Force to turn a generator

[abdelrahman.esmat]

My question is if there was no generator connected the body acceleration will be close 9.8 m/s^2 but if the generator was connected and producing electricity , the body's acceleration will decrease , How to calculate this decrease?

[DamonHD]

This looks like a school homework question: is it?

Rgds

Damon

[abdelrahman.esmat]

No it's not , I put my question in a simple way so it can be understood easily , actually i've trying to reach any mechanical engineer so he can help me with this , no luck so far

[mab]

This is a real "how long's a piece of string" type question, and I think you will need to specify some things (the load on the generator for a start) to get any real answer.


If it were an 'ideal', massless generator, not connected to a load i.e. open circuit then it would generate a voltage that would increase linearly as the mass accelerated. but as there's no load and the generator is 100% efficient (and massless) it would not slow the acceleration at all so it would still be 9.8m/s2.

if it were the same generator with the output shorted then it would stop the mass falling so acceleration would be  zero.

if it were the same generator with a resistive load then the load on the generator would increase with speed, so the acceleration would start at 9.8m/s2 but would decrease until the power going into the load (proportional to the square of the speed of the generator) = the energy coming from the falling mass (proportional to the speed of the mass), at which point the acceleration would be zero and the mass would fall at a constant speed.

[SparWeb]

Okay, you have the attention of that mechanical engineer...  what?  I don't understand your question.  I like complicated questions, with rich amounts of information so that I can understand the context and assumptions you are using.  So you don't have to put it "in a simple way".

Some points to consider: 
The forces area always balanced in a system that doesn't move.
The energy in a system can be converted to and from potential and kinetic energy.  Every conversion pays a penalty.
Friction creates heat.  Heat is energy.  Never ignore it.
A large amount of power, created for a short period of time, is a small amount of energy.

Without knowing any more about your question, I can't say more than broad generalizations.  Welcome more info to discuss further, of course.

[abdelrahman.esmat]

@mab: this is really a hypothetical case, what i really wanted is that i have a body falling , it will gain kinetic energy , how much energy can be converted to electricity , apparently we will need the load in our calculations , so refer to it as L , i dont need the calculations done for me , I want the formula  , because i dont have numbers.
@sparweb: dont worry , you wont find anything that complicated , still in senior high school .
Anyway, like i told mab , the main question was how much of the kinetic energy of a falling body can be extracted as electricity ?
i just made up this design to make a linear movement turn a generator

[Flux]

Probably the exact form of the equipment will determine the limit.

Let.s stick with mechanical units, for electricity you need to add in the generator efficiency.

Wind power is assumed to be capable of extracting 59.3% ( Betz), in reality no one has reached that.

The best Hydro turbines manage over 90%. The water wheel nearly fits your diagram and that can manage near 80%.

Not sure if this helps.
Flux

[abdelrahman.esmat]

I think it will have an efficiency close to the water turbine , wind turbine efficiency is limited by betz law which ( i think so ) wont apply in this case

[Mary B]

Science or math problem...

[tanner0441]

Hi

Looking at your sketch I can't help wondering if this is to be a single shot device or to provide continuous output. What ever energy produced by the falling weight will be totally consumed to get the weight back up to the top. A more simple method would be a weight on a string or chain wrapped wound a pulley or shaft, clock designs spring to mind, this would also remove the frictional losses of a second set of bearings, and the friction of the chain over lower sprocket.

It would be a simple matter to work out the torque on the shaft from the diameter of the pulley and weight used to turn it, though the weight would be variable as the weight of the chain as it pays out should be added into the equation.

Brian

[abdelrahman.esmat]

@tanner: i have this video about pulleys and torque , i will watch it and try to apply it , but we will have a problem still , what's the required torque to turn the generator?

[dnix71]

Old style clocks didn't run on batteries or even wind up. My father has one that has a falling weight. The weight hooked on a closed loop of chain. Gravity pulling the weight down pulled the chain driven gears to drive the clock. When the weight reaches bottom you simply lift it to the top of the chain again to keep it going.

If you hooked weights to your gen belt you could measure the distance the weight fell and calculate the mechanical work done as it fell that fixed distance and compare that to the output of the generator to determine efficiency.

[tanner0441]

Hi

Measuring torque is easy. Put an arm on the generators shaft at a set distance along the arm attach a spring balance pull on the spring balance and note the reading when the generator starts to turn. That is your starting torque and has to be considered in any formula you employ.

If this is just an academic problem take a small  permanent magnet motor, connect it to a drill and at measured speeds take readings. Next attach a thread to the shaft attach a weight to the thread and let it drop. Problem..... The thread needs to be long enough to reach terminal velocity and run beyond that point long enough to take a stable reading. That will allow you to take the unloaded reading, you can then load the generator and take several readings until you have enough readings to answer your question.

If you want to arrive at your answer mathematically then you need a lot more information about your generator and test rig.

I posted a thread on here a couple of years ago on measuring the torque required to turn a generator for a given load.  Drive the generator with an electric motor mounted on a hinge and restricting the movement with a spring a set distance from the pivot point, measure the deflection on the spring at different loadings on the generator.  You could measure the torque from the motor with a prony brake with different weights and then match those reading by loading the generator.


Brian