Dynamic braking for hoists

Follow these directions to a fail-safe system.

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By Thomas Barkand and William Helfrich, U.S. Dept. of Labor, Mine Safety and Health Administration

PlantServices.com

Hoists and elevators can injure or kill. Accidents can occur on counterweighted hoisting systems if the mechanical brake fails while the cage is empty. The counterweight falls; the cage overspeeds and crashes. Direct-current hoist motors prevent this type of accident if equipped with passive electrical braking systems known as dynamic braking. Installing a dynamic brake requires minimal modifications to the control system and modest expense.

Hoists and elevators have safety features to prevent the cage from falling. Safety catches activate if the brakes or wire ropes fail. Safety catches, however, are not normally installed on the counterweight.
Many hoisting systems rely solely on the mechanical brakes to stop the cage in an emergency. Under normal operation, the electrical drive equipment controls the speed of the hoisting system while the mechanical brakes only hold the cage at a stopped position. The frequency with which the mechanical brakes are exercised is minimal when compared to the constant use of the drive equipment. However, in an emergency, the majority of hoists in the United States rely on the mechanical brakes to stop the hoist. The assumption is that the mechanical brakes are 100 percent reliable when the electrical drive becomes inoperative.

History proves that this is not a good assumption. Accidents occurred when the emergency stop button was pushed--an action that defeated the retarding effort of the hoist motor--when the mechanical brakes were inoperative. This allowed the overhauling load to free-fall, with the final speed limited only by inertia and frictional forces. The high-speed crashes at the travel limit cause extensive mechanical damage and fatal injuries.

The direct-current motors on elevators and hoists can prevent the failure because the electrical drive and control system can limit the speed of the falling overhauling load. This electrical source of braking retards the free-fall speed when the mechanical brakes fail.

Dynamic braking exploits the ability of the direct-current drive motor to act as a generator. The motor requires torque and kinetic energy of the falling load to generates electricity that is dissipated as heat in a resistor. The retarding torque limits the speed of the falling overhauling load. The amount of retardation and the final speed of the cage depend on the motor terminal characteristics and the resistance value of the dynamic braking resistor.

Hoist motor performance
The direct current motor circuit has the motor armature in series with the power supply. This power supply is either a generator or SCR bridge that converts line voltage to a variable direct current voltage that controls the speed of the motor. The field of the motor is normally supplied from a separate source--either fixed (constant potential) or variable (field weakening)--that controls the speed of the hoist motor.
A shunt-wound direct current motor can operate as either a motor or generator. It operates as a motor when it produces torque in the same direction as shaft rotation. The motor operates as a generator when the direction of motor torque opposes shaft rotation, as when a load overhauls motor torque, thus reversing shaft rotation. Then, the direct current generator--a.k.a. hoisting motor--converts the energy of the overhauling load into electrical power to be returned to the grid. This regenerative braking actively pushes power into the ac power system instead of dissipating it as heat.

The motor is said to be in the powering mode when motor action is taking place. In the inverting mode, generator action is taking place. If raising the load produces positive shaft rotation, then the four quadrants of operation are defined. The motor torque and direction are directly proportional to the armature current and voltage, respectively.

The constant-speed unbalanced hoisting system operates in quadrants 1 and 4. Quadrant 1 represents motor speed and torque acting in the same direction. Thus, the motor supplies a positive motive force to the load. Quadrant 4 represents the negative direction and positive torque of a motor that is developing a braking force. During deceleration and acceleration, the hoisting system operates in quadrants 2 and 3, respectively.

Constant-speed counterweighted (balanced) hoisting systems operate in four quadrants. When the counterweight is heavier than the load, the hoisting system operates primarily in quadrants 2 and 3. The motor acts as a generator for part of the hoist cycle and as a motor for another other part, depending on the load.

The ability of the motor to return power to the grid is what allows the motor to provide a braking force. Under normal operation, the motor control circuit provides both motive power and braking power. The mechanical brakes are called upon only to provide a very low-speed stop at the top or bottom or, in case of an emergency, to provide complete stopping. The mechanical brakes are called upon very infrequently to completely stop the hoist. However, when they are called upon, they must provide 100 percent of the stopping force. This places a severe burden on the mechanical brakes at a critical time.
Normally, motors do not experience debilitating failures. However, failures to the motor control circuits and the related power supply do occur. The motor will not develop torque to continue motoring or regenerating if the field supply fails, if the power supply for the motor fails or if utility power is lost. This warrants a system that uses the retarding capabilities of the hoist motor.