FIGURE 1673 Speed-torque curve of repulsioninduction motor
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During running, the speed is constant and is xed by the frequency and number of poles The speed is not affected by change of load or eld strength Changing the eld, however, does change the power factor If load is applied beyond the breakdown point, the rotor falls out of step and stops, since it has no torque below synchronous speed Synchronous motors are being used for a greater variety of services each year because they improve the power factor They must therefore be designed for their particular duty A normal motor for industrial service has approximately 50 to 100 percent starting torque, 100 percent pull-in torque, and 175 percent pullout torque Pull-in torque is that value developed by the squirrel-cage winding at a speed high enough to put on the eld, usually about 95 percent Pull-out torque is the maximum the motor will carry without falling out of step Motors with high starting torque, up to 175 percent, are sometimes necessary for certain applications, such as ball- or rod-mill drives High starting torque means a motor of larger size to meet this particular requirement or a special type of motor One method is to have the motor connected to the load through a magnetic clutch Sometimes this clutch is built with the motor; sometimes it is separate With this arrangement, the motor starts without load After it is up to speed and the eld is applied, the clutch is energized, bringing the load up to speed This allows the motor to use its pull-out torque when starting the load A supersynchronous motor is sometimes used in which the rotor is solidly coupled to the load and the stator is placed in bearings When power is applied to the motor, the stator revolves and comes up to synchronous speed The eld is applied and the motor is synchronized A brake is then applied to the stator, bringing it to rest The rotor, being held in synchronism with the stator, starts and comes up to speed By this method, the pull-out torque is also available for starting the load A synchronous motor has the inherent characteristic of having its power factor dependent on the load and eld strength For a given load, an increase in eld strength gives a leading power factor, and a decrease in eld strength gives a lagging power factor Figure 1674 shows the relation between line current and eld excitation and the resulting power factor In Fig 1674, curves 1, 2, and 3, line current is plotted against excitation The low point of each of these V-curves gives unity power factor at that line current
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FIGURE 1674 Excitation pf curves
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The eld current necessary to give unity power factor at normal line current is the normal eld current of the motor Curves A, B, and C are curves of equal power factor In order to hold the power factor at unity or leading, it is necessary to increase the eld with load For a motor with xed excitation, the power factor varies with the load With normal excitation and light load, the power factor is leading; as the load increases, the power factor reaches unity, and then becomes lagging As stated in Sec 88, the impressed voltage is equaled by the induced voltage plus the IR drop This statement is nearly true of synchronous motors, but there are other variables Since the speed is xed, the induced voltage varies with the eld strength The third member of the equation, the IR drop, is now superseded in importance by the impedance drop ZI, which is always nearly 90 out of phase with the current Figure 1675a shows these voltages, where E is the applied voltage and e is the back emf or induced voltage Figure 1675a is for unity power factor, and thus the current I is in phase with the applied voltage, and IE equals power or kilowatts If when holding constant load the eld is increased, then e is increased and the diagram becomes as shown in Fig 1675b Since the power remains constant, the current in phase with the applied voltage remains the same Therefore, the line representing the current, times cos , must equal the original current ZI is always nearly 90 out of phase with I and must increase as I increases, to I1 or Z11 ZI / cos Completing the triangle gives e1 The angle represents the power factor and in this case is leading If the eld is decreased, then e must decrease to e2, which may be determined by the same reasoning This, shown in Fig 1675c, gives a lagging power factor Synchronous motors are often used for power-factor correction The motor usually does some mechanical work and is given a horsepower rating on this basis It is then designed to carry a leading current so as to offset or neutralize the lagging current required by induction motors The motor is rated 050 or other power factor to show that it can deliver leading current Sometimes synchronous motors are used running idle on the line All their capacity is then used to give leading current They are then known as synchronous condensers
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