Dynamic Braking
Dynamic braking is an electrical braking method where a drive motor operates as a generator during deceleration, converting kinetic energy of the moving load into electrical energy. This energy is dissipated as heat in a resistor (rheostatic braking) or returned to the supply line (regenerative braking). It is used in AC and DC motor-driven equipment like hoists, cranes, and locomotives for rapid deceleration and overhauling-load speed control.
On shop floors, dynamic braking is implemented in motor-driven machinery such as hoists, cranes, conveyors, and elevators to achieve rapid deceleration or control overhauling loads. In a VFD system, when a motor slows, it generates energy that raises the DC bus voltage; a brake chopper switches an external dynamic braking resistor into the circuit to dissipate this energy as heat. This method is critical for safety and productivity, reducing coast-down times and wear on mechanical brakes. It supports compliance with OSHA machine guarding standards by minimizing hazardous motion, and with WorkSafeBC requirements for braking redundancy in hoists, though it is supplementary to mechanical fail-safe brakes.
How does dynamic braking interact with WorkSafeBC’s requirement for two independent hoist braking systems?
WorkSafeBC requires two independent braking systems for hoists: one automatic in neutral and one on power loss. Dynamic braking is an electrical retarding system that limits speed but does not fully stop the load, as per MSHA guidance. It is supplementary to a mechanical fail-safe brake, which must be present and independent to meet the requirement.
How does OSHA view dynamic braking in relation to machine safeguarding and stopping time?
OSHA has no specific motor-stopping standards but requires protection from hazardous moving parts. Dynamic braking is an engineering control that shortens stopping time, supporting compliance with 1910 Subpart O. Employers must ensure stopping time is sufficient to prevent injury during foreseeable access and maintain the system to avoid extended coast-down.
What key design parameters must be verified when engineering a dynamic braking system for an AC VFD?
Critical parameters include energy per braking event (kinetic and potential), duty cycle, maximum DC bus voltage, and resistor ohmic value/power rating. Safety aspects include physical guarding for hot DBRs, over-temperature protection, and ensuring dynamic braking does not interfere with mechanical brake operation. UL 508 compliance is required for DBRs.