The selection of inverter capacity is a critical and complex task that requires careful consideration. It's essential to ensure proper matching between the inverter and the motor. Choosing an inverter with too small a capacity can lead to reduced effective torque output, disrupt normal system operation, and even cause damage to the equipment. Additionally, it may increase harmonic distortion in the current, which could raise overall equipment costs.
**Principles for Inverter Capacity Selection**
When choosing an inverter, the following basic principles should be followed:
**1. Matching Principle**
- **Power Matching**: The inverter’s rated power should match the load’s requirements. For example, motors with the same power might require different inverter capacities depending on their load characteristics. A square-torque load (like a fan) typically needs a smaller inverter than a constant-torque load.
- **Voltage Matching**: The inverter’s rated voltage must align with the motor’s rated voltage.
- **Current Matching**: For standard loads like centrifugal pumps, the inverter’s rated current should match the motor’s. For special loads, such as deep well pumps, the inverter must account for the motor’s maximum current and overload capability.
- **Torque Matching**: This becomes important when using constant-torque loads or gearboxes.
**2. Economic Consideration**
It’s crucial to perform both technical and economic analyses to choose a control scheme that meets application needs while offering a good cost-performance ratio.
**3. Situation-Specific Analysis**
Different applications require unique evaluations. When selecting an inverter, consider the motor’s specifications, the type of load, and operating conditions.
- Choose the inverter based on the product specification, especially for standard loads like fans or pumps.
- Select the appropriate inverter size—first or second gear—based on the motor’s capacity.
- If the inverter is selected based on rated power, ensure that the motor’s rated current is not exceeded. Otherwise, the inverter might be undersized.
- Using the motor’s rated current as a basis for inverter selection may result in oversized inverters, leading to inefficient operation and wasted resources.
- Consider the motor’s maximum operating current and overload time, especially in fluctuating load scenarios. The inverter should not exceed its rated current at peak times or allow overloads above 150% for more than 1 minute.
**4. Proper Inverter-Motor Matching**
Ensure the motor’s rated or maximum operating current is less than the inverter’s rated current. For example, if you’re using a high-load application, you may need a larger inverter to accommodate fluctuations and prevent overheating.
**5. Impact of Motor Starting Current**
Reducing inverter capacity may affect the starting and acceleration currents. To mitigate this, consider adding an output reactor to smooth out current spikes and improve system stability. Adjust acceleration/deceleration times and set lower U/f ratios during startup to reduce stress on the inverter.
**Steps for Inverter Capacity Selection**
1. Understand the load type and current behavior, then calculate or plot the load current over time.
2. Pre-select an inverter based on the calculated values.
3. Verify the pre-selected inverter for overload and starting capabilities. If it passes, finalize the selection; otherwise, repeat the process until a suitable option is found.
Smaller inverters are more economical, provided they meet all operational requirements.
**Inverter Capacity Selection Methods**
There are several approaches to selecting the right inverter capacity:
**(1) From the Current Perspective**
The inverter’s rated current is the most critical factor in determining its load-handling capability. Ensure the load current does not exceed the inverter’s rated current. Understanding the motor’s parameters and process conditions is essential. For instance, submersible pumps and wound-rotor motors have higher rated currents than standard squirrel-cage motors, and some industrial motors can handle short-term stall conditions. Total load current must remain within the inverter’s limits.
**(2) From the Efficiency Perspective**
System efficiency is the product of the inverter and motor efficiencies. To maximize efficiency:
- Match the inverter power to the motor power.
- Choose an inverter slightly larger than the motor power if the motor operates under heavy load or frequent start-stop cycles.
- If the motor has surplus power, consider a smaller inverter, but check for potential overcurrent issues.
- Adjust energy-saving settings when inverter and motor power differ.
**(3) From the Power Calculation Perspective**
For continuous operation, the inverter must meet the following criteria:
1. Load Output: PCN ≥ PM / η
2. Motor Capacity: PCN ≥ k * 3 * Ue * Ie * 10^-3
3. Motor Current: ICN ≥ k * Ie
Where:
- PCN = Inverter capacity (kVA)
- PM = Motor shaft output power (kW)
- Ue = Motor rated voltage (V)
- Ie = Motor rated current (A)
- η = Motor efficiency (~0.85)
- cosφ = Motor power factor (~0.75)
- k = Current waveform compensation coefficient (usually 1.05–1.1)
By following these guidelines, you can ensure safe, efficient, and cost-effective inverter selection.
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