# How To Calculate Dynamic Braking Resistor for Vfd

Calculating the dynamic braking resistor for a Variable Frequency Drive (VFD) involves determining the required resistance value and power rating. Here’s a step-by-step guide:

### 1. Determine the System Parameters:

- **Motor Power (P)**: in kW or HP.

- **DC Bus Voltage (Vdc)**: typically provided by the VFD manufacturer.

- **Braking Torque (Tb)**: as a percentage of the motor’s rated torque.

- **Braking Time (Tb_time)**: the duration for which braking is applied.

- **Deceleration Time (Td)**: the total time to decelerate the motor.

### 2. Calculate the Required Braking Power:

The braking power can be estimated using the motor power and the braking torque.

\[ P_b = P \times \frac{Tb}{100} \]

Where:

- \( P_b \) = Braking power in kW

- \( P \) = Motor power in kW

- \( Tb \) = Braking torque as a percentage of rated torque

### 3. Calculate the Resistance Value:

The resistance value can be calculated using the DC bus voltage and the braking power.

\[ R = \frac{Vdc^2}{P_b \times 1000} \]

Where:

- \( R \) = Resistance in ohms

- \( Vdc \) = DC bus voltage in volts

- \( P_b \) = Braking power in kW

### 4. Calculate the Power Rating of the Resistor:

The power rating of the braking resistor should handle the energy dissipated during braking.

\[ P_r = P_b \times \left(\frac{Tb_time}{Td}\right) \]

Where:

- \( P_r \) = Power rating of the resistor in kW

- \( P_b \) = Braking power in kW

- \( Tb_time \) = Braking time in seconds

- \( Td \) = Deceleration time in seconds

### Example Calculation

Assume the following values:

- Motor Power (\( P \)) = 10 kW

- DC Bus Voltage (\( Vdc \)) = 600 V

- Braking Torque (\( Tb \)) = 150% of rated torque

- Braking Time (\( Tb_time \)) = 10 seconds

- Deceleration Time (\( Td \)) = 15 seconds

1. Calculate the braking power:

\[ P_b = 10 \times \frac{150}{100} = 15 \text{ kW} \]

2. Calculate the resistance value:

\[ R = \frac{600^2}{15 \times 1000} = 24 \text{ ohms} \]

3. Calculate the power rating of the resistor:

\[ P_r = 15 \times \left(\frac{10}{15}\right) = 10 \text{ kW} \]

### Summary:

- **Resistance**: 24 ohms

- **Power Rating**: 10 kW

This resistor will effectively dissipate the braking energy, ensuring safe and efficient braking. Always ensure that the resistor's specifications meet or exceed these calculated values and consult the VFD manufacturer's guidelines for additional recommendations.