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The importance of inverters in electric vehicle applications

Apr 06, 2021

The importance of inverters in electric vehicle applications

The core technology of new energy vehicles that is different from traditional fuel vehicles is the "three electrics"-electric drive, battery, and electronic control. Among them, the inverter device has become important in the field of electric vehicles. Without it, electric vehicles can't run at all, and the performance of the inverter directly affects the price of electric vehicles. So what is this little thing for? Learn about.

First popularize the basic knowledge of Sanden, DC and AC:

Among them, the electric drive is composed of three parts: transmission mechanism, motor, and inverter.

Briefly introduce AC and DC:

The characteristics of alternating current AC: both size and direction change periodically. Alternating current is a dangerous voltage in the domestic and civil voltage of 220V and the general industrial voltage of 380V. Its most basic form is sinusoidal current, and the standard frequency of AC power supply in our country is specified as 50Hz.

The characteristics of direct current DC: the direction does not change over time. Direct current is generally widely used in various small household appliances such as flashlights (dry batteries) and mobile phones (lithium batteries). Dry batteries (1.5V), lithium batteries, accumulators, etc. are called DC power supplies, all of which are lower than 24V.

If we want to truly understand the role of the inverter, we must first know the principle of the on-board power battery.

New energy vehicles can run because the motor drives the wheels, and the power of the motor comes from the battery, but the power battery is stored in direct current, and the motor uses alternating current. AC motors must rely on sine wave alternating current to drive rotation. But what the vehicle-mounted power battery can output is direct current, and the role of the inverter is to convert direct current into sine wave alternating current, and it also controls the speed and torque of the AC motor. Therefore, if you want to convert DC to AC operation, you must rely on inverters.

Therefore, for an electric vehicle equipped with an AC induction motor, the high-voltage DC output from the battery pack must be converted into a sine wave AC with a controllable amplitude and frequency through an inverter to drive the vehicle.

The sine wave is obtained through the evolution of the square wave. First understand the formation of a lower wave. Please see the circuit diagram, this magical circuit is called Full Bridge Inverter, a full bridge inverter circuit. Its structure is very simple, consisting of four switches (S1-S4). A and B are the positive and negative poles of the output terminal of the circuit.

Through the switch control, the current flow is reversed, and by continuously closing the switch, a square alternating current is generated. The frequency of our daily household 220V power supply is 50Hz, which means that we need to switch 100 times per minute. No one can control such a high frequency, so it needs to be connected to a field effect tube, such as an IGBT or MOSFET. This electronic component can switch thousands of times per minute.

Through the switch control of the field effect tube, the square wave we need can be obtained, but what we want is a sine wave. Here is a technical term-pulse width modulation.

Imagine if we averaged the pulses per unit time, it would become?

This is a graph that is very close to a sine curve. The more precise the pulse, the higher the switching frequency, and the smoother the resulting curve. We can get a smooth sine wave curve by modulating the pulse train through a comparator.

There is also a method called heavy voltage inversion technology-adding capacitance and inductance in the circuit to smooth the curve. Capacitors are used to smooth the voltage curve, and inductors are used to smooth the current curve. It is like adding a small-capacity reservoir (secondary buffer) to the circuit. The capacitor is equivalent to a battery that can be charged and discharged instantaneously. It can absorb voltage pulses and smooth the output curve. As mentioned above, only one set of voltages can achieve sine wave output. If multiple sets of voltages are used for modulation, a more precise sine wave curve can be obtained, and the control accuracy is also more precise. This method is mostly used in wind turbines or electric cars.

To put it simply, the Power Inverter is a kind of power converter that can convert DC12V direct current into AC220V, which is the same as the mains, for general electrical appliances. It is a convenient power converter for vehicles. If the inverter of an electric vehicle can support a higher voltage, the corresponding voltage charging current is larger and the power is larger, which means that the same current is charged, and the charging power can be scaled up, that is, the charging time will be shortened.

If the support voltage of the inverter is increased, the heat generated by the inverter during the corresponding charging will increase, so it is necessary to solve the heat dissipation problem of the IGBT module in the inverter. This is a key issue to improve the charging efficiency. At present, Toyota in Japan This research is more in-depth, such as its application of silicon carbon technology.

In addition, the performance of the inverter directly determines the performance of the motor, which is also the core technology of major new energy automobile companies. Therefore, the mastery and breakthrough of inverter technology will become the core technology of new energy vehicle products just like the gearbox technology in the era of fuel vehicles. With the development of a new generation of semiconductor power devices, it can be seen that IGBT and SiC are the mainstays of future motor control systems and charging piles.

IGBT is an inverter module in the electric drive system, inverting the direct current of the power battery into alternating current and providing it to the driving motor. It accounts for about 40% of the cost of new energy vehicle motor drive systems and on-board charging systems, and its performance directly determines the energy efficiency of the vehicle.

The loss of SiC power devices is about 50% of that of Si devices, and is mainly used to realize small-weight and light-weight drive systems such as electric vehicle inverters.