Choosing the right inverter braking resistor is essential to ensure effective braking and prevent damage to the equipment. The first step is to calculate the braking power and draw an accurate braking curve. Based on this curve, you should determine the braking cycle and the required braking power. Once these values are established, you can select the appropriate braking resistor by considering factors such as reference voltage and resistance. It’s important to note that the selected resistor must not be lower than the value specified in the manufacturer's manual; otherwise, it could directly damage the inverter.
Sometimes, the braking power might not be clearly defined, or you may want to prioritize safety. In such cases, it's advisable to choose a resistor with a higher braking capacity. For example, Siemens' MM4 series braking resistors are designed for a 5% braking cycle. When using these, make sure to set parameter P1237 to match the braking cycle, and set P1240 to 0 to disable the DC voltage controller.
Calculating the braking cycle can sometimes be confusing. A 5% braking cycle means that the resistor can handle 100% of the braking power for 12 seconds, after which it needs to cool down for 228 seconds. If the braking time is shorter than 12 seconds or the power consumed is less than 100%, the inverter will calculate the i²t value of the resistor. If the braking period exceeds 5%, the 440 model allows for a longer cycle, but precise calculation can be challenging. For instance, if a frequency converter brakes five times per minute for 2 seconds each time at 50% power, it’s generally recommended to choose a slightly larger resistor than the theoretical calculation and adjust the braking cycle accordingly in parameter P1237.
Here’s an example for better understanding: Suppose a 7.5 kW inverter requires 5 brake cycles per minute, each lasting 2 seconds with 50% braking power. This equals 40 seconds of braking in 240 seconds, which translates to 20 seconds of equivalent time. The braking cycle would then be approximately 8%. The calculated braking power would be around 625W, so a 750W resistor is chosen, and the braking cycle is set to 10% in parameter P1237.
The formula used is: (2 * 5 * 50%) / 60 = 7.5. Based on this, you should always choose a resistor with a higher rating to ensure reliability and safety.
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