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Electromechanics
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Fractional Horsepower Motors A small motor, as de ned by the American Standards Association (ASA) and the National Electrical Manufacturers Association (NEMA; see 17) is a motor built in a frame smaller than that having a continuous rating of 1 hp, open type, at 1700 to 1800 rev/min Small motors are generally considered fractional-horsepower motors However, since the determination is based on frame size and on a given speed range, the classi cation of a motor is not always obvious Let us give two examples 1 Consider a 3 -hp, 1,200-rev/min motor This 4 motor is not considered a fractional-horsepower motor, because of its frame size If the same frame size were used for an 1,800-rev/min motor, it would produce a rating of more than 1 hp Thus, it is considered an integralhorsepower motor of 075 hp 1,800 = 1125 hp 1,200 of 1,700 to 1,800 rev/min, it is classi ed as an integral-horsepower motor 2 Consider now a 125-hp, 3,600-rev/min motor This motor is classi ed as a fractionalhorsepower motor, in spite of the fact that its power output is actually greater than 1 hp If the same motor were used at a speed of 1,800 rev/min, it would produce a rating of less than 1 hp: 125 1,800 = 0625 hp 3,600
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Thus, we see once again that some attention must be paid to the speed of operation of the motor in determining its classi cation The term fractional horsepower relates more to the physical size of the machine than to the actual power output rating
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In other words, since the motor is capable of integral-horsepower performance at speeds
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The Universal Motor If it were possible to operate a DC motor from a single-phase AC supply, a wide range of simple applications would become readily available Recall that the direction of the torque produced by a DC machine is determined by the direction of current ow in the armature conductors and by the polarity of the eld; torque is developed in a DC machine because the commutator arrangement permits the eld and armature currents to remain in phase, thus producing torque in a constant direction A similar result can be obtained by using an AC supply, and by connecting the armature and eld windings in series, as shown in Figure 1821 A series DC motor connected in this con guration can therefore operate on a single-phase AC supply, and is referred to as a universal motor An additional consideration is that, because of the AC excitation, it is necessary to reduce AC core losses by laminating the stator; thus, the universal motor differs from the series DC motor discussed in 17 in its construction features Typical torque-speed curves for AC and DC operation of a universal motor are shown in Figure 1822 As shown in Figure 1821, the load current is sinusoidal and therefore reverses direction each half-cycle; however, the torque generated by the motor is always in the same direction, resulting in a pulsating torque, with nonzero average value As in the case of a DC series motor, the best method for controlling the speed of a universal motor is to change its (rms) input voltage The higher the rms input voltage, the greater the resulting speed of the motor Approximate torque-
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N Load current N S
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Positive half-cycle Load current
Torque
Time
Torque N
Time Negative half-cycle
S Load current S
Load current
Time
Armature Series field
Circuit diagram
Figure 1821 Operation and circuit diagram of a universal motor
AC operation DC operation Torque V 1 >V 2 >V 3 >V 4
Torque
V4 Speed
V1 V2 V3 Speed
Figure 1822 Torque-speed curve of a universal motor
Figure 1823 Torque-speed characteristics of a universal motor
speed characteristics of a universal motor as a function of voltage are shown in Figure 1823
EXAMPLE 187 Analysis of Universal Motor
Problem
Find the following quantities for a universal motor: 1 2 3 4 Back emf Power output Shaft torque Motor ef ciency
Part III
Electromechanics
Solution
Known Quantities: Motor operating data and circuit parameters Find: Eb ; Pout ; Tout ; (ef ciency) Schematics, Diagrams, Circuits, and Given Data: Motor operating data: 120 V; 60 Hz; 2 poles; 800 rev/min; 1785 A (full load); pf = 0912 (lagging) Circuit parameters: Rf = 065 ; Xf = 12 ; Ra = 136 ; Xfa = 16 Assumptions: Use the circuit model for the series motor described in 17 The
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