Techniques used in Stopping DC Motors

Stopping of a motor is actually a form of speed control. Methods used to stop DC motors are in a way similar to speed control techniques. To stop a motor, you must accelerate in the direction opposite to that in which it is moving.

There are two techniques of electrically stopping a DC motor, namely:

• Dynamic braking method.
• Plugging technique.

Dynamic braking method is the most commonly used technique for stopping DC motors. To stop a DC motor using this method, the magnetic field remains in place, but the armature voltage supply is replaced by a resistor. Thus, the motor becomes a generator, and its kinetic energy is converted to electric current that bums off in the resistor. Small motors can be easily stopped in milliseconds using this method.

Plugging is another technique used to stop DC motors. As in dynamic braking, the magnetic field must be retained until the motor comes to a stop. Unlike dynamic braking, the armature is connected to a DC supply of opposite polarity. This results in a dramatic acceleration in the opposite direction. Armature current due to the (reversed) armature DC supply is further increased by the current due to the existing counter-electromotive force (CEMF). Braking is rapid; however the high current is hard on the armature.  This type of braking is therefore used only for emergency braking or for the braking of motors specially built for this type of extra armature current.

Plugging carries another potential problem, once plugging stops a motor, it will accelerate the motor in the opposite direction. Motors that are stopped by plugging need to have a zero speed switch mounted on the motor shaft or on the load. A zero speed switch contains an inertial switch that disconnects the armature voltage supply when the motor speed reduces to a near stop.

These electrical braking techniques will not hold the motor in the stopped position, therefore if positive braking is needed to hold a load steady at the stopped position, then either a mechanical brake can be used to very rigidly hold a load wherever it happens to stop or a positional servo system can cause the motor to stop at given position.

Related articles:

• Speed Control of a DC Compound Motor
• Pulse Width Modulation (PWM) Speed Control of a DC motor
• Speed Control of a DC Series Motor
• The General Principle of using a Servo Motor for Position Control
• DC Motor Power Op-amp Speed Controller
• DC Shunt Motor Speed Control