Evaluating Electric Motor Technology

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Electric (Magnetic) Motors (and generators) consume over 60% of all electricity produced and on the average, are operating at less than optimum efficiency or performance. Considering these prodigious facts and today’s acute sensitivity to energy conservation, new electric motor technologies are emerging that advertise the ultimate in efficiency and performance. Fortunately, there is advice, which is based on simple Magnetic Motor Physics and Magnetic Motor Practices, for evaluating new magnetic motor technologies.

Magnetic Motor Physics

All electric motors are categorized as either Singly-Fed Electric Machines or Doubly-Fed Electric Machines. The theoretical level of performance between the two categories of the electric machines is determined by well established electromagnetic relationships between Flux Density and Current Density. The basic set of simplified relations are:

• Air-gap Flux Density Relation [FluxDensity = uo x MMF/AirGap] is the product of the Permeability Constant of air (uo), the Magneto-Motive-Force (or MMF), and the inverse of the air-gap depth (length). MMF is the product of the current in the winding and the number of winding-turns.
• Lorentz Relation [Force = MMF x Length (Cross-Product) β] defines Force (or Torque) as the Cross-Product between the Flux Density (β) in the air-gap, the length of the winding, and the MMF. Basically, force (or torque) is proportional to the product of the current density vector, the air-gap flux density vector, and the Sine of the angle between the vectors (with ninety degree angle being optimal).
• Faraday’s Law [Voltage = N x d/dt(MagneticFlux] defines voltage as the product between the number of winding-turns, N, and the time derivative (speed) of the air-gap magnetic flux. Magnetic Flux is the product of the air-gap area and the air-gap Flux Density, β.

In accordance with the basic simplified relations and with a given air-gap Flux Density:

• Force (or Torque) is Current.
• Speed (or Movement) is Voltage (or Frequency).
• Electrical Loss (or Heat) is Current Squared.
• Power is the product of Speed (or Voltage) and Force (or Current).
• Doubly-Fed electric motors have twice the constant torque speed range as Singly-Fed electric motors for a given Voltage (or Frequency) of operation.
• Doubly-Fed electric motors electronically condition half (or less) of the power as Singly-Fed electric motors.

There is a motor invention that received substantial development funding by advertising the motor invention operated according to Lorentz Relation and is different from all other motors, which operate according to Faraday’s Law. But without speed (or Faraday’s Law) and force (or Lorentz Relation) occurring simultaneously, there is no useful work or electromechanical power. If the so-called motor invention does what was advertised, it may have been a very good Brake but certainly not an electromechanical converter (or electric motor).

Advice: All new electric (magnetic) motors belong to either the Singly-Fed or Doubly-Fed category and obey the same electro-magnetic relationships for that category. Don’t let someone tell you differently!

Advice: Motor experts inadvertently make mistakes, too! Get the results of at least two competing evaluations before forming a conclusion. Make sure the evaluator(s) do not have any conflict of interest with the subject or with their own pet project. Throughout technology history, ignoring this simple advice has kept meaningful technology from a timely market introduction.

Magnetic Motor Practices

Through the years, electric motors have constantly improved in power density and performance because of advances in magnetic material, construction, control (i.e., electronic), and manufacturing techniques mainly to reduce heat while improving flux and current densities. The motor physics has remained unchanged.

• In practice, all magnetic motors have a core of magnetic steel to reduce the length of the air-gap magnetic flux path around the active winding set. But magnetic steel has a flux saturation limit and any Flux density produced by the winding set, which is above this limit, will revert to following the longest flux path. As a result, the MMF would have to increase by an impractical amount (hundreds of times higher) to make an effective difference in core Flux Density. Any motor technology can increase current for more torque and power but what is the practical justification.

Advice: Prove the electromagnetic characteristics of the new motor do not violate the practical limits of the day for motor construction, manufacturing, and electro-magnetic materials. Then verify the characteristics of the new motor that do violate the standards of the day, are within acceptable risk.

• Without considering other dependencies, all motors are designed to achieve the highest possible flux density in the air-gap, which is within the flux saturation limit of the magnetic core.

There is a motor invention that increases flux density by up to four units by redirecting the flux from a number of permanent magnets by means of an electric winding (electromagnet). After considering the saturation limits of magnetic steel, the concept would not be useful until a quantum leap in magnetic core material and flux density of the Permanent Magnets were invented. Most importantly, Permanent Magnets are passive devices and do not create energy (or power). Redirecting the flux of a permanent magnet requires outside work by an electric winding set, which actively participates in the energy conversion process, and as a result, supplies the full power of the machine as done for any electric motor. So what makes this machine standout? Where's the beef?

Advice: Always ask, "Where's the beef." Make sure the basic rules are not violated, such as Conservation of Energy. "You can't get something from nothing!"

• Resonant switching (or soft switching) is a well proven and beneficial electronic switching technology for power electronic circuits but is difficult to implement with certain circuit topologies. In contrast, resonanting motor circuits show high currents or voltages that must be controlled to avoid circuit destruction or high electrical loss. Also, any motor will exhibits high torque with high currents if the proper torque angle is maintained and excessive heat is removed. So what is the value of adding bulky (low frequency), extraneous capacitors to a motor circuit to promote resonance? Where's the beef?

Advice: Be careful of attractive (or mystical) terms, such as resonance, unless these terms are well understood both in purpose and implementation.

• Permanent Magnets are magnetized (or demagnetized) by an electromagnet (with MMF). It is only reasonable to assume Permanent Magnets exhibit a flux density that is less than what is achievable with an electromagnet (i.e., a motor winding set), at least for the foreseeable future. Accordingly, fully electromagnet electric machines (no permanent magnets) can achieve higher torque and power than PM motors, if the vector angle between current and flux is precisely controlled and heat is not considered a motor design factor. Keep in mind, Permanent Magnets do not produce work and as a result, permanent magnet electric motors require an active AC winding set (like all electric machines) to produce work. By the way, under certain conditions the active AC winding set can destructively demagnetize the Permanent Magnets by producing a higher flux, as well.

Advice: Do a little research on the technology being sold. Understand the technical Pros and Cons of competing technologies.

• Superconductor motors can achieve extremely high air-gap flux density because the ultra high MMF in the field winding is not limited by resistance (or heat). In practice, Superconductor motors are only operational as motor systems that need exotic materials, refrigeration, construction, and electronic control techniques with formidable efficiency and power density limits of their own. Following a similar analysis, brushless Permanent Magnet motors are useless without a complementary electronic controller, which significantly affects efficiency, size, and cost as a motor system. By the way, superconductor motors use a conventionl AC winding set (with comparable resistance to any other motor), and are not directly efficient because of "zero" resistance (in the superconductor field winding).

Advice: When evaluating cost, size, and efficiency of a motor, include the effects of all components that would be required for practical operation. You'll be surprised how these extraneous components affect efficiency, size, and cost in the real situation.

• By the way, electronic control will always improve energy efficiency by tuning the motor to the load dynamics. The most efficient motors are electronically controlled.

Advice: Following the research and development trend of electric motor experts, invest in problem solutions for Doubly-Fed Electric Machines, such as the Brushless Wound-Rotor Doubly-Fed Electric Machine.