Electrical Motors Braking Methods
Electrical braking may be achieved by the following:
• Counter current braking or plugging
• Regenerative braking
• Dynamic or rheostatic braking.
Electromechanical braking may be achieved through an electromechanical brake.
Counter current braking or plugging
This is accomplished by momentarily connecting the motor in a forward direction, when the motor is already running in the reverse direction. It is accomplished in DC motors, by reversing the armature supply leads, so that the motor draws a current to develop a reverse torque, to oppose its already existing rotation.
The motor acts as a brake and comes quickly to rest but has the tendency to accelerate in a reverse direction. If the reversal is not required, the supply to the motor should be cut off at zero speed.
In case of a DC motor, this is achieved by reversing the polarity of the supply voltage to the armature; however, in case of AC motors the phase sequence is interchanged.
This method of braking is also used to maintain a constant speed when the load tries to accelerate the rotor to high speeds.
This method is inefficient because of the power loss in resistors, used for limiting the current due to an interconnection. The mechanical energy is converted to heat and additional power is required. At the same time, if for a large motor, sudden torque is applied, in the reverse direction by plugging, then it may result in damaging the machinery. This may result in a high current flow through the system.
In electrical braking, it is possible to convert the kinetic energy of the rotating parts to electrical energy and return it back to the mains or dissipate it in a resistance.
The braking is called regenerative when the energy is returned to the mains supply. This method uses the motor as a generator during braking, developing a retarding torque, which acts on the running motor and halts it. The kinetic energy and potential energy, minus the losses of the motor are returned to the mains and the motor runs at a constant speed. Thus, regenerative braking eliminates the tendency of the load to accelerate the motor.
Dynamic or rheostat braking
The braking is called dynamic or rheostatic braking when the energy is dissipated as heat in a resistance. In either case, the machine operates as a generator. Electric machines are capable of smooth transition from motor to generator action. In dynamic braking, the motor must be switched to the load or braking resistor keeping the field constant.
Electromechanical friction braking
This is an external brake, used for stopping the motor as well as holding the motor at a single position. It consists of a solenoid along with a drum brake arrangement. When the motor is running, the solenoid is energized; so, it keeps the brake shoes away from the rotor shaft. As the motor is turned off, the solenoid is de-energized and the brake shoe acts on the rotating shaft. There is a braking action due to the friction between the brake shoe and the rotating shaft.
This kind of brake holds the load at one position, even after the motor stops. It is therefore used in applications that require the motor to be held at one position with the load, like in crane applications.
These brakes require more maintenance than purely electrical braking, because of the wear and tear on the brake shoe mechanism.