Document a1y1960bVakEbe42pDNLkOg5B

X k ArmOtlirS iiom fto* of currant 1 Equipped with Ward-Ueonord speod control, this 7000-hp dc motor, ot 40*rpm boie speed, reveries to 40 rpm in two sec *_ A Generator feeds ormature of controlled motor. Motor ~ changed by varying ormoture voltoge with generator I V' Ward-Leonard System Assures ExaH *%totcr been amffgtnorfftod bocouu motor fitaffttueutt ^ ormeaurt conductors (A) Noimol operation Regenerative braking or pump baek occurs when generator voitoge drops and connected motor hos lood with high WRs ioi A Generator field control for reversing motor.. If generator * . is large, vorloble voltage exciter is used for field supply Speed Control For DC Motors Dc ihunt motors, using Word* Leonard speed-control system, suit drives which call for wide speed variation or the maintenance of exact speed, regardfess of load changes By HENRY OMAN Engineer, AHis-Chalman Mfg. Co. Milwaukee, Wli. Squmrel-cace tNPUCTion motors ere edequete for most industrial drives. They are simple, relatively Inexpensive and reliable. However, there are some applications that require exactly con stant speed-under conditions of variable load. Other drives require an unusually wide range of speed. For these, and other special applications where exact control is needed, the Ward-Leonard system, although 57 years old. Is hard to beat. The basic patent, issued to the late Harry Ward Leonard on November 24, 1891, covered speed variation of a dc motor by changing its armature voltage. Variable armature voltage was obtained by varying the 6eld of on intermediate generator. Since that time many pat ents have been issued covering modifi cations and refinements. How It Works. Output voltage of a dc generator is produced by rotating ormoture conductor* through lines of flux created by stationary field poles. Amount of voltage depends on rate at which these lines of flux are cuL By simply adjusting rheostat in generator shunt-field circuit, it is possible to in crease or decrease current In the field, and hence the flux. Thus, output trollage of the d-c generator can be varied, os shown in Fig. 2, by simply adjusting field rheostat. A do motor has a speed characteristic described approximately by the for mula; K x Armature Voltage Speed s= " 1 " Field Flux K, the constant, depends upon elec-' trical design of the motor. Field flux can be varied by adjusting the motor field current with a rheostat. If field current Is held constant, then motor speed varies directly with armature voltage. If annatuit voltage U held constant, speed varies approximately in versely with field current. Obviously, it isn't desirable to reduce field current of a motor to tero with armature voltage on the machine, since speed would tend to approach infinity. It is interesting to note that speed in an ideal machine Is not influenced by load torque, although torque does have a slight effect on the speed of a practi cal do motor. However a motor can be designed so variation in speed caused by load fluctuations is small The Ward-Leonard speed-eon system is rnsde by simply conn* generator to motor as in Fig. 1 speed can be adjusted either by, ing generator or motor field. A pro' designed motor will stay close loi speed under all load cojidlUorix *j tional controls ean be incorpotsl* generator field circuit to keep' speed within a value which d~: vary from its set speed by more 0.1 percent of the motors rar peed.. Generator of a Ward-L system is usually driven by a r ous or induction motor, or by a,; mover such as a diesel engine or turbine. Reversing Service. With the Leonard speed-control system, U i as easy to reverse a machine to change speed. From the riR (or the speed of a dc motor, Figis apparent that if armature fo is negative, the speed of the tnot$ become negative (reverse). A potentiometer type of field permits reverting generator fie . 4a. In the poaitiorf shown, poten ter contacts place +80 volts oj t tor field. If contacts ate rotated. terelockwUe to tho horizontal \ no voltage la impressed on g*M field, and hence no current flo*ther counterclockwise rotation o tentiometer contacts Impresses n* voltage on generator-field, output voltage then reverses, ei tor starts going backwards. This reversing feature is os. 91 (544) POWER .Saptsffh*. ttuful tn large blooming mills where nal voltage and motor back emf repre e steel ingot is sent through the rolls sents voltage required for forcing the ertl times to reduce It to a long load current through motor armature com. It Is especially suitable for circuit This drop within armature is iesel-elcctric marine propulsion drives normally quite low, about six to ten ;bere it Is necessary to reverse the volts for a 240-v motor. repelier to stop or maneuver the vee- Now suppose generator voltage is For. this type of application, a fly wheel is often installed on the supply ing motor-generator set which is usually driven by a wound-rotor induction mo tor. When a regenerative. Impulse oc curs, flywheel is speeded up slightly by generator. When a sudden load peak 'Soaetimes, when generators sod mo- . etc large, variable-voltage exciters used in field circuit, Fig. 4b. The strolling rheostat or potentiometer sa then be relatively small, and there Do need for power-consuming ro* tot* in main generator or main mo* [ field circuits. The Ward-Leonard cm operates just os effectively with * w ** k circuits as it would fh * constant-voltage supply and rheo- U In tba field circuit Furthermore, Aexciter ean have special fields which *. . * Pfeclse control of speed, torque Power. *lerartre Broking. Another impor- ; M,ure of Ward-Leonard system is ^*enerUvc--braking or pump-back suddenly reduced by changing genera occurs, flywheel slows.down slightly. tor field rheostat setting.-If motor is Hence power requirement for the drive connected to a mechanical load haring * is smoothed out. high inertia. It will not slow down in Often automatic slip regulator* are stantly. Consequently, motor baek emf -installed in secondary of wound-rotor will remain at a high value until motor induction motor which drives generator. and its connected load alow down. If If input current to wound-rotor motor motor back eraf U higher thon terminal becomes high, the slip regulator adds voltage of generator, then current-will resistance to rotor circuit, causing mo flow in the reverse direction, and flow tor to alow down slightly. This permits of power will be from motor to genera flywheel to give up energy to do gen tor. erator. When Input current to wound- . Aa a result, the motor will absorb rotor motor drops off, the slip control mechanical power from Us load and reduces secondary resistance permitting pour it back Into the generator. Gen motor to catch up in speed by putting erator will then try to speed up its energy into flywheel. prime mover; and when an alternating fcoaontic Unitarians. Ward-Leonard current motor Is used to drive genera drives are expensive because (l) de tor, it will actually pul power back into machines are more expensive than oc; Pcr*^on of the drive is t. i- ^JB- where generator volt- *My higher than the bock line. This process is known as regenera tive braking. Flywheel Mg Seti. Sometimes load of a Ward-Leonard drive is intermittent (2) it takea three machines to drive one load rix, an ao motor, do generator and a de motor. Certainly if a woundrotor motor or a mechanical speed- f. orce mo,or- This back r7* ore<* i*1 vlia, U result of molr~#lur* con{*uciors cutting lines t*bf .8ttCra,ed ** motot fieW- Prodfn!tlt,UTe curTent and back eraf wf represenu electrical power oonverted into mechanical and there ere frequent regeneration im pulses. A typical example is a revers ing metal-rolling mill, where periodi cally inertia of rolls has to be obsorbed by motor, and then supplied again when rolls start turning the other way. On top of that, a peak in power require changer can do tho job more economi cally, U ahould be used. Keep in mind physical limitations of equipment it self aa well aa control type required. However, (here are many drive* which call for precise control, a wide variation in speed, or exact speed re meat occurs every time an ingot or gardless of load. For these drives, the w.ooco between generator termi bloom hits the rolls. Ward-Leonard system is suitable. *.Ssplmbr 1948 (547) 91