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The speed-control circuit of FIG. 2 has much more to offer than might be apparent from casual inspection. Although no obvious connection between output and input is discernible, this half-wave circuit nevertheless incorporates feedback. Because of this, the speed of the universal motor at any setting of potentiometer P1 does not deviate greatly over a wide torque range. Thus, the “natural” characteristics of the universal motor are electronically modified. This is highly significant, because it exemplifies a unique advantage of electronic control; it enables the motor designer to produce a mo tor with optimized cost, commutation, and flexibility. The actual speed/torque behavior then can be manipulated electronically. In this case, the motor with the poorest speed regulation can be converted to near constant speed performance.
Feedback occurs in the following manner: When the SCR is not conducting, the rotating motor continues to generate a counter EMF, which is polarized to inhibit triggering of the SCR. In order for the SCR to trigger, a voltage that is equal to its triggering voltage plus the counter EMF of the motor must be applied to its gate circuit. It receives this voltage from the low-impedance output of emitter-follower stage Q1. Stage Q1, in turn, samples the triggering voltage from the divider network comprising R1, P1, CR2, C1, and C2. Transistor Q1 functions as a current amplifier and is instrumental in extending the low-speed range of the motor where excessive loading of the divider network tends to be detrimental. At any given setting of potentiometer P1, the voltage applied to the SCR gate by Q1 can be considered a reference voltage. The SCR gate circuit “compares” this reference voltage with the counter EMF of the motor. Suppose that the motor is operating and additional mechanical load is imposed on its shaft. In the manner of series machines, it will at tempt to slow down drastically and consume more torque-producing current. However, such a slowdown is accompanied by decreased counter EMF. This, in turn, enables the SCR to conduct with a lower output voltage from Q1. Such a lower volt age is available earlier in the ac cycle, so the SCR now delivers an increased average voltage to the motor. The motor develops increased torque, which accelerates it back to the vicinity of its previous speed.
In the event that the load on the motor is relaxed, the attempt of the motor to speed up is counteracted by a sequence of events opposite to those described above. Because of the electronic control, the universal motor is forced to behave in a manner similar to that of the dc shunt motor. The latter machine is often referred to as a “constant-speed” motor.
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