Summary and Key Terms in Engine Braking Systems

♦ The most commonly used procedures for braking short-circuited rotor three-phase asynchronous motors are: byelectric brake, to  against the current and bydirect current injection.

♦ For braking three-phase asynchronous motors, three types of electro-braking systems are used, all based on a movable plate attached to the motor shaft and braking shoes or bands that act upon it: electromagnet, electro-hydraulic, and motor-integrated brake. In the first two cases, the plate is fixed to the motor shaft and the shoes are actuated either by an electromagnet or an electro-hydraulic drive; while in the case of the motor-integrated brake, the plate slides and presses against the braking band when the motor is disconnected from the power supply.

♦ The electromagnet brake is used in small and medium power motors; the electrohydraulic brake can be used in all types of motors, mainly medium and large power motors; the brake incorporated into the motor is usually used in machine tools, elevators and any other small power machine that requires rapid braking.

♦ In counter-current braking, the principle of the reversing motor is used, reversing two phases of the power supply and generating a powerful braking torque. This system can be effectively used to achieve rapid deceleration of motors carrying inertial loads, such as in machine tools.

♦ In DC injection braking, the motor is disconnected from the power line and two terminals of the stator are immediately connected to a DC source; under these conditions, the rotor rotates relative to a fixed magnetic field and its slip creates a braking torque.

♦ The most commonly used braking procedures in this type of motor are: By electrobrake, hypersynchronous braking, countercurrent braking, by direct current injection, braking by direct current self-excitation, braking by unbalanced supply.

♦ In electrobrake braking the procedure used is the same as that of a short-circuited rotor, in hypersynchronous braking the asynchronous motor operates at a higher speed than the synchronous speed, the motor works as an asynchronous generator, opposing a braking torque.

♦ In counter-current braking, two phases of the power supply are reversed. The braking torque can be adjusted by modifying the value of the rotor resistors.

♦ In DC injection braking, the same procedure is used as in the short-circuited rotor motor.

♦ In DC injection braking, the stator acts as the exciter and the rotor as the alternator. It seems convenient to use this alternating rotor voltage and rectify it to power the stator, thus achieving self-excitation braking.

♦ If the supply voltages applied to the windings of the three phases of the stator are unbalanced, the operation is disturbed and in some cases the motor torque is reversed, working in braking mode.