7 3/8 x 9 1/4 T echnical / Build Your Own Electric Vehicle / Leitman / 373-2 / Chapter 5
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Build Your Own Elec tric Vehicle hp 5 (146.19 3 50)/375 5 19.49 or approximately 20 hp Only about 20 hp is necessary—at the wheels—to propel this pickup truck along at 50 mph on a level road without wind. In fact, a rated 20-hp electric motor will easily propel a 4,000-lb. vehicle at 50 mph—a fact that might amaze those who think in terms of the typical rated 90-hp or 120-hp internal combustion engine that might just have been removed from the pickup. The point here is to condition yourself to think in terms of force values, which are relatively easy to determine, rather than in terms of a horsepower figure that is arrived at differently for engines versus electric motors, and that means little until tied to specific force and speed values anyway. Another point (covered in more detail in Chapter 6’s discussion of electric motors and Chapter 9’s discussion of the electrical system) is to think in terms of current when working with electric motors. The current is directly related to motor torque. Through the torque-current relationship, you can directly link the mechanical and electrical worlds. (Note: The controller gives current multiplication. In other words, if the motor voltage is one-third the battery voltage, then the motor current is slightly less than three times the battery current. The motor and battery current would be the same only if you used a very inefficient resistive controller.)
Calculation Overview Notice that the starting point in the calculations was the ending point of the force value required. Once you know the forces acting on your vehicle chassis at a given speed, the rest is easy. For your calculation approach, first determine these values, then plug in your motor and drivetrain values for its design center operating point, be it a 100-mph speedster, a 20-mph economy flyer, or a 50-mph utility vehicle. A 50-mph speed will be the design center for our pickup truck utility vehicle example. In short, you need to select a speed, select an electric motor for that speed, choose the RPM at which the motor delivers that horsepower, choose the target gear ratio based on that RPM, and see if the motor provides the torque over the range of level and hillclimbing conditions you need. Once you go through the equations, worksheets, and graphed results covered in this section, and repeat them with your own values, you’ll find the process quite simple. The entire process is designed to give you graphic results you can quickly use to see how the torque available from your selected motor and drivetrain meets your vehicle’s torque requirements at different vehicle speeds. If you have a microcomputer with a spreadsheet program, you can set it up once, and afterwards graph the results of any changed input parameter in seconds. In equation form, what we are saying is Available engine power 5 Tractive resistance demand Power 5 (Acceleration 1 Climbing 1 Rolling 1 Drag 1 Wind) Resistance Plugging into the force equations gives you: Force 5 Fa 1 Fh 1 Fr 1 Fd 1 Fw Force 5 CiWa 1 Wsin f 1 Cr Wcos f 1 CdAV2 1 CwFd You’ve determined every one of these earlier in the chapter. Under steady-speed conditions, acceleration is zero, so there is no acceleration force. If you are on a level surface, sin f 5 0, cos f 5 1 and the force equation can be rewritten as
