AC motor drives and DC motor drives are two major types of drives, both of which have their characteristics and advantages. But do you know how to make a choice between these two drives? The following will answer for you.
The DC drive is a speed control system applied to DC motors. The speed of a DC motor is proportional to the armature voltage and inversely proportional to the motor flux, so the armature voltage or field current can be used to control the speed. There are many different types of DC motors, and their control methods are also different.
Generally, a DC drive converts alternating current (AC) to direct current (DC) to run a DC motor. Most DC drives use several thyristors (also called SCRs) to produce a half-cycle DC output from a single-phase AC input (known as the half-bridge method). More advanced uses 6 SCRs to generate a DC output from a three-phase AC input (called a full bridge). Therefore, in the full-bridge method, we have two SCRs for any input phase.
SCR is like a "one-way" switch controlled by a gate input. Applying a low voltage to the gate activates the SCR. When voltage is applied to the network with different input phase angles, the output current will change, so that the drive can control the speed of the motor. In order to check the motor speed and compensate if necessary, most DC drives require a tachometer as motor feedback. A tachometer or "tachometer" is basically a small permanent magnet DC motor connected to the main motor shaft. A higher engine speed will produce a higher voltage in the tachometer, which the frequency converter will refer to to ensure that the engine is running at the correct speed set by each user. Small DC motors have a permanent magnetic field, while large DC motors have a separate coil inside the motor, called a magnetic field, which eliminates the need for permanent magnets in the engine. A DC drive with an exciter output usually has a separate smaller circuit to power the excitation coil.
On the input side, the AC drive is similar to the DC drive in that the AC input is set to DC by SCR or a simple bridge rectifier. According to the phase frequency of the AC input, the DC output is half a cycle, so the AC drive uses a capacitor bank to stabilize and smooth the DC voltage. In the output part of the driver, the motor is powered by 6 output transistors or IGBT modules. In simple terms, the inverter converts AC input power to DC power, and then converts the DC power back to AC power to power the motor.
In addition, the AC input takes 50 or 60 Hz as the cycle. When the driver converts the generated DC voltage back to AC current, for more complex drivers, it uses a carrier frequency of at least 2KHZ to 100KHZ. Therefore, the output current can be easily increased by tens or hundreds of times without burning the motor coils. This function also allows the AC motor to change gears quickly without any problems. Inverters usually have a variety of feedback, from simple 2-wire incremental encoders to very high-resolution resolvers or absolute encoders, which can help the drive accurately and very accurately calculate the speed, speed, and motor shaft angle. Some more powerful drives have a third cycle called regeneration. When the motor changes from a very high speed to a lower speed (or even zero speed) in a short time, the circuit converts the inertia of the motor and the load into alternating current and sends it back to the inlet line. This saves electricity and improves energy efficiency.
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