GENERATORS AND MOTORS 7.66 DIVISION 7 2. Stalling of induction motors that are operating close to full load or slightly over full load 3. Objectionable dips in the light output of lamps and flickering of lamps 4. In the case of small private generating plants, danger of overloading of gen- erators. Each electric power company has definite rules for permissible starting currents for motors supplied from its system. At present there is no standardization of these rules, each company having its own set of rules. Some of these rules limit the starting current on a basis of an allowable percentage of full-load current of the motor. With one of the newer rules, called the increment method, the maximum starting current of any motor must not be greater than a certain number of amperes per kilowatt of the customer’s maximum-demand load. 133. The speed regulation of a motor is the percentage drop in speed be- tween no load and full load based on the full-load speed. no-load speed Ϫ full-load speed Percent speed regulation ϭϫ100 (3) full-load speed DIRECT-CURRENT MOTORS 134. Direct-current motors. All dc motors must receive their excitation from some outside source of supply. Therefore they are always separately excited. The interconnection of the field and armature windings can be made, however, in one of the three different ways employed for self-excited dc generators. A dc motor may be designed, therefore, to operate with its field connected in parallel or in series with its armature, producing, respectively, a shunt or a series motor. If the machine is provided with two field windings, one connected shunt and the other series, it is a compound motor. The speed of a shunt motor connected directly to the line is nearly constant from no load to full load, while the speed of a series motor drops off rapidly as the load is increased. If a series motor were operated at no load with normal voltage, it would attain a dangerous speed, so high in most cases that it would throw itself apart by centrifugal force. A series motor should never be operated at no load unless there is sufficient external resistance connected in series with the motor to limit its speed to be a safe value. For this reason a belt drive should never be used with a series motor. Standard compound motors are designed to operate with the shunt and series fields connected so as to aid each other (cumulative). Their operating characteristics therefore are a compromise between those of shunt and series motors. Their speed drops off considerably as the load is increased, but not nearly so much as with series motors. A compound motor will not run away under no load. The amount of change of speed from no load to full load depends upon the strength of the series field. The stronger the series field, the greater the change in speed from no load to full load. Special compound motors with their field windings connected to buck each other (differential motors) are used sometimes for special applications. If the series field is of just the proper strength, a differential compound motor will Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. GENERATORS AND MOTORS GENERATORS AND MOTORS 7.67 operate at a more constant speed than a shunt motor, but it tends to be unstable under overload conditions. Many shunt motors have a weak series-field winding in addition to the shunt- field winding. The series-field winding is connected cumulatively and consists of only a very few turns. The purpose of the series-field winding is to counteract partially the effect of the armature current upon the speed of the motor. The ar- mature current tends to reduce the strength of the magnetic field of the motor. Therefore, as the load on the motor is increased, the decrease in the strength of the field may cause a rise in the speed. A motor with a rising-speed characteristic is unstable. A few series-field turns will sufficiently counteract the demagnetizing effect of the armature current and stabilize the speed characteristic of the motor. These motors, although actually cumulative compound motors with a very weak series field, are called stabilized shunt motors. Typical characteristics for shunt, series, and compound motors are shown in Figs. 7.71, 7.72, and 7.73. FIGURE 7.71 Typical characteristics for a FIGURE 7.72 Typical characteristics for a se- shunt motor. ries motor. 1This material is reproduced with the permission of the National Electrical Manufacturers Association from the NEMA Standards Publication MG 1-1993 copyright ᭧ 1995 by NEMA. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. GENERATORS AND MOTORS 7.68 DIVISION 7 FIGURE 7.73 Typical characteristics for a FIGURE 7.74 Torque characteristics for dc compound motor. motors. The speed regulation due to variations in load from rated load to no load of standard dc motors which meet NEMA1 requirements will not exceed the following values: 135. Torque characteristics of dc motors. The torque developed by any one of the three types of dc motors, either at starting or while running, depends upon the product of the magnetic flux and the armature current. Since in a shunt motor the flux is nearly constant at all times, the torque varies nearly directly with the value of the armature current. In a series motor the armature current flows directly through the field winding, so that the flux is not constant but varies almost directly with the value of the armature current. The torque of a series motor, therefore, varies approximately as the square of the armature current. Doubling the armature current will make the torque 4 times as great. The cumulative compound motor has a torque characteristic that is a compromise between that of the series motor and that of the shunt motor. The torque will increase faster than the increase in armature current but not so fast as the square of the armature current. The way in which the torque varies with the armature current for a cumulative compound motor depends upon the relative strength of the series and shunt fields. The curves of Fig. 7.74 indicate the relation between torque and armature current for the different types of dc motors. The maximum value of starting torque that it is satisfactorily possible for any dc motor to produce is limited only by the commutating conditions. These usually limit the current to approximately 200 percent of full-load current for a shunt motor and 250 percent of full-load current for a series or compound motor. The corre- sponding torques are approximately 200 percent of full-load torque for a shunt motor, 300 percent for a compound motor, and 400 percent for a series motor. There is no value of pullout torque for a dc motor. If the load on a dc motor were continually increased, the motor would gradually slow down as the load in- creased until finally the load would become so great that the motor would gradually stop. Of course, it would never be feasible in practice to overload the machine to such an extent that it would stop, and long before such a load was reached the fuses in the circuit would blow. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. GENERATORS AND MOTORS GENERATORS AND MOTORS 7.69 136. Method of starting dc motors A dc motor of any capacity, when its armature is at rest, will offer a very low resistance to the flow of current, and an excessive and perhaps destructive current would flow through it if it were connected directly across the supply mains while at rest. Consider a motor adapted to a normal full-load current of 100 A and having a resistance of 0.25 ⍀; if this motor were connected across a 250-V circuit, a current of 1000 A would flow through its armature; in other words, it would be overloaded 900 percent, with consequent danger to its windings and also to the driven machine. For the same motor, with a rheostat having a resistance of 2.25 ⍀ inserted in the motor circuit, at the time of starting the total resistance to the flow of current would be the resistance of the motor (0.25 ⍀) plus the resistance of the rheostat (2.25 ⍀), or a total of 2.5 ⍀ . Under these conditions exactly full-load current, or 100 A, would flow through the motor, and neither the motor nor the driven machine would be overstrained in starting. This indicates the necessity of a rheostat for limiting the flow of current in starting the motor from rest. An electric motor is simply an inverted generator or dynamo. Consequently when its armature begins to revolve, a voltage is generated within its windings just as a voltage is generated in the windings of a generator when driven by a prime mover. This voltage generated within the moving armature of a motor opposes the voltage of the circuit from which the motor is supplied and hence is known as a counter-emf. The net voltage tending to force current through the armature of a motor when the motor is running is, therefore, the line voltage minus the counter- emf.