dynamic braking(redirected from Rheostatic brake)
dynamic braking[dī¦nam·ik ′brāk·iŋ]
A technique for braking in which mechanical energy is converted to heat or electrical energy in order to slow or stop motion. An all-mechanical dynamic brake consists of rotating vanes that circulate a viscous fluid in a manner that generates heat. This is one way that the power of the wind is harnessed for space heating. An electric dynamic brake consists of an electric dynamo in which the mechanical energy is converted to electric form, and either converted to heat in a resistor or returned to the supply lines. Typically, electric braking is accomplished with the same machine that serves as the drive motor. Electric dynamic braking is employed in electric vehicles, elevators, and other electrically driven devices that start and stop frequently. See Wind power
The most common type of dynamic braking will be explained for a direct-current (dc) motor. To accomplish braking action, the supply voltage is removed from the armature of the motor but not from the field. The armature is then connected across a resistor. The electromotive force generated by the machine, now acting as a generator driven by the mechanical system, forces current in the reverse direction through the armature. Thus a torque is produced to oppose rotation, and the load decelerates as its energy is dissipated, mostly in the external resistor, but to some extent in core and copper losses of the machine. See Direct-current motor
Electric braking can also be accomplished by causing the energy of the rotating system to be converted in the armature to electrical energy and then returned to the supply lines. This mode of operation, called regenerative braking, occurs when the counterelectromotive force exceeds the supply voltage.
Interchanging two of the lines supplying a three-phase alternating-current (ac) induction motor also produces braking. In this case, called plugging, the direction of the electromagnetic torque on the rotor is reversed to cause deceleration. Both the energy of the system and the energy drawn from the supply lines are expended in copper and core losses in the machine as heat. The power lines must be disconnected when the rotor comes to rest. See Alternating-current generator, Electric power generation, Electric rotating machinery, Induction motor