RELIANCE ELECTRIC INVERTRON DBU Dynamic Braking Unit

Installation, Connection, and Protection: Details Determine Safety

Proper installation and wiring are the cornerstone of reliable DBU operation.

Short circuit protection: If the freewheeling diode inside the DBU is damaged, the DC bus capacitor of the inverter may discharge to the DBU. Therefore, quick fuses (aR type, 660V) must be installed on the DC+(terminals 45-45) and DC - (terminals 47-47) connection lines between the frequency converter and DBU. The manual provides a detailed list of fuse and fuse holder models (e.g. DBU-100 requires 100A fuse).

Wiring specifications and inductance control:

Inverter to DBU connection: IGBT switch action will generate instantaneous voltage spikes (Δ U) on the inductance of the connection line. To ensure that Δ U does not exceed 200V (damped by the internal RC absorption circuit of DBU), this distance must be shortened as much as possible. For single machine drives, the total cable length must not exceed 1.5 meters (DBU-200/-400 is particularly strict). The wires should be twisted together or shielded multi-core cables should be used to reduce inductance.

Connection from DBU to braking resistor: The time constant τ=L/R formed by the inductance (L) and resistance value (R) of the resistor itself and its connecting wire must be less than 20 μ s. High inductance resistors (such as wound ceramic resistors) or long cables will limit this distance. Heat resistant cables (≥ 90 ° C) should be used, and the appropriate cross-sectional area should be selected based on the root mean square braking current.

Grounding and shielding: The armor layer of all shielded cables must be reliably grounded at both ends through appropriate EMC cable joints to form a continuous grounding path, which is crucial for meeting CE/EMC requirements.

Additional protection for braking resistor: The thermal switch on the DBU heat sink only monitors the IGBT temperature. If the DBU fails (such as IGBT through), the rectified AC power will be directly applied to the braking resistor, and the IGBT will not generate heat, making it impossible for the thermal switch to detect. Therefore, it is strongly recommended to install an independent thermal switch on the brake resistor radiator and connect its contacts in series with the thermal switch contacts of the DBU to achieve dual protection. Another solution is to use the switch output of the optional diagnostic card BUD for monitoring.

System Design: Braking Power and Resistance Calculation

Reasonable selection of DBU and braking resistor requires precise calculation.

Calculate the required braking power P:

P [kW] = (J * Δn * n) / (91200 * tB)

Among them, J is the total inertia (kg · m ²), n is the starting speed (rpm), Δ n is the speed change (rpm), and tB is the braking time (s). During linear deceleration, the initial braking power is the highest, and this peak power is the key basis for selection.

Calculate the minimum value of braking resistance Rmin:

For 380/415V systems: R=439/P [kW]

For the 460V system: R=534/P [kW]

The selected actual resistance value should be greater than or equal to Rmin, while considering the influence of resistance on the duty cycle of DBU, and the duty cycle should be as high as possible at the maximum braking power.

Provide complete specifications to the resistor supplier, including resistance value, maximum braking power/current and duty cycle, average power and duty cycle, braking time and cooling time, maximum DC bus voltage (800V), and L/R time constant of the resistor.

Example: A roller conveyor driven by 20 2.5kW motors, with a total power of 50kW, needs to brake within 1.8 seconds. Calculated to require 50kW braking power, with a resistance value of R=534/50 ≈ 10.7 Ω, select 10 Ω. Corresponding to a short-term power of 75kW, DBU-100 (75kW) is selected. The specifications of the braking resistor are: 10 Ω, 50kW, 1.8s braking, 28s cooling, and 800V withstand voltage.

BUD diagnostic card: status monitoring and intelligent feedback

For the DBU xx DIS model and all DBU-400 models, a powerful BUD diagnostic card is built-in, which works independently of the DBU main regulator and provides multiple monitoring:

Status indication: Two LEDs respectively indicate "DC bus voltage>50V" and "braking current>0".

Switch output S1-2: a transistor switch that provides a unique logic state:

Closed: When UDC>50V and braking current IB=0 (indicating that the DC bus is charged but in standby or electric mode).

Disconnected: When UDC>50V and IB>0 (braking), or when UDC<50V.

This output can be used to monitor the status of the DC bus and whether the DBU function is normal. It is independent of the frequency converter and is an important safety monitoring point.

Analog output A5-6: Provides a 0-20mA signal proportional to the power consumption P on the braking resistor. The scale depends on the jumper setting, braking resistance value, and current PWM duty cycle. After being powered by an external 24V power supply, this signal can be connected to a PLC or instrument to monitor the level of braking energy consumption in real time, achieving advanced energy management or load analysis.