There are generally three types of electric braking for motors : regenerative braking, dynamic braking, and stop braking. Of these three methods, stop braking offers the fastest stop, but it can be very demanding on electrical and mechanical components. Therefore, while not the most commonly used braking method, it is suitable for certain applications.
The blockage of electrical and mechanical components may be more severe, but it provides faster braking than dynamic braking methods.
Blocking—sometimes called "reverse current braking"—is possible in both DC and AC induction motors. For DC motors, blocking is achieved by reversing the polarity of the armature voltage. When this happens, the back electromotive force (EMF) voltage is no longer opposite to the supply voltage. Instead, the back EMF and supply voltage operate in the same direction, opposite to the motor's rotation, causing the motor to stop almost immediately. The reverse current generated by the combined supply voltage and back EMF is very high, so a resistor is placed in the circuit to limit the current.
A DC motor circuit for normal motor operation (left) and a DC motor circuit for blocking (right). Note that in blocking operation, the armature voltage is reversed, and a resistor is added to the circuit.
For an AC induction motor, the stator voltage is reversed by swapping any two power supply wires. The magnetic field then rotates in the opposite direction, and the motor slip (the difference between the speed of the rotating stator magnetic field and the speed of the rotor) is greater than 1 (s>1). In other words, the rotor rotates faster than the rotating stator magnetic field. The torque is opposite to the direction of motor rotation, producing a strong braking effect.
Slip is a fundamental characteristic of AC induction motors; it is the difference between the speed of the stator's rotating magnetic field and the rotor's rotational speed. During normal motor operation, the rotor's speed is slower than the stator's rotating magnetic field.
When the motor speed is zero, if it is not disconnected from the power supply, the motor will begin to reverse or rotate in the opposite direction. In some applications, reversing the motor's direction is the goal. However, when using a stop to brake the motor, when the motor speed is zero, a zero-speed switch or stop contactor is used to disconnect the motor from the power supply.
One of the potential problems with blocking as a braking method (especially with very short braking times) is the difficulty in braking at zero speed. Another drawback of blocking is that the sudden stop caused by blocking places high mechanical shock loads on the motor and connecting equipment. Blocking is also a very inefficient stopping method, thus generating a significant amount of heat.
Despite these drawbacks, blocking is used in equipment such as elevators, cranes, presses, and rolling mills, where a motor that stops quickly (or does not reverse) is necessary.
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