

03202017, 12:41 PM

Junior Member


Join Date: Mar 2017
Posts: 15


Re: Horsepower confusion
If I may provide a short quote from the book, this is where the author summarises the proposed method:
"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."
The book even goes through the procedure for a ford pickup using 50 mph as the main operating speed. He then, as I have done, calculated the 'required horsepower' and comes to a value of 22hp and in the end chooses the FB14001 advanced dc motor which I am sure you are familiar with.
He then simply states that from the performance curve of the motor driven at 120 volts, and using the hp= torque x rpm /5252 formula, the motor produces roughly 22 hp at 4600 RPM at 25ftlbs of torque and 170 amps.
So now he is able to calculate the wheel rpm and the overall gear ratio. My problem still lies in reading the performance curves for different motors/finding the 5hp point and reading the RPM. However once I have done this and FINALLY chosen my motor, I will be able to move on with choosing the gear ratio and then onto battery selection and all that other good stuff.

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03202017, 02:43 PM


Senior Member


Join Date: Apr 2008
Location: Ohio, USA
Posts: 7,330


Re: Horsepower confusion
Hi Kid,
Been a long time since I read it. The problem with that design logic is that with his 120V battery, it will essentially take forever to accelerate to 50mph and with any battery sag, headwind or grade, hopeless. You need to design your 50mph point for less than full voltage, say 90V, and use the controller to reduce the 120V (battery) to 90V to cruise at 50 mph (4600 RPM). That way you have torque available to accelerate and voltage headroom to produce 4600 RPM at a bit higher torque in case it's loaded more by wind or grade.
Remember that the RPM vs Torque curve is for the stated voltage only. Once you use your controller to reduce voltage to the motor, any RPM under that curve is available for a given torque value. At the lower RPM (and voltage) will be lower power and less battery current.
My opinion,
major

03202017, 03:57 PM

Administrator


Join Date: Dec 2008
Location: Southland New Zealand
Posts: 4,510


Re: Horsepower confusion
Hi Kid
You are overthinking this  those calculations will give a "minimum" power  you need to ensure that you have at least twice that in order to be usable
You are NOT designing to a point  you are designing to clear a bar
When you drive YOU  the driver  select how much power you want at that point
UP TO the limit of the motor/controller
Just make sure that that limit is high enough

03212017, 05:12 PM

Junior Member


Join Date: Mar 2017
Posts: 15


Re: Horsepower confusion
I (think) I get what you're both saying. Basically choose a motor where I reach my design center speed (sorry to keep using that phrase) of 30mph but still have more in the "tank" to overcome hills etc.
Let's just take this motor right here: http://www.cems.uvm.edu/~aero/AC24_2007.pdf
I am trying to determine the right overall gear ratio, and produce a torque available graph (wheel torque) based on the readings from the power curve.
So, I need to choose an overall gear ratio based on the motor RPM at 30mph ie, that 4.5hp value I calculated. This is why I need to use this speed value, so that I can refer to the power curve and determine the RPM of the motor at this speed (based on the torque required value/hp value I previously calculated) in order to find the overall gear ratio. Because 30mph is the speed at which the vehicle will mostly be running at, it is being designed purely for city use.
I want to do this so that I can then use these equations to generate the torque available graph for that motor:
Torque(wheel) = torque(motor) / overall gear ratio x n (transmission efficiency)
Speed(vehicle in mph) = (RPM(motor) x 60) / (overall gear ratio x revs/mile)
Quote from the book: "With all the other motor torque and RPM values you can then calculate wheel torque and vehicle speed using the following equations." The equations are the ones I have specified above, so it is saying I need to take the torque values from the curve and their respective RPM values. This is also a major point of confusion for me. Because like what was said before, the graph is a curve of the peak voltage? How do I read these values from the curve. I am too confused about these curves but I would really like to understand this method. Just a thought but perhaps it is more suited to choosing a DC motor? I am quite intent on an AC motor.
Then I can plot a graph of the torque available and the torque required against vehicle speed, I have already calculated torque required in the excel spreadsheet, which I attached in one on my first posts. This will allow me to determine if the motor will do what I want. These torque required values are for level, incline and acceleration by the way. This is the method used in the textbook, it just doesn't offer specific details about the power curve.
Sorry if I'm going in circles, I feel like I have been for the past 2 weeks now. Again I really appreciate your help and patience
Last edited by TheKid; 03212017 at 05:18 PM.

03212017, 09:49 PM


Senior Member


Join Date: Apr 2008
Location: Ohio, USA
Posts: 7,330


Re: Horsepower confusion
Hi Kid,
You need to plot your load curve on the motor speed torque curve. Choose your GR and tire size and convert that spredsheet into RPM and Nm. Similar to Brant on figure 69 page 158 in the edition I have. He converts the motor curve to wheel torque and mph. Same result.
Being able to see the motor curve and load curve together will give a view of equilibrium and a available torque across the whole speed range.
Regards,
major

03212017, 10:28 PM


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Join Date: Aug 2012
Location: Heart of Dixie
Posts: 717


Re: Horsepower confusion
If you are designing a vehicle, then the first thing you need to decide is the size of the wheels and which tires you plan to run. This will be a constraint to help determine the gear ratio. Here's a few questions:
1. What size wheels and tires?
2. How much does your vehicle weighwhat is your design center weight target?
3. What is the maximum design center speed that you require?
4. How much acceleration do you requirehow fast do you need to reach your maximum speed? e.g. 0 to xx mph in 10, in 20, or in 30 seconds
5. How fast do you need to reach you design center speed?
Once you answer these questions then you will have some performance factors to help determine energy and power requirements.
Now consider the motor curve plotted as a torquespeed curve as shown here. [edit: didn't notice it was metric units]
Motors don't generate speed, they generate torque based upon the current. The motor will turn at a speed determined by the voltage and the current required to counteract the sum of the load torques (aerodrag, friction, Rolling Resistance, etc).
For example let's say you need 50 mN of torque to make your vehicle run on the road at 30 mph. The chart shows that this motor can make 50 mN of torque and turn, depending upon the voltage, at any speed from zero up to a maximum of ~5700 rpm at 156V.
So current and voltage will determine the torque and speed, and these are also used to determine the size of battery pack, energy and power required to meet your range and speed and performance design center targets.
Last edited by kennybobby; 03232017 at 04:21 AM.

03222017, 06:30 AM


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Join Date: Apr 2008
Location: Ohio, USA
Posts: 7,330


Re: Horsepower confusion
KB, see attachment on #4.

03222017, 06:07 PM

Junior Member


Join Date: Mar 2017
Posts: 15


Re: Horsepower confusion
Major, I am trying to do this for the AC24: http://www.cems.uvm.edu/~aero/AC24_2007.pdf
and for the AC20:
http://www.hpevs.com/Site/images/tor...ial%20peak.pdf
What the book says is this: " From these figures or curves, you can derive the RPM at which your electric motor delivers closest to its rated horsepower. Using this motor RPM figure and the wheel RPM figure, which gives you the wheel RPM from your target speed and RPM, you can determine your best gear or gear ratio from:
Overall gear ratio = RPM (motor)/RPM (wheel)
This—or the one closest to it—is the best gear for the transmission in your selected vehicle to use; if you were setting up a onegearonly EV, you would pick this ratio.
With all the other motor torque and RPM values you can then calculate wheel torque and vehicle speed using the following equations for the different overall gear ratios in your drivetrain..."
kennybobby, great stuff this really helped with my understanding. Like major said, one of my previous messages containts a pdf attachment that will answer most of the questions you have posed. What I am hoping to achieve is to create a torque available graph, using values derived from the power curve of the motors. This is what the book I am following does with the FBI4001 DC motor. I feel like I am still missing something. For example, for the AC24 motor, I am not entirely sure which RPM value to use to calculate the best overall gear ratio. Is it the "nominal" speed value or the "maximum" speed value.
Also, relating to the second bold sentences above in the quote, it seems that what I need to do is simply read off the RPM at 10 ftlbs, then 15 ftlbs etc until 74 ftlbs. However the curve doesn't show an RPM value for 10 ftlbs at this voltage?
many thanks

03222017, 07:46 PM


Senior Member


Join Date: Apr 2008
Location: Ohio, USA
Posts: 7,330


Re: Horsepower confusion
Quote:
Originally Posted by TheKid
... it seems that what I need to do is simply read off the RPM at 10 ftlbs, then 15 ftlbs etc until 74 ftlbs. However the curve doesn't show an RPM value for 10 ftlbs at this voltage?

You could extrapolate and get like 12,000RPM, but it really is irrelevant because it would break something if it could go that fast. Please do as I suggested and draw the load curve on the motor speed torque graph.
For a "normal" speed or whatever you call it, just pick 3000 RPM and 30 mph, figure the GR, draw the curve, and then adjust from there.
major

03232017, 07:34 PM

Junior Member


Join Date: Mar 2017
Posts: 15


Re: Horsepower confusion
I have come up with a torque required or available vs speed graph. Major were you suggesting that I did it in reverse to what Brant does?
Here is the graph I made (attached), few details to explain first:
Wheel RPM at 30mph = 443
Gear Ratio 1 = 6000/443 = 13.54
Gear Ratio 2 = 4500/443 = 10.16
Gear Ratio 3 = 3000/443 = 6.77

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