Braking Resistor
A braking resistor is a high-wattage electrical resistor connected to the DC bus of a variable-frequency drive (VFD) to dissipate regenerated energy from a decelerating motor as heat. It prevents DC-link overvoltage, enabling controlled, faster braking without damaging the drive. Typically low ohmic value and high power rating, it is used with a brake chopper transistor that switches it onto the DC bus when voltage exceeds a threshold.
On the shop floor, braking resistors are installed on VFD-driven hoists, cranes, conveyors, machine tools, and elevators to manage regenerative energy during deceleration. When a stop is commanded, the motor regenerates, raising DC-bus voltage. The drive's brake chopper switches the resistor onto the bus, converting energy to heat and limiting voltage. This allows rapid, controlled stops without overvoltage trips. Safety considerations include hot surfaces up to 350°C requiring guarding, clearance from combustibles, and warning labels. Regular maintenance checks for overheating, loose connections, and resistance values are critical to ensure emergency stop performance and compliance with safety standards.
How do I determine if a braking resistor is required for a given VFD application?
A braking resistor is required if the desired braking time is shorter than the natural coast-down time without a resistor, causing DC-bus voltage to exceed drive limits. Calculate minimum braking time without resistor based on motor inertia and load torque. If desired time is less, calculate braking torque and peak braking power for worst case (max speed to standstill). Select a resistor with peak power rating equal to or greater than peak braking power and continuous power adequate for the cycle, with resistance within drive's permissible range.
How is the resistor ohmic value selected, and what happens if it is too low or too high?
Ohmic value is selected using DC-link voltage during braking and desired braking current, often via formulas like R = 2U/I. Too low resistance causes excessive current, risking braking transistor failure or overcurrent trips. Too high resistance reduces braking current, leading to insufficient torque and longer stopping times, potentially failing safety requirements. Manufacturers specify a minimum permissible value that must not be undercut.
How does the dynamic braking function interact with safety standards for hoists and mobile equipment?
WorkSafeBC requires hoists to have two independent braking systems. In VFD-driven hoists, mechanical brakes provide one system, while electronic braking via VFD with a braking resistor can serve as the other, especially during controlled deceleration and loss-of-power scenarios. For mobile equipment, braking resistors must support required deceleration rates and stopping distances per ISO standards. SafeDesk programs should tie resistor design and maintenance to braking performance verification, and any changes trigger re-assessment against regulatory requirements.