With the rapid advancement of power electronics technology, new control theories have continuously emerged and evolved, driving significant progress in frequency conversion technology. In recent years, as the functionality of inverters has expanded and improved, their application fields have broadened considerably. Initially used for speed control in fans and pumps, inverters are now widely applied in various industrial scenarios that demand high precision and fast response, making them essential components in modern automation systems.
The ABB ACS150 series inverter is specifically designed to meet the diverse needs of the machinery industry. It offers a wide range of applications through optimized configuration options, making it suitable for controlling devices such as fans, pumps, gates, material handling systems, and conveyor belts. This inverter is known for its reliability and performance, especially in environments where precise speed regulation is critical.
### 1. Frequency Conversion Principle
The rotational speed of an induction motor can be calculated using the formula:
$$ n = \frac{60f(1 - s)}{p} $$
Where:
- $ n $: Motor speed (in RPM)
- $ f $: Supply frequency
- $ s $: Slip rate
- $ p $: Number of pole pairs
From this equation, it’s clear that the motor speed is directly proportional to the supply frequency. By adjusting the frequency, we can effectively control the motor speed. When the frequency ranges from 0 to 50 Hz, the motor's speed adjustment range becomes quite extensive. Frequency conversion involves changing the frequency of the power supplied to the motor, allowing for smooth and efficient speed control.
The inverter operates by first converting the AC power from the grid into DC power using a rectifier. The DC power is then converted back into AC with adjustable frequency and voltage to drive the motor. The inverter circuit typically consists of four main parts: rectification, intermediate DC link, inverter, and control. The rectifier usually uses a three-phase bridge configuration, while the inverter section employs IGBT-based three-phase bridges to generate PWM waveforms. The intermediate DC link serves to filter, store energy, and manage reactive power.
In practical applications, frequency converters must adjust both frequency and voltage. If only the frequency is changed without adjusting the voltage, the motor may suffer from magnetic saturation or weak magnetization, potentially leading to overheating or damage. To maintain constant magnetic flux, the voltage should be adjusted proportionally with the frequency. However, at low frequencies, even with reduced voltage, the AC impedance decreases, causing waveform distortion and reduced torque.
### 2. Inverter Selection
Selecting the right inverter is crucial for ensuring the stable operation of the entire system. The choice depends on factors such as the type of mechanical equipment, load characteristics, speed range, accuracy, starting torque, and environmental conditions. The concept of "fitness" refers to achieving the best cost-performance ratio while meeting the specific requirements of the application.
#### 2.1 Square Torque Load
Fans and pumps typically operate under square torque loads. Most of these applications can be handled by inverters operating in U/f=const mode, where the output voltage and frequency are kept in a fixed ratio. Below are key considerations when selecting such inverters.
##### 2.1.1 Avoid Overloading
Fans and pumps are generally not prone to overloading. When selecting the inverter, ensure its capacity is slightly higher than the motor’s. While the overload capability of the inverter is lower, care must be taken during parameter setup. Since the torque is proportional to the square of the speed, exceeding the motor’s rated frequency can cause excessive torque, leading to motor overload. Therefore, the maximum operating frequency must be strictly limited.
##### 2.1.2 Matching Acceleration and Deceleration Times
Due to the high inertia of fan and pump loads, matching the inverter’s acceleration and deceleration times with the system’s start and stop requirements is critical. Calculating the appropriate time based on load parameters helps prevent overcurrent or overvoltage trips. If production demands require shorter start-up times, the inverter may need to be redesigned accordingly.
##### 2.1.3 Avoid Resonance
Changing the motor speed via frequency can sometimes lead to resonance with the load system’s natural frequency, causing vibrations that may damage equipment. To avoid this, the inverter’s jump frequency and width should be set according to the system’s resonance frequency.
##### 2.1.4 Pressure and Water Hammer Effects
Pump loads are susceptible to pressure fluctuations and water hammer effects. Proper inverter settings are necessary to mitigate these issues. For example, limiting the minimum frequency during low-speed operation can prevent cavitation, while proper deceleration and shutdown procedures can avoid reverse rotation and potential damage.
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