Hub motor

[bystander]

Take a gander at this photo:

http://kaden.watch.impress.co.jp/cda/static/image/2007/08/05/ox_b1.jpg

http://kaden.watch.impress.co.jp/cda/static/image/2007/08/05/ox_b1.jpg

It's a close up of a vehicle used to promote AA batteries in Japan. Translated article here (http://translate.google.com/translate?u=http%3A%2F%2Fkaden.watch.impress.co.jp %2Fcda%2Fnews%2F2007%2F08%2F06%2F1146.html&langpair=ja%7Cen&hl=en&ie=UTF-8&oe=UTF-8&prev=%2Flanguage_tools).

The interesting part is the resemblence of the wheels to PT wheels. But what are those red / orange things in the hub?

They're windings of a hub motor. A motor built into the wheel's hub. Direct drive. No gearbox.

Something some of us would like to see on a future PT model.

That big box is holding 192 AA batteries, so you can see the wheels approx. p-133 sized, and yes, they are bald for high performance on track conditions. (also note the presta style air valve on the outside side of the other wheel, maybe something there too, although I would think it would be a potential problem when mixing with pedistrians or doorjams.)

[Sal]

Intriguing.

We already think that the HT / PT were utilitarian and minimal. Imagine the prospect of hub motors, no gearboxes to get in the way or wear out, etc.

Great find.

I'm no engineer, but doesn't the gearbox reduce the rapid spin of the motors to a slower rate of turn to "manage" the 100x sec correction... would this "finesse" be possible with direct drive with no reduction?

-Sal

[bystander]

Intriguing.

We already think that the HT / PT were utilitarian and minimal. Imagine the prospect of hub motors, no gearboxes to get in the way or wear out, etc.

Great find.

I'm no engineer, but doesn't the gearbox reduce the rapid spin of the motors to a slower rate of turn to "manage" the 100x sec correction... would this "finesse" be possible with direct drive with no reduction?

-SalMy understanding of how things work by examining the block diagrams of the PT related patents is that the motors spin so fast that the processors do not sense the rotation directly. As far as I can tell, the motor's rotational rate is converted to a DC voltage, and that voltage is sampled well above the 100x sec rate, and then adjustments are made to the drive current at the 100x sec rate.

I am fairly confidant that the "finesse", as you call it, is still possible using hub motors. I think the biggest obstacle to adopting them is the cost for the precision required at the power level needed.

ps, if I'm not mistaken, there's part of a gas strut adjacent to the wheel. Having the motors/gearboxes not embedded in the chassis allows for independent wheel suspension. Possibly, it may also allow for wheels to tilt into turns along with the leansteer mechanism. Wouldn't either one of those developments be an interesting addition?

[KSagal]

My other question is, "what is the payload" of this vehicle?

One other advantage of a gearbox is that it allows for greater RPM of the electric motor, which equates to more horsepower available for the payload.

Still, even an image of the whole device may be interesting...

[bystander]

My other question is, "what is the payload" of this vehicle?
Translated article here (http://translate.google.com/translate?u=http%3A%2F%2Fkaden.watch.impress.co.jp %2Fcda%2Fnews%2F2007%2F08%2F06%2F1146.html&langpair=ja%7Cen&hl=en&ie=UTF-8&oe=UTF-8&prev=%2Flanguage_tools).

One other advantage of a gearbox is that it allows for greater RPM of the electric motor, which equates to more horsepower available for the payload.
A hub motor is naturally a lower speed, higher torque device. It's drawback is that efficiency is lost as the number of poles rises.

In general,

Let's say the motors in the PT have two "poles" or sets-of-windings and the gearbox has a reduction factor of 24x.

An equivalent hub motor would have 48 "poles" to establish a comparable hp rating at a similar power draw.

If a PT motor has some other number of "poles" then the equivalent hub motor would need 24 times that number to be a "drop-in" replacement.

If the number of poles becomes impractically high, one can lower the pole count and re-design the winding wire diameter and voltage through the wire (system battery voltage) to adjust the hp. But such alterations can only be taken so far until you end up with a motor that isn't as efficient as a high speed motor plus gearbox.

Also, the bearings on a hub motor need to be a bit beefier because there is no elastomeric link in the power train to shield the motor bearings from shock.

So the hub motor's time may have not come quite yet.

[quade]

One other advantage of a gearbox is that it allows for greater RPM of the electric motor, which equates to more horsepower available for the payload.

Uh . . . no.

Disregarding friction losses, and all other things (like voltage and amperage) remaining constant, the horse power in is the same as horse power out.

What you might be confused with is the concept of gear ratios and torque.

Let's say I have a 1/3 hp engine mounted on two wheels . . . the human body on a 10-speed bicycle (http://en.wikipedia.org/wiki/Bicycle_gearing). The hp remains constant, but I want to change gear ratios depending on whether I want to accelerate from a standing start, cruise on flat pavement or climb a steep hill. Engine hp and torque remains essentially constant. Wheel hp remains essentially constant, but torque changes dramatically depending on the gear ratio selected. The trade off is wheel rpm for torque.

This is why when you see the guys in spandex climbing a hill, their legs are pumping 80 rpm(? -- essentially the same as in cruise), but they're going up the hill much slower. Guys that try to stay in higher gears and muscle it out at lower rpm, end up with knees like mine.

Another example would be a 201lb person lifting a 2000lb weight 1 foot in the air via a pulley system. Obviously it can't be a single pully attached to a hard point near the ceiling, but needs to be an arrangement of several pulleys working together known as a compound pulley (http://en.wikipedia.org/wiki/Pulley). The potential energy in either system would be the 201lb person and would (sort of) remain constant, but if the 201lb person had pulleys arranged in such a way that he could pull 10 feet of line through them, he could (in theory) lift the 2000lb weight 1 foot.

[KSagal]

Quade I appreciate the information...

I raced bicycles for 20 years. I know the difference betweed spinning and power pedaling. (At least that is what my group called the techniques you described)

I know of your need to correct everything I say, but a gearbox is indeed a device that allows for more power to be available to drive a payload from a given powersource. I said it and it is so...

You may not like the way I said it, but I believe it is so.

Logic demands that if it provided no advantage, than every electric device would be direct drive instead of transmission driven. The very fact that so many devices use transmissions proves my point.

[quade]

. . . but a gearbox is indeed a device that allows for more power to be available to drive a payload from a given powersource. I said it and it is so...

Let science textbooks everywhere be corrected! Karl has spoken! :)

I do not correct you to correct you, but rather to provide correct information to others that may read what you've written and may have an incomplete understanding of how some things may actually work.

I can't say I even paid any attention to the fact that the words came from you.

[hellphish]

Karl never takes things personally, not on these forums, no sir...

I for one appreciate the technical discussion, but its sad that we can't get some engineers to agree on something.
(like the difference between HP and torque)

[quade]

. . . but its sad that we can't get some engineers to agree on something.

Well, I'd say that the majority of engineers in the world actually do agree on the definition of horsepower!

To wit;

One horsepower is the ability to lift 33,000 pounds, one foot, in one minute.

It's possible to shift the numbers around and move half the weight, twice the distance, in the same time or twice the weight, half the distance in the same time. For instance lifting 16,500 pounds two feet in one minute is still one horsepower, as is lifting 66,000 pounds, 1/2 foot, in one minute. Gearing systems allow these conversions to take place, but the actual horsepower remains the same.

[KSagal]

Well, I'd say that the majority of engineers in the world actually do agree on the definition of horsepower!

To wit;

One horsepower is the ability to lift 33,000 pounds, one foot, in one minute.

It's possible to shift the numbers around and move half the weight, twice the distance, in the same time or twice the weight, half the distance in the same time. For instance lifting 16,500 pounds two feet in one minute is still one horsepower, as is lifting 66,000 pounds, 1/2 foot, in one minute. Gearing systems allow these conversions to take place, but the actual horsepower remains the same.


Perhaps you would be willing to explain why there is a 28x gear reduction transmission betweeen the electric motor of the seg and the wheel, instead of having the elelctric motor hooked directly to the wheel, in a one to one rotation manner (Direct drive)


Thank you for your comments. Have a nice day.

[quade]

Perhaps you would be willing to explain why there is a 28x gear reduction transmission betweeen the electric motor of the seg and the wheel, instead of having the elelctric motor hooked directly to the wheel, in a one to one rotation manner (Direct drive)

I'm not really able to explain the hundreds of design decisions and tradeoffs made by the designers. I can only guess as to why they made some of their decisions.

Among the many of which and of primary concern would be the costs involved. Costs in both money and efficiency. The motor in the Segway, while very high quality, is much less expensive than one of these hub motors would be of the same quality.

Your question about why not a direct 1:1 relationship of motor to wheel is a bit like asking why you don't use a track bike for hill climbing. The simple answer is because it just doesn't make much sense. The motor is most efficient with a certain load and rpm. If you tried to use the exact same motor without the gearing and just tried to overpower it to make up for the torque loss at low rpm, you'd just burn it out. Generally speaking, traditional electric motors are more efficient at higher speeds.

[bystander]

Logic demands that if it provided no advantage, than every electric device would be direct drive instead of transmission driven. The very fact that so many devices use transmissions proves my point.Electric motors have a response curve, that is, you can chart mechanical power delivered vs. electric power consumed. Generally, this curve will be low at low RPMs and increase with raising RPMs. It will peak at a certain point, and then fall due to various factors such as friction, electrical losses at higher currents or operating temperature, etc.

So there is a "sweet spot" on the chart where one gets the best efficiency out of the electric motor. Once these parameters are determined, a designer looks at the application at hand and determines what RPM range is required. If the electric motor is used best at 7200 RPM, and the application calls for a RPM of 300, the solution is to use a gear reduction of 24x. Note that one has to account for the mechanical losses of the gearbox when looking at the overall picture.

And yes, the reducing gearbox "amplifies" the available torque. But all that determines is how small a motor can be used in the first place. A designer usually figures how much torque is needed first, then works backwards to determine what gearing and motor hp (and related losses) will give the amount needed at the end of the power train.

One can reduce the RPM of a motor by reducing the operating voltage or current. But by reducing RPMs in this way, one also slides down the efficiency curve described earlier. Even if one redesigns the motor with larger coils, up to the point where the hp would be equivalent to the original specification, you'd still be operating at a low point on the efficiency curve. And that would be a significant waste of power.

But by redesigning the motor with more "poles", or sets-of-windings (instead of just bigger windings), one can obtain an efficiency curve with higher output at lower RPMs. This means the gear box can be omitted from the power train.

Technical information on the vehicle from the original post is sketchy, but I think it operates at about 100 km/hour. Also, I do not know the hp rating of the pictured hub motor. If it is in the 1.5 - 3 hp range, it should be adequate.

If the wheel diameter is about 40 cm (guessing), then the RPMs is about 1333. This is about 4 times that of a PT, so I conclude this particular hub motor would not be well suited for the PT unless there were a 4x reducing gearbox included in the drive train. Or one could increase (redesign the motor) the number of "poles" by 4. But it appears that the windings are already pretty closely packed, so that might not be practical.

So, summing up, use of a reducing gearbox is often the best way to accomplish efficient transfer of electrical power to mechanical power. But there are situations where it can be designed out of the power train when facing certain design constraints, such as compactness.

[bystander]

Karl never takes things personally, not on these forums, no sir...

I for one appreciate the technical discussion, but its sad that we can't get some engineers to agree on something.
(like the difference between HP and torque)Well, there's always google:

http://www.google.com/search?hl=en&q=torque+curve

Comparing torque vs hp is similar to comparing watts vs watthours. A watt is a measure of power for an instant, a watthour is an accumulation of those "instants" for a set period of time.

Likewise, torque is a snapshot of available power at any given instant. hp is a measure of how much work that power has done over an interval of time. In other words, hp is the "area under the curve" to torque's "point on the curve".

Just my way of looking at it, offered as an example. Not trying to "correct" anyone.

[Five-Flags]

Another way to look at the effects of a gearbox to "amplify" torque is a simple electric clock motor (remember them, with hands even).

You could stop the armature by touching it with a feather, but you would need a pipewrench on the output shaft to stall it. Just trading rotational speed for torque. :)

[KSagal]

So, from what everyone is saying, it appears to me, that the use of a transmission may be a method for taking a specific electric motor, and improving the amount of horsepower that can be applied to the payload...

I will concede that it will not generate more power, but will assist in applying that power to the payload.

Further, I was speaking functionally and in laymans terms about an motor producing more horsepower at higher rpm. That was not complete, it will more efficiently produce more horsepower given a reasonable supply of current.

I have been properly admonished. I never should have said what I said, because even though I am an electromechanical engineer who makes my living by understanding these topics, I thought is was safe to offer up a single sentence which made sense to me, and conveyed what I wanted to say, and still fits my vernacular, but left me open to the braying of those who have a need for more technical accuracy than I.

I believe that the emphasis was placed on the word horsepower, and not on the message that transmissions are a method of delevering that power, whatever you call it, to the payload. The wheels in this case...

[hellphish]

No need to explain yourself Karl, this is a discussion about torque and power, not semantics and personal attacks. I saw your layman's description to be perfectly adequate. The only problem, if you want to call it that, is that this is the science and technology subforum, so there probably aren't many laymen here :) :) 8>) Just nerds :)

[Madsegs]

Reduction gearbox AND motor could be placed in the wheel hub......
But then you have the most expensive single parts exposed to the easiest damage/valnerable spot.
Batteries could also be placed in the wheel hub, giving great ground clearance......

The lighter the moving part (wheel) the easier it is to stop and reverse (change direction) with less power.

[hellphish]

Good point. A hub motor and a planetary gearbx would be very compact.