Uses Tesla DC motors

Why does a Tesla car use an AC motor instead of a DC motor?


I was just watching a mega factory video and was wondering why are they using an AC motor that requires an inverter instead of a DC power that may be fed directly from their DC battery. The introduction of an inverter means higher costs (weight, control, etc.).

Are there any reasons for that? What are the differences between an AC and a DC motor that may have led to this decision? Also does anyone know what type of motor is used in other electric cars?





Reply:


You are asking about the technical tradeoffs when choosing a traction motor for an electric vehicle application. The description of the full design trading area goes well beyond what can reasonably be summarized here, but I will outline the main design tradeoffs for such an application.

Since the amount of energy that can be chemically stored (i.e. in a battery) is very limited, almost all electric vehicles are designed for efficiency. Most transit motors for automotive applications have a peak power between 60 kW and 300 kW. Ohm's law states that the power losses in cables, motor windings and battery connections P = I 2 R are. Halving the current reduces the resistance losses by four times. As a result, most automotive applications run at a nominal DC link voltage between 288 and 360V nom (There are other reasons for this selection of tension, too, but let's focus on losses). The supply voltage is relevant in this discussion as certain motors, such as. B. brush direct current, due to commutator arcs have practical upper limits for the supply voltage.

Ignoring more exotic motor technologies like switched / variable resistor, there are three main categories of electric motors used in automotive applications:

Brush DC motor : mechanically commutated, only a simple DC chopper is required to control the torque. While brushed DC motors may have permanent magnets, the size of the magnets makes them unaffordable for traction applications. As a result, most DC traction motors are series or shunt wound. In such a configuration, windings are present on both the stator and the rotor.

Brushless DC motor (BLDC): electronically commutated by an inverter, permanent magnets on the rotor, windings on the stator.

Induction motor : electronically commutated by inverter, induction rotor, windings on the stator.

Below are some basic generalizations regarding tradeoffs between the three engine technologies. There are many examples that defy these parameters. my goal is just to share what i would consider to be notices for this type of application.

- efficiency:
Brush direct current: Motor: ~ 80%, DC regulator: ~ 94% (passive reverse), NET = 75%
BLDC: ~ 93%, inverter: ~ 97% (synchronous return or hysteretic control), NET = 90%
Induction: ~ 91%: Inverter: 97% (synchronous return or hysteretic control), NET = 88%

- Wear / maintenance:
Brush DC: Brushes are subject to wear and tear; require regular exchange. Warehouse.
BLDC: Bearing (service life)
Induction: Bearing (service life)

- Specific costs (costs per kW), including inverters
DC brush: The low motor and controller are generally inexpensive.
BLDC: High performance permanent magnets are very expensive.
Induction: Moderate inverters add an additional cost, but the motor is inexpensive

-
Heat Dissipation Brush DC: The Windings on the rotor make it difficult to dissipate heat from both the rotor and the commutator in high-performance motors.
BLDC: Windings on the stator ensure easy heat dissipation. Magnets on the rotor have a weak heating induced by eddy currents.
Induction: Windings on the stator ensure that the stator emits heat directly. Induced currents in the rotor may require oil cooling in high-performance applications (running in and out via the shaft, not splashed).

- Torque / speed behavior
Brush DC: Theoretically infinitely fast torque, torque decreases with increasing speed. For brushed DC vehicle applications, 3 to 4 gear ratios are generally required to cover the entire vehicle class and top speed range. I drove a motorized 24 kW DC electric vehicle for several years that was able to light up the tires from a standing start (but struggled to get to 65 mph).
BLDC: Constant torque up to the base speed, constant power up to the maximum speed. Automotive applications can be implemented with a transmission with a gear ratio.
Induction: Constant torque up to the base speed, constant power up to the maximum speed. Automotive applications can be implemented with a transmission with a gear ratio. It can take hundreds of ms for the torque to build up after the current is applied

- Miscellaneous:
Brush direct current: Problems with the commutator arc can occur at high voltages. Brushed DC motors are canonically used in golf cart and forklift (24V or 48V) applications, although newer models are inductive because of their improved efficiency. Regenerative braking is difficult and requires a more complex cruise control.
BLDC: The Magnet cost and the assembly issues (the magnets are VERY powerful) make BLDC motors useful for lower powered applications (like the two Prius motor / generators). Regenerative braking is essentially free.
Induction: The motor is relatively cheap to manufacture and power electronics for automotive applications have become significantly cheaper over the past 20 years. Regenerative braking is essentially free.

Again, this is just a high-level summary of some of the key design drivers for engine selection. I purposely foregone a certain power and torque as these vary much more with the actual implementation.







... and now why Tesla Uses induction motors

The other answers are excellent and depend on the technical reasons. After following Tesla and the electric vehicle market in general for many years, I want to answer your question, why Tesla Induction motors used.

background

Elon Musk (co-founder of Tesla) comes from Silicon Valley (SV) and thinks that "go fast and things break" is the mantra. When he moved out of PayPal for hundreds of millions, he decided to tackle electric vehicles (space exploration and). In SV-Land, time and speed are everything to get things done and he's looked around to find something to use as a starting point for an early start.

JB Straubel was a like-minded engineer (both Space and EV) who contacted Musk shortly after Musk made his interest in Space and EV public.

During her first lunch, Straubel mentioned a company called AC Propulsion that had developed a prototype electric sports car with a kit-car frame. Already in the second generation it had recently switched to lithium-ion batteries, had a range of 250 miles, offered a lot of torque, could go between 0 and 60 in less than 4 seconds, but what was most important for this discussion was used: - you guessed it - AC drive (Induction motor).

Musk visited AC Propulsion and was very impressed. He spent a few months trying to convince AC Propulsion to commercialize the electric vehicle, but at the time they were not interested.

Tom Gage, the president of AC Propulsion, suggested that Musk team up with another suitor made up of Martin Eberhard, Marc Tarpenning and Ian Wright. They agreed to pool their efforts, with Musk becoming chairman and overall director of product design, Eberhard CEO and Straubel CTO of the new company they named "Tesla Motors".

The answer

So there you have it, Tesla mainly uses induction because the first working prototype Musk saw used it. Indolence (no pun intended ... ok, a little) explains the rest ("If it ain't broke ...").

Well why AC Propulsion used it in their Tzero prototype, see the other answers ... ;-)

If you want the full story go here or here.





It's hard to say what the exact reasons the engineers were for not being a member of the design team, but here are a few thoughts:

  1. Both engines need similar drives. Brushed DC motors can be powered directly from a battery, but the type of motor you see in an electric vehicle is a brushless DC motor. The drives for an induction motor and a brushless DC motor are very similar. Controlling an induction motor is likely to be more complex in general.

  2. Brushless DC motors have magnets in the rotor. This is more expensive than an induction rotor with copper. In addition, the magnet market is very volatile. On the other hand, in an induction motor, much more heat is generated in the rotor due to I²R losses and core losses.

  3. The starting torque on the brushless motor is usually higher than on asynchronous motors.

  4. The peak efficiency of brushless motors is generally higher than that of induction motors, but I think I read somewhere that Tesla got a higher average efficiency with its induction motor than with a brushless motor. Unfortunately, I can't remember where I read this.

  5. Many people are now researching switched reluctance machines. The last couple of auto conferences I've been to have been all about mock reluctance. You don't need magnets and the efficiency of these types of motors looks promising. Everyone would like to get away from magnets in motors.

Like I said, I doubt anyone can answer your question, except for the engineers at Tesla. But I suspect that it probably has something to do with my point 4), but I don't know for sure. I'm sure the magnet price volatility has something to do with it too.







The answer comes from the Tesla employees themselves to the article Induction versus Brushless DC Motors

This part is particularly noteworthy:

With an ideal brushless drive, the strength of the magnetic field generated by the permanent magnets would be adjustable. When maximum torque is required, especially at low speeds, the magnetic field strength (B) should be maximum - so that the inverter and motor currents are kept at the lowest possible values. This minimizes the I²R losses (current² resistance losses) and thereby optimizes the efficiency. The B field should also be reduced at low torques so that eddy and hysteresis losses due to B are also reduced. Ideally, B should be set in such a way that the sum of the eddy, hysteresis and I² losses is minimized. Unfortunately there is no easy way to change B with permanent magnets.

In contrast to this, induction machines have no magnets and B fields are “adjustable” because B is proportional to U / f (voltage to frequency). This means that the inverter can reduce the voltage at low loads, so that the magnetic losses are reduced and the efficiency is maximized. When operated with an intelligent inverter, the induction machine has an advantage over a brushless DC machine - magnet and line losses can be dealt with in such a way that the efficiency is optimized. This advantage becomes more and more important as the performance increases. With brushless direct current, the magnetic losses increase proportionally with increasing machine size and the partial load efficiency decreases. With induction, the losses do not necessarily increase as the machine size increases. Therefore, induction drives can be the preferred approach when high performance is desired.

Permanent magnets are expensive - around $ 50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle because of the very large forces acting when something ferromagnetic comes near them. This means that induction motors are likely to retain a cost advantage over PM machines. Due to the field weakening ability of induction machines, the inverter performance and costs seem to be lower, especially for high-performance drives. As rotating induction machines generate little or no voltage when de-energized, they are easier to protect.


ALL rotating electric motors are AC motors. All of them.
Basically, they do the same thing. The difference is how the direct current is converted to alternating current and how it is used to then achieve a standard result.

The only electronic DC motor is the brush motor. The direct current is converted into alternating current by the rotating commutator and the fixed brushes. Aside from this motor, all others will require some form of DC to AC conversion. The brush motor is generally unattractive because the mechanical DC-AC converter (commutator) is relatively expensive and relatively short-lived.

For a Tesla or other electric vehicle, the choice is not direct current or alternating current, but which form of alternating current motor best suits the design goals in a cost-effective manner.

The Tesla will use what it does because it is the most cost-effective way to meet design goals.


The votes indicate that a number of people agree with Marcus and feel that the above answer is incorrect. A few thoughts and a look at my answers in general may indicate a lack of understanding among the downvoters.

All rotating electric motors are AC motors

  • If you think this point is wrong, then you need to think more carefully about what an electric car can do overall.

Let's see if the downvoters have the courage to read the following and then remove their downvotes. It doesn't matter to me. To the extent that you are misleading other people, it matters a lot.

ALL rotating electric motors require a controller to in some way apply AC power to the motor.
The distinction between AC motor and DC motor is useful in some contexts, but in an automobile, which is a closed system that starts with a source of DC power and ends with a rotating electric motor, the distinction is wrong and not useful. The car is a closed system. Somewhere in the system is a controller that converts the direct current into another form. It does not matter whether it is mounted in the rotor, stator or rotor, in the motor housing, on the housing or elsewhere in the car.

In a brushed DC motor, the regulator is a mechanical switch that is attached to the end of the motor shaft. This regulator is called a commutator, but is functionally a regulator that draws DC power and creates a DC magnetic field when it comes to the windings in the motor.

A "brushless DC motor" wound with a permanent magnet rotor stator is functionally very similar to a brushed DC motor, with the commutator being replaced by electronic switches and sensors that pick up the supplied direct current and apply it to various fields so that they can track their tail Rotor turns. Again it's an AC motor with a controller. Just ask a winding. The sensors are in the actual engine, and the switches may be next to the actual engine or far away.

A squirrel cage induction motor adds complexity by using the rotation of a nest of low resistance windings in the stator field to induce voltage in the rotor bars and create a magnetic field that turns the rotor so that it chases the rotating AC field applied to the stator windings. Again, it has a monodirectional (but sinusoidally varying) direct current during any portion of the drive sequence. It is as much a mixed DC and AC system as any other.

One could reluctantly describe motors with variable eddy currents - rather the same, but different. It is an AC motor with a controller that generates it from direct current.

The distinction is irrelevant and trivial. The real question is, "Why is Tesla using this particular form of engine more than any other?" Wordin shows that this is not just a semantic, but a lack of understanding

  • ... who need the Power Inveter, instead of DC, the more direct of them DC battery.The introduction of inveter means more costs (weight, controller, etc.) ...

The only "DC motor" that does not require an inverter or electronic switching system is the mechanical brush motor. These are so unsuitable for the task of lightweight variable-speed drives that there are only a few that are used in modern electric car designs. All other types of electric motors that don't have an inverter have some electronics instead of an inverter.


I said, ROTARY "Electric motors are AC motors because you can probably make a brushless DC motor with only switched DC operation, although that would use copper and magnets inefficiently. You could do that with a rotary motor, but without a real world motor in series production it would do so .







DC motors cannot keep up with the power density of AC machines. The maximum field strength that even the best magnets can achieve is 2.5 Tesla across the air gap. Thorough engineering is required to achieve this, especially if you want to spin fast for high power density. Induction machines easily produce 3+ Tesla without any grief from magnets and stupid tolerances. They obviously don't do this as efficient DC machines, but who said sports cars are efficient in full swing? Kg for kg The AC induction machine is the most powerful of all machine types. Buy a sophisticated inverter that runs at high speeds.




The real reasons they use induction motors for their cars are:

  1. Asynchronous motors are cheaper
  2. Asynchronous motors require little maintenance (no brushes)
  3. Asynchronous motors are lighter
  4. The new technology for speed control of induction motors is now available (variable voltage, variable frequency) and is easy to mass-produce

IMHO, AC Propulsion (Tesla Motors) uses AC power because a mechanically commutated DC motor that meets the high "drawdown" ratio of a vehicle application is more complex than an electronically commutated AC motor. Without this high reduction ratio, the physical size of the motor, which only generates raw torque, would be intolerable. The induction motor instead of the PM motor is not only more financially stable, but also more stable from a technical point of view. Magnets can and will be damaged. The electromagnetic field coils in the rotor are not as strong and as they show the energy density is similar.

I take a major exception to the obvious consensus that "All electric motors are AC" and base my argument on unipolar motion, not a full revolution of the motor.

Within a unipolar movement, alternating current is only really required if a current flow has to be induced in a parasitic winding such as in the rotor of induction motors. Otherwise, only one commutation is required.

This argument can best be shown by observing a motor at a standstill. Only motors without PM or wound fields that are induction motors need AC power to generate the field current that creates the reactive magnetic field.

All other motors only have to supply the stator with direct current in order to generate the full torque at standstill. Wound field motors often use AC to create the field, but they do well with DC too, probably with even more torque than AC.

My PM servo motors might cut the DC current to control the power, but they just cut the DC current and don't invert it every time you chop. Put a mechanical commutator on top of the AC PM servo motor and it will work on DC power. Right, not as efficient, but not because of the lack of a sinusoid. It is also limited in top speed without a mechanical brush feed.

Take some time to consider the stall characteristics of a double-wound motor, one that obviously "contains AC only" when supplied with DC power, and maybe you can understand my argument. Only if you want to push each pole in addition to pulling do you need to provide AC power. Otherwise you will only need direct current and often only direct current, even if the power supply is alternating current.

slate


All: Brushed machines are limited to 48V to avoid arcing. In contrast, a brushless machine can be operated with a 240 V battery without any problems, with the voltage being increased to 480 V or higher by a DC boost converter placed between the battery and the motor. With such a high voltage, similar to that used in most hybrid or plug-in cars today, the cruise control losses are minimized in relation to the total power transmitted, thereby promoting high efficiency.



In fact, Tesla uses synchronous electric motors that use both AC and DC power. If the motor is operated by AC only, it is an asynchronous motor, which is unpredictable due to the slip in the electromagnetic field when a voltage is induced in the rotor (the output speed is slower than the speed of the motor) electromagnetic field formula: Revolutions per minute = frequency * 60 / pole pairs per phase - slip in speed).

In a synchronous motor, an AC has an enlarged stator coil (like a conventional induction motor), but it also has a DC enlarged rotor (as opposed to an induction motor). This allows the output speed to reach the theoretical maximum speed (the speed of the sun), creating a predictable and effective engine for use in vehicles. (Formula: revolutions per minute = frequency * 60 / pole pairs per phase).

Tesla can then investigate this and use an ESC (Electronic Speed ​​Controller). An ESC is a circuit board that converts some of the DC power from the battery to AC power, converts the square waves to sine waves, changes the frequency and amplitude according to the signals from the accelerator pedal, and sends the processed power to the stator. It also changes the amplitude of the DC power to the rotor in accordance with the AC power to the stator.



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