Motor power output

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s8nsmum
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Motor power output

I have been looking through this forum as I am am about to convert a motocross bike to electric and thought I would post in the hope that someone could help me out. I have a fairly good grasp of basic electronics as I used to build fighting robots but am totally confused by the contradictory information I am finding relating to the two motors I am trying to decide between.

The two motors are the Etek (inc Etek-R or other versions) and the perm PMG 132. From what I can see both produce around 15Hp at 48V and i have read that both can handle 72V. Surely this means they are pretty much the same? However popular opinion appears to be that the perm is far more powerful. I'm confused!!!!!

In case it makes a difference I have been thinking of using as many Thunder Sky lithium cells as I can fit in the frame and the most powerful speedo I can afford/build.

Thanks in advance,

Joe

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Re: Motor power output

This site: http://www.thunderstruck-ev.com/perm132.htm offers a pretty good set of figures for comparing the two.

My understanding is that the Perm is more efficient and is more capable of handling higher power ratings. However, it suffers from only working well in a single direction of rotation. I don't know if the Etek has the same issue or not.

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LinkOfHyrule
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Re: Motor power output

You should be able to fix the "one direction only" thing by altering the timing on the brushes. You don't have to do this with the Etek because it's neutrally timed.

Anyway, I think the Perm is more powerful. The Etek is rated to 48V and has been run up to 72V. The Perm is rated at 72V and I think has been at least tested at 96V.

The kind/size of cells doesn't really have an impact on motor performance unless the batteries are being pushed pretty hard or you're using them in a really high current situation (e.g. Killacycle).

I'm jealous that you can afford that much lithium ;).

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andrew
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Re: Motor power output

It looks like the PMG 132 has a slightly higher official continuous current rating, but I doubt it is very significant.

I think the most significant difference is the operating speed (or rpm per volt). A lower operating speed is most likely desirable due to ease of gearing. Also the PMG can't be run counter to its designed-for direction of rotation practically. I remember someone posting that they contacted Perm about it, and it can only be run at a reduced load when reversed.

The etek may not be neutrally timed FYI. And even if it was, you'd want to advance it slightly anyway for the given direction of rotation. It can be run in either direction at full load okay, but you'd want to break in the brushes for the direction of rotation first because they may tilt slightly in their holders and not get full contact initially.

P.S. Here's a pic of the PMG 132 and an arrow showing the direction of rotation as CW.

[url=/forum-topic/motorcycles-and-large-scooters/587-my-kz750-electric-motorcycle-project]KZ750 Motorcycle Conversion[/url]
[url=/forum-topic/motorcycles-and-large-scooters/588-fixing-my-chinese-scooter]900 watt scooter[/url]
Pic from http://www.electri

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Re: Motor power output

The PMG-132 can't be run the other way even if the brush timing is altered? Really? Anyone know the technical reasons for this?

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Frxdy
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Re: Motor power output

I guess if I had to get one of those two and money were no object I'd get the Perm. It seems to be a bit tougher electrically. But... I have a question. If you can afford bunches of LiFepo batteries, why are you choosing a lower power motor such as these? When someone speaks of buying all the Lifepo that will fit it reads to me that money is not a large object. I'd be looking at more horsepower.

jdh2550_1
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Re: Motor power output

I'd be looking at more horsepower.

The PMG is about the best power you can get in this size of package. So, it might be the restraint is size of bike rather than depth of pocket.

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Frxdy
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Re: Motor power output

"The PMG is about the best power you can get in this size of package."

You make a good point. Hopefully Joe will come back & tell us more of his hopes of speed and range.

s8nsmum
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Re: Motor power output

Wow, I didn't expect to get quite such a response!

As for the speed and range;

My aim is to end up with a motocross bike that will be able to keep up with a 2-stroke Suzuki RM-250 and a 4-stroke RMZ-450 around the farm. Hopefully with 40Ah lithium cells the bike will be able to run for an hour if ridden gently and about half an hour if pushed hard. The only reason I can afford the nice cells is because the project will be sponsored :).

If all goes well then I will convert it to road use and take it on my daily commute (only 15 mins by push bike across West London).

The main problem I can see is fitting the cells into the frame of the bike. I want to keep them as low as possible to keep the bike stable but also want to have the motor near to bottom to make connection to the gearbox easier (yes I want to keep the gearbox), this is why I thought the perm and etek were good choices (can anyone suggest a more powerful alternative?). If I can't fit enough cells in then I will have to run on a lower voltage and sacrifice power :(.

Note: I really am interested in another system that would provide more power!!!!

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Re: Motor power output

The Advanced DC (ADC) line of motors is likely the next step up in power for you to look at. However, they are significantly larger and heavier (the Perm is a pancake motor - the ADC is the more traditional shape (sausage?)). I'm using an ADC on my CB-750 project. Note that the bigger ADC's are likely more robust (more thermal mass and thus less prone to burning up)

I'm curious - what are your plans for keeping your gearbox?

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Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

s8nsmum
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Re: Motor power output

I was inspired when I saw this;

http://www.evalbum.com/popupimg.php?5973

I am planning on doing something similar but probably gearing up into the gearbox (a perm at 72V no load speed is less than 4000rpm, much lower than a petrol equivalent). I want to keep the gearbox so I can maintain a good top speed while not frying my motor trying to get it off the line (and sacrificing acceleration).

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Re: Motor power output

You got a sponsor? Lucky. Mind telling :)?

I think they're right. About the only motors past the Etek are the ADC line. They're much heavier, though.

Anyway, what were the engine specs of your original bike?

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s8nsmum
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Re: Motor power output

The project is going to be sponsored by the company I work for unless we get some external sponsors (because my boss is a motocross fanatic). I haven't got the rolling chassis yet, I am probably going to go for a full on race bike, where the engine has gone (they are only rated for 11 hours!!!!), probably a 125 or maybe a 250 (to keep weight down). I like the look of the Suzuki RM/RM-Z but it will all depend on what type of chassis the cells fit into easiest.

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Re: Motor power output

Hmmm, after seeing that pic I'm taking back all I said about gearboxes on motorcycles (over on another post). Do you have the link to the main page for that pic? How do they do lubrication? Isn't the engine oil and gearbox oil all the same in this sort of set up? What about the clutch? So many questions! ;-)

In terms of gearing - if you were to gear the output of your PMG up by 3 say a 30 tooth sprocket on the PMG and a 10 tooth sprocket on the input to the gearbox then you'd have 12,000RPM going into the gearbox (which is probably in the region of the original) and could keep the internal gearing the same. However, you'd probably have so much torque at that point that you'd rip the clutch to shreds }:)

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chas_stevenson
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Re: Motor power output

However, you'd probably have so much torque at that point that you'd rip the clutch to shreds.

You don't need a clutch with an electric motor so who cares about the amount of torque.

s8nsmum
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Re: Motor power output

Here is the full page for the geared bike.

http://www.evalbum.com/1159

I also have lots of concerns regarding the practicality of running the origonal gearbox but I know that if I manage it then the bike will be far superior to a non geared one. Unfortunately it is beginning to look like I won't have enough space to mount all the cells inside the frame with the gearbox taking up so much room. Also having 5 or 6 gears is a bit excessive, 2 or 3 would be all the bike needs. I really want to leave the gearbox in, but it might be a bit to difficult. Only time will tell........

I wonder how a 48V geared setup would compare to a 72V non-geared one?

I hadn't really thought about having to uprate the clutch (or anything else on the bike apart from the engine), I hope the perm won't smoke it too quickly!!!!!!!!!

Joe

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Re: Motor power output
However, you'd probably have so much torque at that point that you'd rip the clutch to shreds.

You don't need a clutch with an electric motor so who cares about the amount of torque.

You do if you want to shift gears...

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

jdh2550_1
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Re: Motor power output

Have you seen this guys other bike: http://www.evalbum.com/148

Now that is COOL 8)

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Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

chas_stevenson
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Re: Motor power output

John,

There is no need for a clutch in an electric vehicle. To shift you just let off the throttle and shift. The only time you would need a clutch is if the transmission has no synchronizers. I can even shift a gas vehicle without a clutch. The only time you have to have a clutch for a gasser is to start and stop.

Just to be clear [size=17]"NO clutch is needed for an electric vehicle to shift"[/size]

Chas S.

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Re: Motor power output

I have to admit I only use the clutch to shift up on a bike, I don't bother when changing down. I would probably keep the clutch whan I converted the bike (unless it is excessivly large or complex) and then remove it if I found that I could do without. Removing the clutch would also allow me to put both brakes on hand levers (I hate foot brakes!!!). Hmmmm, the more I think about it the more I like the idea of removing it straight away, maybe it will have to be binned after all.

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Re: Motor power output

Yeah, I knew there'd be several people that would post "I don't use the clutch when I shift". I too can shift without a clutch - usually I do it for upshifting (because with downshifting I tend to favor feathering the clutch for engine braking). But I only do it with upshifting when playing around. If I want to ride fast I use the clutch (not much of a clutch but enough to ensure the shift occurs exactly when I want it to).

However, most mere mortals (unlike us lot of motorcycling gods) would prefer a clutch ;-)

If you want to have any sort of smooth ride and you have a gear box then a clutch makes sense - shutting off the throttle between each shift is (for most people) going to be a herky-jerky mess (that's a technical term).

You (personally) might not need a clutch - but most other folks do benefit from one. Either that or a CVT.

;-)

Oh yeah, and if clutches aren't needed then why do race bikes have them? And, no, it's not just for starting off!

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Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

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Re: Motor power output

Oh yeah, and if clutches aren't needed then why do race bikes have them? And, no, it's not just for starting off!

Well I do not race bikes so I can't answer that question.

If you want to have any sort of smooth ride and you have a gear box then a clutch makes sense - shutting off the throttle between each shift is (for most people) going to be a herky-jerky mess (that's a technical term).

This I can agree with, I guess I was not thinking about all the folks out there that do not know how to shift without a clutch. I drive a stick a lot and very seldom use the clutch but then I know my vehicle very well.

I will concede that the average person may rather have a clutch but I still think they are not really needed on an electric vehicle. I would think the use of a clutch would just use energy for no good reason.

You (personally) might not need a clutch - but most other folks do benefit from one. Either that or a CVT.

John, I think you have a great solution, the CVT. But how efficient are they? For an electric they might be great, you could bring the motor RPM to the sweat spot, most efficient RPM, and use the CVT to adjust your speed.

IMO,
Chas S.

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Re: Motor power output

"John, I think you have a great solution, the CVT. But how efficient are they? For an electric they might be great, you could bring the motor RPM to the sweat spot, most efficient RPM, and use the CVT to adjust your speed."

Not very. Most are somewhere between 30% and 60% efficient (I think). Switching gears for a CVT isn't really a good trade off when you have the torque an electric motor provides.

The efficiency gained by having the motor run in it's most efficient/powerful spot is more than lost throught the CVT's inefficiency.

The only CVT I know of that would make this practical is a NuVinci, but it isn't near powerful enough to handle an Etek or PMG-132.

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Re: Motor power output

Link - do you have any sources of info on CVT efficiency? Most of the Chinese gas scooters use CVT - are they all sacrificing 40% to 70% of their power? I'm off to google "CVT efficiency" :-)

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Re: Motor power output

Couldn't find any figures on CVT efficiency - but did find this:

http://www.snowtechmagazine.com/articles/2001/clutch/cltchtun.php

The CVT clutch system is one of those brilliant but simple devices that manufactures, purchasing agents, and engineers like to see. It is simple to assemble, cheap to build, and it is very efficient in power transfer when properly calibrated<.

So, Lynk - what makes you think they're inefficient? Have you been infiltrated by the manual transmission lobbyists? ;-)

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Ray_T
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Re: Motor power output

A cvt is essentially a v-belt transmission that changes pulley diameters:

http://www.energyideas.org/default.cfm?o=h,g,ds&c=z,z,537

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Re: Motor power output

Probably because I'm mixing them up with torque converters. Again.

My bad.

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Re: Motor power output

Here's my understanding of CVT and the areas that can introduce inefficiency:

As Ray_T says there's two pulleys that change their effective dimension. The front one is called the primary or variator and the rear one is called the secondary or ????

The front pulley changes it's effective dimension by having a slanted wall pulley that changes it's width one side is fixed and the other "slides in" as RPM increases. As it slides in the belt is pushed outwards thus creating a larger effective diameter. This "sliding in" is caused by the centrifugal (or is it centripetal?) force. The amount of that force is controlled by weights in the pulley - these weights can be changed to give a different rate of change for different RPM conditions.

The rear pulley uses the same slant wall design but works the other way around - it is held in it's closed/largest diameter state by a spring. As load increases the belt forces the pulley apart and the belt begins to ride closer to the center of the pulley. The amount of force required is controlled by a spring - this spring can be changed to give a different rate of change for different load conditions.

Inefficiency occurs if: (a) the belt slips, (b) the CVT "maxes out" too soon or (c) the CVT never "maxes out". (b) & (c) can lead to a more pronounced case of (a).

Now, if one were to use an electric motor spinning at up to 4000 rpm rather than an ICE spinning up to 8000 then I don't know if one can adjust a CVT for use in this situation. I think you can - because in both cases we're talking about reduction of engine speed to wheel speed - so the CVT would have to be configured such that it doesn't use the full range of travel (lighter weights in the front and/or a firmer spring in the rear).

Does that sound feasible?

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
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Re: Motor power output

It's doable.

I remeber thinking about doing this a while back. I was researching CVTs and torque converters for this purpose.

Somewhere during my browsing, I came across someone who was doing the same thing with a go-kart. His problem, however, was that most go-kart CVTs were also clutches (or something like that), meaning that they would only engage above a certain RPM. Eventually, he found a CVT that he could disable that on, and make it work at 0 RPM. I don't know how it turned out, however.

I problem I do see with it, however, is the incredible torque motors give at low RPM. If you were to try to use one, you might have a serious problem with belt slippage, if not screw up the belt.

There are other kinds of CVTs than the pully kind, remember. I know of ones that have two disks (one rotates it's edge against the other's side) and cone drives (which have two identical cones with a disk that tilts and changes the ratio between them). You still might have the belt slippage/destroy-age problem, though.

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jdh2550_1
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Re: Motor power output

I'm only interested in using the CVT originally fitted to a gas scooter. I'll likely try it and just see what happens - if it doesn't work and I mangle the CVT then no worries, I'll just go back to direct gearing.

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

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Re: Motor power output

I'm only interested in using the CVT originally fitted to a gas scooter. I'll likely try it and just see what happens - if it doesn't work and I mangle the CVT then no worries, I'll just go back to direct gearing.

It's been done look at this scooter http://www.evalbum.com/1394

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