Instantaneous braking of DC motors refers to the rapid and abrupt stopping of the motor’s rotation. This is achieved by applying a reverse voltage or a short-circuit across the motor terminals to counteract its forward motion. The purpose of instantaneous braking is to bring the motor to a complete stop as quickly as possible, often for safety or precision control reasons.
Here’s how instantaneous braking of a DC motor typically works:
- One common method of instantaneous braking is to quickly reverse the polarity of the voltage applied to the motor. For instance, if the motor is running with a positive voltage, a negative voltage is applied abruptly to the terminals. This effectively applies a strong opposing torque to stop the motor’s rotation.
- Another technique used in instantaneous braking is dynamic braking. In dynamic braking, the motor is momentarily disconnected from the power supply, and its terminals are short-circuited. This allows the motor to act as a generator, converting its kinetic energy into electrical energy, which is dissipated as heat in the form of braking resistance. Dynamic braking is particularly useful in applications where the motor has a significant amount of inertia.
- In modern motor control systems, electronic braking methods are often employed. This involves using semiconductor devices such as transistors or power diodes to control the motor’s voltage and current, enabling precise and rapid braking control. Electronic braking methods are especially common in adjustable-speed drive (ASD) systems and can provide more sophisticated braking control.
Instantaneous braking is useful in various applications, including:
- Emergency stops in machinery to prevent accidents.
- Precision control in robotics and automation where the motor needs to stop precisely at a given position.
- Rapid braking of electric vehicles or trains for safety reasons.
- Quickly stopping conveyor belts or other material handling equipment.
To ensure a robust starting torque, the shunt winding in the circuit remains continuously connected to the power source. When the start button is engaged in this setup, the contactor (M) becomes energized, initiating the motor’s operation. The (M) contact seals the start button, enabling uninterrupted operation, and the power circuit contacts (M) are closed, establishing a connection to the mains. Consequently, a current flows from top to bottom through the motor windings, setting the motor in motion in the indicated direction. As the motor rotates, either due to friction or centrifugal force on the shaft, the sudden stop switch (A) closes. Typically, the normally closed (M) contact remains engaged until the contactor G is not energized, allowing the motor to operate continuously until the stop button is pressed.
Upon pressing the stop button, the current to the (M) contactor is severed, disconnecting the inductor from the power source. Subsequently, the previously open (M) contact closes, and the inertia of the motor keeps the sudden stop switch (A) closed. As a result, the contactor G is energized, and its contacts (G) close, reestablishing a connection to the power source. This leads to a current flowing from the armature to the upper part, generating a reverse rotational torque. Consequently, the rotational speed of the arrow’s rotating end diminishes rapidly. When the speed reaches zero, the (A) sudden stop switch is activated. The (G) contactor is de-energized, causing its contacts (G) to open, thereby disconnecting the inductor from the mains. This mechanism brings the motor to a swift halt without causing it to reverse its rotation.