I. Differences between DC speed controllers and frequency converters
(I) Working Principle
DC speed controllers achieve speed regulation by changing the voltage between the motor's electrodes or by changing the number of electrodes. DC speed controllers are electrode-based speed controllers, so their speed range is limited by the number of electrodes. They are commonly used in low-power load applications such as machine tools.
Variable frequency drives (VFDs) achieve speed regulation by changing the frequency of AC power supply voltage. They convert electrical energy from AC power into the electrical energy required by the motor, thus enabling a wide range of speed adjustments. Furthermore, due to their ability to provide variable frequency speed control and active reactive power compensation, VFDs are becoming increasingly important in power systems and are considered a crucial means of power system regulation and energy conservation.
(II) Characteristics
Due to their different working principles, DC speed controllers and frequency converters have different characteristics in practical applications.
1. Control performance
DC speed controllers offer fast response and high reliability, but their speed control accuracy is lower and their speed range is smaller. In contrast, frequency converters provide high precision, a wide speed control range, and better steady-state and dynamic characteristics, making them suitable for various control requirements.
2. Energy Consumption
DC speed controllers operate with unidirectional devices, cannot regenerate electrical energy, generate a lot of heat, and have low efficiency. In contrast, frequency converters, by employing energy feedback technologies such as PWM converters, can regenerate a portion of electrical energy, reducing energy consumption and improving efficiency.
3. Applicable load
DC speed controllers are commonly used for speed regulation of loads in the low power range. Variable frequency drives (VFDs), on the other hand, are suitable for speed regulation of various loads, especially for high-power speed control applications.
II. Voltage Testing Methods for DC Speed Controller Repair
The synchronous input voltage is a 50Hz AC sinusoidal voltage. After current limiting (amplitude limiting) by R5 and R6 and rectification by P2, it becomes a 100Hz pulsating DC. Due to the clipping effect of the diode on the input side of U1, a trapezoidal voltage is actually formed at point a (between pins 1 and 2 of U1), with the trough of the trapezoidal wave corresponding to the zero-crossing point of the power grid. The peak value of this voltage is the forward voltage drop of the diode on the input side of U1, which is approximately 1.2V. Because the zero-crossing time of the power grid is extremely short, but with the threshold voltage of the LED on the input side of U1 and the limitation of the forward voltage drop of P2, the low-level time of the trapezoidal wave is "widened". It can be estimated that the voltage between pins 1 and 2 of U1 is slightly lower than 1.2V, and should be approximately 1V DC.
The input diode of U1 is mostly forward-biased, and the output transistor is mostly conducting, with only a brief cutoff at the zero-crossing of the mains power. Therefore, the peak voltage at point B is +9V (at the moment U1 is cut off), and the lowest voltage is 0V (ignoring the saturation voltage drop of the internal transistor of U1). The waveform is close to a rectangular wave, but may have a certain rising slope. Thus, the voltage at point B to ground is close to 0V and below 1V, with an estimated DC voltage of approximately 0.4V. Furthermore, the method for detecting whether U1 outputs a synchronization pulse voltage is simple and effective: short-circuit pins 1 and 2 of U1 with tweezers; if the voltage at point B rises to +9V, and then drops below 1V after releasing the tweezers, it can be determined that the U1 circuit is good, the synchronization voltage is being input normally, and the synchronization pulse signal is being output normally.
The N1 voltage comparator circuit has noise reduction and input voltage shaping functions. Its output is a rectangular pulse voltage corresponding to the waveform at point c, and the measured DC voltage value should be approximately 0.4V. The DC voltage at pin 9 of N1 should be 5.4V, the voltage division value of R12 and R13, with an output of 0.4V. To determine if this stage transmits signals normally, when pins 9 and 10 are shorted with a wire or tweezers, pin 8 should show 0V. Releasing the short circuit should result in a rise to 0.4V. If the output voltage is measured using an AC setting, shorting pin 8 with tweezers should result in 0V. Releasing the short circuit should result in a significant rise in the output voltage at pin 8, such as 3V or higher. The sawtooth wave forming capacitor C3, controlled by the charging and discharging of V4 and V3, forms a sawtooth wave voltage at point d. The maximum amplitude (peak value) of the sawtooth wave voltage can be referenced to the highest voltage value given for the speed (it should be lower than the given maximum value), approximately 7V. The minimum voltage amplitude is approximately 0V (ignoring the saturation conduction voltage drop of V3). If the triangular gap and the area of the sawtooth wave are considered approximately equal, then the measured DC voltage at point d should be around 3.5V. A voltage above 7V or below 1V indicates that a sawtooth wave voltage is not formed at that point. The presence or absence of this voltage can also be determined by shorting the two input pins of N1. When shorted, the voltage at point d rises to above 7V; when the shorted is removed, it drops to around 3V, indicating that the sawtooth wave is being output normally at that point.
N2 is an adjustable pulse width processing circuit. When the sawtooth wave voltage at pin 5 is normal, adjusting the speed set voltage at pin 6 should cause the output pin to change in the opposite direction to the set voltage. When the set voltage is 0~8V, the output should change approximately from 8~0V. By adjusting the voltage at pin 6 and measuring the corresponding change at the output pin, it is easy to determine whether the circuit is working properly.
