Explore the essential principles of electric machines and the innovative coil switching technology through this insightful blog. Discover how coil switching enhances electric vehicle operation and the way Exro Technologies’ Coil Driver™ addresses e-mobility challenges, offering optimal performance solutions. With detailed examples and analyses, grasp the critical balance between torque, speed, and efficiency in electric machine design.
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The rapidly advancing landscape of electric vehicle technology continually pushes the boundaries of innovation to enhance efficiency and performance. One concept that has been around since the early days of electric machines is coil switching technology. In this comprehensive blog, we will explore the fundamentals of electric machines, including torque, voltage, their relationship, and the physical characteristics of coils. We will then delve into the intricacies of coil switching technology and its benefits in variable speed applications, particularly in electric vehicle operation.
Overview and Background
Coil switching technology, or the reconfiguration of windings during the operation of an electric machine, has been recognized for its potential advantages since the inception of electric machines. A well-known example is the star-delta starter, in which the machine starts in a star configuration and transitions to delta once in operation. Regardless of the technique employed - be it star-delta, series-parallel, or a combination thereof - the impact of altering the number of effective turns, amp-turns, and volts per turn is identical.
Exro Technologies did not pioneer the concept of coil switching technology, but its Coil Driver™ architecture has made it a practical and cost-effective solution. By integrating both inverter function and coil switching within a single power electronics system, Exro has dramatically reduced the total silicon needed.
This blog focuses on synchronous or asynchronous machines driven by a motor controller, also known as a traction inverter. E-mobility applications, which require a wide range of speed and torque, are particularly well-suited for coil switching technology due to the inherent design challenges and compromises associated with these applications. However, fixed-speed, line-operated machines interact differently with their supply and are beyond the scope of this article.
To fully comprehend the advantages of coil switching technology, we must first establish a foundational understanding of electric machine operation and system-level constraints. First, we will cover the basics of electric machines, discussing the relationship between inverter current, stator coil turns, and torque production. We will also examine two popular types of electric machines: Surface Permanent Magnet (SPM) and Interior Permanent Magnet (IPM) machines, highlighting their unique characteristics and the design trade-offs involved in choosing between them.
Next, we will explore the concept of coil switching technology and its impact on electric machine performance. We will investigate how varying the number of turns in the stator winding can affect torque and power production, as well as the implications of magnetic core saturation on machine efficiency.
Throughout this article, we will use detailed examples, graphs, and explanations to illustrate the complex relationships between various design factors in electric machines. By the end, you will have gained a technical understanding of the challenges and considerations involved in designing electric machines that achieve the perfect balance of torque, speed, and efficiency, and the role that coil switching technology plays in optimizing their performance.
Fundamentals of Electric Machines
In electrical systems, voltage and current are the two fundamental quantities that collectively accomplish work. These quantities have different impacts on electric drive (e-drive) systems: current is responsible for producing torque in electric motors, while supply voltage sets constraints on speed.
Voltage and current stem from a property called "charge," represented by "q" in the realm of physics. A charge is an intrinsic characteristic of certain subatomic particles, such as electrons and protons, which comprise our world. Electrons are elementary particles carrying a negative charge, whereas protons possess a positive charge. Since protons are typically confined within atomic nuclei, discussions about "electricity" usually involve the interaction of electrons with other electrons.
Voltage, or potential difference, arises due to an imbalance of charge (electrons) between two locations. This charge imbalance results in a potential difference, which in turn creates an electric field (E). The electric field exerts a force on the electrons, similar to "pressure," that strives to equalize the potential difference by redistributing the charge.
Electrons can move through conductors, such as metals, to achieve this charge rebalancing. The movement of electrons is called "current." In essence, voltage is the force potential that moves electrons, and current is the flow of electrons due to this force. The relationship between charge difference (voltage), charge flow rate (current), and the opposition to the flow of electric charge (resistance) is defined by Ohm's law:
V = I * R
Ohm's law demonstrates that voltage (V) is directly proportional to the product of current (I) and resistance (R). This fundamental law helps us understand the behavior of electric circuits and the factors influencing the flow of current, which is essential for the proper functioning of electric machines.
Torque and Lorentz Force
Building upon the understanding of voltage, current, and resistance, we can now explore the concept of torque in electric machines. Torque is the rotational force generated by the interaction of current-carrying conductors within a magnetic field. To fully grasp the concept of torque and its relationship with current, voltage, and magnetic field, we need to examine the Lorentz force and how it applies to electric machines.
The Lorentz force describes the force “F” experienced by a charge “q” moving at a velocity “v” through an electric field “E” and a magnetic field “B”, as shown in the following equation:
F = qE + qv x B
Most electric machines rely primarily on the interaction of moving charge with a magnetic field, so the effects of the electric field can be ignored. This simplification leads to the equation:
F = qv x B
As velocity is defined as distance over time (v = L/t), we can replace “v” with “L/t”, where “L” is the length of the current path (i.e., wire) where the magnetic field “B” is present:
F = q(L/t) x B
Since current is a measure of "charge flow rate" (I = q/t), we can make another substitution, resulting in the fundamental equation for motor designers:
F = BIL
This equation describes the force “F” experienced by a wire of length “L” in a magnetic field “B” while carrying a current “I”. In electric machines, the wire is perpendicular (90 degrees) to the magnetic field, which allows us to eliminate the cross product (x) from the equation.
When this wire is attached to a lever mounted on a shaft, it generates a rotational force, or torque.
For instance, if the force has a magnitude of 1 Newton and the radius is 1 meter, the result is 1 Nm of torque. In most machines, there are multiple wires per slot and multiple slots, so the total rotational force produced by the machine depends on the sum of the forces exerted by these wires.
In electric machines, the total force (torque) is produced not by the current in a single wire but by the cumulative current from all the slots. This leads us to the concept of ampere-turns. For instance, if a slot contains 10 wires, and each wire carries 100A in the same direction, then the combined current for that slot is the sum of the currents from all 10 wires, totaling 1000 Amperes. If these wires form a 10-turn coil, with each wire in the slot connected in series through an external circuit, then a 10-turn coil carrying 100A will generate the same force as a lone wire carrying 1000A. This illustrates the principle of ampere-turns.
To calculate the ampere-turns in a slot, we simply multiply the number of turns “N” a coil has by the current flowing in the wire of that coil. As a result, the same force can be created by using one turn carrying 1000A, 10 turns carrying 100A, or 100 turns carrying 10A. In a given electric machine, each of these three configurations will produce identical torque.
Having discussed torque and its relationship with current, magnetic fields, and ampere-turns, we now turn our attention to voltage and its role in electric machines. Voltage is crucial to understanding the interaction between the electric and magnetic fields in a motor, especially when it comes to the concept of back EMF.
Voltage and Back EMF
In the previous section, we focused on the static force generated by current-carrying conductors in a magnetic field. However, static force alone is not sufficient for a machine to do work (i.e., force x distance). We need the shaft to turn, allowing the wire or the magnets to move relative to one another. In brushed DC machines, the wire moves while the magnets remain stationary. However, most modern machines use stationary wires and moving magnets to eliminate issues with brushes wearing out. The key aspect is the relative movement between the magnetic field and the wire.
According to Newton's third law, every force will have an equal and opposite reaction. Therefore, both the wire and the magnets experience the force described above, albeit in opposite directions. However, when the wire shifts in relation to the magnetic field, another fundamental physics principle becomes relevant: Faraday's law of electromagnetic induction.
Faraday's law states that a wire in a changing magnetic field “B” produces an electromotive force (EMF), i.e., voltage, which depends on the rate at which the field changes. As an electric machine rotates and the magnetic field is exposed to changes, the wire generates a voltage to counteract this changing magnetic field.
Similar to the concept of ampere-turns, the total voltage produced by a coil is proportional to the number of turns it has. For instance, if one turn produces one volt at a fixed speed, two turns will produce two volts, three turns will produce three volts, and so on. The voltage induced is intrinsically tied to the rate at which the magnetic field alters. Thus, if the rotor's speed is doubled, the magnetic field's rate of change doubles in tandem, and correspondingly, the voltage produced in the wire doubles.
This induced voltage is known as the back EMF and is generally specified for permanent magnet machines in terms of volts per RPM. All electric machines produce back EMF during rotation and torque production, even if they are not permanently excited, such as induction machines. However, permanently excited machines like permanent magnet motors produce voltage whenever they rotate, regardless of their operational state. This characteristic presents several safety challenges in electric machine design and operation.
The Interplay of Torque and Voltage in Electric Machines
Having established the relationships between torque, voltage, and their respective dependencies on the number of turns and phase current or rotational speed, let's explore how these factors come together in the operation of electric machines, particularly permanent magnet or externally excited synchronous machines. While induction and reluctance machines also fundamentally produce torque with current, and back EMF with speed, their mechanisms are more complex and beyond the scope of this article.
We have identified two key relationships for electric machines:
Torque is produced proportionally to the number of turns (N) and the phase current (I).
Voltage is produced proportionally to the number of turns (N) and the rotational speed.
The characteristics of an electric machine producing larger voltages as the rotation speed increases have critical implications. At some point, the machine generates more voltage than the drive system or power supply can deliver. Once this speed is reached, the motor-drive combination cannot produce torque above this speed. To push current into the machine, the machine's induced voltage must be lower than the maximum voltage available on the drive. Otherwise, the current flows backward, and the machine acts as a generator.
This critical speed is referred to as the "knee speed" or "base speed." If an operation above this point is required, a control strategy called "field weakening" is employed. In the next section, we will delve deeper into the concept of field weakening and its role in electric machine operation.
The Concept of Field Weakening
In an electric machine, the torque production is primarily a function of the ampere-turns generated by the stator winding and the strength of the magnetic field produced by the rotor's magnets. As the machine speed increases, the back electromotive force (BEMF) also increases, and at the knee speed, it reaches the maximum voltage available from the inverter. Beyond this point, the voltage constraint limits further torque production at constant power.
Field weakening, also known as flux weakening, aims to overcome this limitation by actively reducing the magnetic field strength in the air gap between the stator and the rotor. This reduction in the air-gap magnetic field results in a decrease in the back EMF generated at higher speeds, allowing the machine to operate beyond the knee speed within the voltage constraints of the inverter or power supply. Consequently, this control strategy enables the machine to maintain constant power operation over an extended speed range.
Implementing Field Weakening
Field weakening is implemented by adjusting the current vector supplied to the stator windings. In machines with a separately controlled rotor winding, such as wound rotor induction machines or synchronous machines with wound field rotors, the field weakening can be achieved by directly controlling the rotor current. However, in permanent magnet machines, the rotor field is fixed, and field weakening is realized by manipulating the stator current.
Vector control or field-oriented control (FOC) techniques are commonly used to implement field weakening in permanent magnet machines. In FOC, the stator current is decomposed into two components: the torque-producing current (Iq) and the magnetizing or field-weakening current (Id). By adjusting the “Id” component, the magnetic field in the air gap can be weakened or strengthened, effectively controlling the machine's speed range.
In the following section, we will discuss the physical characteristics of coils and how they influence the performance of electric machines, further building on the foundation established so far.
Physical Characteristics of Coils: Understanding Resistance and Inductance
In electric machines, the magnetic interaction with the rotor is not the only crucial aspect to consider. Two key electrical characteristics of a coil of wire, resistance (R) and inductance (L), significantly impact the machine's performance and efficiency.
These resistances and inductances in the coils are essentially “parasitic” components that sit in series with the circuit between the power source “V”, the inverter, and the EMF, ie the voltages generated by the wire interacting with the rotating magnetic field.
Electrical resistance is defined as the opposition to the flow of electric current in a conductor. In this context, resistance is purely a loss element with no useful function and needs to be minimized. Current flowing through a resistor generates both voltage and heat. The voltage across the resistance reduces the voltage available to the magnetic circuit (EMF) as the current increases, causing the speed-torque curve to round off at the knee speed. Moreover, the heat generated by current flowing in resistance consumes electrical energy, reducing the total power available for conversion into mechanical motion and negatively impacting the efficiency of the drive system.
Inductance, defined as the property of an electrical conductor that opposes changes in current due to the creation of a magnetic field, is another characteristic of electric machines. Inductance is a passive element that also reduces the available voltage to the load but introduces an additional complication compared to resistance. The impedance, or "AC resistance," of an inductor is frequency-dependent. As the machine speed and frequency increase, the impedance also increases. Consequently, inductance progressively reduces available voltage as speed increases, which is undesirable.
Furthermore, the voltage induced across an inductor is 90 degrees out of phase with the driving current. This out-of-phase relationship means that inductance also reduces the power factor. A lower power factor implies that some of the stator current is not performing useful work on the shaft but is instead merely cycling the magnetic field within the inductor.
Inductance arises because a current-carrying wire also produces a magnetic field, which interacts with the rotor, creating interesting effects. The amount of magnetic field generated by a given current is determined by the number of turns in the coil and the reluctance of the magnetic circuit. The resulting value is called inductance. A higher inductance value indicates that more magnetic field is produced from a given current. The coil's physical construction and the magnetic circuit created by the iron, magnets, and air gaps in the machine determine the magnetic field generated by the stator's current.
In summary, understanding and optimizing resistance and inductance in electric machines are crucial for ensuring efficient and reliable operation. Both characteristics significantly influence the performance, design, and control strategies of various electric machine applications. Providing a technical understanding of these concepts enables the development of more effective and energy-efficient electric machines.
Magnetic Materials in Permanent Magnet Machines: Hard and Soft Magnetics
In permanent magnet machines, two distinct types of magnetic materials play vital roles in their operation and performance: hard magnetic materials and soft magnetic materials. Understanding these materials' characteristics is essential for designing efficient and reliable electric machines.
Hard Magnetic Materials
Hard magnetic materials, such as the magnets used in electric machines, possess the unique property of maintaining their magnetization unless specific limits are reached. When these limits are surpassed, the magnets may lose their magnetization, which is undesirable in electric machine applications. This means that while permanent magnets produce a magnetic field, they do not aid the creation of magnetic fields due to external influence like the current in the windings. The relative permeability of hard magnetic materials is approximately 1, which means to external fields they behave like empty spaces. Air, magnetically speaking, is a close approximation of empty space.
Soft Magnetic Materials
In contrast, soft magnetic materials, like the stator and rotor iron, are more responsive to external magnetic fields. These materials temporarily magnetize in the same direction as the external field, effectively amplifying the produced field. However, this temporary magnetization only exists while the external field is present. Soft magnetic materials exhibit high relative permeability, often several thousand times larger than that of air.
The interplay between hard and soft magnetic materials in permanent magnet machines directly affects the inductance of the magnetic circuit. While the stator and rotor iron (soft magnetic materials) in the flux path increase inductance, any permanent magnets or air (hard magnetic materials) in the flux path reduce it.
The characteristics of these magnetic materials play a significant role in determining the differences between surface-mounted permanent magnet (SPM) and interior permanent magnet (IPM) machines, which will be discussed further in the next section.
Comparing Surface Mounted and Interior Permanent Magnet Machines: Field Weakening and Performance Trade-offs
In permanent magnet machines, surface mounted permanent magnet (SPM) and interior permanent magnet (IPM) machines differ in their stator inductance generation, which determines the magnetic field created by the current in the stator windings. This difference influences the machines' field weakening capabilities and results in trade-offs between high-speed operation and torque density.
Surface Mounted Permanent Magnet (SPM) Machines
In SPM machines, the magnets are directly in the stator flux path, resulting in a large effective air gap and minimal influence of the stator current on the magnetic field produced by the rotor magnets. This limited influence on the rotor field prevents torque production above the knee speed as the induced voltage from the rotating magnet flux impedes current flow. Consequently, SPM machines exhibit relatively poor field-weakening performance.
Interior Permanent Magnet (IPM) Machines
IPM machines, on the other hand, have a different geometry, allowing the stator field to influence the rotor field by burying the magnets inside the rotor iron. This improved flux path from the stator to the rotor enables the introduction of stator current at an angle, allowing the total field to be reduced and subsequently reducing the voltage produced at a given RPM. The inverter can then inject current to facilitate the machine's operation at higher RPMs than the knee speed.
Below are two Finite Element Analysis (FEA) results of an IPM machine, one at low RPM with no field weakening (high total flux) and one at high RPM, deep in field weakening, with much lower total flux in the machine.
Field Weakening in IPM Machines
As mentioned before, field weakening employs a portion of the phase current to generate a magnetic field in the stator that partially counteracts the field produced by the rotor's magnets. The outcome is a reduced total magnetic field within the machine, leading to decreased voltage and torque production, as both are proportional to the total magnetic field “B”. However, this allows the machine to operate well above the knee speed. In interior permanent magnet (IPM) machines with field weakening, a classic motor speed-torque curve is achieved:
As illustrated in the above figure, when operating above the knee speed and in field weakening, the power curve becomes more or less constant, resulting in a steady power level. This region of operation is referred to as the "constant power speed region" (CPSR).
Field weakening is paramount in many electric drive applications, especially those requiring a broad operational speed range without sacrificing power. In the absence of field weakening, the speed of an electric machine would be limited by its inherent characteristics and the voltage constraint of the driving electronics. This limitation would render many applications, such as electric vehicles and certain industrial drives, inefficient or even infeasible. Field weakening allows these applications to harness high-speed operation while maintaining a consistent power output. By extending the usable speed range of the machine, it ensures that the application can meet diverse operational demands and adapt to varying conditions. In essence, field weakening provides the flexibility and adaptability necessary for modern electric drives, ensuring that they deliver optimal performance across their entire speed range.
There is one more operating region in an electric machine, which occurs when the voltage loss due to inductance begins to limit the machine's performance. It is essential to note that the AC resistance of an inductor increases with frequency and, consequently, speed. This operating region is called the "voltage limited" region. In the example provided, this region is not depicted because the speed does not reach a high enough level. However, if the machine's operation were simulated at 20,000 RPM, the voltage limited region would become visible.
Operating Regions of Electric Machines
Electric machines generally have three main operating regions:
Constant torque: In this region, the back electromotive force (BEMF) is low, and the machine torque is limited by the inverter current or available ampere-turns.
Constant power (field weakening): Here, the inverter advances the angle of the current to control BEMF, maintaining the maximum stator current even as torque decreases.
Voltage limited: In this region, the rising impedance of the stator inductance combined with the machine BEMF prevents the inverter from delivering full current, resulting in a decrease in total current and power.
SPM vs IPM: Design Trade-offs and Considerations
You might be wondering why anyone would choose to build SPM machines if IPM machines demonstrate superior field weakening capabilities. The reason lies in the trade-offs involved in each design. While IPM machines offer better high-speed operation, burying the magnets inside the rotor significantly reduces the total flux available for producing magnetic torque. This reduction, often by more than half, limits the torque density achievable with a given volume of magnets.
The design choice between SPM and IPM machines, therefore, comes down to balancing high-speed operation with torque density. Allowing the stator field to easily reach the rotor magnets in IPM machines creates a flux leakage path for the magnets themselves, reducing the overall torque density.
The only reason IPM machines remain viable despite this massive reduction in magnet flux is the introduction of "saliency" in the rotor. Saliency refers to the non-uniformity in the rotor iron shape resulting from the embedded magnets. This unique shape enables IPM machines to generate torque not only from the magnetic forces of the magnets and current in the wire but also from the reluctance force. In this way, the rotor is pulled by the rotating stator field, similar to how a fridge magnet is attracted to a steel door.
In IPM machines, both magnetic and reluctance forces contribute to torque production. If an IPM machine were built without magnets but with slots cut out for them, it would still operate as a torque-producing machine. However, it would then be considered a Synchronous Reluctance Motor, which has approximately half the torque density of an IPM machine.
Essentially, the choice between SPM and IPM machines depends on the specific application requirements and involves a trade-off between high-speed operation and torque density.
Balancing Ampere-Turns, Voltage, and Speed in Electric Machines
In summary, the primary takeaway from this discussion is the crucial role of ampere-turns in producing torque in electric machines. Ampere-turns, the product of the available inverter current and the number of turns in the stator coils, can be achieved through a combination of a large number of turns with low inverter current or fewer turns with a higher inverter current.
Voltage, or back EMF, produced by an electric machine is also dependent on the number of turns. More turns result in a larger voltage at a given speed, while fewer turns produce less voltage, allowing for higher-speed operation with a given DC supply voltage.
When designing an electric machine, the engineer must carefully consider the available inverter current and voltage. Striking the right balance between these factors is essential to achieve a compromise between producing high torque at low speeds and maintaining torque production at high speeds. This balance ultimately determines the performance and efficiency of the electric machine, making it a critical aspect of the design process.
Introducing Coil Switching Technology
Up to this point, we discussed the fundamental principles of electric machines, focusing on the role of ampere-turns, voltage, and speed in determining torque and power. Now, we can delve deeper into the intricacies of electric machine design, specifically examining coil switching technology and the constraints imposed by inverters.
When designing an electric machine, it is essential to consider the limitations of the inverter, which typically involve the maximum phase current it can deliver and the maximum available voltage on the DC link. As we discussed, the balance between these factors, along with the number of turns in the stator winding, plays a crucial role in determining the performance and efficiency of the electric machine.
In this section, we will explore how electric machines can transform the electrical power produced by an inverter into mechanical power at various combinations of speed and torque. By understanding the relationship between these elements and the constraints of the inverter, engineers can optimize their electric machine designs for specific applications.
Coil Switching and Inverter Constraints
When examining an inverter or motor controller, there are two primary constraints: the maximum phase current it can deliver and the maximum available voltage on the DC link. Electrical power is the product of voltage and current. As an inverter has limitations on both current and voltage, this determines the maximum power it can supply.
An electric machine essentially functions as an electro-mechanical transformer. To elaborate, just as a transformer can convert one voltage and current into various combinations of output voltages and currents based on the turns ratio between the primary and secondary, an electric machine can transform electrical power produced by an inverter into mechanical power in numerous combinations of speed and torque. This transformation is achieved by simply altering the number of turns while adhering to the conservation of power principle. In the context of an electric machine, the stator winding corresponds to the transformer's primary winding, and the rotor's rotating field is analogous to the secondary.
Consider a machine that generates 200 Nm of torque with a knee speed of 2000 RPM, resulting in approximately 42 kW of mechanical power. If we want this machine to generate fewer volts and spin faster, we need to decrease the number of turns. Suppose we want the machine to spin twice as fast, requiring half the number of turns. However, as torque production is proportional to turns, reducing the turns by half while maintaining the same current (since we haven't changed the inverter, only the machine) means the torque is also halved. As mechanical power is proportional to torque multiplied by speed, doubling the speed while halving the torque yields the same power output – in this case, 100 Nm and 4000 RPM still equal 42 kW. Thus, no laws of physics are violated.
The first machine will produce 42 kW at 2000 RPM, while the second will generate the same power at 4000 RPM, given the same input voltage and current. If an electric vehicle has fixed gearing such that 2000 RPM equates to 50 km/h, the first machine will accelerate the vehicle swiftly to 50 km/h but will struggle to go much faster. In contrast, the second machine will accelerate at half the rate but can reach 100 km/h or higher.
The Impact of Varying Turn Counts on Electric Machines
In our discussion so far, we've explored how the number of turns in an electric machine affects ampere-turns and the produced voltage. Now, let's visualize what this looks like by examining the behavior of an electric machine with different numbers of turns, while keeping the phase current and DC link voltage constant with the same inverter.
Turn Counts and Torque Production
Consider an IPM machine with 3, 4, 6, 9, 12, and 18 turns in its stator winding. These numbers may seem odd, but they are chosen to maintain the same total slot fill with the given wire, keeping the total copper area in the slot constant. This is crucial because it ensures that the copper loss for a given torque remains the same, which is important when we look at efficiency.
As the graph illustrates, more turns with the same current produce higher ampere-turns, resulting in increased torque production. However, as the turn count goes up, the produced back EMF (BEMF) also increases, causing the knee point to shift downward in speed.
This graph reveals another important effect: the saturation of the magnetic core. Although more ampere-turns yield greater torque, they also drive the magnetic material into higher flux densities. For example, increasing the turn count from 3t to 4t should theoretically result in a 1.33x (4 divided by 3) increase in torque, which aligns closely with the simulated 1.35x increase. However, when comparing 3t to 18t, the theory predicts a 6x increase in torque, but the simulation shows only a 4x increase due to saturation.
The 18-turn winding combined with 340Arms phase current generates approximately 6,100 A-t per slot. This large number was chosen intentionally to demonstrate the effects of saturation. While magnetically, machines can produce more torque even in saturation, each machine design has a practical upper continuous torque limit based on the cooling system's ability to remove heat from the windings. The cooling system's capability and the windings' thermal mass also determine the duration of "overload" torque application, with "overload" referring to any torque higher than the machine's continuous thermal rating.
Turn Counts and Power Production
This graph demonstrates that while the 18-turn winding produces more power early on in the RPM range due to higher torque, it sacrifices peak and high-speed power. This is because the increased resistance and inductance, which follow a turns-squared relationship, generate four times the voltage loss, limiting the total electrical power available to turn the shaft. Consequently, the knee point is lower than the ratio of turns would suggest: the 18-turn example has a knee point of 1,000 RPM, while the 9-turn example has a knee point of 2,500 RPM—half the turns but 2.5 times the knee speed.
Impact of Coil Switching on Efficiency
In this section, we will discuss the impact of coil switching on motor efficiency by comparing the efficiency maps of the same electric machine connected to a traditional 3-phase inverter and the Exro Coil Driver™, built on patented coil switching technology. This comparison will demonstrate how coil switching technology can enhance motor efficiency by dynamically adjusting turns to optimize performance in various operating conditions.
The efficiency maps presented here depict the performance of the same electric machine under two different scenarios. The middle graph illustrates the original 3-phase machine connected to its inverter, while the graphs on the right and left show the machine's performance when connected to the Coil Driver™. These maps essentially compare the same machine with different turns.
In a standard system that cannot dynamically and under load change turns, a trade-off must be made, selecting a middle ground for the closest match to performance. However, with coil switching technology, the motor can be optimized for high starting torque and then essentially "switch gears" to achieve highly efficient high-speed operation with improved power.
By allowing the electric machine to adapt to different operating conditions, coil switching technology enables it to maintain peak efficiency over a broader range of speeds and torques. As a result, electric vehicles that require a wide range of speed and torque capabilities can benefit from improved performance and reduced energy consumption.
In Conclusion: The Complex World of Electric Machines and Design Trade-offs
To wrap up our exploration of electric machines, it's essential to take a moment to summarize the key points discussed in this comprehensive blog article.
First, we delved into the fundamental concepts of electric machines, specifically focusing on the relationship between inverter current, stator coil turns, and torque production. We learned that high ampere-turns can be achieved either with a large number of turns and low inverter current or fewer turns and high inverter current. We also examined how the number of turns affects the back EMF produced by a machine, with a larger number of turns generating a higher voltage at a given speed. Machine designers must balance these factors when choosing the number of turns in order to produce high torque at low speeds or maintain any torque at high speeds.
We then explored the distinctions between Surface Permanent Magnet (SPM) and Interior Permanent Magnet (IPM) machines. While IPM machines excel in field weakening and high-speed operation, SPM machines offer superior torque density. This trade-off arises from the fact that the ease of reaching rotor magnets in SPM machines also creates a flux leakage path for the magnets themselves. IPM machines, however, introduce saliency in the rotor, allowing them to produce torque from both magnetic and reluctance forces.
Next, we examined the concept of coil switching in relation to inverters and motor controllers. Electric machines, acting as electro-mechanical transformers, can convert the electrical power produced by an inverter into various combinations of mechanical speed and torque. This is achieved by changing the number of turns in the stator winding, with the stator analogous to the primary winding of a transformer and the rotor's rotating field akin to the secondary.
We also explored the impact of varying turn counts on electric machines while keeping the phase current and DC link voltage constant. By looking at an IPM machine with varying stator winding turn counts, we observed how more turns with the same current produce higher ampere-turns and torque, but also lead to magnetic core saturation as flux densities increase. Additionally, we discovered that higher turn counts result in higher resistance and inductance, generating more voltage loss and affecting power production.
In summary, designing electric machines involves a series of complex trade-offs between torque density, high-speed operation, inverter current, coil turns, back EMF, and magnetic core saturation. Understanding these intricacies enables designers to make informed decisions about the type of electric machine that best suits a particular application. More importantly, this article informs readers about the benefits of coil switching technology and demonstrates how coil switching technology is poised to replace standard 3-phase drive systems, as it offers a much greater level of design flexibility to electric motor and vehicle manufacturers.
We hope this article has provided you with a technical understanding of electric machines and their design considerations. To learn more about electric machines and how our expertise can help you find the best solution for your specific needs, we invite you to visit our website and explore our portfolio of traction inverters built on patented coil switching technology. Our team of professionals is always available to answer your questions and guide you through the complex world of electric machines. Additionally, you may be interested in our related industry insight articles, such as the three-part series on the evolution of traction inverters and advancements in traction inverter technology. Don't hesitate—visit us today and discover how we can help you harness the power of electric machines to achieve optimal performance and efficiency.