Sometimes a horse is just a horse

Angelique and Kerosine, You both have won, Case closed. Bert

 

This isn't a matter of win or lose, it's just a matter of myth + fiction + misunderstanding vs proven facts, since everyone is entitled to have a own opinion, but no one is entitled to have own facts in a healthy discussion.

For myself I was mostly reading, which gave openings to dig deeper, so from the postings here, as well as some other finds, I've learned quite a lot.

I'll consider that as a win . . .

So thanks all contributors, including Bert, since without controversy there wouldn't have been any discussion to learn from.

 

Don't know, who knows who likes to contribute any info in the future, or discussion, but I've buried the hatchet anyway . . .

Mutual ?

 

Torque is not the rate of change of an angular momentum
The units for torque are static, pounds times feet or feet times pound,
When we are discussing torque in a shaft, moment and torque are the same thing, the units are the same and are static, pounds times feet or feet times pound.
Both terms, as it applies to the magnitude of the twist in the shaft, are the same.

Generally, ( and lets just assume a 2 dimensional plane, ie a plane of X and Y axis just to keep it simple) if you had many forces acting on a plate in the X and Y plane, but in various directions in the plane AND YOU ARE ONLY TRYING TO DETERMINE IF THE PLATE WILL TURN with these forces applied to the plate. You pick any spot on the plate, usually a point that has as many forces whose locations run through the same point, just for ease of calculation because you know that the moment arm is zero if the force is through the point that you picked.
Then you sum the moments of the forces, ie the perpendicular distance of the force from the point and deal with the direction of twist, say positive is clockwise, negative is ccw.
When you have summed the moments of forces, you will be able to determine the twist in the plate (I am intentionally ignoring that the forces can and would cause the plate to move up or down or side to side)

Engineers, at least in North America, would call this Summing the Moments.

When you get into a shaft, which is basically a round plate, but long, the math would be the same, we normally refer to this as Torque. Which is static.

I have always been of the mind that this confusion comes from the fact that engine curves have static Torque Curve values displayed on an Engine Horsepower curve and leads people to think that peak torque is what is important and not horsepower.

Torque does not move anything, it is static. Most dynos are able to read the amount of twist that an engine/motor can put into a shaft. Or rather the amount of twist that the engine/motor
could put into the shaft if the shaft was stationary. Pounds feet is the correct engineering term for Torque, though people often use foot pounds. The same thing for sure but further confusion occurs as foot pound is the measure for work. If you lift one pound, one foot straight up, you have done one foot pound of work. (the faster you do it, the higher the rate of work and hence the higher the horsepower value, horsepower being the rate of doing work)

In a frictionless gear train, it would be easy to create 500 pounds.feet of torque with either a 1 horsepower motor or 100 horsepower motor, as torque is a static force.

 

Torque is a subset of moments

 

We may have here a confusing misunderstanding. I am under the impression that you, Barry take "twisting" in material as the torque subject, while we are having it over torque applied to a shaft, which changes, when an electrical current is applied in an magnetic field. I am afraid to agree with Gonzo. Bert

 

Barry, thanks for a good reply - except the units I find clumsy, being a SI system man myself.

I think a big part of the confusion comes from cars. A broad powerband is often more usable and gives you power where you often need it. Equally in race cars compromising some of the absolute peak power for more power in lower revs can mean much better transitions when shifting gears or coming of corners.
Thus statements like "torque wins races and horsepower sells cars" (rough quote attributed to Carrol Shelby I believe), are taken out of context. Shelby for sure understands what power is and I am convinced never meant that power is a useless attribute.
These statements of torque being the holy value have also found harbor in Harley riders' and 80's american V8 owners' who have hard time accepting that Japanese 1000cc sportbike at half cost provides double the power or that a less than 2000cc suped up turbo Civic can outperform muscle cars. (Don't take this as me hating american machines, I do not).

F1 car engine (2010 ish) at 18,000 rpm has about the same torque as my 2004 VW 1.9 TDI. I bet it can make more things happen my old Golf can even dream of.

 

Hi everybody, I have never been so confused than with this original statement in the first thread who is a manufacturer of electrical e-motors. Most likely brushless electrical DC motor. Al what I read are tractors, F1 cars, Ferari's, V8 engines, Volkwagens and torque when is shaft get twisted, which is indeed static. However we are, according to me, discussing torque created by an electrical current in a magnetic field, which creates a torque on the shaft. The formula is different. The original topic was whether the electrical torque on the propeller can be better than mechanical torque by explosions in a diesel or petrol/gas engine. Kerosene has answered already that a boat needs the maximum torque at higher revolutions of the propeller. E-motors supply torque at different revolutions and confuses customers and everybody else. torque is torque 1Nm = 1Nm electrical or mechanical torque. The e-motor manufacturers are not giving info at the propeller shaft but as input in Watts. Unless Ocean Volt and Torqeedo are willing to give their power at the propeller shaft, we will always have this confusing debate. Over and above, we have many foreigners who sometimes express themselves with the wrong words or phrases and culture, which adds additional confusing. I love e-Motors and know what can and cannot be done with them. Barry, thank you that you used Capitals in emphasising a particular point. I am out of this discussion, unless we move back to torque created by an electrical current. Bert

 

Torque created by hamsters, electric current or sequntial combustions is no different. E-motor marketers try to explain it otherwise. That is why have (repeatedly) these discussions.

 

Consumed P (Power in Watt) = U (Voltage in Volt) × I (Current in Ampere)

↓ Answer 1

P shaft = Consumed P minus the (estimated) yield losses.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

The below goes for every shaft, regardless the nature of the power source.

P = T × ω

P = Power in Watt

T = Torque in Nm

ω = angular velocity = 2 π n/60 - where - π = Pi (3.14159265.......)

(ω = angular velocity symbol represented by the lowercase omega)​

n = revs per minute (divided by 60 is revs per second)

so - P = T × ω - becomes - P = T × 2 π × n/60

T = P / (2 π × n/60)

↓ Answer 2

T shaft = P shaft / (2 π × n/60)

 

There is certainly a "confusing misunderstanding" here as there are some FACTS supplied to you from some of the contributors as well as OPINIONS. My short comments about torque/moment was just a condensed summation of engineering courses in statics and dynamics which I tried to present. Technically, torque/moment in a shaft that we are discussing here
could be referred to as a "couple" in the shaft. As a couple does not produce any side force in a shaft whereas depending on how you are applying torque to a shaft, ie a torque wrench or a moment to a shaft,( a single force acting at a distance from the shaft) can produce a side force in the shaft. In other words if you were designing a shaft that only had the torque applied by a single force acting off the center of the shaft, the shaft diameter would be different than a shaft with a couple applied if designing to the same stress value in a shaft

Re your comment, that I take "twisting in a material (shaft) as a torque subject". Guilty!! We are dealing with a shaft, and torque is the commonly used term. A twist in a shaft for this thread.

For you to say that somehow "torque applied to a shaft, changes when an electrical current is applied in magnetic field" is incorrect. As the goal is to provide a spin a shaft turn a prop,with some degree of torque/twist in the shaft, it does not matter how the twist gets into the shaft. Others have said the same thing in their posts.

Torque is a static measurement, one pound feet of torque is the same as 1.36 NM. You do not see units of rpm, angular momentum, angular velocities. The instant that you produce say 1 pound-feet of torque or 1.36 NM of torque/moment/couple, WITH A TURNING SHAFT you are creating horsepower. I will just use foot pounds for the torque value. (old habits die hard)

Different dynos measure a static torque in a shaft, a simple friction brake lifting a moment arm that changes the readout of a simple spring scale, strain gauges on the shaft measuring the amount of twist in the shaft, light pickups on two spots on the shaft, and many more. As our lab time was developed to get an understanding of torque, (moment,couple) rpm, strain, and horsepower, we took the static torque (force distance) values using the simple brake dyno, the strain gauge resistance values for the strain dyno, and a time interval on the light pickup dyno AND THEN applied the RPM to come up with horsepower curves for steam and gas turbines, reciprocating gas and diesel engines and electric motors.

Comparing electric motors to gas engines with the electric motor proponents saying that they are more efficient, better, when using different criteria, 1/3 hp (248 watts) motor to a
3 hp (2235 watt) gas engine is ludicrous. (another contributor said the same)

Different props, speeds, etc.

I will describe a picture, (the boxes are just used to isolate the three components if you were designing a system, control boundaries. Each box would have a net input, and a net output, ie the motor would have watts input, and could be watts output if you want to use SI units, but I will use Horsepower) ( the output of the motor box would be the input of the shaft box)

Three boxes, the one on the left is your electric motor, out of this box is a center box with a shaft in it, and on the right another box with the prop in it.

At .0001 rpm, the prop will miniscule horsepower, (at this point those readers enamored by torque might say low torque, yes low torque BUT as torque is static and we have began to turn the shaft, we must deal with horsepower) as the prop increases in rpm (throughout a normal working rpm range just to keep cavitation etc out of the conversation) the prop will require more horsepower. HP =Torque x rpm /5252

The prop does not care how the hp is given to it. Gas, diesel, steam turbine, mice in a cage, or an electric motor. )The center box, with the shaft, does also not care what is providing the horsepower, (though shafts could be different with gas, diesel, electric, turbine, length of shaft, natural frequency of vibration, nodes, etc)

Now to the Electric motor control box or boundary. Power input, power output. Bert, you mention that you can produce high max torque with your electric motor throughout a wider
rpm band
ASSUME one prop for this discussion and frictionless drive train . The electric motor will only create the AMOUNT OF TORQUE AT A SPECIFIC RPM, HORSEPOWER, that the prop requires for that rpm. Period

As with a gas engine, while an engine is capable of producing a max horsepower curve for particular rpm, in a boat, the power that an engine, motor produces is what the prop requires at that rpm. A prop that requires 200 hp at 1500 prop rpm, will require that you apply enough fuel, current to produce 200 hp. Even if the engine/motor is able to produce 300 hp at that rpm

EFFICIENCIES
Throughout various electric vs gas/diesel threads, efficiencies are thrown around with comments like, the electric motor is more efficient than a gas or diesel, or, or, or
Efficiency is unitless. And is a comparison between a base, which can be a theoretical number or between two engines, motors, props, the list goes on.

When a person says that an engine is 20% more efficient than another, you need to define what efficiency you are talking about. With a gas engine, there are about 5 that I can think of without grabbing a text, volumetric, adiabatic, energy, heat, combustion, and there are more.

 

Well in the brushless motor we calculating torque, probably also in other electrical motors with the following formula as earlier mentioned.

N * mu * current magnetic field * 2 * radius
mag field = ----------------- or N * mu * current = magnetic field * 2* radius or N = --------------------------------
2 * radius mu * current
MY apology, it seem I am not able to project the formula properly after posting my reply.

With a sensors built in a brushless motor and Pulse Width Modulation, the current is sky high and a fraction of a second later zero. The magnetic field is constant while the current is changed all the time due to the changes in frequency and speed.

During the interval that no current is applied, the torque has changed. I have difficulty in believing that Torque is static as it all the time changes. I really mean it that we can discuss it for years but we have to force somehow to release industry standards by Ocean Volt and Torqeedo.

I looked at a few other statements.
what creates torque in an electric motor - Google Search https://www.google.co.za/search?q=what+creates+torque+in+an+electric+motor&rlz=1C1GGRV_enZA751ZA751&oq=Torque+in+an+elctrica+motor&aqs=chrome.2.69i57j0l2.10154j0j8&sourceid=chrome&ie=UTF-8

But the difference in voltage does not affect the torque. To get more torque to go up a hill, you need to supply your motor with more current. A motor that can take more current (and a battery and motor controller that can supply more current) will give you more torque to help you up the hill.

How are current and voltage related to torque and speed of a brushless motor? https://electronics.stackexchange.com/questions/39387/how-are-current-and-voltage-related-to-torque-and-speed-of-a-brushless-motor

How can torque be static when the torque changes all the time. For me that is dynamic. The magnetic flux of the magnets are static. However the current changes all the time.

Barry, Kerosene, you both are probably nice people, but none of you have convinced me. I am out of this discussion. For me it is dynamic and hope that I have been able to supply the necessary proof. I think we all are talking about something different and we will never be able to come to a mutual conclusion.
Bert

 

Marine propulsion application is different from cars and other uses. It is tied to the efficiency curve of a propeller. Therefore, high torque at zero or low RPMs is not as important. I would like to see a graph comparing the thrust and speeds generated by electric, diesel and gas engines pf the same power output ratings, using the same propeller.

 

Post #57 - - ‘‘ . . . . I am out of this discussion . . . . ’’

That's remarkable Bert, since I think in post #55 I've met your condition to stay, which you set in post #53.

 

I'm not sure if the same propeller would give a fair comparison here, since it could be fairer to give each kind of engine/e-motor a propeller that is optimised for that engine's/e-motor optimum ~ power ~ torque ~ rev ~ range.