40墨 DESIGN OF MACHINERY CHAPTER 2 Linkages have the disadvantage of relatively large size compared to the output dis- placement of the working portion;thus they can be somewhat difficult to package.Cams tend to be compact in size compared to the follower displacement.Linkages are rela- tively difficult to synthesize,and cams are relatively easy to design (as long as a com- puter is available).But linkages are much easier and cheaper to manufacture to high precision than cams.Dwells are easy to get with cams,and difficult with linkages.Link- ages can survive very hostile environments,with poor lubrication,whereas cams cannot. unless sealed from environmental contaminants.Linkages have better high-speed dy- namic behavior than cams,are less sensitive to manufacturing errors,and can handle very high loads,but cams can match specified motions better. So the answer is far from clear-cut.It is another design trade-off situation in which you must weigh all the factors and make the best compromise.Because of the potential advantages of the pure linkage it is important to consider a linkage design before choos- ing a potentially easier design task but an ultimately more expensive solution. 2.16 MOTORS AND DRIVERS Unless manually operated,a mechanism will require some type of driver device to pro- vide the input motion and energy.There are many possibilities.If the design requires a continuous rotary input motion,such as for a Grashof linkage,a slider-crank,or a cam-follower,then a motor or engine*is the logical choice.Motors come in a wide va- riety of types.The most common energy source for a motor is electricity,but compressed air and pressurized hydraulic fluid are also used to power air and hydraulic motors. Gasoline or diesel engines are another possibility.If the input motion is translation,as is common in earth-moving equipment,then a hydraulic or pneumatic cylinder is usual- ly needed. Electric Motors *The terms motor and Electric motors are classified both by their function or application and by their electrical configuration.Some functional classifications (described below)are gearmotors,ser- engine are often used interchangeably,but they vomotors,and stepping motors.Many different electrical configurations as shown in do not mean the same Figure 2-31 are also available,independent of their functional classifications.The main thing.Their difference is electrical configuration division is between AC and DC motors,though one type,the largely semantic,but the universal motor is designed to run on either AC or DC. "purist"reserves the term motor for electrical, AC and DC refer to alternating current and direct current respectively.AC is typ- hydraulic and pneumatic ically supplied by the power companies and,in the U.S.,will be alternating sinusoidally motors and the term engine at60 hertz (Hz),at about±120，±240，or±480 volts(V)peak.Many other countries for thermodynamic devices supply AC at 50 Hz.Single-phase AC provides a single sinusoid varying with time,and such as stcam engines and 3-phase AC provides three sinusoids at 120 phase angles.DC current is constant with internal combustion time,supplied from generators or battery sources and is most often used in vehicles,such engines.Thus,your automobile is powered by as ships,automobiles,aircraft,etc.Batteries are made in multiples of 1.5 V,with 6,12, an engine,but its and 24 V being the most common.Electric motors are also classed by their rated power windshield wipers and as shown in Table 2-5.Both AC and DC motors are designed to provide continuous ro- window lifts are run by tary output.While they can be stalled momentarily against a load,they can not tolerate motors. a full-current,zero-velocity stall for more than a few minutes without overheating
* The terms motor and engine are often used interchangeably, but they do not mean the same thing. Their difference is largely semantic, but the "purist" reserves the term motor for electrical, hydraulic and pneumatic motors and the term engine for thermodynamic devices such as steam engines and internal combustion engines. Thus, your automobile is powered by an engine, but its windshield wipers and window lifts are run by motors. Linkages have the disadvantage of relatively large size compared to the output displacement of the working portion; thus they can be somewhat difficult to package. Cams tend to be compact in size compared to the follower displacement. Linkages are relatively difficult to synthesize, and cams are relatively easy to design (as long as a computer is available). But linkages are much easier and cheaper to manufacture to high precision than cams. Dwells are easy to get with cams, and difficult with linkages. Linkages can survive very hostile environments, with poor lubrication, whereas cams cannot, unless sealed from environmental contaminants. Linkages have better high-speed dynamic behavior than cams, are less sensitive to manufacturing errors, and can handle very high loads, but cams can match specified motions better. So the answer is far from clear-cut. It is another design trade-off situation in which you must weigh all the factors and make the best compromise. Because of the potential advantages of the pure linkage it is important to consider a linkage design before choosing a potentially easier design task but an ultimately more expensive solution. 2.16 MOTORS AND DRIVERS Unless manually operated, a mechanism will require some type of driver device to provide the input motion and energy. There are many possibilities. If the design requires a continuous rotary input motion, such as for a Grashof linkage, a slider-crank, or a cam-follower, then a motor or engine* is the logical choice. Motors come in a wide variety of types. The most common energy source for a motor is electricity, but compressed air and pressurized hydraulic fluid are also used to power air and hydraulic motors. Gasoline or diesel engines are another possibility. If the input motion is translation, as is common in earth-moving equipment, then a hydraulic or pneumatic cylinder is usually needed. Electric Motors Electric motors are classified both by their function or application and by their electrical configuration. Some functional classifications (described below) are gearmotors, servomotors, and stepping motors. Many different electrical configurations as shown in Figure 2-31 are also available, independent of their functional classifications. The main electrical configuration division is between AC and DC motors, though one type, the universal motor is designed to run on either AC or DC. AC and DC refer to alternating current and direct current respectively. AC is typically supplied by the power companies and, in the U. S., will be alternating sinusoidally at 60 hertz (Hz), at about ±120, ±240, or ±480 volts (V) peak. Many other countries supply AC at 50 Hz. Single-phase AC provides a single sinusoid varying with time, and 3-phase AC provides three sinusoids at 1200 phase angles. DC current is constant with time, supplied from generators or battery sources and is most often used in vehicles, such as ships, automobiles, aircraft, etc. Batteries are made in multiples of 1.5 V, with 6, 12, and 24 V being the most common. Electric motors are also classed by their rated power as shown in Table 2-5. Both AC and DC motors are designed to provide continuous rotary output. While they can be stalled momentarily against a load, they can not tolerate a full-current, zero-velocity stall for more than a few minutes without overheating
KINEMATICS FUNDAMENTALS DC Motors AC Motors Permanent Magnet Shunt-Wound Series-Wound Single Phase Universal Motor Polyphase Compound-Wound Induction Synchronous Synchronous Induction Squirrel Cage Wound Rotor Split Phase Shaded Pole Shaded Pole Capacitor Start Repulsion Hysteresls Two-Value Capacitor Repulsion Start Reluctance Wound Rotor Permanent Split Capacitor Repulsion Induction Permanent Magnet Squirrel Cage FIGURE 2-31 Types of electric motors Source:Reference (14) DC MOTORS are made in different electrical configurations,such as permanent magnet (PM),shunt-wound,series-wound,and compound-wound.The names refer to the manner in which the rotating armature coils are electrically connected to the station- ary field coils-in parallel (shunt),in series,or in combined series-parallel (compound). Permanent magnets replace the field coils in a PM motor.Each configuration provides different torque-speed characteristics.The torque-speed curve of a motor describes how it will respond to an applied load and is of great interest to the mechanical designer as it predicts how the mechanical-electrical system will behave when the load varies dynam- ically with time. PERMANENT MAGNET DC MOTORS Figure 2-32a shows a torque-speed curve for a permanent magnet (PM)motor.Note that its torque varies greatly with speed,ranging from a maximum (stall)torque at zero speed to zero torque at maximum (no-load)speed. This relationship comes from the fact that power forgue X angular velocity.Since the power available from the motor is limited to some finite value,an increase in torque re- TABLE 2-5 quires a decrease in angular velocity and vice versa.Its torque is maximum at stall (zero Motor Power Classes velocity),which is typical of all electric motors.This is an advantage when starting heavy loads:e.g.,an electric-motor-powered vehicle needs no clutch,unlike one pow- Class HP ered by an internal combustion engine which cannot start from stall under load.An en- gine's torque increases rather than decreases with increasing angular velocity. Subfractional <1/20 Figure 2-32b shows a family of load lines superposed on the torque-speed curve of a PM motor.These load lines represent a time-varying load applied to the driven mech- Fractional 1/20-1 anism.The problem comes from the fact that as the required load torgue increases,the Integral >1 motor must reduce speed to supply it.Thus,the input speed will vary in response to load
DC MOTORS are made in different electrical configurations, such as permanent magnet (PM), shunt-wound, series-wound, and compound-wound. The names refer to the manner in which the rotating armature coils are electrically connected to the stationary field coils-in parallel (shunt), in series, or in combined series-parallel (compound). Permanent magnets replace the field coils in a PM motor. Each configuration provides different torque-speed characteristics. The torque-speed curve of a motor describes how it will respond to an applied load and is of great interest to the mechanical designer as it predicts how the mechanical-electrical system will behave when the load varies dynamically with time. PERMANENT MAGNET DC MOTORS Figure 2-32a shows a torque-speed curve for a permanent magnet (PM) motor. Note that its torque varies greatly with speed, ranging from a maximum (stall) torque at zero speed to zero torque at maximum (no-load) speed. This relationship comes from the fact that power = torque X angular velocity. Since the power available from the motor is limited to some finite value, an increase in torque requires a decrease in angular velocity and vice versa. Its torque is maximum at stall (zero velocity), which is typical of all electric motors. This is an advantage when starting heavy loads: e.g., an electric-motor-powered vehicle needs no clutch, unlike one powered by an internal combustion engine which cannot start from stall under load. An engine's torque increases rather than decreases with increasing angular velocity. Figure 2-32b shows a family of load lines superposed on the torque-speed curve of a PM motor. These load lines represent a time-varying load applied to the driven mechanism. The problem comes from the fact that as the required load torque increases, the motor must reduce speed to supply it. Thus, the input speed will vary in response to load
6 DESIGN OF MACHINERY CHAPTER 2 Speed Speed Operating points Varying load paads 100 100 75 75 50 50 25 00 100200300400 Torque 0 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 FIGURE2-32 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 torgue-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 sensitivity 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 Figure 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 operating range, but stalls very quickly when the load exceeds its maximum overload capacity 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 detectors 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 tendency 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
100 100 100 四 60 % 60 40 40 40 20 30 20 0 0 0/ 0 100200300 400 0 100200300400 0 100200300400 %of Rated Torque of Rated Torque of Rated Torque (a)Shunt wound (b)Serles wound (c)Compound wound FIGURE 2-33 Torque-speed curves for three types of DC motor Ibeir speed sensitivity is greater than a shunt-wound but less than a series-wound motor and it will not run away when unloaded.This feature plus its high starting torque and soft-start capability make it a good choice for cranes and hoists which experience high inertial loads and can suddenly lose the load due to cable failure,creating a potential run- away problem if the motor does not have a self-limited no-load speed. SPEED-CONTROLLEDDC MOTORS If precise speed control is needed,as is often the case in production machinery,another solution is to use a speed-controlled DC mo- tor which operates from a controller that increases and decreases the current to the mo- tor in the face of changing load to try to maintain constant speed.These speed-controlled (typically PM)DC motors will run from an AC source since the controller also converts AC to DC.The cost of this solution is high,however.Another possible solution is to provide a flywheel on the input shaft,which will store kinetic energy and help smooth out the speed variations introduced by load variations.Flywheels will be investigated in Chapter 11. AC MOTORS are the least expensive way to get continuous rotary motion,and they can be had with a variety of torque-speed curves to suit various load applications. They are limited to a few standard speeds that are a function of the AC line frequency (60 Hz in North America,50 Hz elsewhere).The synchronous motor speed ns is a func- tion of line frequency I and the number of magnetic poles p present in the rotor. %,=1204 (2.17) p Synchronous motors "lock on"to the AC line frequency and run exactly at synchronous speed.These motors are used for clocks and timers.Nonsynchronous AC motors have a small amount of slip which makes them lag the line frequency by about 3 to 10%. Table 2-6 shows the synchronous and non synchronous speeds for various AC mo- tor-pole configurations.The most common AC motors have 4 poles,giving nonsynchro-
1beir speed sensitivity is greater than a shunt-wound but less than a series-wound motor and it will not run away when unloaded. This feature plus its high starting torque and soft-start capability make it a good choice for cranes and hoists which experience high inertial loads and can suddenly lose the load due to cable failure, creating a potential runaway problem if the motor does not have a self-limited no-load speed. SPEED-CONTROLLEDDC MOTORS If precise speed control is needed, as is often the case in production machinery, another solution is to use a speed-controlled DC motor which operates from a controller that increases and decreases the current to the motor in the face of changing load to try to maintain constant speed. These speed-controlled (typically PM) DC motors will run from an AC source since the controller also converts AC to DC. The cost of this solution is high, however. Another possible solution is to provide a flywheel on the input shaft, which will store kinetic energy and help smooth out the speed variations introduced by load variations. Flywheels will be investigated in Chapter 11. AC MOTORS are the least expensive way to get continuous rotary motion, and they can be had with a variety of torque-speed curves to suit various load applications. They are limited to a few standard speeds that are a function of the AC line frequency (60 Hz in North America, 50 Hz elsewhere). The synchronous motor speed ns is a function of line frequency f and the number of magnetic poles p present in the rotor. (2.17) Synchronous motors "lock on" to the AC line frequency and run exactly at synchronous speed. These motors are used for clocks and timers. Nonsynchronous AC motors have a small amount of slip which makes them lag the line frequency by about 3 to 10%. Table 2-6 shows the synchronous and non synchronous speeds for various AC motor-pole configurations. The most common AC motors have 4 poles, giving nonsynchro-
DESIGN OF MACHINERY CHAPTER 2 TABLE 2-6 nous no-load speeds of about 1725 rpm,which reflects slippage from the 60-Hz synchro- AC Motor Speeds nous speed of 1800 rpm. Figure 2-34 shows typical torque-speed curves for single-phase (1<and 3-phase Poles Sync Async (3<AC motors of various designs.The single-phase shaded pole and permanent split m rpm capacitor designs have a starting torque lower than their full-load torque.To boost the 2 3600 3450 start torque,the split-phase and capacitor-start designs employ a separate starting circuit that is cut off by a centrifugal switch as the motor approaches operating speed.The bro- ken curves indicate that the motor has switched from its starting circuit to its running 4 1800 1725 circuit.The NEMA*three-phase motor designs B,C,and D in Figure 2-34 differ main- b 1200 1140 ly in their starting torque and in speed sensitivity (slope)near the full-load point. GEARMOTORS If different single (as opposed to variable)output speeds than the 900 850 standard ones of Table 2-6 are needed,a gearbox speed reducer can be attached to the motor's output shaft,or a gearmotor can be purchased that has an integral gearbox.Gear- 10 720 690 motors are commercially available in a large variety of output speeds and power ratings. The kinematics of gearbox design are covered in Chapter 9. 12 600 575 SERVOMOTORS are fast-response,closed-loop-controlled motors capable of pro- viding a programmed function of acceleration or velocity,as well as of holding a fixed position against a load.Closed loop means that sensors on the output device being moved feed back information on its position,velocity,and acceleration.Circuitry in the motor controller responds to the fed back information by reducing or increasing (or re- versing)the current flow to the motor.Precise positioning of the output device is then possible,as is control of the speed and shape of the motor's response to changes in load or input commands.These are very expensive devices which are commonly used in ap- plications such as moving the flight control surfaces in aircraft and guided missiles,and in controlling robots,for example.Servomotors have lower power and torque capacity than is available from non servo AC or DC motors. Permanent split capacitor Split phase Capacitor Design B 100 start 100 Design C 80 Shaded 80 60 pole 60 Design D 40 40 20 0 100200300 400 100200 300 400 %of Rated Torque %of Rated Torque (a)Single phase (b)Three phase National Electrical FIGURE 2-34 Manufacturers Association. Torque-speed curves for single-and three-phase AC motors
nous no-load speeds of about 1725 rpm, which reflects slippage from the 60-Hz synchronous speed of 1800 rpm. Figure 2-34 shows typical torque-speed curves for single-phase (1<\»and 3-phase (3<\»AC motors of various designs. The single-phase shaded pole and permanent split capacitor designs have a starting torque lower than their full-load torque. To boost the start torque, the split-phase and capacitor-start designs employ a separate starting circuit that is cut off by a centrifugal switch as the motor approaches operating speed. The broken curves indicate that the motor has switched from its starting circuit to its running circuit. The NEMA * three-phase motor designs B, C, and D in Figure 2-34 differ mainly in their starting torque and in speed sensitivity (slope) near the full-load point. GEARMOTORS If different single (as opposed to variable) output speeds than the standard ones of Table 2-6 are needed, a gearbox speed reducer can be attached to the motor's output shaft, or a gearmotor can be purchased that has an integral gearbox. Gearmotors are commercially available in a large variety of output speeds and power ratings. The kinematics of gearbox design are covered in Chapter 9. SERVOMOTORS are fast-response, closed-loop-controlled motors capable of providing a programmed function of acceleration or velocity, as well as of holding a fixed position against a load. Closed loop means that sensors on the output device being moved feed back information on its position, velocity, and acceleration. Circuitry in the motor controller responds to the fed back information by reducing or increasing (or reversing) the current flow to the motor. Precise positioning of the output device is then possible, as is control of the speed and shape of the motor's response to changes in load or input commands. These are very expensive devices which are commonly used in applications such as moving the flight control surfaces in aircraft and guided missiles, and in controlling robots, for example. Servomotors have lower power and torque capacity than is available from non servo AC or DC motors