The Great "Gearing vs Hub Motor" Debate

[Matt Gruber]

This weighs only 5 lbs and costs $250 and produces 1200 Watts peak power which with gears is all you really need. (and very high efficiency too because it's brushless)

http://www.thunderstruck-ev.com/transmagnetic.htm

a 5# motor has no appeal to me.
i want it heavy to dissipate heat. for my mbike 10-15# is light enough. over 20# is too heavy for it imo, however.
...unless it had some special feature i wanted

 

[Malcolm]

Seems like people are arguing about different horses for different courses here. The transmagnetics motor is a good choice for safe's custom-built machine. You can set up the gearing exactly the way you want and get a very lightweight combo. The Puma and crystalyte motors look as if they make great plug and play solutions, but aren't you defeating the point if you take them out of the wheel and start adding gearing systems?

 

[safe]

The Puma and crystalyte motors look as if they make great plug and play solutions, but aren't you defeating the point if you take them out of the wheel and start adding gearing systems?

That was my argument.

If you are going to use chains and gears why use a hub motor to power it when other solutions are better for such situations?

I liked that one response:

"All the extra weight is better because it dissapates heat."

...talk about inventing "justifications".

A well designed motor and proper gearing would eliminate heat as a problem... my current Unite motor never gets hot because I downshift when it's in that "bad heat" zone and so it never needs to do "bad things" in "bad situations". People who use fixed gears deal with heat because.... well... because you have no gears to prevent heat.

If I really overload the motor with extra voltage and current then heat might be a factor (since I'd be going beyond the normal capacity) but I plan to use forced air cooling in such a case.

 

[safe]

At a listed 7.5 lbs and 4hp, the Perm 080 has similar peak performance to the 5-7hp x5/Puma.

That PMG 080 is a nice motor, but it revs too high for a direct gear and chain solution. (there's no way to gear it down enough from 7000 rpms to make it work)

I'd have to say the NUMBER ONE argument for a hub motor is LOW RPMS. (it makes gear selection simple)

How much does the Puma cost?

 

[xyster]

I haven't seen the puma hubmotor sold by itself. In the U.S. you'll find it bundled with a reportedly crummy controller for ~$600 here:
http://www.texaselectricbikes.com/catalog/electric-bike-motor-p-163.html
and
http://superkids.stores.yahoo.net/500wabmcelbi.html

This hubmotor also works with better crystalyte controllers.

 

[xyster]

A nice, brief review of the nuvinci hub CVT:
http://phil.veloblog.ch/post/7/333

 

[Matt Gruber]

i bet the nuvinci is strong!
i like heavy strong parts! MACHO

 

[Matt Gruber]

SAFE said "That PMG 080 is a nice motor, but it revs too high for a direct gear and chain solution. "


Razor figured how to do it. Safe u need to study more!

STEP 1: it cant be done.(safe is here)
.....2. its too weak.
.....3. i knew it would work all along.

 

[safe]

Razor figured how to do it..

You can run at lower voltage, but your efficiency goes down. So what do you gain? If you can get the same overall efficiency from a $75 Unite motor as from a $400 PMG 080 (running at reduced voltage) then I don't see the wisdom in that.

The ONLY way to make that motor be of any value is if you can run it so that you get the efficiency advantages of full voltage, but then it revs to 7000 rpms and the 12 / 114 gearing limitation that the go kart sprockets provide just can't deal with it. The MAXIMUM rpm the go kart sprockets can handle is about 4000 rpm. The LAST possible way around it would be to use an internal multispeed hub that GEARS DOWN to less that a one-to-one relationship in the lower gears.

I'm all "ears" for a real solution to this...

 

[xyster]

I'm all "ears" for a real solution to this...

How about using more than two gears to get the ratio necessary.

little=====BIGlittle=====BIGlittle======BIG

 

[Matt Gruber]

safe
i need to see the efficiency, and power, of an over-volted unite 1020
please post
+6v
+12v
+18v

Seems to me voltage gives electric "gears".
VOLTS vs. GEARS

 

[safe]

Rather than getting into charts let me explain the basic problems....

There are a couple of things:

First - Voltage establishes peak rpms.

Second - Current limits establish WHERE in the powerband the "peak power" is attained. A low current limit makes the "peak" happen near the maximum rpm. A high current limit makes the "peak" happen near the middle of the rpms.

Third - High torque at low rpms is very inefficient with electricity so if you want to climb a hill the low rpms literally "suck" your battery empty and turn your motor into a "space heater". Even with the PWM current warping effect the efficiency is in the 50% range at low rpms. (best efficiency is around 80% nearer to the maximum rpms)

If you "juggle" all these things in your head you begin to appreciate the struggles that the simple "Voltage" mentality runs into. So matter which way you "turn" you end up either building a "tractor" or an "indy car" and have to live with the consequences.

There is no "simple" way out except a bigger motor (like Xyster has done) and with the bigger motor you need a "monster" battery pack (to handle the massive power drain) and to get all that you need a "fat wallet".

 

[Matt Gruber]

well, that's not my experience.
I've been over volting my 1020 since august, and i'm super happy.
In fact it only draws 1-2 amps in stores(low speed stop & go), either way.
and it goes faster, 20 v 17 on the path, and doesn't overheat.

 

[safe]

The physics hasn't changed any.

In your application you might have found a combination that works fine for your needs, but my point is that when you start into projects where you are looking to get into the "50 mph Club" one day you have to get serious about understanding how all these things work at the fundamental level.

If taking a 500 Watt Unite motor and overvolting it a little to give more top end speed works for you, then great. I was planning to do the same by taking a 750 Watt 36 Volt and going 48 Volt with it on one of my bikes. But if I had the choice of a "perfect motor" that could run at below it's limits and had high efficiency I'd take it. That "Transmagnetics" motor looks very good to me... (but it costs a lot more)

 

[safe]

Was fiddling around with some Twin 1200 Watt motor simulations and realized something profound that I had not pondered that much before. When you use gears it really changes things. With gears you can move the "efficiency peak" around to match your mph. On a fixed hub motor you are permanently tied to your powerband and this presents a real problem at low speeds. Low mph/rpm on a hub motor places you into the low efficiency areas of the powerband. You can compensate a little if you have an ammeter and are really careful about using even less current than the current limit allows (so you are using almost no voltage down low in order to be more efficient) but you are still fighting a less than desirable situation.

Anyway... this is the chart of an "ideal motor" (definition: the "efficiency peak" is equal to the "power peak"... you can't get any better than this for a given motor efficiency) and how it's efficiency looks across the MPH spectrum. In this case I was using an "ideal" Twin 1200 Watt setup running at 17 amps per motor.

See how in the low mph areas the efficiency really sucks?

At 10 mph the efficiency is running about 50%. (and this is ideal) The good efficiency areas don't even exist until you are past 20 mph. So unlike with gears where you can "gas it" off the line and be in the good efficiency areas all the time, the hub motor has a real "issue" with quick starting. Quick starts on a hub motor are really bad news for battery usage... (much worse than if you are geared right)

 

[Leeps]

Actually its off topic but talking about overvolting got me thinking. Suppose we had a 120 or so volt battery connected to a 36 or so volt motor using a synchronous switching(key here) motor controller. Have a feedback of the actual motor speed going back to the motor controller. Of course this means you would have to build your own motor controller.
benefits would be very low current drain from the battery and all the wiring before the motor controller. peukert effect would no longer be an issue or would be greatly reduced. Within the motor controller only the flyback mosfet and forward would have to deal with large average currents.
The motor would also be able to produce maximum torque at full rpm, meaning no reduction in acceleration as you approach maximum speed.
Downside would be battery balancing and charging such a battery.
Just random thoughts
Joe

 

[xyster]

Can you explain for us electrical apprentices what a "synchronous switching(key here) motor controller" is?

 

[Leeps]

Synchronous switching means that the controller doesnt use a flyback diode. When the main mosfet switches off theres another mosfet connected in parallel with the motor that switches on.
This is more efficient than a diode because with a diode you have a 0.7 volt loss when its conducting(when the pwm is in its low cycle), mosfets are ohmic with a low resistance.
In this setup you would always be at relatively low duty cycles meaning that the flyback device is conducting most of the time, thus a flyback diode would make for an inefficient system. The synchronous switching method would bring your efficiency back to that of a regular motor controller.
Joe

 

[safe]

Start a new thread guys... this is the "Hub Motor vs Gears" debate.

 

[fechter]

Synchronous switching also gives you regen. My old DE controller has synchronous switching. Getting the FET timing right is tricky. Too fast and it starts to shoot through.

What Safe really wants is an induction motor. No rpm limit due to battery voltage. Too bad the controllers are wallet busters.

 

[Leeps]

I forgot that detail, if your controller has regen it definetely is synchronous, the designs go hand in hand. But fechter what do you think about my idea in using a high voltage battery stepped down to the motor voltage. Seems like there are some benefits to it. Not worth building a controller for it but still.
joe

 

[Lowell]

When I get back home I'm going to do some time to speed tests, but I don't think my X5 hub spends much time at the very low speed ranges at WOT. Even going up hills it pulls 60km/h with ease.

 

[safe]

When I get back home I'm going to do some time to speed tests, but I don't think my X5 hub spends much time at the very low speed ranges at WOT. Even going up hills it pulls 60km/h with ease.

Put into simple terms:

If going from zero to 15 mph takes "X" amount of energy at 50% efficiency with a fixed gear bike whose voltage based power peak is 50 mph.

Then going fron zero to 15 mph on a geared bike in first gear where the power peak shows up early and that makes the efficiency better, more like 70%, then the geared bike will always save some portion of "X" energy with each stop and start.

The difference may or may not be small (a good simulation experiement... though complex to do properly), but I can't think of any time when having gears would ever be worse... it's always better to accellerate at peak power and efficiency than not... it seems clear to me...

You've given me an idea... calculate the ACTUAL difference in energy between the two options... this might take a while...

 

[safe]

The Preliminary Results

Here's how I calculated acceleration. I took the torque at the hub in Newton Meters and figured out how much acceleration it would create on a bike/rider combination of 325 lbs. I then knew the speed increments in my spreadsheet and calculated how long it would take to accelerate the bike/rider to get to the next increment. Now I have the time in seconds and I apply the "Power(in)" wattage to that time interval. I then seek out within each gear ratio when the speed crosses 15 mph. From this I can sum either total wattage or total time to achieve acceleration of the mass.

So this is what I found:

If you compare my future first gear of my 1200 Watt 8-speed to it's fifth gear (which translates as the same gear as a hub motor that can do 50 mph for a top speed) then you get these findings:

First Gear: 0-15 mph in 5 seconds with a energy "cost" of 10 kiloWatts. (10,000 Watts)

Fifth Gear: 0-15 mph in 9 seconds with a energy "cost" of 14 kiloWatts. (14,000 Watts)

The net "waste" for the "hub motor" gearing is 4 kiloWatts or an excess waste of power to the tune of 38%. The reason that the wattage numbers "seem" high is that this is the "Power(in)" figure which focuses on the losses due to low efficiency, while the accelleration is based on the torque, which is a Power(out) figure. This way you "see" what's really being lost.

It's possible there are errors in all this since it's my first pass at it, but the numbers look in the general ballpark. The times seem slow though... five seconds to get to 15 mph? I'll be going back over this stuff and figuring how to make ACCELERATION a regular part of my spreadsheet and then publish it later on. These are just the "sneak preview" figures that should be taken with skepticism of course.

 

[xyster]

Fifth Gear: 0-15 mph in 9 seconds with a energy "cost" of 14 kiloWatts. (14,000 Watts)

The net "waste" for the "hub motor" gearing is 4 kiloWatts or an excess waste of power to the tune of 38%.

Energy and power are not interchangeable terms as you've been using them.

Your sim may show an average power use of 14kw to go from 0-15mph (which sounds way high), but the energy used is power X the time the power is used for, like in kilowatt-hours.