62 DESIGN OF MACHINERY CHAPTER 2 Speed Speed Operating points Varying load 100 paads 100 75 75 5 50 25 保 do 0 100200300400 Torque Torque 0 100200300400 %of Rated Torque %of Rated Torque (a)Speed-torque characteristic of a PM electric motor (b)Load lines superposed on speed-torque curve FIGURE 2-32 DC permanent magnet (PM)electric motor's typical speed-torque characteristic variations in most motors,regardless of their design.If constant speed is required,this may be unacceptable.Other types of DC motors have either more or less speed sensitiv- ity to load than the PM motor.A motor is typically selected based on its torque-speed curve. SHUNT-WOUND DC MOTORS have a torque speed curve like that shown in Fig- ure 2-33a.Note the flatter slope around the rated torque point (at 100%)compared to Figure 2-32.The shunt-wound motor is less speed-sensitive to load variation in its oper- ating range,but stalls very quickly when the load exceeds its maximum overload capac- ity of about 250%of rated torque.Shunt-wound motors are typically used on fans and blowers. SERIES-WOUND DC MOTORS have a torque-speed characteristic like that shown in Figure 2-33b.This type is more speed-sensitive than the shunt or PM configurations. However,its starting torque can be as high as 800%of full-load rated torque.It also does not have any theoretical maximum no-load speed which makes it tend to run away if the load is removed.Actually,friction and windage losses will limit its maximum speed which can be as high as 20,000 to 30,000 revolutions per minute (rpm).Overspeed de- tectors are sometimes fitted to limit its unloaded speed.Series-wound motors are used in sewing machines and portable electric drills where their speed variability can be an advantage as it can be controlled,to a degree,with voltage variation.They are also used in heavy-duty applications such as vehicle traction drives where their high starting torque is an advantage.Also their speed sensitivity (large slope)is advantageous in high-load applications as it gives a "soft-start"when moving high-inertia loads.The motor's ten- dency to slow down when the load is applied cushions the shock that would be felt if a large step in torque were suddenly applied to the mechanical elements. The synchronous AC motor and the speed- COMPOUND-WOUND DC MOTORS have their field and armature coils connected controlled DC motor are in a combination of series and parallel.As a result their torque-speed characteristic has exceptions. aspects of both the shunt-wound and series-wound motors as shown in Figure 2-33c.variations in most motors, regardless of their design. * If constant speed is required, this may be unacceptable. Other types of DC motors have either more or less speed sensitiv￾ity to load than the PM motor. A motor is typically selected based on its torque-speed curve. SHUNT-WOUND DC MOTORS have a torque speed curve like that shown in Fig￾ure 2-33a. Note the flatter slope around the rated torque point (at 100%) compared to Figure 2-32. The shunt-wound motor is less speed-sensitive to load variation in its oper￾ating range, but stalls very quickly when the load exceeds its maximum overload capac￾ity of about 250% of rated torque. Shunt-wound motors are typically used on fans and blowers. SERIES-WOUND DC MOTORS have a torque-speed characteristic like that shown in Figure 2-33b. This type is more speed-sensitive than the shunt or PM configurations. However, its starting torque can be as high as 800% of full-load rated torque. It also does not have any theoretical maximum no-load speed which makes it tend to run away if the load is removed. Actually, friction and windage losses will limit its maximum speed which can be as high as 20,000 to 30,000 revolutions per minute (rpm). Overspeed de￾tectors are sometimes fitted to limit its unloaded speed. Series-wound motors are used in sewing machines and portable electric drills where their speed variability can be an advantage as it can be controlled, to a degree, with voltage variation. They are also used in heavy-duty applications such as vehicle traction drives where their high starting torque is an advantage. Also their speed sensitivity (large slope) is advantageous in high-load applications as it gives a "soft-start" when moving high-inertia loads. The motor's ten￾dency to slow down when the load is applied cushions the shock that would be felt if a large step in torque were suddenly applied to the mechanical elements. COMPOUND-WOUND DC MOTORS have their field and armature coils connected in a combination of series and parallel. As a result their torque-speed characteristic has aspects of both the shunt-wound and series-wound motors as shown in Figure 2-33c