[mechman600]

Hi all. I'm very impressed by the cumulative knowledge on this Forum. There are some very impressive builds out there!

I have wanted to build an EV for years. I was planning on doing it a while back but never got around to it. Now I'm getting excited about it again. As a "training EV" I want to convert my '78 Honda Hawk. I'll use a Mars 0708 or EMC-R PM motor and 48 volts with 4 floodies. Keep it cheap and simple.

Now my questions, mainly about gearing. I want to be able to do 50 mph. The EMC-R is rated at 3700 rpm @ 48V no load. Obviously I don't want to gear it for 3700 rpm @ 50 mph. What RPM should I gear it for my desired top speed? AKA...how much RPM sag/droop in RPM will it need before there is sufficient torque to achieve my 50 mph?

Torque questions: In a typical DC motor, what does the torque curve look like? Is max torque at 100% throttle, zero rpm? Does the torque stay the same all the way up the rev range, or does it decrease in a linear fashion until 3700 rpm (in the ETEK-R's case), where the available torque is basically zero?

Power draw questions: I imagine that at full throttle, full batt pack voltage is applied to the motor. Is the highest amp draw at zero rpm, full throttle? Does the amp draw stay the same all the way up the rev range, or does it decrease in a linear fashion until 3700 rpm? I remember learning the basics about DC motors in my apprenticeship (starter motors): as the rpm increases, electromotive force (induced voltage opposing the flow of current) increases, which basically self governs the motor at a certain rpm and limits current flow as the revs get higher. I'm wondering how this plays out in an EV. Maybe I just need to drive/ride one. Anyone here near Surrey, BC? lol.

Sorry for the many questions. But going by what I've read here, there are some of you who love answering them! Thanks in advance.

James

 

[major]

Hi mech,

Cool , some technical questions.

I want to be able to do 50 mph. The EMC-R is rated at 3700 rpm @ 48V no load. Obviously I don't want to gear it for 3700 rpm @ 50 mph.

You're correct about that. The only time that would happen is downhill or like a 70 mph wind on your back

What RPM should I gear it for my desired top speed? AKA...how much RPM sag/droop in RPM will it need before there is sufficient torque to achieve my 50 mph?

What you need is a motor speed torque curve and need to know the voltage sag in the battery at load. If you knew the power required at 50 mph and had the motor curve (which included battery sag), it would be simple to find.

I assume both these motors are brushed PM motors??? Then the speed torque and current torque and therefore speed current relationships are all linear. And also a good approximation of battery sag is linear. But we don't know the slopes Sometimes for PM motors they will supply you a factor for this call Kv. Which is the RPM per volt (at no-load). This would be helpful.

O.K. That didn't answer your question. So let's try this. Say at 50 mph the actual voltage from the battery is 46 volts. And at that torque, the actual 48V motor speed would have dropped to 3500 RPM. Multiply 3500 times 46/48 to get 3350 RPM. Give that a shot and test it on the bike. Leave yourself some room with those sprockets so you can change a couple of teeth to adjust it if needed.

Torque questions: In a typical DC motor, what does the torque curve look like? Is max torque at 100% throttle, zero rpm? Does the torque stay the same all the way up the rev range, or does it decrease in a linear fashion until 3700 rpm (in the ETEK-R's case), where the available torque is basically zero?

Torque is proportional to armature current and linear with a PM motor. You are going to have a PWM controller. So the throttle is going to set the PWM duty cycle, which is the percentage of voltage applied to the motor. However there is another factor governing this PWM duty cycle called current limit. So, at a standstill, without a current limit, if you apply full voltage to the motor it would draw stall current (like 2000A) and produce maximum torque, probably breaking something. But with the current limit, at standstill, when you give it full throttle, the controller will set the PWM to limit the current, say to 400A.

Now the torque the motor will produce at 400A is independent of RPM. And if 400A is the maximum current, that will be the maximum torque. At standstill (0 RPM) the 400A and maximum torque will only require a low voltage to the motor, so a low duty cycle PWM on the controller. As the motor accelerates and the RPM increase, the motor voltage needs to increase, so the controller will increase the PWM, all the while the motor will produce maximum torque. This continues as a constant torque acceleration until the PWM hits 100% duty cycle. This will be at the maximum power point. Beyond that, the vehicle and motor will continue to accelerate (increase RPM), however the current will start to decrease. As the current decreases, the torque will decrease and the acceleration will decrease, although you will continue to increase speed, just at a lower rate. Until the RPM increase and current decrease reach an equilibrium point with the motor natural characteristic, namely 3350 RPM, if my guess was correct.

Power draw questions: I imagine that at full throttle, full batt pack voltage is applied to the motor.

Not when in current limit. See above.

Is the highest amp draw at zero rpm, full throttle?

This will be the highest motor current, but not the highest battery current.

Does the amp draw stay the same all the way up the rev range, or does it decrease in a linear fashion until 3700 rpm?

On an acceleration from zero, full throttle, the motor current stays constant at current limit until you reach 100% PWM (full voltage to the motor) and then the current decreases until equilibrium.

On acceleration from zero, full throttle, the battery current starts low and increases until you hit 100% and then is equal to motor current and decreases in the same fashion as motor current.

I remember learning the basics about DC motors in my apprenticeship (starter motors): as the rpm increases, electromotive force (induced voltage opposing the flow of current) increases, which basically self governs the motor at a certain rpm and limits current flow as the revs get higher.

Yes, and this plays out once you come out of current limit (100%) and is why the current drops as you approach top speed.

I hope that didn't totally confuse you

major

 

[madderscience]

Howdy, and welcome.

You might seriously consider doing a 72V system instead of 48V. Much more conducive to higher speeds, and at 72V you aren't going to be spending that much more. A higher voltage system will allow you to get torque even at 3700rpm. You probably won't want to run that motor under no load at 72V though. kaboom.

I would purposely gear it low, aim for maximum safe motor RPM at something like 10% above your fastest desired speed. So in your case, 55mph or so. This gives you some margin but keeps available torque up there. It is a motorcycle, so changing gear ratios should be fairly easy if you find you want it to have longer legs.

I know it is true for series wound motors and I presume it is similar for permanent magnet motors as long as you aren't saturating the permanent magnet fields: torque is basically proportional to motor amps, regardless of RPM. There are extreme situations where this breaks down, but something else will probably break first. Look for a "torque constant" rating for your motor. this maps amps to nm (or foot pounds, etc). Note that since horsepower is proportional to rpm * torque, this means you will need higher and higher voltages across the motor as it speeds up to maintain the torque output. Somebody may even have published a torque curve for it, which would provide even better detail.

If the battery were connected straight to the motor, then you would get maximum torque (and lots of smoke and melting metal) at stall speed. With a controller in between, at full throttle torque will be basically constant from stall up to the point where the full battery voltage is delivered to the motor. The motor controller limits its output to whatever its maximum rating is to protect itself and the motor.

You will see the most amps out of the batteries someplace in the middle of the RPM range of the motor. This "sweet spot" (peak HP) will occur when the motor is spinning just fast enough to let the controller deliver its full amp and voltage rating to the motor, but not so fast that the back EMF in the motor prevents that amount of power from actually flowing through it.

If your are in BC, try to get to a VEVA (vancouver EVA) meeting. They are a very active group and I am sure they have several motorcycles for you to look at.

Good luck

 

[mechman600]

You both answered my questions perfectly. I even learned something - about controller current limit being in control until more voltage is required in the higher revs. Never really thought about that before.
Thanks!!