Page 23 - Issue 3 2023

TECHNICAL

electrolyte made of solid material instead of liquid. Solid electrolytes enable optimised Brush excited synchronous motors
safety of the battery as well as the use of new types of anodes, such as lithium-metal In these motors the rotor DC excitation
anodes. The SSB is more stable, has a higher energy density than the already high Li- current is provided via brushes. The
ion battery (with estimates ranging from 25% to 100% increase), uses readily-available lack of permanent magnets avoids the
materials, and offers lower flammability and faster charging. SSBs’ batteries are expected issues of rising rare-earth costs and the
to become the standard technology for many EPMs in the future. Some major car environmental impact of mining for
manufactures are already changing their strategies to use SSBs. the material. This scheme also makes
it possible to vary the strength of the
Traction inverter (TI) and controller rotor’s magnetic field, which enables
The traction inverter serves several functions further optimisation. Power is required to
• AC power for the motor energize the rotor, making these motors
• Charge control of the battery during regenerative braking less efficient, notably at low speeds.
Several manufacturers have introduced
The traction inverter is a variable frequency inverter that controls the power fed to the these motors in recent models. The
motor as well as the speed of the motor. Driver controls send signals to the controller brushed motor has the drawback of
which adjust the vehicle’s speed by changing the frequency of the AC power from the requiring maintenance of the brushes,
inverter to the motor. Recent developments in power electronics have allowed higher but this has been taken into account in
voltages to be used in TI, with 800 V now becoming common. The TI is also used to feed the design of the motor.
regenerative braking power back to the battery.
EPM charging
Controller EPM charging grid impact
The controller sends signals to the inverter appropriate to the required action and the EPM charging requires additional
existing condition of the motor or vehicle. This unit controls the speed and torque of generation capacity on the grid, and much
the motor by controlling the frequency and voltage fed to the motor from the inverter has been said of the additional generation
and responds to driver signals and vehicle feedback. The controller implements various required to power EVs. Using the distance
algorithms to achieve the required actions. an average South African motorist
travels per day and the average energy
Motor consumption for EPMs, the average daily
An EPM may have a single motor, two motors, or a motor on every wheel. The three energy consumption for EPMs ranges from
basic classes of motor used in a BEV are, asynchronous induction motors, synchronous 3,75 to 8,25 kWh. If 10% of the seven
permanent magnet motors and brush current excited AC synchronous motors, with million passenger vehicles in South Africa
variations within each class. EV motors are high speed devices, running at speeds of up were converted to EPMs, the average daily
to 20 000 RPM. Development is focussed on even higher speed motors, which allows a additional electricity required to charge
reduction in motor size. these vehicles would amount to 700 000
x 5 kWh = 3,5 GWh or 0,5% of the total
Induction motors daily electricity production.
The induction motor was commonly used in early EVs. AC induction motors are inherently Converting 100% of the three million
asynchronous, and rotor speed is controlled by varying the alternating current’s commercial vehicles to electric would
frequency. At light loads, the inverter controlling the motor can reduce voltage to reduce result in an average additional load of
magnetic losses and improve efficiency. Peak efficiency is slightly lower than other 3 000 000 x 20 kWh/day = 60 GWh or
designs, but induction motors often achieve higher average efficiency. Another small 10% of the daily electricity production.
trade-off is a slightly lower starting torque than other motors. Induction motors are The additional instantaneous load
commonly used because they don’t use rare-earth magnets and are relatively cheap to on the grid would depend on how many
manufacture. vehicles were charging simultaneously
and the rate of charge, which would
Permanent magnet synchronous motors (PMSMs) depend on the type of charger. Assuming
PMSMs use permanent magnets in the rotor to provide the rotor field, and field windings 500 000 EPMs were being charged
produce an electric field in the stator. The rotor spins at the same speed as the magnetic simultaneously at 7 kW, the additional
field in the windings and speed is controlled by varying the frequency fed to the motor. load on the grid would amount to 3,5
Advantages are high power density and a strong starting torque. The PMSM has become GW. Charging may result in additional
the motor of choice in most of today’s EPMs. Both radial flux and axial flux versions are midmorning and early evening peaks
used. PMSMs make use of magnets that use rare earth materials of which there are both unless a system of smart controlled
a cost and environmental issues. charging were to be introduced.

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