Regardless of the motor type, a load reactor is generally recommended if the cable length exceeds 90 m. If the distance exceeds 150 m, a specially designed filter is usually recommended. In EMI-sensitive environments, using a load reactor for all applications is usually good practice. Load reactors are often designed for use with specific drive models. For example, the Omron model 3G3AX-RAO04600110-DE load reactor is rated for 11 A and 4.6 mH and designed for use with 400 V three-phase 5.5 kW motors driven by the company’s 3G3MX2- A4040-V1 VFD. Braking resistors and thermal overloads In addition to a load reactor, a braking resistor and thermal overload shutdown device can be essential additions to the output side of a VSD/VFD. Braking resistors enable maximum transient braking torque by absorbing the braking energy. Most braking resistors dissipate the energy, while some are used as part of a regenerative braking system that captures and recycles the energy. Dissipative braking resistors are rated for specific applications. The Schneider Electric VW3A7755 8
Figure 3: Definition of percentage of energy dissipation (ED%). (Image source: Delta Electronics)
Ω braking resistor can dissipate up to 25 kW, while the Delta Electronics BR300W100 100 Ω braking resistor is rated for 300 W. Braking resistor applications are defined using a percentage of energy dissipation (ED%). The defined ED% ensures the resistor can effectively dissipate the heat generated during braking. ED% is defined relative to the peak dissipation, the braking interval (T1), and the overall cycle time (T0) in Figure 3. Depending on the severity of the braking, ED% is specified to ensure adequate time for the brake unit and brake resistor to dissipate the heat generated by braking. If the brake resistor heats up due to inadequate thermal dissipation, its resistance increases, reducing the current flow and the brake torque absorbed. Braking resistors can be defined by various dissipation cycles like: ■ Light braking, where the braking power is limited to 1.5
times the nominal torque (Tn) for 0.8 s every 40 s. Used with machines with limited inertia, like injection molding machines ■ Medium braking, where the braking power is limited to 1.35 Tn for 4 s every 40 s. Used with machines with high inertia, like flywheel presses and industrial centrifuges ■ Severe braking where the braking power is limited to 1.65 Tn for 6 s and Tn for 54 s every 120 s. Used with machines with very high inertia, often accompanied by vertical movement, like hoists and cranes In addition to a braking resistor, most systems include a thermal overload unit connected to the brake resistor as a safety precaution, like the ABB Control TF65-33 thermal overload relay. The thermal overload unit protects the resistor and drive system from too frequent or too strong braking. When a thermal
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