This article mainly explains why a reactor is needed before a servo amplifier . First, it introduces the function and principle of the servo amplifier, and then explains why a reactor is needed before a servo amplifier. Follow along to learn more.
The function and principle of servo amplifiers
The function of a servo amplifier is to combine and amplify multiple input signals and feedback signals. Depending on the polarity of the combined signal, it outputs a corresponding signal to control the servo motor to rotate forward or backward. When the input signal and feedback signal are balanced, the servo motor stops rotating, and the output shaft of the actuator stabilizes at a certain position.
The servo amplifier group consists of a preamplifier, trigger, thyristor main circuit and power supply, as shown in the figure below.
To accommodate complex multi-parameter adjustments, the servo amplifier is configured with three input signal channels and one position feedback signal channel. Therefore, it can simultaneously accept three input signals and one position feedback signal. In simple single-parameter adjustment systems, only one input channel and one feedback channel are used.
In the servo amplifier, the preamplifier combines the three input signals and one feedback signal into a deviation signal, which is then amplified into a voltage signal U22-21 for output. This output voltage is simultaneously converted into trigger pulses by trigger 1 (or 2) to control the thyristor in the thyristor main circuit 1 (or 2) to conduct, thereby applying AC 220V power to the windings of the two-phase servo motor and driving the two-phase servo motor to rotate. When Δ1>0, U22-21>0, trigger 2 and main circuit 2 operate, and the two-phase servo motor rotates forward; when Δ1<0, trigger 1 and main circuit 1 operate, and the two-phase servo motor rotates in reverse. The circuit composition and parameters of the two sets of triggers and the two sets of thyristor main circuits are exactly the same. Therefore, when the input signal and the position feedback current If are balanced, the output U22-21 of the preamplifier is approximately 0, neither trigger outputs a trigger pulse, the thyristors in main circuits 1 and 2 are blocked, the power supply to the two-phase servo motor is disconnected, and the motor stops rotating.
Therefore, it can be seen that the servo amplifier is equivalent to a three-position contactless relay and has a large power amplification capability.
Analysis of why a reactor is needed before a servo amplifier
Servo amplifiers are a crucial component of motion control systems, providing adjustable power to servo motors and enabling high-precision positioning of the transmission system . They are currently high-end products in transmission technology. Servo amplifiers, which can cost anywhere from several thousand to tens of thousands of yuan, should be used with extreme care to extend their lifespan as much as possible.
Sudden voltage changes in the power grid and operational overvoltages can cause current surges, harmonic interference, and voltage spikes, all of which can severely damage sensitive components in the servo amplifier circuit. Adding a reactor (called a line reactor) before the servo amplifier can protect the drive unit from these shocks, as shown in Figure 1.
Figure 1 Incoming line reactor
1. Reactors and their functions
A reactor, as the name suggests, is a device that acts as an impedance in a circuit. It is made by winding a wire into a solenoid and inserting an iron core inside the solenoid; this results in a large inductance, thus creating a reactor, which is essentially an inductor.
The ratio of the voltage drop across the reactor to the line voltage is called the reactance, which characterizes the reactor's parameters. The reactor's impedance and reactance (%impedanceofactor) are shown in the formula in Figure 2. Typical reactance values are 3% or 5%.
Figure 2 Impedance calculation and reactance formula
As can be seen, the higher the frequency of the current passing through the reactor, the greater the impedance. This is the principle behind how reactors can smooth voltage spikes and suppress harmonics (click the blue text to see harmonics).
Furthermore, as anyone who has studied harmonics knows, servo amplifiers and frequency converters are significant sources of harmonics and major culprits in power grid pollution. Adding a reactor can both block interference from the power grid and protect the servo amplifier, and also reduce the pollution of the power grid by harmonic currents generated by the servo amplifier's rectifier unit.
2. Reactor symbol
In electrical diagrams, reactors are represented by the same symbol as inductors, L. Figure 3 shows the main power input circuit diagram of the FANUC βiSVSP servo amplifier, where L1 is the input reactor.
Figure 3. Main power input circuit diagram of a servo amplifier
3. Output reactor
A reactor is installed between the frequency converter and the motor, and is called a load reactor or output reactor to protect the motor, as shown in Figure 4.
Figure 4 Output Reactor
The detailed principle is shown in Figure 5.
Figure 5. Schematic diagram of the connection between the reactor and the drive system
How effective are reactors in smoothing current, and how effective are reactors in this process? As shown in Figure 6, reactors can smooth transient voltage spikes and suppress harmonics.
Figure 6 Comparison of effects with and without reactor
4. Power Filter
Electromagnetic interference (EMI) is also a major problem for servo drive systems. Therefore, a power filter is installed at the input of the power module along with the line reactor to suppress electromagnetic interference, as shown in Figure 7.
Figure 7 shows the use of a power filter.
FANUC servo amplifier manuals often mention ACreactor and AClinefilter. Do you understand their function now? Why add a reactor before the servo amplifier? It's to suppress harmonic interference from the power grid, smooth out spikes in the power supply voltage, improve the power quality of the servo amplifier, and protect sensitive components in the circuit.
