Most ac generators used in Power Stations tend to be of the "Flux Cutting" types. Their efficiencies range from less than 33% to slightly more than 60%. In their efficiency equations, they tend to stress more on Power Factor correction. They tend to neglect the effect of
"Lorentz Force " - F = BILN , which can be very huge, acting in the OPPOSITE direction to the rotation of the rotor. They have to use steam-turbines to overcome this force. The question is: Why "Lorentz Force" term not included in the Efficiency equations? If "Lorentz Force" is very difficult to solve, why not use "Magnetic Induction" type ac generators in Power Stations.

Best Regards,
UNGKU KAMAL.

 

UNGKU KAMAL,

"Flux cutting" ac generators are usually called synchronous generators, or, alternators. Their efficiency--measured by their ability to convert one form of power (torque multiplied by angular velocity) to another (watts)--is usually better than 90% at power factors close to unity (1.0). The efficiency you seem to be referring is the overall efficiency of the generator and its prime mover (turbine; reciprocating engine; etc.) at converting the energy input to the prime mover, in the form of fuel or steam, into electrical power--which can vary by as much, and be as low, as your numbers indicate.

The advantage of a synchronous machine over an induction machine is that the power factor of the output of the synchronous machine can be controlled by varying the excitation applied to the machine's rotating DC field(s). Induction machines, most of them anyway, do not have this ability, and, in fact, represent a reactive "load" on the other generators with which they are connected. If not operated in parallel with other generators, induction machines used as generators require some source to induce the necessary voltage to produce a magnetic field on the rotor.

It is suggested you research synchronous generators (or, alternators) and induction generators and their characteristics and operation using your preferred Internet search engine for more details. A great place to start is wikipedia.org.

Again, generators are electric machines for converting torque into amperes. Motors are electric machines for converting amperes into torque. Wires are used to connect the sources of torque (prime movers--turbines, reciprocating engines, etc.) to the load being driven by electric motors. Electricity is just the way torque is transmitted from one place to another--using wires.

And, there is virtually no difference between motors and generators from a mechanical perspective. Synchronous electric machines can be either motors or generators. Induction electric machines be either motors or generators. There are fewer induction generators because induction machines aren't well suited to being driven by large prime movers. There aren't many synchronous motors because they require brushes and slip-rings (which require maintenance!) and sources of excitation--which make them expensive to build, operate and maintain for use as small electric motors.

Hope this helps!

 

Hello CSA,

Thanks for the reply. Yes, you're right, the 33% is the overall efficiency of the Power system.

But what I really would like to know is: Where does the “Lorentz Force” term fit into the Efficiency equations for ac generators.

I've searched the Internet, including Research papers presented by University Professors, they tend to neglect “Lorentz Force” term in their Efficiency equations.

If I may say so, in your reply, you also didn't mention anything about “Lorentz Force”.

I would like to know, how many percent of the Input Power (from steam-turbines) is taken up by the “Lorentz Force”, in typical ac generators used in Power Stations.

The other thing, which I would like to clarify, what I actually meant by “Magnetic Induction” ac generators, they are the “New” types, using Neodymium rare-earth magnets. Apart from providing very high Flux density across the air-gap, they can act as “Buffers” , against ac magnetic fields produced by the iron core in the Output coils. Preferably, should use the “Powder” type Neodymium magnets, to reduce Eddy currents.

On the other hand, electro magnets, can easily be induced by ac magnetic fields, produced by iron core at the Output coils, resulting in very high ac voltages being generated in the dc Field coils.

However, the only snag in using Neodymium magnets is that they cannot withstand very high temperatures, such as in Power Station systems. They are also very expensive, and difficult to obtain in large quantities. They are produced mainly by countries like Japan and China.

So, it looks like we are back to square one.

That's why I would like to know, in typical ac generators used in Power Stations, how many percent of the Input power from the steam-turbines goes to “Lorentz Force”.

I suspected it could be as high as 65% of Input power. That's why you need steam-turbines to overcome it. If “Lorentz Force” can be “solved”, the Power Station people will be laughing all the way to the Bank.

Best Regards,
UNGKU KAMAL

 

UNGKU KAMAL,

I, too, searched the Internet after your post trying to recall what my twisted university professor tried to teach us about Lorentz Law.

I'm as confused now as I was then. I'm even more convinced my twisted professor was just using it to confuse us--as he was very prone to do. Many things that he tried to teach me I have since learned were very obscure and tangential issues, and the things he could have explained better weren't because of the time spent on things like Lorentz Law.

I don't understand what it is you think that torque from a prime mover (a steam turbine is one type of prime mover) has to overcome when driving a generator. While generators are not 100% efficient at converting torque to amperes, they are in the 90 percent range, and I would imagine that include any losses for Mr. Lorentz and his legal statement.

I would imagine if whatever losses were attributable to Mr. Lorentz' postulate were significant there would be more people working on resolving them. Perhaps there are ways to resolve the "problem" but they are too expensive--and the losses which do occur are insignificant.

A lot of people would really like it if someone (you?) could explain Lorentz Law and its applicability to modern generators and how modern generators might be designed to limit its affects.

Since you seem so well-steeped in Lorentz Law, why don't you explain it to us, and tell us exactly what losses you believe are attributed to it. I might even change my mind about that twisted professor (not likely, though).

Looking forward to hearing from you. Perhaps others might respond after you explain the phenomenon.

 

> Since you seem so well-steeped in Lorentz Law, why don't you explain it to us, and tell us exactly what losses you
> believe are attributed to it. I might even change my mind about that twisted professor (not likely, though).

While dormant for almost 100-years, the Lorentz-Force Motor (LFM) has received renewed attention beyond the educational science project often known as the “homopolar motor”. In this concept we are speaking of rotational machines, not linear motors. The principle has been exhaustively used in conventional loudspeaker driver design, and also in specialty applications for angular motors with a limited range. A comprehensive background is offered in an article published in 2005 by the engineering team from Bodine, who acquired and commercialized LFMs: http://machinedesign.com/archive/lorenz-force-motors

Similar designs are developed by KET under license from Lynx. None of these design seem to have reached tremendous attention from the industry, publicity, or known commercial break through. A number of reasons have kept traditional motors, AC induction motors, BLCD motors, and lately Switched-Reluctance Machines in the lead, continuously improving in efficiency. Improving, and more expensive, magnetic materials, significant advancements in power electronics, low-cost processors and core speeds well beyond the need to process rather slow motion control applications have enabled a fresh look at Lorentz force principle. As discussed earlier in this thread, rare earth or neodymiums, have improved flux densities at the expense of temperature sensitivity. It’s cost has increased by 711% since 2011, but market forces and WTO intervention may improve the situation, see http://europa.eu/rapid/press-release_MEMO-14-236_en.htm

As far the technical implications are concerned, any motor is subject to the physical principals that govern induction. As most of us will have learned during physics class, applying the “right-hand-rule” yields a visual demonstration of the force on a conductor, whose current produces a magnetic field surrounding it. Maximum force applies to electrons when each surrounded by an electric field, perpendicularly crossing the flux lines of another field, such as generated by a permanent magnetic field. Technical literature has detailed the effect of an electron and it’s electric field crosses magnetic flux lines, breaking and recombining it. Those interested in further detail shall review:

The law of Biot-Savart, a magnetic field generated by currents in wires Ampere’s law, the effect of a current on a loop of flux which it threads Force law, the force on an electron moving through a magnetic field Faraday's law, the voltage induced in a circuit by magnetic flux cutting it.

Since the change-of-flux-field strength is responsible for the accumulative forces applied on each free electron harnessed inside the wire, and vice versa current induction, LFMs are notoriously poor in creating “electron flow” (as generator) or force/torque (as motor), reflected in their low EMF, which also characterizes the machines motor/torque/back-EMF constant. Thus, the LFM is a very poor generator, at least when considering the typical rotation speeds such generators are coupled to, such as wind turbines (60rpm horizontal axis, 300rpm vertical axis, 1200-1800rpm diesel or NG driven). Of course, velocity on the outside of a larger radius improves induction due to tangential speed, as it did to “pole density” for large wind and hydro generators, leading to the typical pan-cake motor. Such designs become unacceptably large, heavy, and costly. Alternatively, when rotational speed can be increased, the induction improvement effect can be captured. Thus, the LFM can be actually of great interest for high-speed machines, e.g. gas turbines, flywheels, or high-speed grinding tools. However, concurrent designs have a significant challenge in accurately controlling the current for motor operation, or timely efficient rectification for generator operation: reversal/rectification in current direction each time the flux polarity changes requires very accurate timing. While the elimination of core iron is welcome and reduces magnetic (hysteretic) losses, eliminates “cogging”, it poses a timing challenge for machines running 10’s, if not 100’s of kRPMs. A second challenge has to be mastered: because a typical inductor loop develops canceling forces in each leg of the loop that is crossing the flux path. Mechanical construction to route the wire accordingly makes for uneconomical assembly processes. Thirdly, a large area of wire will be just burning up power resistively without contributing to the conversion to force, and making efficiencies even worse. Consequently, energy conversion efficiencies of LFMs have been sub-par. Lastly, since the motor constant kT (Nm/A) and backEMF constant (V/s) are physically the same. Lower back-EMF also means lower torque, which only can be compensated with more “Amp-turns” of resistive wire in a higher density flux field. Such increase in flux can saturate iron core material. More copper, similar to the magnetic flux conductivity of air, increases the air gap required between rotor and stator to hold the wire and still allow for motion. And since magnets themselves are a very poor conductor of flux, magnetic circuit design has been a challenge.

Many of these challenges have been addressed, mitigated, or minimized, with a new, patent-pending, architecture that completely eliminates high-speed switching - and commutation. The second and significant benefit of the new design is a multiple-peak efficiency curve, similar to an electronics equivalent of the automotive transmission: making up for lower maximum efficiency and the LFM’s lower torque constant. It should become apparent that there is no magic pill and even applications of LFM technology have limited scope. Wide load and speed operating ranges, challenging for conventional electrical machine, would work exceptionally well with a “reconfigurable LFM”. Start-up proven engineers for high-voltage/high-current power electronics, embedded software architects/programmers experienced in digital power control, business advocate realizing the benefits of the above design, or investors comfortable in this space and excited to participated in a new venture that is lead by a seasoned team, formerly from Sundstrand, Honeywell and Texas Instruments, should contact me for further details: [email protected]

 

Bert Wank,

Great--all of it. But it seems to address Lorentz Force <b>Motors</b>, not the forces attributable to Lorentz' Law in generators, which is what Ugu Kumal was asking about.

Interesting, and an interesting development, though. Worth watching.

Doesn't change my opinion of my twisted university electrical/electronics professor.