The term braking means, to reduce the speed of any rotating equipment like a vehicle, locomotive. There are mainly two types of braking;
- Mechanical braking
- Electrical braking
In mechanical braking, the energy of the machine is converted in terms of heat. Therefore, it is a waste of energy. It requires frequent maintenance of the element which is a direct connection with the machine. Hence, the mechanical braking is not a more reliable method.
But also, mechanical braking used in some applications because the initial cost is very high, it can be installed in all types of machines, and it is easy to implement any kind of system.
In this article, we will discuss electrical braking and different types of electrical braking. The electrical machine can work as motoring, generating, and braking mode.
To reduce the speed of the electrical machine, the motor operates under braking mode. In different types of motor, working under braking mode is different.
There are three types of braking;
- Plugging or Dynamic braking
- Rheostatic braking
- Regenerative braking
Plugging or Dynamic Braking
The connection diagram with a DC shunt motor to explain the Dynamic breaking is as shown in the below figure. The switch S is double pole double through a switch. And it is connected either 1 and 1’ connection or 2 and 2’ connection.
If the switch S is connected at 1 and 1’ connection, the DC motor running in simple condition. And during this condition, the right-hand part (supply) remains inactive.
When the baking is initiated, the switch changes its connection to 2 and 2’ points. Now, in this condition, the armature of the DC shunt motor is disconnected from the left-hand supply. During this condition, the armature current will be;
Here, the direction of Ia reverses its direction and producing torque Te in the opposite direction to the current speed N. The armature current decreases as decrease the back EMF (Eb) with time and speed.
But the armature current Ia will never be zero because of the voltage V. And stopping of a motor in this condition is faster compared to the rheostatic braking.
If the motor keeps running on the position of 2 and 2’ even after zero speed, the machine will start to increase speed in a reverse direction and operating as a motor. Hence, it must be taken care of to disconnect the position of switch S after the motor speed is zero.
Regenerative Electrical Braking
If the back EMF of DC motor is more than the supply voltage, the motor operates in regenerative mode. In this condition, the direction of the armature current is reversed and impose torque in opposite to the direction of rotation.
In normal motoring mode, the back EMF is less than the supply voltage. In this condition, the armature current drawn from the supply. And back EMF, Eb = kфn1.
Now, the question is how to enter the motor in regenerative mode from the motoring mode. This only is done when the mechanical load itself becomes active.
Let’s take an example of a locomotive that runs on the road. It moving on a plain road and the machine works as a motor. The speed of the motor is n1 rpm. When the road has a downward slope, due to the gravitational force of the motor, the speed increase to n2 rpm.
And in this condition, the back EMF Eb= kфn2. This back EMF is more than the supply voltage. The direction of armature current reverses and motor operators in the regenerative braking mode.
In this mode, the motor will not stop, but it will help to avoid the risk of dangerously high speed.
Rheostatic Electrical Braking
Consider the DC motor is connected with switch S as shown in the below figure. Here, the switch S is a single pole single through a switch and it can be connected either position-1 or position-2.
When the switch is connected at position-1, the motor runs at speed of n rpm. And the back EMF Eb= kфn. The polarity of back EMF is opposite to the supply voltage in motor mode and the torque is in the same direction of rotation n.
When the switch is connected at position-2, the armature is disconnected from the supply. And terminated by the resistance Rb. The field coil is still energized from the supply.
Speed of a motor cannot change instantaneously. Hence, the back EMF maintains its polarity. The direction of the armature current will be reversed as compared to the direction of the armature current of the motor mode.
In this condition, the motor behaves as a generator and dissipating power in the resistance Rb. And the torque is in opposite direction to the speed n.
The speed n decreases with respect to time and consequently, the back EMF and armature current is also decreasing.
The braking torque will be highest when the switch changes its position. It decreases progressively and becoming zero when the machine comes to the stop.
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