to produce a torque, which rotates the rotor in same direction as magnetic field so that relative speed decreases. The speed of rotor gradually increases and tries to catch up with the speed of rotating magnetic field, but it fails to reach synchronous speed, because if it catches up with speed of magnetic field, relative speed becomes zero and hence no EMF will be induced in the rotor conductors, the torque becomes zero. Hence, rotor will not be able to catch up with the speed of magnetic field but rotates at a speed N r which is slightly less than the synchronous speed [3]. 62

7 3.2.4 Equivalent Circuit Fig.3.4 Per phase equivalent circuit of induction motor The equivalent circuit [3] is as shown in Fig From the equivalent circuit diagram the various power expressions can be written as follows: Input power = 3V s I s cos Φ (3.2) Stator copper loss = P ls = 3 I 2 SR s (3.3) Core Loss = (3.4) Power across air gap = p g = (3.5) Rotor copper loss: P lr = 3 R r (3.6) Output power: P o = P g - P lr = 3 R r ( ) (3.7) Since the output power is the product of developed torque T e and speed ω m, T e can be expressed as T e = T e = R r ( ) = (3.8) 63

8 From the equivalent circuit, the approximate equivalent circuit can be obtained as shown in Fig 3.5, where the core loss resistor R m has been dropped and the magnetizing inductance L m has been shifted to the input. This approximation is easily justified for an integral horsepower machine, where (R s + j L ls )<< L m [5]. The performance prediction by the simplified circuit typically varies within 5 percent from that of the actual machine Fig. 3.5 Approximate per phase equivalent circuit of IM In Figure 3.5, the current I r is figured out by: I r = (3.9) Substituting Equation (3.9) in (3.8) yields T e = (3.10) A further simplification of the equivalent circuit of Fig 3.4 can be made by neglecting the stator parameters R s and L ls. This assumption is not unreasonable for an integral horsepower machine, particularly if the speed is typically above 10 percent. Then, the equation (3.10) can be simplified as 64

9 T e = 3 ) (3.11) where, ω sl = sω e The air gap flux can be given by (3.12) in a low-slip region, equation (3.11) can be approximated as T e = (3.13) where, >> Equation (3.11 ) is important because it indicated that at constant flux, the torque T e is proportional to slip frequency, or at constant slip frequency, torque T e is proportional to Ψ 2 m Slip The slip can be defined as the difference between the synchronous speed and actual speed of the machine. It can be expressed in the percentage. Based on this slip speed, the voltage induced in the rotor winding changes, which in turn changes the rotor current and also the torque. As slip increases, the rotor current and the torque also increases. The rotor moves in the same direction as that of the rotating magnetic field to reduce the induced current (Lenz s law). The slip can be expressed as given below [1][2]: (3.14) or Slip S = = (3.15) or rotor speed N r = Ns (1-S) (3.16) Synchronous speed is given by Ns = (3.17) Where P represents the number of poles and f is stator frequency in Hz 65

10 Therefore equation 3.16 becomes, Rotor speed N r = (1-S) (3.18) Thus, the speed of an induction motor depends on slip S, stator frequency f and the number of poles P for which the windings are wound. 3.3 Different Speed Control Methods From equation 3.18, the speed of IM can be varied by varying the slip S or number of poles p or frequency of supply. The different methods of speed control of induction motor can be broadly classified in to scalar and vector control methods. In this work, scalar control methods are used. Hence only details of scalar methods are discussed here. The explanation of vector control method is beyond the scope of this thesis. The scalar methods of speed control [4] can be classified as Stator voltage control Frequency control Stator voltage and frequency control i.e Volts Hertz control Rotor voltage control The first three methods are the basic methods of speed control and are explained in detail as follows: Stator voltage control method A very simple and economical method of speed control is to vary the stator voltage at constant supply frequency. The three-phase stator voltage at line frequency can be controlled by controlling the switches in the inverter. As seen from the equation (3.10) the developed torque is proportional to the square of the stator supply voltage and a reduction in stator voltage will produce a reduction in speed. Therefore, continuous speed control may be obtained by adjustment of the stator voltage without any alteration in the stator frequency. 66

11 T e = (3.19) The Torque speed curves with variable stator voltage [3] control are shown in Fig 3.6. Fig.3.6: Speed-Torque characteristics with variable stator voltage The salient features of stator voltage control method are: For low-slip motor, the speed range is very low. Not suitable for constant-torque load. Poor power factor. Used mainly in low power applications, such as fans, blowers, centrifugal pumps, etc. 67

12 3.3.2 Frequency control method The torque and speed of induction motors can be controlled by changing the supply frequency but keeping the voltage constant. If the frequency is decreased keeping voltage constant, then saturation of air-gap flux takes place. At low frequency, the reactance will decrease and the motor current may be too high. If the frequency is increased above its rated value, then the air gap flux and rotor current decreases correspondingly, the developed torque also decreases. Due to these reasons this method of control is rarely used. The torque speed characteristics with frequency control [3] are shown in Fig.3.7. Fig.3.7 Speed-Torque characteristics with variable frequency control Volts Hertz (V/F) control method The constant V/F control method is the most popular method of Scalar control. If an attempt is made to reduce the supply frequency at the rated supply voltage, the air gap flux Ψ m will tend to saturate, causing excessive stator current and distortion of flux wave. 68

13 Therefore, the region below the base or rated frequency should be accompanied by the proportional reduction of stator voltage so as to maintain the air gap flux constant. If the ratio of voltage to frequency is kept constant, the flux remains constant. By varying the voltage and frequency the torque and speed can be varied. The torque is normally maintained constant while the speed is varied. This arrangement is widely used in the locomotives and industrial applications. The purpose of the volts hertz control scheme is to maintain the airgap flux of AC Induction motor constant in order to achieve higher run-time efficiency. The magnitude of stator flux is proportional to the ratio of stator voltage & the frequency. If ratio is kept constant the stator flux remains constant & motor torque will only depends upon slip frequency. In variable-frequency, variable-voltage operation of a drive system, the machine usually has low slip characteristics (i.e low rotor resistance), giving high efficiency. In spite of the low inherent starting torque for base frequency operation, the machine can always be started at maximum torque as indicated in Fig 3.8.The absence of high in-rush starting current in a direct-start drive reduces stress and therefore improves the effective life of the machine. By far the majority of variable-speed ac drives operate with a variable-frequency, variablevoltage power supply. Fig 3.8 shows the torque-speed characteristics [3] of the machine with constant V/F control method. Other than the variation in speed, the torque-speed characteristics of the V/F control reveal the following: The starting current is low. The stable operating region of the motor is increased. Instead of simply running at its base/ rated speed (N B ), the motor can be run typically from 5% of the synchronous speed (N S ) up to the base speed. The torque generated by the motor can be kept constant throughout this region. 69

14 Since almost constant rated torque is available over the entire operating range, the speed range of the motor becomes wider. User can set the speed as per the load requirement, thereby achieving the higher efficiency. One of the most advantages is soft start capability in which motors are ramped up to speed instead of being abruptly thrown on line. This useful feature reduces mechanical stresses on the motor and leads to lower maintenance cost as well as a longer motor life. Because of above reasons V/F control method is proposed in this work. Fig.3.8 Speed-Torque curves with constant V/F ratio 70

15 3.4 Conclusion The construction, different parts of induction motor i.e stator and rotor, different types of induction machines, advantages of squirrel-cage induction motor over wound-rotor induction motor, the working principle, equivalent circuit of induction motor, various power expressions, slip, the different speed control methods of induction motor and the speed v/s torque characteristics are discussed in this chapter. 71