Posted with permission from saveonenergy.com
The electric motor has become a little more well known and appreciated over the past few years with its improving integration into electric cars.
With most people understanding and appreciating the impact that their pollution has on the climate, there has been a greater demand for auto manufacturers to create cars that can help to improve our environment, or at the very least, do less harm.
It is through this demand for growth and development that some of the world’s greatest inventors have refined the electric motor to now work better and be more efficient than it ever was before.
Let us understand this wonderful innovation that is propelling the electric vehicle revolution.
The stator is made of three parts - a stator core, conducting wire and a frame. The stator core is a group of steel rings that are insulated from one another and laminated together. These rings have slots on the inside of the rings that the conducting wire will wrap around to form the stator coils.
Simply put, in a three phase induction motor, there are three different wire types. You can call these wire types Phase 1, Phase 2 and Phase 3. Each wire type is wrapped around the slots on opposite sides of the inside of the stator core.
Once the conducting wire is in place within the stator core, the core is placed within the frame.
The rotor is made of three parts as well - a rotor core, conducting rods and two end rings. High grade alloy steel laminations make up the cylindrical rotor core that has what looks like a rod running through the center of it. On the outside of the rotor core there are slots that either run parallel to the rod-like bar in the center of the rotor core or slightly twisted to form diagonal slots. If the stator core has diagonal slots on the outside of the core, it is called a squirrel cage rotor.
A three phase, four pole induction motor uses squirrel cage rotors. Along the diagonal lines in the core, conducting rods are placed to make rotor winding. End rings are then placed on either side of the core to short circuit all the conducting rods that were placed in the diagonal lines of the rotor core.
Once the rotor and stator have been assembled, the rotor slides into the the stator and two end bells are placed on either side. These end bells are made of the same material as the stator frame and are used to protect the motor from both sides.
If you’re an electrical engineer, you know how an electric motor works. If you aren’t, it can be extremely confusing, therefore, here’s the simplified
explanation (or the “how an electric motor works for dummies” version) of how a four pole, three phase AC induction motor works in a car.
It starts with the battery in the car that is connected to the motor. Electrical energy is supplied to the stator via the car’s battery. The coils within the stator (made from the conducting wire) are arranged on opposite sides of the stator core and act as magnets in a way. Therefore, when the electrical energy from the car battery is supplied to the motor, the coils create rotating magnetic fields that pull the conducting rods on the outside of the rotor along behind it. The spinning rotor is what creates the mechanical energy need to turn the gears of the car, which, in turn, rotate the tires.
Now, in a typical car that isn’t electric, there is both an engine and an alternator. The battery powers the engine, which powers the gears and wheels. The rotation of the wheels is what then powers the alternator in the car and the alternator recharges the battery. This is why you are told to drive your car around for a period of time after being jumped - the battery needs to be recharged in order to function appropriately.
In an electric car, there is no alternator. So, how does the battery recharge then? While there is no separate alternator, the motor in an electric car acts as both the motor and an alternator. That’s one of the reasons why electric cars are so unique. As referenced above, the battery starts the motor, which supplies energy to the gears, which rotates the tires. This process happens when your foot is on the accelerator - the rotor gets pulled along by the rotating magnetic field, requiring more torque. But what happens when you let off of the accelerator?
When your foot comes off the accelerator, the rotating magnetic field stops and the rotor starts spinning faster (as opposed to being pulled along by the magnetic field). When the rotor spins faster than the rotating magnetic field in the stator, this action recharges the battery, acting as an alternator.
To further simplify this process - imagine pedaling a bike up a hill. To get to the top of the hill, you need to pedal harder and may even have to stand up and expend more energy to rotate the tires and reach the peak of the hill. This is similar to pressing down on the gas. The rotating magnetic field pulling the rotor behind it creates the resistance (or torque) needed to move the tires and car. Once at the peak of the hill, you can take it easy and recharge while the wheels move even faster to take you down the hill. In the car, this happens when you let your foot off the gas and the rotor moves faster and feeds electrical energy back into the power line to recharge the battery.
The conceptual differences behind these two types of currents is seemingly fairly obvious. While one current is consistent, the other is more intermittent. However, things are a bit more complicated than just that simple explanation, so let’s break these two terms out in a bit more detail.
Direct Current (DC)
The term direct current refers to electricity that moves in a singular and consistent direction at all times. Furthermore, a direct current’s voltage maintains a regular polarity, that is, one that does not change.
Think of how batteries have clearly defined positive and negative sides to them. They use direct currents to send the same voltage out on a consistent basis. In addition to batteries, fuel cells and solar cells also produce direct currents, while simple acts like rubbing certain materials together can produce DC as well.
In keeping with our battery concept, when considering the positive and negative sides of the battery, it is important to note that the direct current always flows the same direction between the positive and negative side. This ensures that both sides of the battery are always positive and negative.
The term alternating current defines a type of electricity characterized by voltage (think water pressure in a hose) and current (think rate of water flow through the hose) which vary with respect to time. As the voltage and current of an AC signal change, they most often follow the pattern of a sine wave (in the image above, the sine wave is shown in the right voltage graph). Due to the waveform being a sine wave, the voltage and current alternate between a positive and negative polarity when viewed over time. The sine wave shape of AC signals is due to the way in which the electricity is generated.
Another term you may hear when discussing AC electricity is frequency. The frequency of the signal is the number of complete wave cycles completed during one second of time. Frequency is measured in Hertz (Hz) and in the United States, the standard power-line frequency is 60 Hz. This means that the AC signal oscillates at a rate of 60 complete back-forth cycles every second.
So why is this important?
AC electricity is the best way to transfer useable energy from a generation source (i.e., a dam or windmill) over great distances. This is due to the alternating nature of the AC signal which allows the voltage to be easily stepped up or stepped down to different values. This is why your home's outlets will say 120 volts AC (safer for human consumption) but the voltage of a distribution transformer which supply power to a neighborhood (those cylindrical grey boxes you see on the power line poles), might have voltage as high as 66 kVA (66,000 volts AC).
AC power allows us to construct generators, motors, and distribution systems from electricity that are far more efficient than direct current, which is why AC is the most popular energy current for powering applications.
What does polyphase mean?
Whenever you have a stator that houses multiple unique windings per motor pole, you are dealing with polyphase. It is most common to expect a polyphase motor to be made up of three phases, but there are motors that utilize two phases.
A polyphase system uses multiple voltages to phase-shift apart from each one in order to go intentionally out of line.
What does three phase mean?
Based around Nikola Tesla’s basic principles defined in his polyphase induction motor put forth in 1883, "three phase" refers to the electrical energy currents that are supplied to the stator via the car’s battery. This energy causes the conducting wire coils to start to behave like electromagnets.
A simple way to understand three phase is to consider three cylinders, shaped in a Y formation, utilizing energy pointed toward the center point to generate power. As the energy is created, the current flows into the coil pairs inside the engine in such a way that it naturally creates a north and south pole within the coils, allowing them to act like opposite sides of a magnet.
As this technology continues to advance, the performance of electric cars are starting to quickly catch up to, and even outperform, their gas counterparts. While there remains some distance for electric cars to go, the leaps that companies like Tesla and Toyota have made to this point have inspired hope that the future of transportation will no longer be reliant on fossil fuels.
Electric engines impact the environment both directly and indirectly at a micro and macro level. It depends on how you want to perceive the situation and how much energy you want. From the individual standpoint, electric cars don’t require petrol to run, which leads to cars with no emissions populating our highways and cities. While this presents a new problem with additional burden of electricity production, it alleviates the strain from millions of cars densely populating cities and suburbs putting toxins into the air.
From a large scale perspective, there are several benefits to the rise of electric cars. For starters, there is a reduction in noise pollution as the noise emitted from an electric engine is far more subdued than that of a petrol powered engine. In addition, due to the fact that electric engines do not require the same type of lubricants and maintenance that a gas engine does, the chemicals and oils used at auto-shops will be reduced due to fewer cars needing check ups.
The electric engine is changing the course of history in the same way that the steam powered engine and printing press redefined progress. While the electric engine is not paving new grounds in the same vein as these inventions, it is opening up a brand new segment of the transportation industry that is not only focused on style and performance, but also external impact. So, while the electric engine may not be reforming the world due to an introduction of some brand new invention or the creation of a new marketplace, it is redefining how we as a society define progress.
If nothing else is to come from the advancements with the electric engine, at the very least we can say that our society has moved forward with our awareness of our environmental impact. This is the new definition of progress, as defined by the electric engine.
Construction of Three Phase Induction Motor https://www.youtube.com/watch?v=Mle-ZvYi8HA
How does an induction motor work? https://www.youtube.com/watch?v=LtJoJBUSe28
Working Principal of Three Phase Induction Motor https://www.youtube.com/watch?v=DsVbaKZZOFQ
Image from: http://faq.zoltenergy.co/technical/