I. Core Basis for Inverter Brake Resistor Selection

The core function of an inverter brake resistor is to dissipate the regenerative energy of a motor. When the motor is in a deceleration, braking, or heavy-load lowering state (e.g., elevator descent, machine tool emergency stop), it acts as a generator and feeds energy back to the inverter’s DC bus. If this energy is not dissipated promptly, the bus voltage will rise excessively and trigger protection mechanisms. Selection should focus on “how to safely and efficiently dissipate regenerative energy,” with three core criteria: regenerative power, braking time, and allowable temperature rise.

II. Key Parameters to Confirm Before Selection (Prerequisites)

Before selecting a brake resistor, the following four types of basic parameters must be obtained, as they directly determine the resistor’s specifications:

III. Calculation of Core Brake Resistor Parameters (Key Steps)

The essence of brake resistor selection is to calculate two core specifications: resistance value \( R_{b} \) and resistor power \( P_{b} \). The specific calculation methods are as follows:

1. Calculate the Minimum Allowable Brake Resistance \( R_{bmin} \) (Avoid Current Overload)

The resistance value of the brake resistor cannot be too small; otherwise, the braking current will exceed the maximum allowable value of the inverter’s brake unit, burning the brake unit or resistor. The calculation formula is:\( R_{bmin} = \frac{U_{dc}}{I_{bmax}} \)

Example: For a three-phase 380V inverter with a maximum allowable brake unit current of 20A:\( R_{bmin} = \frac{513}{20} \approx 25.65\Omega \)

→ The selected resistor value must be ≥25.65Ω (standard values such as 27Ω or 30Ω are usually chosen).

2. Calculate the Required Brake Resistor Power \( P_{b} \) (Avoid Burnout Due to Insufficient Power)

The resistor power must meet the requirement of “not exceeding the allowable temperature rise during braking.” Calculation methods differ for short-term braking and continuous braking scenarios:

(1) Short-Term Braking Scenario (Most Common, e.g., Machine Tool Emergency Stop, Single Deceleration)

During short-term braking, the resistor dissipates “pulse power,” so selection should be based on average power. The formula is:\( P_{b} = \frac{K \times P_{e} \times t_{b}}{t_{c} + t_{b}} \)

Example: For a 7.5kW constant torque motor with a single braking time of 2s, a braking interval of 28s, and \( K=0.6 \):\( P_{b} = \frac{0.6 \times 7.5 \times 2}{28 + 2} = \frac{9}{30} = 0.3kW \)

→ A resistor with power ≥0.3kW can be selected (standard values such as 0.5kW or 1kW are used in practice to allow a margin).

(2) Continuous Braking Scenario (e.g., Frequent Braking of Elevators or Cranes)

During continuous braking, regenerative energy is generated continuously, so selection should be based on rated power. The formula is:\( P_{b} \geq K \times P_{e} \)

Example: For an 11kW elevator motor with frequent up/down braking and \( K=1.0 \), \( P_{b} \geq 11kW \). If the maximum allowable power of the inverter’s brake unit is 15kW, an 11kW or 15kW resistor can be selected.

3. Verify the Resistor’s Rated Temperature Rise (Ensure Long-Term Stability)

The allowable temperature rise \( \Delta T \) of the resistor (℃, e.g., 150℃ for aluminum-housed resistors) must satisfy:\( \Delta T = \frac{P_{b} \times R_{th}}{S} \leq \Delta T_{max} \)

If the calculated \( \Delta T \) exceeds \( \Delta T_{max} \), increase the resistor power or switch to forced air cooling.

IV. Summary of Selection Steps (4-Step Implementation Method)

Based on the above calculations, practical selection can be performed in 4 steps to ensure no omissions:

Step 1: Confirm Basic Parameters

Collect parameters such as motor rated power, inverter brake unit maximum current/power, braking time, and braking frequency. Clarify whether the braking type is short-term or continuous.

Step 2: Calculate the Minimum Resistance \( R_{bmin} \)

Calculate using \( R_{bmin} = U_{dc}/I_{bmax} \). The selected resistor value must be ≥\( R_{bmin} \), and a standard resistance value should be chosen (common standard values: 10Ω, 15Ω, 20Ω, 27Ω, 30Ω, 50Ω, 100Ω, etc.).

Step 3: Calculate the Required Resistor Power \( P_{b} \)

Select the corresponding formula based on the braking type (short-term/continuous) for calculation. The selected power must be ≥ the calculated value and not exceed the maximum allowable power \( P_{bmax} \) of the inverter’s brake unit. Reference standard power values: 0.5kW, 1kW, 2kW, 3kW, 5kW, 10kW, 15kW, etc.

Step 4: Confirm Additional Specifications

V. Common Selection Misconceptions and Precautions (Avoid Mistakes)

If the resistance value is too small (e.g., the calculated requirement is ≥25Ω, but 10Ω is selected), the braking current will exceed \( I_{bmax} \), burning the brake unit or resistor. The resistance value must first meet ≥\( R_{bmin} \), then match the power.

For scenarios such as frequent elevator braking, selecting a low-power resistor based on short-term braking (e.g., the calculated requirement is 11kW, but 5kW is selected) will cause the resistor to overheat and burn out. Selection must be based on the rated power for continuous braking.

If the inverter has no built-in brake unit, an external brake unit must be selected. At this time, the maximum current of the brake unit must be ≥ the rated current of the resistor (\( I_{b} = U_{dc}/R_{b} \)) to avoid brake unit overload.

In industrial scenarios, it is recommended to allow a 10%~20% margin based on the calculated value:

Brake resistors generate a large amount of heat during operation (e.g., a 10kW resistor generates 36,000kJ of heat per hour). Ensure good ventilation in the installation environment:

VI. Selection Example (Practical Application Scenario)

Scenario: A three-phase 380V, 5.5kW constant torque motor (e.g., a conveyor belt), an inverter with a built-in brake unit (\( I_{bmax}=15A \), \( P_{bmax}=10kW \)), a braking time of 2s, a braking interval of 30s, and an ambient temperature of 25℃.

Selection Process:

Final Selection Result: 36Ω/0.3kW aluminum-housed brake resistor.