Kinetic to Electric Power Conversion

In summary: It's a personal project / investigation into how to turn kinetic power into electric power. I'm not sure if I'm doing it right, so I'm turning to the experts!]In summary, you can calculate the power output of a generator by multiplying the engine's horsepower by the generator's wattage.
  • #1
RoSmith
12
0
Hi all,

Having trawled the web for too long, I don't seem to be able to find any resources surrounding a question I've had regarding turning kinetic power into electric power.

Take the situation where an internal combustion engine's flywheel is connected to an alternator. The idea is that the alternator turns the kinetic energy of the engine into electrical power.

Assuming the load on the engine from the alternator is constant, what is the relationship between the horsepower, rpm and torque of the engine and the volts and ampage produced by the alternator?

This is obviously a hypothetical situation and I know that I'm probably missing some factors that need to be taken into consideration (such as flywheel diameter etc.) but, what are they? How can you calculate the relationship between the two?

Many thanks in advance,



Richard
 
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  • #2
Welcome to PF!

The easiest way to figure this out is to just skip all the intermediate calculations and use a conservation of energy equation to compare power into power out, figuring in an efficiency. If the output at the engine shaft is 1kW and the generator is 90% efficient, then you have 1*.9=.9 kW output. If you want to use horsepower from the engine, you'll of course need to apply the conversion factor for horsepower to kW of .746.
 
  • #3
Hi, thanks :)

The engine will produce a kW output which will be reflected in the electricity output of the generator... but since Watts = Volts x Amps, how can you tell how many amps and volts you're getting in relation to engine torque, rpm and hp? I want to find out the connection between the two... so whether there's an equation;

Engine rpm, horsepower and torque ---------> Volts and Amps produced.

Is there any way of doing that?
 
  • #4
RoSmith said:
The engine will produce a kW output which will be reflected in the electricity output of the generator... but since Watts = Volts x Amps, how can you tell how many amps and volts you're getting in relation to engine torque, rpm and hp? I want to find out the connection between the two... so whether there's an equation;

Engine rpm, horsepower and torque ---------> Volts and Amps produced.

Is there any way of doing that?
No, there is no direct connection. It depends entirely on the specifics of the motor of the generator and those specifics are generally just chosen. Ie, if you go out and buy a 1kW generator that outputs 120V it may have exactly the same gas engine as one that outputs 240V. ByP=VI, the second one puts out half the amperage, but they both put out the same kW.

It really looks to me like you are focusing on things that don't matter. What exactly is this for?
 
  • #5
I want to investigate the conversion of an IC engine's power to electrical energy at source without battery storage. It's a personal project / investigation into passanger car hybrid systems.

I see what you mean about the generators. So, say for example, you had the engine running at 3000 rpm, producing 100 ft/lbs of torque (and concequently 57.1 hp). The generator is absorbing that kinetic power at 100% efficiency (let's dream!) and is pushing out 24 volts. How can you calculate how many amps are being produced by the generator?
 
  • #6
1hp is the imperial unit for power. The Watt is the SI unit. The conversion constant is
746W = 1hp, as stated above

Electrical power is given by Voltage times Current P=VI but Power is Power, in whatever form - being the rate of transfer of energy (also Joules per second)
In Europe they tend to quote engine power in kW aamof.

The efficiency of the system needs to be considered and that will depend upon the specific system but the system you are discussing is far from optimal (it's designed to get Power to the road wheels as well as possible) and I could suggest you won't be getting better than around 60%. That means if you have 100W of lights turned on, you are probably taking around 150W from the engine (about 1/5hp). You can notice the effect of an electrical load - turning on the windscreen heaters in particular - on the tone of the engine at tick-over.

btw torque is not relevant here. it's all about Power. Power is torque times revs but this would be the whole Power and not just what's delivered to the alternator (and, in any case, it's the torque at the crankshaft that's specified).

[Edit: Our posts have crossed and now I see your particular application - if you want an idea of efficiency then look up "motor generator efficiency" or suchlike - I should imagine the specific info relating to automotive systems would be a bit commercial in confidence so you would have to look at more general applications.
How come you're doing a project like this if you haven't been taught about Power?]
 
  • #7
Wait... have I just asked a really stupid question? Reading my post back I've just realized I've given enough information to answer my own question...

If it's producing 57.1 horsepower, that's 42.6 kW (42,600 Watts).

Watts = Amps x Volts, so 42,600 = Amps x 24

So the number of Amps produced is 1,775 Amps?

Can anyone tell me if that's right?

----------------------------------------------------------------------
Hi Sophiecentaur

Thanks. I only mentioned 100% efficiency just to simplify the equation. Is 60% typical? Thanks for the info on torque :)

[Edit: I'll take a look. I have learned about power in automotive systems, I'm just taking it back to basics and, in all honesty, I know very little about hybrid systems. I'm still doing my reading and I just wanted to ask some very broad questions.]
 
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  • #8
RoSmith said:
Wait... have I just asked a really stupid question? Reading my post back I've just realized I've given enough information to answer my own question...

If it's producing 57.1 horsepower, that's 42.6 kW (42,600 Watts).

Watts = Amps x Volts, so 42,600 = Amps x 24

So the number of Amps produced is 1,775 Amps?

Can anyone tell me if that's right?

----------------------------------------------------------------------
Hi Sophiecentaur

Thanks. I only mentioned 100% efficiency just to simplify the equation. Is 60% typical? Thanks for the info on torque :)

60% is only a 'number'. How can you be ignoring the Battery issue if you are looking into Hybrid cars? The overall efficiency, involving batteries is worse, of course.

And, yea - watts is amps times volts and you can do arithmetic :wink:.
 
  • #9
sophiecentaur said:
60% is only a 'number'. How can you be ignoring the Battery issue if you are looking into Hybrid cars? The overall efficiency, involving batteries is worse, of course.

And, yea - watts is amps times volts and you can do arithmetic :wink:.

I won't go into too much detail, but it's looking into using batteries in a supporting role, rather than a vital 'power storage' role... if that makes sense!

Heh I know :wink: 1,775 amps seemed a lot, just wanted to make sure it wasn't 1.775 amps! (And if there wasn't something embarrasingly stupid I was missing!)
 
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  • #10
Perhaps you ought to look into diesel electric railway locomotive design then.

If you are talking of a 40kW load at low voltage then you don't be surprised at needing very high currents. This is why all domestic supplies use 110 or 240V. It means that you can use sensible thicknesses of cables with less loss. It must be a serious consideration in existing hybrid cars. They use around 200V. Look it up.
 
  • #11
I will. I know that some trains have an enormous diesel engine at one end with a generator that powers the electric wheels... I didn't think of that. I'll have a trawl.

I know that EVs try to use as high voltage as possible and then utilise reduction gears in the 'in wheel motors' that power them along. I'm going to have to do some more reading. Thanks for your help, I really do appreciate it! :)
 
  • #12
Re: "reduction gears" - this is not to do with the high voltage supply. You would obviously use motors with the correct voltage rating and operate them at an optimum speed. Reduction gears with a high ratio are inefficient and modern motors can operate at more sensible speeds, I believe. Motor speed control is also a big issue, if you want to do it efficiently. In the old days they used to use different battery connection arrangements plus massive hot variable resistors - not very green!
 
  • #13
Oh... I was told that reduction gears were used in the motors to compensate for the fact that EVs made use of high voltage low amperage systems to avoid weight issues and 'bulking up' of insulation etc. Remembering my sources, I was probably informed wrongly!
 
  • #14
I don't think there is any speed factors for low or high voltage motors. High voltage would certainly be lighter, though. But there are often 'special' factors in new fangled motor technology. Let us know if you find out about it.
 
  • #15
I will... just to clarify... if in the above scenario, the generator was a 120 volt rated unit and it was connected to a 120 volt rated motor... the amount of amperage flowing would be directly proportionate to the speed of the motor, right?
 
  • #16
Omg. Nothing as simple as that. The current will relate to the LOAD and not the speed. A motor will go very fast off-load. The highest current is when the motor is 'stalled'.
When they are spinning, they act as a generator and produce a 'back emf' which is against the sense of the supply, limiting the current. At high speed and high power (uphill or accelerating), the current will be high, of course, but this is often for a minority of time - as with most auto engines..

Look this up. In fact, you need to look a lot of things up. You want to search for "electric motors basics" and suchlike. If you don't then your project could risk being based on some serious misconceptions.
Immerse yourself in motors an you will find some it familiar.
 
  • #17
I'll start doing more reading...

I am really confused (I'm sorry, this is probably basic stuff). I know that if you put load on a motor, it draws more amps from the power source... but what if the kW are already predetermined by the output of the generator? If the generator produces so many kW then the motor will spin relatively regardless of it's load, right? The voltage will be fixed, so the amperage will have to be fixed.
 
  • #18
Like a car engine, a generator will put out only as much power as is demanded of it, regardless of RPM. Consider that you can make a car travel at constant speed at multiple different RPMs.

And consider what happens when you get to a hill: you push on the gas pedal to generate more horsepower and hold your speed. The speed regulator on a gas generator works much the same way.
 
  • #19
When you say the generator will only put out as much power as is demanded of it, that's surely only when the generator as a unit is fitted with sensors to detect when it's current output is insufficient or excessive to the demand? If the IC engine is being given varying power demands by the driver, how does that translate into what the generator gives out in amperage?

So, if the load on the wheels and the motor stays the same, but the driver demands more power from the engine... what will change? I presume the amp output of the generator would increase... so would the speed of the wheels change?
 
  • #20
RoSmith said:
When you say the generator will only put out as much power as is demanded of it, that's surely only when the generator as a unit is fitted with sensors to detect when it's current output is insufficient or excessive to the demand? If the IC engine is being given varying power demands by the driver, how does that translate into what the generator gives out in amperage?

So, if the load on the wheels and the motor stays the same, but the driver demands more power from the engine... what will change? I presume the amp output of the generator would increase... so would the speed of the wheels change?

A common misconception is that a generator or alternator always outputs its maximum amperage (at a given RPM) all of the time. Current only flows if there is a load, and the amount of current varies with the load (for the same voltage).

This says it better than I can -- http://www.generatorguide.net/howgeneratorworks.html :
"If an external circuit is connected to the coil's terminals, this voltage will create current through this circuit, resulting in energy being delivered to the load. Note that the load current in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher current, the larger force must be applied to the armature to keep it from slowing down."​

Many other examples can be found around the 'net. http://www.google.com/search?q=electric+generator+basics

As for formulas, there are many, depending on the specific question.
http://www.google.com/search?q=generator+formulas
 
  • #21
RoS
A static generator will normally be supplying AC and will have a speed regulator to keep the frequency constant. IF there is no load, then the engine will require less fuel to keep it running at its design speed over than under full load. Full load will be specified as the load the set can provide before volts or frequency start to drop.

The above would apply to an engine with 'electric transmission' but, for an automobile alternator, there needs to be a voltage regulator which controls the field current in order to keep the volts right for the DC system - battery, lights etc. Afaik, a hybrid vehicle would need to have its alternator running at variable speed so that you could vary the road speed.

You would need to specify more about your proposal if you wanted more detailed answers.

pantaz's point about generator output power is a very important one. If you think about a Battery, it will deliver current that is defined by its Voltage and the Resistance of the Load. It's the same for a generator. Batteries are usually specified to have a certain maximum current capacity, which tells you the maximum practical available power output.
 
  • #22
Thanks for the links! I really appreciate all your help. I've been doing my reading... Sophiecentaur, you're right I should probably make my questions a little more specific. Below is the kind of situation I'm trying to get my head around. I hope you can see what I've been trying to get at, and I was wondering if you could confirm what I am doing is right...?

If you take the following formulae...
1 HP = 0.746 kW
Watts = Volts x Amps
Horsepower = (Torque x RPM) / 5252

You can deduce that...
Volts x Amps x 7.04 = Torque x RPM


So... Consider a situation where you have an IC engine connected to a 50V rated alternator. If the engine is producing 57.1 horsepower... 100Nm torque at 3000rpm...

Amps produced = (Torque x RPM) / (V x 7.04) = 852 Amps.

So, the generator is producing 50 volts and 852 Amps. This current then flows through a transformer and is stepped up (coil ratio 2:1) to 100 volts and 426 amps.

This power is then transferred to a 100V rated AC motor... How can you calculate the resultant RPM and torque of the AC motor? You have two unknown variables in the equation...

Torque = (Volts x Amps x 7.04) / RPM

Is it just a case that the transformer ratio of 2:1 is reflected in the mechanical output? If so, how do you calculate that?

Sorry I've done baby-steps through my logic... I just want to make sure I'm on the right train of thought.
 
  • #23
Just a reality check. I would say that an 800A alternator plus rectifiers could be quite a challenge. Why do you want such a low voltage when we know that Hybrids use four times that voltage. Why do you think that's the way they do it, bearing in mind that insulation is a real consideration in vehicles?
I can see no point in generating at an inconvenient level of current and then using a transformer to raise the voltage. A high power transformer constitutes a large mass that you could well do without in a vehicle that you are trying to make highly efficient. A 40kVA transformer would cost a bomb, too. Generator and motors (and battery bank) should ideally work at the same voltage.

You need to immerse yourself quite a bit deeper into this if you want to get a half believable project out of it. (Assuming that the people who assess you are worth their salt)
 
  • #24
sophiecentaur said:
Just a reality check. I would say that an 800A alternator plus rectifiers could be quite a challenge. Why do you want such a low voltage when we know that Hybrids use four times that voltage. Why do you think that's the way they do it, bearing in mind that insulation is a real consideration in vehicles?
I can see no point in generating at an inconvenient level of current and then using a transformer to raise the voltage. A high power transformer constitutes a large mass that you could well do without in a vehicle that you are trying to make highly efficient. A 40kVA transformer would cost a bomb, too. Generator and motors (and battery bank) should ideally work at the same voltage.

These aren't real numbers. I've just plugged in hypotheticals to check that the theory behind it is correct.
 
  • #25
I think you need to get familiar enough with the accepted formulae to believe that they work. Your arithmetic looks ok so have confidence- do the sum three times and check you get the right answer, which is what I always do.

I don't know what the accepted practice is for electric traction but I wouldn't normally be working in Torque for the initial calculations. It's only at the final mechanical linkage that torque would come into it. Power is the more 'portable' quantity because it doesn't specify wheel or engine speed.
 

1. What is kinetic to electric power conversion?

Kinetic to electric power conversion is the process of transforming the energy of motion, known as kinetic energy, into electrical energy. This is typically done through the use of a generator, which converts mechanical energy into electrical energy through electromagnetic induction.

2. What are some examples of kinetic to electric power conversion?

Some common examples of kinetic to electric power conversion include wind turbines, hydroelectric dams, and bicycles with generators. In these cases, the movement of wind, water, or the cyclist's legs is converted into electrical energy.

3. How does kinetic to electric power conversion work?

Kinetic to electric power conversion works by using a generator, which contains a magnet and a coil of wire. When the magnet is spun, it creates a changing magnetic field that induces an electric current in the wire. This current can then be harnessed as electrical energy.

4. What are the advantages of kinetic to electric power conversion?

One major advantage of kinetic to electric power conversion is that it relies on renewable sources of energy, such as wind and water, which are constantly replenished by natural processes. It also produces electricity without emitting any harmful pollutants, making it a cleaner and more sustainable option.

5. What are some potential challenges of kinetic to electric power conversion?

One challenge of kinetic to electric power conversion is that it is highly dependent on external factors, such as wind speed and water flow, which can vary and affect the amount of electricity produced. Additionally, the initial cost of building and maintaining these systems can be expensive, although they may ultimately save money in the long run through reduced reliance on traditional energy sources.

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