1.2.6 Maximizing Motor Power - VEX



Activity 1.2.6 Maximizing Motor Power – VEXIntroductionAn engineer often must design a system to get the most out of limited resources. When designing a laptop or mobile phone, for example, an engineering team works to make the battery last as long as possible. A vibrating cell phone uses a motor with an off-center load, and it needs to be designed with a gear train that delivers the most power or the highest efficiency. These design decisions can be made by making only four measurements of a motor. These four measurements describe the most important properties of a particular motor at a given voltage. In this activity you will learn how to make those four measurements. You will also learn how to choose a gear ratio to make a motor deliver maximum power or maximum efficiency for a given task.352742531369000Equipment Winch assembly from Activity 1.2.5(2) 1” standoffs(4) 3/8” 8-32 screws(1) 1/2” 8-32 screw DC power supply or two AA batteries and holdersMultimeterTiming device for timing to 0.01 s precisionVernier force sensor or other device to precisely measure ~5.0 N Breadboard Pair of wires StringClampProcedure251015524257000Reuse the winch assembly from Activity 1.2.5. 12414253429000Set the 1802 power supply to deliver constant voltage. This is accomplished by turning the current knob all the way clockwise. Then use the voltage knob to set the voltage to 3.0 V. Alternatively, provide power with two AA batteries in series.3301365221615101536511366500 Connect the power source to the motor using the breadboard. 67373513271500Measure the no-load speed of the motor. You will time how long it takes the motor to complete n rotations—you will pick the number n. If n is too small, the time will be too short to measure easily. If n is too large, you are more likely to lose count of the rotation. More rotations, however, will increase the precision with which you can calculate the time per rotation. n = number of rotations = _______Time for n rotations = _________ _________ _________Calculate the no-load speed = rotations/time. Using the fact that 2? radians = 1?rotation, convert the noload speed ? to radians per second. This is the fastest that the motor will spin at this voltage.?no-load = No-load speed = _________ rotations/second???? = _________ radians/second Record the no-load current. The power supply displays current, but a multimeter should be used for accuracy. If current greater than the range setting of an ammeter passes through the meter, it blows the fuse of the meter and can damage the meter. Always start with an ammeter range setting greater than the current could be. The power supply gives a rough indication of the current, so you can start at the correct range setting on the ammeter. If the motor is drawing more than 200 mA, use the 10 A dial setting and the 10 A banana receptacle. Ino-load = No-load current = _________ A461772024892000Measure the stall torque of the motor. Make a holder for the force probe. Attach two 1” standoffs to the baseplate using two 3/8” 8-32 screws. Attach one bent 1 x 3-hole bar and one bent 1 x 4-hole bar to the standoffs as shown using two 3/8” 8-32 screws. Use one 1/2” 8-32 screw as a latch for the gate formed by the two bent bars.Thread the string from the winch between the base plate and the winch drum and hook the string on the force probe. Secure the force probe with the gate formed by the bent bars. You will actually calculate the torque by measuring a force and an off-axis distance. 408686070612000Turn on the power supply and note the maximum force registered by the force probe. You will notice that the VEX? motors stall at one force and then “relax” to a lower force. This occurs because VEX motors have thermal protection and cut off the current when the motor gets hot. 4166235-58293000Some motors without thermal protection can be damaged by too much current. The current heats up the motor coils and melts the insulation, short-circuiting coils to each other. Even though VEX motors have overcurrent protection, it is good practice to make this measurement quickly and then turn off the power to the motor. It is okay to let the VEX motor cool for ~10 seconds and measure again. d = distance from axis of rotation to point where force is applied = _______ mF┴ = force to stall the motor = ______ N?stall = stall torque = d × F┴ = ______ N?mRecord the stall current of the motor. Note that VEX motors will stall at a high current, and then the current will drop, once limited by thermal protection. You want to record the initial, higher value of the current. If a current greater than the range setting of an ammeter passes through the meter, it blows the fuse of the meter and can damage the meter. Always start with ammeter range settings larger than the current could be. VEX motors can draw more than 200 mA, so you are likely to need to use the 10 A setting with the 10 A banana receptacle on the multimeter.Congratulations! You have measured everything you need to measure to characterize your motor. You have all of the most important design parameters that describe the motor’s performance for operation at 3 V. Let’s put them in one place, creating a specification sheet, or spec sheet as it is commonly called.Specification SheetMotor model: ________For operation at 3 V:No-load current: ________Stall current: ________No-load speed: ________Stall torque: ________Engineers make many design decisions by calculating important results from these specs. For example, what is the torque at which the motor delivers maximum power? Refer to the presentation Maximizing Motor Power if needed.??=?Motor torque for maximum power = ? stall torque = ________ N?mEngineers often want to select a component based on something they calculate from the specs. Suppose you are selecting a cell phone vibration motor and want at least 200 mW of power. Will this motor do the trick? Calculate the maximum output power of this motor. Note that the speed of the motor at half the stall torque is half the no-load speed.? = Motor speed at maximum power = ? no-load speed = ________ rad/sPout = Motor output power at maximum power = ? × ??= ________ WIt is common for motor output power to be reported in horsepower. Convert the output power to hp. Show your work.Pout = ________ W × __________________ = _________ hpUse the Internet to look up the power output of a power tool or air compressor on the market. Describe what you found out. Compare to your motor.Because you characterized your motor, you can make a lot of design decisions. Suppose you want to use your motor to drive a robot that requires 10 N?m of torque on the drive axle. A gear train will transfer power from the motor to the drive axle. That 10? N?m will be the output torque from the gears. The motor will deliver the input torque to the gears. What gear ratio would you need to use to make this motor deliver maximum power to the drive shaft? GR= τoutτin=Because you measured the specifications for your motor, you can predict how much electrical power it will consume. Let’s perform that calculation when the motor is exerting half of its stall torque—while it is working at maximum output power. How much electrical power is flowing into the motor? Voltage = V = 3.0 VCurrent = I = halfway between stall current and no-load current = _______ AElectrical Power = Pin = I ? V = __________________ = ___________ WSometimes engineers will design a device to be as efficient as possible instead of designing it to be as powerful as possible. How efficient is your motor? Calculate the efficiency of the motor when it is operating at maximum power.η= PoutPin =It was easy to calculate the torque at which the motor would deliver maximum power. (Review: How did you calculate that torque? Look back at step 10.) It is not so easy to calculate the torque at which the motor will deliver power at maximum efficiency. But there is an algorithm for it. An algorithm is a procedure to follow. Steps 11, 15, and 16 walked you through the algorithm for calculating the efficiency for a given torque. What are the steps of the algorithm?207899095186500You could find the maximum efficiency by calculating the efficiency at many different torques and picking the highest efficiency. That would require many iterations of the algorithm! Computers are very good at automating that work. Open the spreadsheet 1.2.6 Motor Power at Constant Voltage and enter the specifications of your motor.61976017843500324358017843500This spreadsheet is automated to calculate efficiencies for 100 torques between 0 N?m and the stall torque you enter. It automatically updates the two graphs to specify how four design parameters for your motor are dependent on the torque delivered by the motor. The first graph shows efficiency (in blue) and motor output power (in red). Discuss these graphs with your lab partner.Paste images of your graphs here.What are the x- and y-axes for each curve? Notice two y-axes on each graph!Color: y-variable vs. x-variableblue:_____________ vs. ______________red: motor power vs. motor torque____green: _____________ vs. ______________purple: _____________ vs. ______________Find the point on the efficiency graph that identifies the maximum efficiency. Indicate this spot on the graph that you pasted into this document in step 18a above. You can indicate the spot on the graph by inserting a circle into this Word document. Following these steps:Select Insert > Shapes > select oval. Right-click the oval.Change fill to “no fill.” 97917085979000Change the outline color to a setting that you think works well to draw attention to a precise location on the graph. Change the line weight to be large enough to be obvious but small enough to be precise. Based on your graphs, what is the maximum efficiency??max= ___________ %At what torque does the maximum efficiency occur? ?max efficiency = ___________ N?mUsing the torque from step 21 as the x-coordinate on the second graph, find the current used by your motor when it is operating at maximum efficiency. Circle this point on your second graph following the method listed in step 19.?max efficiency = ___________ AHow much electrical power does the motor use when it is operating at maximum efficiency?Pelec = I ? V = ______________ = ________ W The efficiency tells you what percent of the electrical energy is transformed to mechanical energy. The rest of the energy becomes heat energy, increasing the temperature of the motor. Power is the rate at which energy is transformed. At what rate is heat energy deposited into the motor when it is operating most efficiently?Pheat = ( 1 - ?max ) Pelec =Conclusion QuestionsExplain what it means to “characterize” a component to be used in a system.In this activity you used math and science to calculate how to use a motor in a design, instead of using trial and error. What are the advantages of using math and science in engineering design? What are the advantages of trial and error? Explain how efficiency is related to electrical energy, mechanical energy, and heat energy. The motor in the design shown at left has the specifications shown at right. What gear ratio should be used for maximum power?0-127000For operation at: 9 V No-load current: 100 mA Stall current: 3 A No-load speed: 20 rad/s Stall torque: 60 N?m ................
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