I. How to completely solve the problem of DC motor speed controller frequency converter failure
In our daily use, we often encounter malfunctions in DC motor speed controllers and frequency converters, and how to handle them becomes particularly important.
In fact, extra care should be taken in the routine protection of variable frequency drives.
Scenario 1: Upon discovering a problem and tripping of the frequency converter, immediately opening it for repairs is risky. Improper operation could lead to electric shock. Even when the frequency converter is not in operation, or even if the power supply has been cut off, the capacitors, power input lines, DC terminals, and motor terminals can still carry voltage. After disconnecting the switch, it is necessary to wait several minutes for the frequency converter to fully discharge before resuming operation.
Scenario 2: If the variable frequency drive system trips, immediately use a megohmmeter to perform an insulation test on the motor driven by the inverter, and then determine if the motor is burnt out. This is also very dangerous, as it can easily burn out the inverter. Therefore, never perform an insulation test on the motor or the cable already connected to the inverter before disconnecting the cable between the motor and the inverter.
Special attention should be paid when measuring the output parameters of a DC motor speed controller. Because the output of the inverter is a PWM waveform, which is rich in high-order harmonics, and the motor torque mainly depends on the effective value of the fundamental voltage, when measuring the output voltage, the fundamental voltage value should be measured first. Using a rectifier voltmeter, the measurement result is closest to the measurement value of a digital spectrum analyzer and has an excellent linear relationship with the output frequency of the inverter.
For higher measurement accuracy, an RC filter can be used. Digital multimeters are simple but susceptible to interference, resulting in significant measurement errors. Output current measurement requires the total effective value, including the fundamental frequency and other higher harmonics; therefore, a moving-coil ammeter is commonly used (under motor load, the fundamental current RMS value and the total current RMS value are not significantly different). When a current transformer is chosen for ease of measurement, it can operate at full capacity at low frequencies; therefore, it is necessary to select a current transformer of appropriate capacity.
To extend the lifespan of a speed controller, the key is to follow the correct usage methods:
1. Place the DC speed controller chassis in a well-ventilated place free from corrosive gases.
2. Check if any internal screws have become loose during transportation.
3. Connect the AC power cord and ensure the excitation voltage is connected correctly (positive and negative). Otherwise, it will cause reverse rotation.
4. At the output terminal of the main armature circuit, the + terminal is connected to the positive terminal of the DC motor main armature, and the - terminal is connected to the negative terminal of the main armature.
5. Connect the positive terminal of the speed feedback to the positive terminal of the tachogenerator feedback signal, and connect the negative terminal of the speed feedback to the negative terminal of the tachogenerator feedback signal.
6. Before operation, turn the manual adjustment potentiometer on the panel to the zero position, then apply the power supply voltage, and then rotate the manual adjustment potentiometer knob to start the armature motor.
II. Differences between DC motor speed controllers and frequency converters
DC motor speed controllers are mostly used in applications that control DC motors, while frequency converters are mostly used in applications that control AC motors.
In high-power (50KW and above) production and construction applications, the advantages of using DC motors are their high output power, stable operation, and good speed regulation performance. Initially, AC motors did not perform as well in these aspects. However, currently, the speed regulation and output power of AC motors have gradually improved. Therefore, in modern designs, frequency converters can be used to drive AC motors instead of DC motor speed controllers, with minimal difference in performance.
Variable frequency speed control can only be used for speed regulation, and cannot achieve precise torque control. The reason is simple: in DC speed regulation, the armature and excitation are not coupled but separate, allowing for precise control of the armature current and excitation current. In AC speed regulation, however, the armature current and excitation current are coupled, making precise control impossible. Although current variable frequency speed control features vector control—that is, using modern control theory to decouple the coupled armature current and excitation current in an AC motor through vector conversion, thereby controlling them—this simulates the principle of DC speed regulation.
However, achieving the control characteristics of a DC motor speed controller is currently very difficult. Therefore, DC speed control remains widely used in industries with high torque requirements, such as rolling mills and papermaking. For speed control alone, frequency converters can currently closely mimic the characteristics of DC speed control because the advantages of AC motors are unmatched by DC motors.
