How Electric Car Motors Work
To fully understand how an EV motor works, we need to first discuss two critical components found inside electric vehicles – the power electronics control system and the auxiliary systems.
Power Electronics & Auxiliary Systems
The primary function of the power electronics controller is to manage the flow of electrical energy delivered by the traction battery to the electric motor. The controller accomplishes this by managing the speed at which the motor turns or by controlling the torque the motor produces.
The secondary function of the power electronics controller is to distribute electrical energy from the traction battery to the auxiliary vehicle systems, such as the lighting, heating, ventilation, and infotainment systems. Rather than the traction battery, a separate auxiliary battery – identical to the ones found in gasoline vehicles – is responsible for powering these systems. This 12-volt battery is kept charged by the DC/DC converter, which converts high-voltage DC power from the traction battery into the low-voltage DC power required to power the auxiliary systems. Therefore, technically, the traction battery powers all the auxiliary systems, too.
When it comes to auxiliary systems, the vehicle thermal management system deserves to be mentioned. One of the biggest determining factors of a safe, long-lasting and functional PEV battery is its ability to effectively maintain a uniform temperature distribution across its cells. Since traction batteries are designed to only operate between a certain temperature range, they will cease to work if there is no thermal system to monitor it. The main function of the thermal management system is to keep the traction battery within this specific temperature range. When an EV accelerates, the battery’s electrical energy is discharged, and heat is generated inside the battery. Since acceleration is often the primary method of discharging the battery, without a proper cooling system the battery will quickly overheat and lead to deterioration.
Indirect Liquid Cooling
Several types of thermal management systems are available on the market, but we will only be discussing indirect liquid cooling systems because they are the superior choice. Most modern PEVs are manufactured with indirect liquid cooling systems for several reasons:
- They are highly efficient systems
- They can store large amounts of heat energy
- They are currently the most compact and lightweight solution
As the name implies, the liquid coolant in this type of system does not have direct contact with the vehicle battery. It is instead circulated through a series of metal pipes either surrounding the battery or embedded between the battery’s cells to transfer the heat away. This method allows the cooling system to consume a small amount of energy from the battery to keep it at an operable temperature. In other words, more of the battery’s energy can be devoted to powering the motor and maximizing the powertrain’s performance all the while being uninterrupted by the weight of the system.
Electric Car Motor Components
Next, let’s break the three-phase induction motor down into its two main components: the stator, which is the stationary part of the motor, and the rotor, which is the moving part of the motor.
The stator consists of three parts: the stator core, conducting wire and frame. The stator core is built by stacking thin, laminated rings and forming them into a hollow cylinder. This cylinder has slots in the hollow interior that allows the conducting wire (typically made of copper) to wrap around and shape the coils. For a three-phase induction motor, a different wire type exists for each of the three phases that forms its own individual coil. The stator core and the coils are both found within the frame, which is simply the exterior of the entire motor.
The rotor also consists of three parts: the rotor core, conducting rods and two end rings. The rotor core is built by stacking thin, laminated discs and forming them into a solid cylinder that has what appears to be a rod running through the center of it. On the exterior of the rotor core, there are similarly shaped slots to the stator core, but these run diagonally across the cylinder instead of parallel to the rod in the center. The alignment of this rotor’s exterior slots is known as a squirrel-cage rotor, which is a popular design choice among many industries. Along these diagonal lines of the rotor core, the conducting rods are inserted, and the end rings are placed on both sides of the core to lock the rods in place. The rotor then slides into the hollow stator core, and two end bells are placed on either side of the rotor core’s center rod.
Finally, let’s introduce one last component called the alternator. In terms of traditional gas vehicle components, alternators are responsible for charging the 12-volt battery while the engine is running. This is the reason why you are recommended to drive your car for a while after you receive a jump-start — the battery needs to be recharged to function again. In BEVs, however, the induction motor also acts as an alternator when you release your foot off of the acceleration pedal. During this time, the magnetic field will stop rotating and the rotor will remain spinning until it eventually stops as well. Therefore, regenerative braking will recharge the battery whenever the rotor’s rotational speed is greater than the magnetic field’s rotational speed.
Now that we have a firm understanding of the components inside an induction motor, let’s see how it functions inside the frame of an EV.
Bringing it All Together
First, we begin with the traction battery that is connected to the motor. As previously mentioned, all of the vehicle’s power is derived from the battery. The electrical energy from the battery is supplied to the motor via the stator. The three copper coils within the stator core are arranged 120 degrees apart from each other and act as magnets. Think of these coils arranged in a “Y” formation. As electrical energy is supplied to the motor, the coils produce a rotating magnetic field that induces current through the connecting rods of the squirrel-cage rotor, thus causing the rotor to spin. This spinning rotor is what creates the mechanical energy needed to turn the wheels of the car. Now let’s pull everything together: when you press your foot on the acceleration pedal, the traction battery powers the motor with electrical energy, which produces the rotating magnetic field in the stator, which spins the rotor via induction, which produces the mechanical energy needed to rotate the tires.