With the increasingly widespread application of frequency converters, Delta frequency converters have seen their market share grow significantly thanks to their strong OEM capabilities and continuous R&D. The selection of frequency converter components plays a crucial role in the user's initial design and selection process. This article mainly introduces the basic principles and methods for selecting braking resistors for Delta frequency converters. 1. Introduction Currently, there are roughly three braking methods for frequency converters on the market: regenerative braking, DC braking, and regenerative braking. Delta frequency converters are uncontrolled rectified voltage source type frequency converters, and their braking methods fall under regenerative braking and DC braking. Regenerative braking is the main form of rapid deceleration or stopping of production machinery during operation using Delta frequency converters; DC braking outputs a DC current to generate torque and force the motor to stop at the moment the motor is ready to start, achieving smooth starting characteristics, or outputs a DC current to generate torque and force the motor to stop when the frequency converter stops, ensuring accurate motor stopping. In variable frequency speed control systems using Delta frequency converters, deceleration is achieved by gradually reducing the given frequency. During the frequency reduction process, the motor will be in regenerative braking state (generator state), causing the motor speed to decrease rapidly with the frequency reduction. During braking, the generated pump voltage causes the voltage on the DC bus to rise. At this time, the frequency converter will control the braking unit to dissipate the increased voltage as heat energy through the braking resistor. To ensure smooth system deceleration, an appropriate deceleration time needs to be set, and suitable braking resistors and braking units need to be selected. Currently, there are many methods for calculating braking resistors. From an engineering perspective, accurately calculating the resistance and power of the braking resistor is not practical in actual applications, mainly because some parameters cannot be accurately measured. The commonly used method is estimation. Since each manufacturer's calculation method is different, the results are not consistent. The calculation method introduced in this article is for reference only; specific situations require analysis and calculation based on each site's usage. 2. Introduction to Braking Resistors The braking resistor is used to dissipate the regenerative energy of the motor as heat energy. It includes two important parameters: resistance value and power capacity. In engineering applications, corrugated resistors and aluminum alloy resistors are commonly used. Corrugated resistors feature vertical corrugations on the surface, which facilitates heat dissipation and reduces parasitic inductance. They also use a highly flame-retardant inorganic coating to effectively protect the resistance wire from aging and extend its service life. This is the type of resistor that Delta originally supplied. Aluminum alloy resistors are easy to install tightly and have easy-to-attach heat sinks. They are aesthetically pleasing, and the fully enclosed aluminum alloy casing provides excellent heat dissipation, vibration resistance, weather resistance, and long-term stability. They are small in size, have high power, are easy and stable to install, and are aesthetically pleasing. They are widely used in harsh industrial environments. [b]3 Calculation of Braking Resistor Resistance and Power 3.1 Braking Utilization Rate ED%[/b] Braking utilization rate ED% is the same as the braking utilization rate ED% in Delta's manual. Braking utilization rate ED% is defined as deceleration time T1 divided by deceleration period T2. The braking utilization rate is mainly to allow the braking unit and braking resistor sufficient time to dissipate the heat generated by braking. When the braking resistor heats up, the resistance value will increase with the temperature rise, and the braking torque will decrease accordingly. Braking utilization rate ED% = braking time / braking cycle = T1/T2 * 100%. (Figure 1) Figure 1 Definition of braking utilization rate ED% Now let's use an example to illustrate the concept of braking utilization rate: 10% braking frequency can be understood as follows: if the braking resistor can consume 100% of the power in 10 seconds, then the braking resistor needs at least 90 seconds to dissipate the generated heat. 3.2 Braking unit operating voltage level When the DC bus voltage is greater than or equal to the braking voltage level (discrimination threshold), the braking unit operates to consume energy. Delta's braking voltage level is shown in Table 1. 3.3 Braking resistor design (1) Engineering design. Practice has proven that when the discharge current is equal to half of the rated current of the motor, the same braking torque as the rated torque of the motor can be obtained. Therefore, the rough calculation of the braking resistor is: In order to ensure that the frequency converter is not damaged, the resistance value when the current flowing through the braking resistor is the rated current is forcibly limited to the minimum value of the braking resistor. When selecting the resistance value of the braking resistor, it cannot be less than this resistance value. Based on the above, the range of resistance values ​​for the braking resistor is as follows: The power consumed by the braking resistor when it is working in a circuit with a DC voltage of: The meaning of power consumption: If the power of the resistor is selected according to this value, the resistor can be connected in the circuit for a long time. The power of the resistor used in the field mainly depends on the braking utilization rate ED%. Because the braking time of the system is relatively short, the temperature rise of the braking resistor is not enough to reach a stable temperature rise in a short time. Therefore, the principle for determining the capacity of the braking resistor is that, under the premise that the temperature rise of the braking resistor does not exceed its allowable value (i.e., rated temperature rise), the capacity should be reduced as much as possible. The rough calculation is as follows: (2) Design example. Based on the above formula, we can roughly calculate the resistance value and power of the braking resistor we need. Taking a Delta VFD075F43A frequency converter driving a 7.5KW motor as an example, the rated current of the 7.5KW motor is 18A, and the input voltage is AC460V. Therefore, the range of values ​​for the braking resistor is: It is advisable to choose a resistor value that is readily available on the market and within the specified power range. A resistance of 75 ohms is recommended. Depending on the actual situation, the calculated power value can be appropriately increased. 4. Conclusion The calculation of the braking resistor value and power is considered from an engineering perspective. Therefore, in practical applications, it is necessary to make appropriate adjustments based on the specific site conditions to ultimately form an economical and suitable selection scheme. [b][align=center]For more details, please click: Delta Frequency Converter Braking Resistor Design[/align][/b]