What is the force that slows a generator down when a load is applied?

[kappa9]

hi,

everyone know that when electrical load is applied to a generator, the generator slows down.

Now my question is what actually slows the generator, what kind of force and how much is that force

this question my sound weired to many but i need a very definite answer.

thanks in advance

 

[Phrak]

It's electrical current. The torque is proportional to the amperage supplied by the generator (for a constant magnetic field generator. Series, field wound generators would be different).

 

[kappa9]

It's electrical current. The torque is proportional to the amperage supplied by the generator (for a constant magnetic field generator. Series, field wound generators would be different).

hi,

thanks for your reply... the scene is torque is supplied by the turbine to the alternator, now please tell me how is amperage related?

 

[Phrak]

Amperage is proportional to torque. Voltage is proportional to angular velocity of the generator shaft.

 

[kappa9]

Amperage is proportional to torque. Voltage is proportional to angular velocity of the generator shaft.

see i want to know as the amperes out of alternator are increased... the shaft slows down... what force is responsible for the slowing of the shaft

i'm looking for the kind of FORCE and not the relationships

thanks

 

[Phrak]

see i want to know as the amperes out of alternator are increased... the shaft slows down... what force is responsible for the slowing of the shaft

i'm looking for the kind of FORCE and not the relationships

thanks

OK. First let's presume that you have some source of power making the shaft turn, such as a hydroelectric dam. You increase the electrical load. That means the load demands more current. But the water is still pushing the shaft around just as hard as before.

The water is pushing the shaft to turn, and the current demand is pushing the shaft the other way to stop it from turning. If the two are balanced, the shaft will keep turning at the same speed. If the forces are not balanced, and the current demand is greater, the shaft will slow down.

The angular inertial of the generator resists a change in angular velocity, but it will still slow down or speed up if the two forces (angular forces, actually) are imbalanced.

 

[kappa9]

OK. First let's presume that you have some source of power making the shaft turn, such as a hydroelectric dam. You increase the electrical load. That means the load demands more current. But the water is still pushing the shaft around just as hard as before.

The water is pushing the shaft to turn, and the current demand is pushing the shaft the other way to stop it from turning. If the two are balanced, the shaft will keep turning at the same speed. If the forces are not balanced, and the current demand is greater, the shaft will slow down.

thanks for the reply... but once again... i want to know what kind of force is causing the stall as the shaft and alternator are not physically attached... so what kind of force stalls the shaft.

 

[russ_watters]

The force is electromagnetism.

 

[kappa9]

The force is electromagnetism.

thanks russ,

if you could tell me how to calculate the value of the force, i'll be really grateful to you.

thanks

 

[russ_watters]

From what information?

 

[kappa9]

Voltage, current, distance from the rotor and number of turns is known

 

[russ_watters]

That's not enough information. What are you trying to do, analyze the performance of an existing generator? If you have operating voltage, amperage, power factor and predicted efficiency for example, you can calculate electrical and mechanical wattage and use the relation of torque and power. Is that what you are after? Mentioning the rotor implies you are trying to design a generator from scratch which is a big engineering challenge.

 

[Phrak]

That's not enough information. What are you trying to do, analyze the performance of an existing generator? If you have operating voltage, amperage, power factor and predicted efficiency for example, you can calculate electrical and mechanical wattage and use the relation of torque and power.

Could you?

 

[russ_watters]

I'm not sure I completed that thought: you could use the relationship between torque and power to find the torque at a given rpm.

 

[kappa9]

I'm not sure I completed that thought: you could use the relationship between torque and power to find the torque at a given rpm.

thanks russ... but i am looking at only the losses and the forces that are causing the slowing down of shaft.

thanks

 

[kappa9]

That's not enough information. What are you trying to do, analyze the performance of an existing generator? If you have operating voltage, amperage, power factor and predicted efficiency for example, you can calculate electrical and mechanical wattage and use the relation of torque and power. Is that what you are after? Mentioning the rotor implies you are trying to design a generator from scratch which is a big engineering challenge.

what parameters are needed to calculate the electromagnetic force causing slowing down of the shaft ?

 

[Phrak]

Russ. I'm told that if I apply the Lorentz force law F = q(B x v) to a generator design without core material, then the theoretical results will be in agreement with the physical implementation. But if core material is added around the winding, the results will be off by a factor of ~100.

 

[kappa9]

Russ. I'm told that if I apply the Lorentz force law F = q(B x v) to a generator design without core material, then the theoretical results will be in agreement with the physical implementation. But if core material is added around the winding, the results will be off by a factor of ~100.

hi phrak,

thanks for the information,
the factor of ~100; is it multiplication factor ?
also isn't lorentz force applicable in motors only?

 

[russ_watters]

Russ. I'm told that if I apply the Lorentz force law F = q(B x v) to a generator design without core material, then the theoretical results will be in agreement with the physical implementation.

I'm not an electrical engineer, so this may just be beyond me, but how exactly do you do that without a computer model of the geometry of the generator?

I still don't know what the point of this thread is, but if it can be done more simply with some specs and operating parameters of the generator, that's the way I'd go...

kappa9, are you trying to predict the performance of an existing generator under a given electrical load?

 

[Phrak]

also isn't lorentz force applicable in motors only?

Motors and generators are reciprocals of each other other. Designing them is an art. Without a ready engineer who specializes in their design, who occasions this forum, you won't get much traction. You might do a title search on amazon.com. Contents range from theoretical to practical to application specific such as home built wing generators.

 

[kappa9]

Motors and generators are reciprocals of each other other. Designing them is an art. Without a ready engineer who specializes in their design, who occasions this forum, you won't get much traction. You might do a title search on amazon.com. Contents range from theoretical to practical to application specific such as home built wing generators.

thanks phrak for information,

actually i was looking for designing a prime mover of a generator at particular set of values.
thats why i was looking at alternator drag force with load.

thanks

 

[kappa9]

Motors and generators are reciprocals of each other other. Designing them is an art. Without a ready engineer who specializes in their design, who occasions this forum, you won't get much traction. You might do a title search on amazon.com. Contents range from theoretical to practical to application specific such as home built wing generators.

also phrak, the lorentz force shouldn't be a problem as for half cycle its opposing force, and for the rest of half cycle its aiding force.

 

[Lsos]

When you turn a generator, you are turning a magnetic field which in turn makes electricity flow through a wire.

Electricity flowing through a wire creates it's own magnetic field. So when the electricity is flowing through a wire, it's magnetic field works AGAINST the generator, slowing it down.

Basically, the electricity that the generator is producing is making a magnetic field which in turn tries to slow the generator down.

The force slowing the shaft down will be the same as the force turning the generator (if it is not accelerating). If you know what the load is, in watts let's say, and the rpm of the generator, then you can easily figure out the torque working against the generator.

Not sure if your intent is to go deeper than that. I'm no electrical engineer so i can't say too much more...