Bigger Motors are Better

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Most of the so called "performance" upgrades that people do have something to do with modifying the controller so as to squeeze that last patch of energy out of the power curve and get that little bit more top end power. The sacrifice comes in the form of heat, all (small) electric motors become less efficient as they allow more current to flow at lower rpms. The more extreme the low end current situation the worse the losses until you have a bike that will get one sort stretch of real power and then run out of juice. (or burnout)

There's a better way.

It's always a better idea to use a larger motor and then run that motor at less than it's power peak. You don't want to run it just SLIGHTLY less than it's peak, but in fact WAY less than the peak almost THREE QUARTERS off of the peak power.

Take a look at the chart of a generic electric motor. The yellow is power output and as you go across the rpms (zero is on the right in this one) the power peaks and then declines:

 

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Okay, so where does all this lead us?

The natural question that someone asks is:

"Well if bigger is better, just how big should I go with the motor?"

Well, I thought just for fun that I would run some calculations on the Etek and PMG132 motors just to see how they might behave in comparision to these little scooter/ebike motors. The PMG132 can output a massive amount of horsepower, as much as 18 hp @ 72 Volts, but if you were to trim the voltage down to 12 Volts you would get a motor that maxed out at about 600 rpms and had just a couple of horsepower. But how much current does it draw? That's the problem, in another one of those cases of "non-intuitive learning" one realizes that for a really big motor:

EFFICIENCY INCREASES WITH HIGHER CURRENT

The reason for this is that if you are trying to use the lower current rates you are used to on the smaller machines with less current you cannot "prime the pump" adequately to get the full motor running in it's "efficiency zone".

So the moral of the story is to get the right motor for the job.

All this controller fiddling is of radically less importance than getting the overall motor size fitted correctly.

 

[knightmb]

Well another point to make is being happy with what you have. I could pump more power into my e-bikes for more speed/distance, but I'm happy with what I have. I wouldn't complain if I had a 72 volt e-bike that could go faster than a car :wink:

I know that a lot of people are building e-bikes that have so much power, it might as well be a motorcycle. I've always thought of e-bikes as a mesh of human and electric power, but some of the e-bikes I've seen go way above the speed that any person could pedal. In my bike club, I've seen riders who can pedal faster and further than a lot of the high end e-bikes I've seen here and at other forums. That's why I'm happy with mine, even given it's limited power and speed (though the range is awesome). When you are riding an e-bike by yourself, you always craze that extra speed. I know I do riding mine sometimes. But, when you ride with a group of "non-electric" riders, you soon realize that what you already have is awesome. That has been the mellow factor for me is my bike club. We ride 35 miles at a time and watching my friends do this non-motorized makes me have that much more respect for them.

No one in the club minds that mine is motorized because I always hang back and carry all the heavy stuff to make everyone's ride that much better. If they want me to break a long head wind, I'm glad to help. If they just want to go fast (or blow away another pace line ahead of us), I'm always glad to help. It gives everyone a good laugh to watch one pace line blow by another at 30+MPH when I'm cutting all the wind resistance for those behind me.

 

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Well I was in a sense agreeing with you. If you have an electric motor of a certain size and a controller that is properly designed for that motor you ought to stick with it. If you want more power then the thing to do is to upgrade from the motor side and you can usually get the same range with more power with the bigger and more efficient motor. I was more or less complaining about the endless quest to squander the resources of the battery (and the possible health of the system) by modifying the controller to increase the amp limit.

Stay "stock" with what you have OR do the smart thing and make changes on the motor side not the controller. It's always better to have a slightly larger motor with less current running through it than the other way around... ALWAYS. (well, I guess if you go overboard with a bigger motor like an Etek then you face the limitations on the other extreme)

Another thing might be to go with an exotic controller design like the "Current Based Throttle". That way you can "have your cake and eat it too" because you can essentially reprogram the controller by the way you use the throttle.

I'm still hoping someone will try that before me, because I'll have a 1200 Watt motor with a 100 amp controller that I'd really like to "tame" a little with a "Current Based Throttle" modification. Given that I've already calculated that the MAXIMUM power output on a 1200 Watt motor occurs at 75 Amps (that's it) the remaining 25 Amps are simply getting thrown away as heat. Not so good. But my controller choices were 40, 60 or 100 Amps, so I figured I'd go for the biggest and then figure out how to restrict it later on...

 

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Here's why a 100 Amp controller for a 1200 Watt motor makes no sense at all. This first graph is of the 1200 Watt motor using 75 Amps which is what should be the "proper" amount:

 

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This next graph is of the 1200 Watt motor using a 100 Amp current limit in the controller. Notice how all the extra power goes to waste because the power curve goes DOWN once you've passed the peak of the parabola for power. This is because the efficiency goes down and those 25 extra Amps simply go to waste by producing heat:

 

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Some interesting thoughts about the PMG132...

If you compare a 1200 Watt motor with the PMG132 and think about how it "should" be used then the efficiency peak and the power peak should be the same, that's the "ideal motor setting". For every motor the value is different and bigger motors are naturally going to be higher and smaller lower. So what is the reality of these motors?

For the 1200 Watt motor the reality of it's true best peak is at a low value of only 20 Amps while running at 48 Volts. In this state the motor produces 22% waste and that is "non-negotiable" so to speak, in other words you can't improve on that. The power is a conservative 750 Watts. (not bad)

The PMG132 is a big motor and can be run all the way up to 72 Volts. Since we would never desire that kind of power and current usage we instead need to evaluate the PMG132 at a level that compares well with the 1200 Watt motor so we can make side by side comparisons.

For the PMG132 we choose a setting of 24 Volts and 75 Amps. We get an output of 1500 Watts of power to the driveshaft (that's the legal limit for an electric bike in most states) and the waste is only 15%. Not only that, but the waste across the entire powerband is low staying near 20% most everywhere.

Okay, so how much many Amps do we need to run through the 1200 Watt motor to EQUAL the power of the "mighty" PMG132?

For the 1200 Watt motor to equal the power output to the driveshaft on the PMG132 you would need to put in 50 Amps of power. The net power loss in this configuration is a whopping 36%.

Bigger motors are better, that's a fact...

Now all you need to do is design a bike that can handle a 25 lb motor. The "Cheetah" is just one existing example:

 

[knightmb]

I won't argue that bigger is better, but I think a lot of people still want to be able to pedal the bike in some way. I would guess that some of the reasoning behind adding more power instead of bigger motors. My personal preference is for an e-bike to be able to do 30 MPH, have pedals that still function and not weight so much as to make it impossible to pedal the bike on a level surface at a good rate of speed. I also like having more range/top speed than acceleration/hill climbing power.

Everyone has their own preference I know. I think the best thing that comes out of all of this is our ability to exchange ideas and designs. Right now, everyone wants more voltage and less weight. Perhaps the next big thing for e-bikes can be motors more tuned to the e-biker needs.

As I see it (and myself desire this), we all want a bike that can climb like a billy goat, and run like a cheetah for top speed. I think the dual-hub motors may be the evolution of it all because you get a "low" speed for hill climbing and a "high" speed for level/top end speed.

Otherwise, we just need one motor that is very efficient at a set amount of power and torque with gears added for shifting from low end climbing to high end speed. I think trying to find the motor that does everything only come as close as those dual-hub motors.

 

[sabrewalt]

Since you guys are wishing to the perfect motor pedal controller combination, I want to add one more thing to the wish list. I am with Knights on 30mph for a top speed and the ability to pedal.

The part I dislike about Ebiking is the knowledge in the back of my head that sooner or later, I gotta go home or I am pedeling a bunch of dead weight. Estorage capacity and fast fill to me are the big challanges need an invention. Since we are wishing.

 

[safe]

The entire area of electric bikes and electric motorcycles is in a state of flux. Some people are trying to take existing machines like full sized motorcycles and adapt them. Others take bicycles and try to adapt them to having a motor. Some are taking those european styled sit up scooters and turning them electric. I'm kind of in my own category because my goal is to get the most performance out of a machine that weighs the least. So on one hand I'm drawing from the motorcycle road racing sector for ideas and on the other I'm drawing from the bicycle world. I'm kind of in between the two worlds.

As for batteries...

My point with this discussion about motor size is that bigger motors use less current for the same power output. In a sense you get "free energy" when you go with a bigger motor. Another way to look at it is if you can cut you losses in HALF it's like adding an extra battery pack. The switch from Lead Acid to NiMh will be a big change (for me) because the Peukert Effect has been difficult to work with. (that's the thing about how a Lead Acid battery ends up wasting half of it's energy because it can't stand being drained fast)

Think of all the waste areas:

For Lead Acid there it's Peukerts Effect. (50% losses are standard)

For Motors there is the poor choice in sizes that can produce losses in the 50% range especially with a "modified" controller that lets out too much current.

So at the end of the day you're lucky to "keep" 25% of the energy that you started your day with. That's a LOT of energy to throw away. My point is that (for some reason) poor technical decisions have been made in the design and construction of ebikes. Why has this happened? Probably because it's cheaper the way things are now, I don't know for sure, but there are occasions when a really great machine gets made, but then it never sells well enough and the company goes bankrupt. The number of bankrupt electric bike companies over the years is amazing... it's a difficult area to exist in. (and the start up costs are the worst headache) Try visiting the Morgue:

http://www.electric-bikes.com/morgue.htm

That's part of the reason I'm looking to build serious speed and performance into the bike. If I were to ever try to market this in the future is has to appeal to the more "sexy" side of things. (as well as be interesting to myself... my most important audience... because if I don't love what I create no one else will) The drab and boring electric bikes just never will change anything. (sport interest might be the way to change habits initially)

I assume you guys all know about this:

http://www.teslamotors.com

 

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This should blow your mind.

In these side by side charts I've calculated the power
output verses what is "wasted" on the smaller 1200 Watt
motor verses the larger PMG132. They both produce the
exact same output.

The "waste" is in red:

 

[fechter]

One thing to keep in mind is a bigger motor generally has a higher no-load current. This means at low power levels (riding slow) a bigger motor will have higher losses. An Etek motor takes about 6 amps just to turn over. My MAC BMC motor takes about 2 amps. For a given maximum power level (ie, max top speed) there will be an optimum motor size.
For a sub-30mph machine, a Crystalllyte or BMC motor will be more efficient than a Perm or Etek. If I want to go 45mph, then a Perm will be better.

 

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One thing to keep in mind is a bigger motor generally has a higher no-load current. This means at low power levels (riding slow) a bigger motor will have higher losses. An Etek motor takes about 6 amps just to turn over. My MAC BMC motor takes about 2 amps. For a given maximum power level (ie, max top speed) there will be an optimum motor size.
For a sub-30mph machine, a Crystalllyte or BMC motor will be more efficient than a Perm or Etek. If I want to go 45mph, then a Perm will be better.

This is all true.

The bottom line on electric motors, both in an industrial setting and a electric vehicle setting, is to match the desired load to the more efficient part of an engines powerband. The "ideal" motor will have it's "peak power" and "peak efficiency" at the same rpm and the load matches your desired load, this can only be done by getting the size right and the controller limit right. If you then have the controller perfectly set to that peak rpm as your current limit you get a straight line for your power curve (rather than a parabola) and the efficiency stays near the optimum for a large part of the powerband. That's the best that electric motors can achieve, near "perfect" power across the entire powerband, with little losses.

My point of this thread was to "expose" as a fraud the idea of increasing the current limit as the best way to get more power. It's just not true. If the motor is undersized for the load you want to pull then increasing the current limit just translates into more waste. In the "worst case scenario" where current is allowed to go above the peak of the "power parabola" you are literally throwing energy away in the form of heat with nothing to show for it. (and you might blow up your system while you're at it)

 

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This is from a spreadsheet that I made that can calculate all the power performance possibilities given a 185 lb rider and a 100 lb bike. The aerodynamics are of a "road racer" style with low frontal area and moderate slipstreaming. The things you can adjust are the total Watt Hours that the battery holds and the overal "Waste Factor" that represents how much of the battery gets lost because of design issues. 25% is a "best case" scenario and I would guess that in the case of a Lead Acid Battery with a small motor and high current limit controller your losses could be in the 75% or greater category. That's a LOT of waste...

The columns are:

Power Output (Watts)
Range (miles)
Slope of Road (0%, 5%, 10%, 15%) and it's Top Speed (mph)

It's interesting how you can go from 500 Watts and 30 mph to 1000 Watts and 50 mph, but to go from 1000 Watts to 2000 Watts you only get 10 more mph out of it. So something around 1000 Watts seems the "ideal" for a normal sized person if you want to get the most out of what can be achieved with battery power at a reasonable cost.

 

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Finding Powerband "Heaven"

This is something that would only get you excited if you've been working on understanding this stuff for a very long time like I have. On this chart there is the Power curve (which is now a set of nearly straight lines) and the Waste curve which has a bottom at the same spot as the peak power. This is what you would call the "ideal" motor configuration. The maximum power and the maximum efficiency are in the same place. Aaaaaah! That looks sooooo nice! 8)

 

[Lessss]

Of course max effeciency doesn't tell the whole story I take it. After all it's more effiecient to push something along on a level surface than it is to push the same load up a 12 deg hill. Up that hill the rider is more concerned with maintaining speed, providing sufficient torque. If that can be done inside the max efficiency GREAT, if not I think we all are willing to see a bit of waste to get us up that thar hill.

 

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A hill changes the load situation.

A load comes about because of:

1. Hills
2. Wind Resistance
3. Rolling Resistance

If your goal is to match the engine size to the load then a hill throws a monkey wrench into one's "perfect world" of flat land calculations. Wind Resistance has everything to do with frontal area and streamlining, but for speeds below 30 mph you can get away with ignoring that. Rolling Resistance is only a problem if you don't have basic bicycle repair skills and otherwise it is fairly constant.

The "ideal" way to deal with a hill is with "gears". By gearing the bike to run in a high gear for flat land and a low gear for hills you can keep a constant torque and deal with different loads. An electric motor is very close in behavior to the human body in that they both have a very narrow "powerband" in which they operate. For a human it's from 60 rpm to about 100 rpm. For an electric motor it varies a great deal, but the parabolic shape of the "power curve" is always the same. (and the best power is at the high rev end of the powerband)

You might say:

"But I have a hub motor"

Yeah, well, you will have to live without many options and so your only way to conquer the hills is by throwing electricity at the problem. It's not an elegant thing to be forced to do, but sometimes it's your only option.

Fast cars and fast motorcycles all have a very narrow powerband and they "rev" to high rpms. This setup is exactly the same for electric motors... they run better at higher speeds (typically) and they prefer being used in a narrow powerband. The cool thing about setting the efficiency peak to the power peak is that it "tricks" the motor into a wider efficiency powerband... the motor always "thinks" it's running at optimal speed all the time because at all the various voltages it's always the "efficiency peak" for that voltage. The downside is that the power gets "more peaky" with a "spike" shape rather than a "parabola" shape", but if the motor is sized correctly it's not a problem.

Anyway... people adapt to circumstances and often don't have the "luxury" of invention. I'm building stuff from the ground up and can afford to look past convention and go after what is the best knowledge available.

So practically, I'd say stick with what you have and be content and maybe get a "Current Based Throttle" setup one day to give you more economy.