The utilization rate of a brake resistor is a key parameter that defines how efficiently the brake resistor is used to dissipate energy during the braking process of a motor drive system. It directly impacts the braking performance and the thermal stress on the resistor.

Brake Resistor Utilization Rate Explained

• The utilization rate specifies the fraction of the brake resistor’s rated power that is actually used during braking.

• A lower utilization rate means the resistor generates less heat, consumes less energy, but results in a poorer braking effect because the braking unit’s capacity is underutilized.

• A higher utilization rate, approaching 100%, means the brake resistor is fully utilized, providing the most effective braking. However, this requires a resistor with higher power rating and thus higher cost and thermal management needs.

• For large inertia loads with slow deceleration, a lower utilization rate is preferred to avoid overheating and to achieve better overall results.

• For loads requiring rapid shutdown, a higher utilization rate is advisable to maximize braking effectiveness1.

Derating Brake Resistor Power Based on Load Characteristics

Once the brake resistor power is calculated, it can be derated (reduced) depending on the braking nature of the load:

Non-Repetitive Braking

• Occurs when the system brakes only once in a long period.

• The brake resistor only dissipates energy during that single braking event.

• Power rating of the resistor can be reduced significantly, even by more than 20%, if braking time is less than 10 seconds and if the resistor’s impact resistance is considered1.

Repetitive Braking

• Seen in machines like lifting equipment or gantry planers that brake frequently.

• The power rating of the brake resistor should be proportional to the braking duty cycle, which is the ratio of braking time to the interval between braking events (tb/tc).

• Smaller duty cycles allow for greater reduction in resistor power rating.

• For long braking times with repeated braking, a safety margin should be left when selecting resistor power1.

Additional Brake Resistor Selection Criteria

• Resistance value must be chosen primarily based on braking torque requirements.

• The resistor power rating is selected considering resistance value and utilization rate to avoid overheating.

• The resistor resistance must be above a minimum value to protect the braking transistor and below a maximum value to ensure it can absorb peak power effectively2.

• Typical formulas for instantaneous power and power rating adjustments involve the DC bus voltage and utilization rate12.

Summary Table: Utilization Rate and Brake Resistor Selection