In this chapter, operation of uncontrolled induction motor drives is exam-ined. We briefly outline methods of assisted starting, braking, and re-versing. Speed control by pole changing is explained, and we describe abnormal operating conditions of induction motors.
3 UNCONTROLLED INDUCTION MOTOR DRIVES In this chapter, operation of uncontrolled induction motor drives is examined We briefly outline methods of assisted starting, braking, and reversing Speed control by pole changing is explained, and we describe abnormal operating conditions of induction motors 3.1 UNCONTROLLED OPERATION OF INDUCTION MOTORS In a majority of induction motor drives in industrial and domestic applications, the control functions are limited to the turn-on and turn-off and, in certain cases, to assisted starting, braking, and reversing When driving a load, an induction motor is supplied directly from a power line and operates with fixed values of stator voltage and frequency The speed of the motor is approximately constant, motors with a stiff mechanical characteristic (i.e., with low dependence of load torque on the speed) having been usually used As already mentioned, such a characteristic is associated with a low rotor resistance, that is, with low losses in the rotor 43 44 CONTROL OF INDUCTION MOTORS Thus, high-efficiency motors, somewhat more expensive than standard motors, are particularly insensitive to load changes Clearly, an uncontrolled motor drive is the cheapest investment, but the lack of speed control carries another price In many applications, a large percentage of the electric energy is wasted because of that shortcoming The most common induction motor drives are those associated with fluid transport machinery, such as pumps, fans, blowers, or compressors To control the flow intensity or pressure of the fluid, valves choking the flow are used As a result, the motor delivers full power, a significant portion of which is converted into heat in the fluid This situation is analogous to that of a car driven with a depressed brake pedal Energy and money savings have been the major reason for the increasing popularity of ASDs, which, typically, are characterized by short payback periods Sensitivity to voltage sags constitutes another weakness of uncontrolled drives Even in highly developed industrial nations such as the United States, the power quality occasionally happens to be poor Because the torque developed in an induction motor is quadratically dependent on the stator voltage, a voltage sag can cause the motor to stall This typically leads to intervention of protection relays that trip (disconnect) the motor Often, the resultant process interruption is quite costly Controlled drives can be made less sensitive to voltage changes, enhancing the "ridethrough" capability of the motor 3.2 ASSISTED STARTING As exemplified in Figure 2.18, the stator current at zero slip, that is, the starting current, is typically much higher than the rated current Using the approximate equivalent circuit in Figure 2.16, the starting current, St, can be estimated as V In the example motor, the starting current, at about 250 A/ph, is 6.3 times higher than the rated current For small motors this is usually not a serious issue, and they are started by connecting them directly to the power line However, large motors, especially those driving loads with high inertia or high low-speed torque, require assisted starting The following are the most conmion solutions In autotransformer starting, illustrated in Figure 3.1, a threephase autotransformer is controlled using timed relays The stator CHAPTER / UNCONTROLLED I N D U C T I O N MOTOR DRIVES ABC- 45 POWER LINE nnn MOTOR FIGURE 3.1 Autotransformer starting system voltage at starting is reduced by shutting contacts and 2, while contacts are open After a preset amount of time, contacts and are opened and contacts shut In impedance starting, illustrated in Figure 3.2, series impedances (resistive or reactive) are inserted between the power hne and the motor to limit the starting current As the motor gains speed, the impedances are shorted out, first by contacts 1, then by contacts POWER LINE H\H\M MOTOR FIGURE 3.2 System with starting impedances 46 CONTROL OF I N D U C T I O N MOTORS In wye-delta starting, illustrated in Figure 3.3, a special switch is used to connect stator phase windings in wye (contacts "w") when the motor is started and, when the motor is up to speed, to reconnect the windings in delta (contacts "d") With wye-connected phase windings, the per-phase stator voltage and current are reduced by in comparison with those for deltaconnected windings The wye-delta switch can be controlled manually or automatically In soft-starting, illustrated in Figure 3.4, a three-phase soft-starter based on semiconductor power switches is employed to reduce the stator current This is done by passing only a part of the voltage waveform and blocking the remaining part The voltPOWER LINE STATOR PHASE WrONGS AND WYE-DELTA SWITCH FIGURE 3.3 Starting system with the wye-deha switch POWER L i e MOTOR FIGURE 3.4 Soft-starting system CHAPTER / UNCONTROLLED INDUCTION MOTOR DRIVES 47 age and current waveforms are distorted, generating harmonic torques, until, when the motor has gained sufficient speed, the soft-starter connects it directly to the power line Various starting programs, such as maintaining a constant current or ramping up the voltage, can be realized In comparison with the direct online starting, all the preceding methods of assisted starting result in reduction of the starting torque This, with certain loads, can be a serious disadvantage As explained later, the variable-frequency starting in ASDs does not have this disadvantage, allowing for high values of the torque As an interesting observation, it is worth mentioning that the total energy lost in the rotor during starting is approximately equal to the total kinetic energy of the drive system in the final steady state This is because the efficiency of power conversion in the rotor is — 5* Again, the variable-frequency starting is superior in this respect, because a low slip is consistently maintained 3.3 BRAKING AND REVERSING In drives requiring rapid deceleration, the motor needs to develop a negative torque for braking, especially in systems with low load torque and/ or high inertia Because the torque depends on slip, a proper change in the slip must be effected Apart from frequency control or changing the number of poles of stator winding, there are two ways to induce a negative torque in an induction machine, plugging and dynamic braking Plugging consists in a reversal of phase sequence of the supply voltage, which is easily accomplished by interchanging any two supply leads of the motor This results in reverse rotation of the magnetic field in the motor; the slip becomes greater than unity and the developed torque tries to force the motor to rotate in the opposite direction If only stopping of the drive is required, the motor should be disconnected from the power line at about the instant of zero speed Plugging is quite a harsh operation, because both the kinetic energy of the drive and input electric energy must be dissipated in the motor, mostly in the rotor This braking method can be compared to shifting a transmission into reverse to slow down a running car The total heat produced in the rotor is approximately three times the initial energy of the drive system Therefore, plugging must be employed with caution to 48 CONTROL OF INDUCTION MOTORS avoid thermal damage to the rotor Low-inertia drives and motors with high rotor resistance and, therefore, with a large high-slip torque (see Figure 2.17) are the best candidates for effective plugging 3.1 To illustrate braking by plugging, consider the example motor driving a load under rated operating conditions The mass moment of inertia of the load is twice that of the motor The initial braking torque and total energy dissipated in the rotor by the time the motor stops are to be determined The rated speed is 1168 r/min Thus, when the speed of magnetic field is reversed, the initial slip, s, is (1200 + 1168)71200 = 1.973 The matrix equation (2.13) is EXAMPLE r230l ["-ZW+yW-SSI 715.457 "Ir , and, when solved, it yields = 261.3 A/ph and I^ = 256.7 A/ph The rotor velocity, co^, is TT X 1168/30 = 122.3 rad/s and the equivalent load resistance, /?L, found from Eq (2.10), is —0.077 (1/ph It is negative, because s > I, and consequently the developed torque r ^ , as calculated from Eq (2.12), is negative too Specifically, _ X (-0.077) X 256.7^ ^ _ TM = Ij^ = -124.5 Nm, which is only two-thirds of the rated torque, while the stator current is 6.6 times the rated value The maximum braking torque using this method occurs at zero speed and equals the starting torque of 227 Nm (see Table 2.2) The corresponding stator current of 250 A/ph (see Section 3.2) is still very high at 6.3 times the rated current The load mass moment inertia is X 0.4 = 0.8 kg.m^, and the energy, E^ dissipated in the rotor is three times the initial kinetic energy of the drive system Thus, ^^^3(A^AM = 3