Battery Industry Segment is Heating Up! Power Supply and Voltage of EV Motor

Type of Motor for EV

The EV shift of the car which Europe, the United States and China are pushing forward also with the backing of the government. In Japan as well, Toyota, Nissan, Mitsubishi Motors and others are at the forefront of developing electric vehicles (EVs) at a rapid pace. I think everyone knows that Nissan has already sold Leaf etc.
Well, EV is powered by a battery and a motor instead of a gasoline engine. This motor is different depending on the manufacturer.
First, we introduce what kind of motor there are. Motors can be broadly divided into "DC motors" that operate on DC power supplies and "AC motors" that operate on AC power supplies.
DC motors are further divided into " Brushed DC motors" and " Stepper motors". On the other hand, there are " Brushless motors" for AC motors. These motors consist of a "rotor" that rotates during the passage of a current, and a "stator" that rotates the rotor. Based on this, we introduce the features of each motor listed above.
At first, we explain a brushed DC motor. The brushed DC motor uses coils (electromagnets) for the rotor and permanent magnets for the stator. As the rotor rotates, the magnetic field changes automatically. It will continue rotating semi-permanently as long as the current flows. Because the structure is simple, the cost is low. Disadvantages include the need for a brush at the commutator that is in contact with the power supply, which can include friction noise and brush wear.

This image is structure of Brushed DC motor.

On the other hand, unlike a brushed DC motor, a Stepper motor uses a permanent magnet for the rotor and an electromagnet for the stator. Place multiple electromagnets on the stator and turn on only one of them.

This image is structure of stepper motor. This image is structure of stepper motor.

Since the rotor approaches the electromagnet in response to the magnetic poles of the rotor, at that moment, the electromagnet is turned off and the electromagnet ahead of the direction of rotation is turned on. By repeating this, the rotor keeps rotating and the rotation speed is controlled. However, for smooth control, many electromagnets are required, and the switching control becomes difficult.

Overcoming these weakness, brushless motors are used in EVs. It operates on a three-phase AC power supply, and contrary to a brush motor, the rotor is a permanent magnet and the stator is an electromagnet.

This image is structure of brushless motor.

In this case, there is no need for a brush, so neither noise nor wear of the brush will occur, and the control of the electromagnet can be (slightly) easier. On the other hand, a mechanism to control the stator side operated with a three-phase AC power source is required. In particular, since it is necessary to apply an alternating current in accordance with the rotational speed of the rotor, a "rotational position sensor" is required to detect how much the rotor is rotating.
In the case of EV, it is also necessary to control the flow of alternating current depending on how the accelerator is depressed, so an accelerator position sensor is also required. Based on the values from these sensors, the ECU (Electronic Control Unit) adjusts the frequency and the amount of current so that the rotation speed, torque, etc. become appropriate.

This image is relationship between drive voltage and torque rotor rotational speed.

Also, the higher the drive voltage, the higher the torque and the faster the rotor will rotate. Each auto manufacturer is continuing development to raise the starting voltage.

Requirements for EV Power Supply

What are the factors required for a power supply for an EV motor?
There are four things categorised roughly. They are voltage, electrical capacity, characteristics and safety.
First is the voltage. Lithium-ion batteries are the mainstream battery cells for EVs, but this is only 3.6 V per cell. Normally, four modules are connected in series and in parallel, and one with 7.2V is used as one module.
Depending on the manufacturer, these modules are connected in series and used as 280V, 360V, etc. For example, at the Nissan Leaf, a neodymium permanent magnet embedded three-phase AC synchronous motor "EM61" is operated at a drive voltage of 345 V.
Second is the electrical capacity. If this is large, the cruising distance will increase, so it is an important factor. It is 40 kWh for the October 2017 model of the Leaf, and the EPA coverage is said to be 241 km.
Overseas, BMW's i3 has 33 kWh and has an EPA coverage of 182 km. The top manufacturer of electric vehicle is Tesla Motors. Their Model S has the smallest one as 60 kWh and the largest one as 100 kWh battery. The model S also supports 110, 220 and 440 V chargeable voltages.

automobile manufacturertype of vehicletypeEV driving mode
(JC08)
EV driving mode
(EPA)
capacity of a batteryquick charge
BMWi3 (94 Ah)BEV39018233Yes
HondaFitEV
(end of life at 03/2016)
BEV22513120Yes
Mitsubishii-MiEV XBEV1809916Yes
Mitsubishii-MiEV MBEV12066
(undisclosed)
10.5Yes
NissanLeaf, New Model (40 kWh)BEV40024140Yes
NissanLeaf, Old Model (30 kWh)BEV28017130Yes
TeslaModel S 75BEV(undisclosed)398.475Yes
TeslaModel S P100DBEV(undisclosed)504100Yes
TeslaModel X 75DBEV(undisclosed)380.875Yes
TeslaModel X P100DBEV(undisclosed)462.4100Yes
Volkswagene-GolfBEV30120135.8Yes

Third, these are only lithium-ion batteries and have inherited of the electrical characteristics. The temperature rises due to charge and discharge, and the output voltage decreases when the temperature exceeds 70 degree Celsius. The internal resistance increases when it falls below -30 degree Celsius.
Finally, there are also safety issues. Liquid leakage may occur due to the use of electrolytes. It is essential to take measures against leaks in the module, as it will cause problems if it occurs while driving. In addition, since the vehicle is also required to have safety in the event of a collision, it is necessary to have a level that does not leak even in this case, resulting in increased weight and cost.
Because it is a very high voltage compared to 100 V for household use, measures such as leakage are also required. The 12 V battery installed in a gasoline vehicle makes it easy being not able to compare. Including motors that require high voltage, EV is a collective entity of parts of various voltages ranging from those requiring high voltage to low voltage. Safety tests also need to be done with power supplies that support various voltages.

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Reference (Japanese site)

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