Whether you’re wealthy enough to drive Lotus’ $2M, 2000hp all-electric sports car or you’re more interested in driving a “family EV”, where the range is more important to you than going from 0 to 186mph in under 9 seconds, as with the Lotus; or even interested in the Canadian-designed Daymak’s Spiritus, a sporty three-wheeler, with a180miles range, which goes from 0 to 60mph in 6.9 seconds, here are some things you need to know about EV power systems. Teviot Technology makes custom-designed lithium battery packs for EVs and other products that may potentially use ultra-smart lithium battery packs. Daymak is a key strategic partner of Teviot in a project in development.
Motors
There are two categories of electric motors used in EVs. AC (Alternating Current) and DC (Direct Current). In both cases, motors convert the electrical energy into mechanical energy, to provide torque to the wheels. In many EVs, the motors are an integral part of the wheels (hub motors).
DC Motors
DC motors are robust and simply controllable. They are more used in light electric vehicles such as scooters and bikes. Typically, there are 2 types of DC motors used in EVs. BDCM (Brushed DC Motors) and BLDC (Brushless DC Motors).
Brushed DC motors come with easy control and provide high torque at low speed. While they are very important as traction motors, BDCM is not widely used in EVs because of their disadvantages, including large size, low efficiency, and high maintenance requirements, due to the brush and collector structure.
Brushless DC motors use an electronic commutator/inverter, instead of brushes. BLDC is inherently low-maintenance compared to conventional “Brushed” DC motors and are efficient and produce high starting torque. They are widely used as “hub motors”.
AC Motors
There are two types of AC motors used in EVs. Synchronous and asynchronous.
In synchronous motors, the rotor and the stator magnetic field both rotate at the same speed
In an asynchronous motor (also known as an induction motor), an electric-powered stator generates a rotating magnetic field. While in a synchronous motor, the rotor itself operates as an electromagnet.
In terms of application, a synchronous motor is seen as the better option for urban driving where there can be a lot of starting and stopping at low speeds, whereas an asynchronous motor is preferable for driving at higher speeds for long periods of time. This is because compared to synchronous AC motors, asynchronous/induction motors don’t provide a high starting torque. However, they are cheaper and more efficient.
AC induction motors have no permanent magnets, no brushes, no commutator rings, no position sensors and are highly reliable, efficient, and require less maintenance. Their design is simple, with rugged construction.
With speed increase, while AC induction motors have slight torque reduction, DC motors have a faster falloff. The AC motor torque property is very close to the ideal characteristic.
Regenerative (Regen) Braking
Both AC and DC motor-driven EVs can benefit from regenerative braking systems. The system is more complicated with DC motors. Regenerative braking systems are designed to recover energy that would be otherwise lost during a braking event. Regenerative braking uses an electric vehicle’s motor as a generator to convert around 60 to 70% of the kinetic energy lost when decelerating, back into the vehicle’s battery. Then, when the car accelerates, it uses much of the energy previously stored from regenerative braking, instead of using its battery-stored energy reserves.
Traction Battery Packs
DC Motor Controllers are needed to regulate the power that flows from the batteries to the motor. This happens in response to commands received from the accelerator pedal. Typically, the input voltage range would be from as low as 48V to 400V for the high end, depending on your motor and controller.
AC motor controllers use inverters, also known as Variable Frequency Drive (VFD) Inverters, to convert the battery DC voltage to a pure sinusoidal voltage. In fact, as the AC motors in EV cars are normally three-phase, these inverters produce three sinusoidal voltages which are 120 degrees out of phase from each other. The AC motor’s rotating speed is controlled by changing the inverter’s alternating current frequency and hence the motor’s magnetic field.
Traction motor battery voltage varies between EVs. While most EVs currently use 350-400V, some newer EVs operate at 800V. The higher the voltage, the lower the conductive losses with more efficient invertors, which also help make for lighter EVs.
With high voltages, more cells must be placed in series, making the power solution more costly, with more complex circuitry required for cell balancing and temperature monitoring. It is simpler to double the battery pack capacity (in Ampere Hours). For an LFP (3.2V cell voltage) battery pack with a nominal voltage of 352V, you’ll need 110 cells in series. All these cells have to be isolated from each other, due to defined “High Voltage” (60V to 1500V DC) Safety regulations. Each cell also must be individually monitored for balancing and safety. That means a lot of monitoring cables, which also must be filtered for noise immunity. Therefore, one needs to look at all the pros and cons, when going for a high pack-voltage design.
EV Energy Consumption
The usable battery capacity also varies from EV to EV. The required energy for an EV can be as low as 16.7KWh for a Smart EQ Forfour and could be as high as 107.8KWh for a Mercedes EQS AMG 553 4MATIC+. The data in this link is worth checking to find out the energy consumption of many currently available EVs – EV energy KWh.
Auxiliary Battery Pack
This is typically a 12V lead-acid or Lithium battery pack (30Ah is enough for most EV applications), kept always charged. It is used for powering safety-critical systems such as doors, breaks, and the vehicle. In cases when the main traction battery pack has reached its low-voltage limit or has been disconnected while driving, the auxiliary battery will continue powering the computer and the breaks to ensure a safe stopping of the car.
Charging EVs
The EV chargers, also known as EVSE (Electric Vehicle Supply Equipment) are categorized into three levels:
Level 1 Charging – 1KW to 1.4KW
This uses a 120V household outlet. This is the slowest way to charge an EV. It typically adds 3 to 5 miles of range for every hour of charging. This option is acceptable for smaller battery packs (e.g. 25KWh).
Level 2 Charging – 3.9KW to 19.2KW
This uses a 208V to 240V supply. You would typically find this type of charging equipment installed in the workplace, homes, train stations, shopping plazas, and other public places. They add 12 to 80 miles of range per hour. This is 10 times better than Level One chargers.
Level 2 chargers can deliver 80A current and would require a dedicated100A 240V circuit breaker. For homes also, there are 40A or 50A chargers available, which are suited for home wiring.
Level 3 Charging – 24KW to 300KW
Level 3 chargers are the fastest chargers, providing 400V to 900V DC power. They can replenish between 3 to 20miles worth of charge per minute. They add 75-1200 Miles per hour of charge. They are installed in public places and cost tens of thousands of dollars. Tesla refers to them as “superchargers”.
Charger Plugs
Except for Tesla, all level 2 chargers in North America use a “J-plug” (J1772) and Tesla provides a J-Plug interface on their cars. However, other countries have their own standards and unfortunately, there is currently, no universal standard.
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