An alternative and complement to electromechanical braking, dynamic braking uses magnetics and the interplay of variable frequency drives (also known as inverters) and AC motors to slow and stop hoists and bridges.
Brandon Lyndall, Technical Support
Dynamic braking offers a variety of benefits over electromechanical braking alone. For example:
Overspeed protection
Dynamic braking prevents the uncontrolled descent of hoists, thus enhancing the safety of crane and hoist operation. Naturally, as you drive a hoist in the down direction, the load tries to pull the hoist faster than what the AC motor is telling it to do. Variable Frequency Drives (VFDs), which are designed to vary the frequency and voltage supplied to electric motors, sense this condition. They respond by adjusting torque and slowing the motor to control the descent and set the mechanical load brake.
Decreased brake wear
Friction wears down mechanical brake pads. But dynamic braking minimizes the use of mechanical brakes, thus reducing wear and tear and extending the life of brake components.
Through dynamic braking, VFDs slow the motor, minimizing the required work of the electromechanical brake. Downward motion is slowed considerably before the brake sets. In contrast, a contactor-controlled hoist requires the electromechanical brake to stop the load, resulting in greater mechanical stress and brake wear.
Reduced power consumption
Variable frequency drives produce a form of generator action that opposes the motor action. As it controls the load, this retarding force generates energy. This energy basically goes back to the drive in the form of AC. It’s converted to DC, put on the DC bus, and when the DC bus reaches a certain level, the excess energy in the form of heat dissipates via a brake chopper, an electrical switch, to dynamic braking resistors on the crane.
Regenerative braking for reduced energy consumption
An alternative to resistors, a regenerative braking system returns the excess energy to the electrical grid, reducing net power consumption. In a regenerative system all drives of the crane run off a common DC bus and all regenerated energy returns to the common DC bus. The excess energy is inverted from DC to AC and synched up with the network grid.
Regenerative braking is most common in larger cranes, which typically consume more power than other lifting equipment. For instance, coker cranes, charge cranes and port cranes. Overall, resistors are the more common alternative, as many applications don’t have room for the extra panels that are required to convert DC to AC.
Regular inspections vital to safe operation
While dynamic braking reduces wear on brake components, regular brake inspections and maintenance are still critical. If a drive were to fault, a holding brake must be in optimal condition to stop the load.
In addition, motors, resistors and cooling fans for resistors and motors must be checked on a regular basis. Resistors and fan fins must be kept clean. Dust can cause fires and dust on fan fins can hamper their ability to dissipate heat generated by motors.
