An open circuit in the inverter's braking resistor will affect the braking of the driven mechanical load. Because the braking resistor is open, the electrical energy applied across it cannot be released, effectively rendering the braking function ineffective.
Here's a brief introduction to the function of the braking resistor in a frequency converter, also known as a brake resistor. What is the purpose of the braking resistor in a frequency converter? When the motor driven by the frequency converter stops, the motor may not stop rotating immediately due to inertia. At this time, the motor acts like a generator, and the electrical energy generated by the generator is applied to the internal modules of the frequency converter, potentially causing damage or destruction. The braking resistor is used to dissipate this electrical energy.
★Under normal circumstances, for frequency converters with an output power below 7.5KVA, the braking resistor is installed inside the frequency converter (built-in); for frequency converters with a power greater than 7.5KVA, the braking resistor is generally external. For example, Delta frequency converters with a power of several hundred watts also require an external braking resistor.
The selection of the braking resistor for a frequency converter depends on the output power, braking time, and number of braking cycles. This includes the resistance value and power rating of the braking resistor. For example, an external braking resistor for a 5.5KVA frequency converter requires a resistance value ≥22Ω, but the power rating is 800W. Such a high-power resistor is bulky and generates a lot of heat during operation. Therefore, it cannot be directly connected to the frequency converter. When installing it, its heat generation should be taken into full consideration, and it should be kept away from the frequency converter.
For a 55KVA frequency converter, its braking resistor consists of two 5.5KW resistors connected in series, resulting in a total power of 11KW. It is recommended that the braking resistor value be ≥5Ω×2. Furthermore, it requires forced cooling using a professional axial fan, and all connecting wires at both ends must be heat-resistant.
The functions of the main circuit wiring terminals of the three-phase frequency converter are shown in the figure below.
Generally, PB and (+) are the terminals for connecting an external braking resistor. There is no positive or negative polarity when connecting the braking resistor; simply connect a wire with a sufficient cross-sectional area between these two points. Additionally, for the braking resistor, you need to access the function code to set the DC braking time and the DC braking wait time during shutdown.
★Generally, an open circuit in the braking resistor is caused by selecting a braking resistor with insufficient power and poor heat dissipation.
During braking, almost all of the motor's regenerative energy is consumed by the braking resistor. Here is a recommended calculation formula: U×U/R=Pb, where U is the voltage for stable braking of the system (the value of U varies for different systems; for a 380Vac system, it is generally taken as 700V), and Pb is the power of the braking resistor.
