Page 291 - Solutions Manual to accompany Electric Machinery Fundamentals

P I 2   0.5R    17.32 A 2   0.427   140 W
AG,B 1 B

P AG  P AG,F  P AG,B  3132 W 140 W  2992 W
P 2992 W
  AG   28.57 N m

ind
 sync  1000 r/min  2 rad    1 min 
  1 r     60 s  
Note that the induced torque is proportional to the square of the terminal voltage.
9 -7. What type of motor would you select to perform each of the following jobs? Why?

(a) Vacuum cleaner (b) Refrigerator
(c) Air conditioner co mpressor (d) Air condition er fan
(e) Variable-speed sewing mach ine (f) Clock

(g) Electric drill
SOLUTION
(a) Univer sal motor—for its high torque

(b) Capacitor start or Capacitor start a nd run—For its high starting torque and relatively constant
speed at a wide variety of loads
(c) Same as (b) above

(d) Split-phase—Fans a re low-starting-torque applications, and a split-phase motor is appropriate
(e) Universal Motor—Direction and speed are easy to control with solid-state drives

(f) Hysteresis motor—for its easy starting and operation at n sync . A reluctance mo tor would also do
nicely.
(g) Un iversal Motor—for easy speed control with solid-state drives, plus high torque under loaded
conditions.
9-8. For a particu lar application, a three-phase stepper motor must be capable of stepping in 10° increments.
How many poles must it have?
SOLUTION From Equation (9-1 8), the relationship between mechanical angle and electrical angle in a
three-phase stepper motor is
2
  
m
P e
  60
so P  2 e  2    12 poles
 m  10  
9-9. How many pulses per second must be supplied to the control unit of the motor in Problem 9-8 to achieve
a rotational speed of 600 r/min?
SOLUTION From Equation (9-20),

1
n  n pulses
m
3P

so n pulses  m  3 P n   3 12 poles 600 r/min   21,600 pulses/min  360 pulses/s
9-10. Construc t a table showing step size versus number of poles for three-phase and four-phase stepper motors.

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