Hello all, I am really excited about building an EV but am not sure how to procede. I am a Mechanical engineering student at Texas A&M University and I am realy comfortable building an IC bike, but want a challenge and I think EV builds are really cool, (especialy custom builds) plus it is waaaaay different than anything I have ever done.
As the username suggests, I am new to all EV technology... I have my mind set on building a vehicle somewhere between an electric bicycle and an electric motorcycle. I have alot of questions about several topics so I figured I would just post them all and see how much information I can get.
My bike is still in the design phase but I have most of the parts that I was planning to use. The list is as follows:
Motor: Etek-R
Controller: Curtis 1204-411
Batteries: Yet to be determined, but planning on SLA batteries with 20AH or so...
Voltage: 48V
Frame: Madified Pit bike frame on fat Mountain bike tires
Transmission: Three gears, First has a ratio of 1:15, secong 1:10, third, 1:5 (ratio = rear wheel turns to motor turns)
Curb weight: Ballpark of 100 pounds, up to about 120 pounds
Project purpose:
Look enough like a bike so that I can ride to and on campus(I am a college student) and not get pulled over for riding a motorcycle on campus(hence the mountain bike tires).
If more info is needed please let me know and I will post it too.
My Questions are...
First, will the motor only pull the amperage it needs to carry the load, or will it use the amperage I put in it with the throttle?
Can I do a calculation to relate amps to acceleration, or do I just need to test and figure it out?
Second, how do I relate AH to range?
I know that the higher the AH the longer the range, but can I compute a range based on AH I have, or do I just have to test to find out? I absolutely have to have a range of about 5 miles so that I don't have to charge up in the middle of the day.
Third, how much torque is typical of a bike like this on? ie how much torque will I need to accelerate like a standard bicycle?
I'm sure every answer I get will lead to more questions, but I'm sure that the wisdom here will be a tremendous help.
Thanks in advance for all of the help!!
As an engineering student I guarantee you will enjoy this book: Build Your Own Electric Vehicle ... it's full of design ideas and formulas about torque and power and whatnot as they apply to electric vehicles.
You're unlikely to need gearing at all.
The controller takes care of adjusting amps sent to the motor, based on your throttle setting. It's up to you where you set the throttle. The controller or motor doesn't have an ability to read your mind and determine your true needs.
- David Herron, The Long Tail Pipe, davidherron.com, 7gen.com, What is Reiki
hiya,
if you are using a good controller:
the motor gets whatever amps the controller gives it.
if you are using an analogue based controller (like the curtis range of DC controllers)
the motor will pull however much current corresponds with the voltage applied.
amps at the battery (say 100A) x volts at the battery (say 48v) = power in watts (4800W at the battery)
multiply it by 0.8 because you lose some getting it to the wheel = 3840W.
acceleration (in m/s^2) = power (in W) / (speed (in m/s) x mass (in kg))
you may need to convert the units to imperial if you are more familiar with those (lb, mph, etc)
in general:
range = (battery Ah x battery voltage) / wh/km (or wh/mi)
wh/km is something that tends to get measured after the fact.
careful though, the usable Ah capacity of lead acid at the rates we use in EVs is half that of rated.
so a battery rated at 20Ah at the 10hr rate is only good for 10Ah if used at 10A.
the average person can give around 400W for a short period of time (10 sec?) and around 200W continuously.
20mph is about as fast as you would expect to go on a pedal operated bike.
thats 8.9 m/s
at 400W, that is ~45N applied at the tyre.
multiply by the tyre radius to get torque.
your average pedal bike doesn't take much power to move, as long as you don't go particularly fast.
the power needed to overcome wind resistance increases with the cube of the speed.
that tends to limit top speed more than anything else.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
First, alot of people told me I wouldn't need gearing, but I have to have something on this project mechanically driven... So I have something to work on when the electrical stuff gets to deep... haha
So then I vary the amperage with the throttle and control essentially how many watts the motor puts out, which controlls acceleration... Just like a gas engine varies power with amount of gas in the engine, controlling acceleration... Thats how I thought it worked, but wanted to make sure I wasn't missing something.
So does the good vs. analogue comment mean that the curtis isn't what I need? It is a golf cart controller, so I figured it would do what I needed it to. Do I need a different controller?
What in the world is a wh/m? Is that motor specific? How do I find out what it is? I dont want a rock solid range number, I just want assurance that I'm buying big enough batteries, so I would like to have a general idea.
What is typical for a full blown electric motorcycle as far as AH go, for a 48V system? What ballpark should I be looking in?
RE :"First, will the motor only pull the amperage it needs to carry the load"
I disgree with some of the others answers. YES you are partially correct in that statement, the controller delivers the VOLTAGE, the motor will take more amps if the load is higher because then it will be at a lower speed (and less back EMF, higher amps). This is why electric motors need less gearing than petrol ones.
Re Gearing. Gearing becomes useful particularly if using a low powered motor. This because you can get max power out a motor around 75% of top RPM, three gears as you suggest might be helpful. E.G. I have a 250w scooter which was much improved by having a lower gear for hills.
In contrast I also had a 1500watt scooter that didnt need any gearing for hills or anything, because it had a more powerful motor, and therefore didnt need to run at its max power output to get up slopes!
Thats correct for only basic controllers (which all analogue DC ones seem to be) where your throttle signal changes only the max voltage applied to the motor.
the motor current limit is a fixed value (usually set in hardware).
Thus for that setup, the throttle controls speed, and the motor pulls the amps to hold that speed.
its kinda like driving your car with just cruise control, and not being able to adjust your accelerator.
the major draw back to that approach is the poor acceleration control (big lurch at start).
for controllers that have the throttle signal limit max motor amps (all digital controllers do this) also limit motor torque.
thus for that setup, the throttle controls torque/acceleration, and you accelerate to whatever speed corresponds to your torque setting.
This the same function as the throttle on any combustion engine.
The curtis will probably run the bike ok, but you won't know if the lurch at start up will cause you problems until you have built it.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
So what I am getting is that analogue controllers vary the voltage with a constant current and digital controllers vary current with a constant voltage... Thats a handy piece of imformation. I don't think a lurch when i start will be a big deal since the gearing will be so high the lurch will probably be almost unnoticable.(maybe wishful thinking)
So then does that mean that the batteries have a set run time on them... If the amperage is set to a number, say 25 amps, and i get a 25 AH battery system, then the batteries only have probably 45 minutes or so of runt ime in them... By that train of thought, I could run the motor as hard as I wanted or as soft as I wanted for 45 minutes, because after that time there is no more amperage left in the bateries. As I type that I realize that if you ran a 48V system at 25 Amps and 24V then you could possibly run it for twice as long(since there is only half of the cells putting amperage out), but I would think that the bateries would always put out 48V and that the controller would just reduce it down, so the power would be lost. So then could the amps be set lower to get the range higher? This whole system seems really counter intuitive to me!!
How much would a digital controller cost(assuming I have an analogue controller)? Would that give me any more tange than an analogue controller?
Current and Voltage are linked.
So if a digital controller limits current it effectively does it by reducing voltage
its slightly more complicated than just V=IR because uve got the back EMF voltage, but in essence its not that one type of controller is limiting voltage and the other current, by the laws of physics at a given RPM you can only get a certain current through at a certain voltage
What is back EMF? How does the relationshiop work, does more voltage require less current to produce the same wattage or does a higher voltage allow a higher current?
wh/km is a measure of (on average) how much energy is needed to push your vehicle 1km (or one mile).
that value depends rather more on the vehicle, how heavy it is, how aerodynamic, etc.
its much easier to measure it than calculate it.
not quite:
analogue controllers will hold a fairly constant voltage, unless the current drawn at that voltage is beyond the current limit.
if that is the case, the voltage is varied to hold the current down at the current limit, until the voltage reaches the throttle set point.
digital controllers will hold the current constant at your throttle setting, varying the voltage to do so.
both types of controllers act as "buck" converters.
essentially, they trade volts for amps while holding constant power in = power out + losses
volts x amps = power
so if you have a 48v battery, and the motor needs 20A @ 24v, then the controller will pull 10A from your 48v battery.
the type of motor you chose is a permanent magnet.
so motor voltage = rpm x constant + motor current x constant
torque = motor current x constant
the value of the constants depend on the particular motor.
the run time of a battery is determined by how much usable energy (wh = Ah x V) is in them and how much you use per mile.
basically you don't know for sure how much range you have until you can measure how much energy it uses.
you can't measure how much energy it uses until you have built it.
tricky conundrum huh :)
either controller type will give you the same range.
I would start with a set of junk batteries just to measure how much energy you use.
as far as performance goes, the sky is the limit.
my own bike runs at 125V (soon to be 145V).
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
[the type of motor you chose is a permanent magnet.
so motor voltage = rpm x constant + motor current x constant
torque = motor current x constant]
What constant is this? Are they the same constant? I have seen a torque amp constant, is that what you are talking about?
On that note, the way I now envision it working in my head (loosly speaking) is that the throttle controls not only the amperage but the voltage also, which controls power... But then where does the torwue amp constant come into play? Is the torque amp constant different for every voltage? If the voltage varies, does the constant vary? As the constant varies, the amps needed varies... seems like a GIANT circle!! Can somebody help me with this confusion?
Lets look at a simple electric circuit with say a resistor. V=IR
i.e. if u increase amps u increase volts - they are locked together. So all this talk about one controler adjusting amps and one volts is misguided as both factors are linked. It is circular as you say!
The more complicated situation with motors is its not like a fixed resistor. As it spins faster is produces a back voltage, which in effect increases its resistance to current.
The modulation of controllers works by turning the voltage on and off very quickly to reduce the volts (and therefore amps) at lower throttle.
At any RPM, there can only be one current that fits that voltage. If the battery of controller cant match that current what will happen is a lower current will go through, AT a LOWER voltage!
maybe the equations are better written as:
motor voltage = rpm (or speed) x volts/speed constant + motor current x motor winding resistance
torque = motor current x torque/current constant
the rpm x volt/speed constant is the back emf
both controller types adjust voltage directly.
its the control/logic section that is different.
the analogue variety has the throttle varying the pwm directly.
so with a 48v pack:
0% throttle = 0V
25% throttle = 12v
50% = 24v
etc
the current limit is enforced in hardware (to protect the switching devices) and is permanently set at the controllers rated current.
take a 48v 100A analogue controller.
you apply 25% throttle to take off from a stand still.
the acceleration looks something like this:
motor:
speed voltage current/torque/acceleration (they're all linearly related)
0 2V 100A = full torque/acceleration
10% 6V 100A = still full acceleration
22% 10.5V 100A = still full acceleration
23.5% 12V 50A = half acceleration
25% 12v 0A = no acceleration
30% 14.4V 0A = no acceleration (the switching devices are off separating motor from battery since motor voltage is beyond the 12/25% throttle setting)
the effect is you get full acceleration even at very low throttle input.
a digital controller attempts to control the current instead of the voltage.
so the same situation:
speed voltage current
0 0.5V 25A = 25% of max acceleration
10% 4.5V 25A = 25% of max acceleration
22% 9V 25A = 25% of max acceleration
25% 12.5V 25A = 25% of max acceleration
30% 14.9V 25A = 25% of max acceleration
in effect the digital controller is completely linear.
some digital controller varieties also limit the voltage proportionally aswell as the current.
so in the above scenario, the voltage limit would be 12v, and behave like the analogue controller in the first example.
however, the current would still be limited to the 25% as that is the throttle setting.
the controllers are designed like this, not so much because they are digital or analogue, but rather because of the industry they were designed for.
the analogue controllers are of old (ancient? early 90s) design and are very basic.
They were intended for slow moving EVs (think milk float or forklift) where speed control was important, but acceleration control was not.
doing that in analogue is easier than in digital.
however, for road going EVs, the requirements are more onerous, we want:
motor voltage and current limits, battery voltage and current limits, speed limits, precharging, contactor control, etc.
the more complicated the requirements, the easier it is to do it digitally.
almost all new designs will be digital.
Matt
Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah in July 2010. Done 194'000km
Those are both really great explanations! The basic idea is that no matter what, both types of controllers use the same wh to accelerate/maintain speed... Thats good, now will I need a different controller than the curtis because from what I understand it is analogue? Would I do better with getting a digital controller and dumping the curtis? I think it was made to go in Golf carts, which are constantly starting and stopping and most golf carts are pretty smooth about it. That makes me think that the curtis might be digital or at least a high end analogue controller.
So does the wattage that the motor puts out vary, or is it always going to be 8kW? That is alot of current all the time! I would think that the motor has 8kW in it, but it doesn't always have to use all of it, like an IC engine. How does this work?
The next thing I need some serious guidance on is the AH I should be looking at... My problem is that I would like the bike to be able to carry 2 people. It probably won't do it very often or for very long distance, I'm talking about less than 2 miles here. BUT, it also needs to look enough like a regular bike so that I can get away with riding on campus... It is actually going to have functional pedals!! The only time they will function is the show the campus rent-a-cops that it is a functioning bike!! So putting a huge group of batteries would be less than ideal... What is typical for a bike with a curb weight around 120lbs?