Register to reply

Kinetic to Electric Power Conversion

Share this thread:
RoSmith
#19
Mar1-12, 04:17 PM
P: 12
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?
pantaz
#20
Mar2-12, 04:55 PM
P: 588
Quote Quote by RoSmith View Post
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=elect...nerator+basics

As for formulas, there are many, depending on the specific question.
http://www.google.com/search?q=generator+formulas
sophiecentaur
#21
Mar2-12, 05:19 PM
Sci Advisor
PF Gold
sophiecentaur's Avatar
P: 11,919
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.
RoSmith
#22
Mar3-12, 10:56 AM
P: 12
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.
sophiecentaur
#23
Mar3-12, 11:10 AM
Sci Advisor
PF Gold
sophiecentaur's Avatar
P: 11,919
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)
RoSmith
#24
Mar3-12, 11:12 AM
P: 12
Quote Quote by sophiecentaur View Post
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.
sophiecentaur
#25
Mar3-12, 11:18 AM
Sci Advisor
PF Gold
sophiecentaur's Avatar
P: 11,919
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.


Register to reply

Related Discussions
Conversion of Mass to kinetic energy. Introductory Physics Homework 2
Conversion of EPE to Kinetic Energy in Magnetism Classical Physics 0
Pneumatic Power Conversion Mechanical Engineering 3
Transfer and Conversion of pressure to Energy (Kinetic) General Physics 0
Regarding the conversion of kinetic energy to mass Special & General Relativity 14