. A text-book of electrical engineering;. gned to have its maximum efficiencyat a small load. We pass now to the consideration ofa motor the field winding of which isconnected directly across the constantpressure supply, but the armature ofwhich has an adjustable resistance inseries with it (Fig. 190). We makethe striking, discovery that, whetherloaded or running light, the current isin no way affected by a change in the adjustable resistance. If, however,we turn to equation (98), on page 200, we see that the armature currentmust be determined entirely by the load, since the magnetic flux is c
. A text-book of electrical engineering;. gned to have its maximum efficiencyat a small load. We pass now to the consideration ofa motor the field winding of which isconnected directly across the constantpressure supply, but the armature ofwhich has an adjustable resistance inseries with it (Fig. 190). We makethe striking, discovery that, whetherloaded or running light, the current isin no way affected by a change in the adjustable resistance. If, however,we turn to equation (98), on page 200, we see that the armature currentmust be determined entirely by the load, since the magnetic flux is the load on the motor remains constant, the armature current will re-main constant, however much the series resistance may be varied. Onsuddenly decreasing the resistance there is, it is true, a momentary increaseof the armature current. This causes the motor to exert a greater torquethan that required to overcome the load, with the result that the armatureis accelerated and runs permanently at a higher speed. This increase of. Fig. 190 65. Motor with Constant Excitation 207 speed, however, causes an increase in the back , which reduces thearmature current to its former value corresponding to the load. The onlyway to vary the armature current of a constantly excited motor is to varythe load. The speed, on the other hand, is very dependent on the resistance R inseries with the armature. The applied terminal pressure V has now toovercome the back and cover the pressure drop in both the armatureand the resistance R. We have then V=E + I^.R^ + I„.R,or E=V-I^. In this equation V and Ra are constant, and I^ is also constant so longas the load remains unchanged. It follows, therefore, that a change in thevalue of R will produce a large effect on E, and consequently on the speed. This becomes much clearer if we neglect the small drop of pressure inthe armature, and assume that the pressure across the brushes is exactlyequal and opposite to the back
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