Electric Braking in DC Motors: Types, Working Principles, and Applications
What is Electric Braking?
A running motor can be brought to rest quickly using either mechanical braking or electric braking. Mechanical braking relies on physical brake shoes whose effectiveness depends on surface conditions and wear. Electric braking of motors provides smoother, more controlled deceleration by converting the motor's kinetic energy into electrical energy.
Electric braking offers several advantages over mechanical braking, including reduced wear and tear, better control, and in some cases, energy recovery back to the power supply.
Types of Electric Braking in DC Motors
The electric braking of DC motors can be classified into three main types:
- Dynamic braking (also called rheostatic braking)
- Plugging (also called reverse current braking)
- Regenerative braking
1. Dynamic Braking (Rheostatic Braking)
DC Shunt Motor Dynamic Braking
In dynamic braking of DC shunt motors, the armature is disconnected from the power supply while the field winding remains connected. A variable resistor (rheostat) is then connected across the armature terminals.
Working Principle:
- The motor's inertia continues to rotate the armature even after armature is disconnected from the power supply.
- The machine now operates as a dc generator, producing electrical energy
- Current flows through the connected rheostat, dissipating energy as heat (I2R losses)
- The braking force can be controlled by adjusting the resistance value
DC Series Motor Dynamic Braking
For DC series motors, the procedure differs slightly:
- The motor is disconnected from the supply
- Field connections are reversed to maintain the same current direction through the field winding
- A rheostat is connected in series with the circuit
Applications: Commonly used in electric vehicles, cranes, and hoists where smooth deceleration is required.
2. Plugging (Reverse Current Braking)
Plugging or Reverse Current Braking involves reversing the armature connections while keeping the field connections unchanged, causing the motor to attempt rotation in the opposite direction.
Working Principle:
- Armature terminals are reversed. Due to which, applied voltage (V) and back EMF (Eb) act in the same direction
- As, applied voltage and Eb are now in same direction, total voltage across armature resistance increases significantly
- Therefore, armature current increases substantially.
- High braking torque is produced due to increased current
- To prevent damage to the motor from this surge of current, a variable resistor is connected in series with the armature to limit the current to a safe level.
Advantages:
- Provides greater braking torque compared to dynamic braking
- Faster stopping action
- Simple implementation
Applications: Widely used in elevators, machine tools, printing presses, and applications requiring rapid stopping.
3. Regenerative Braking
Regenerative braking is the most energy-efficient braking method, particularly suitable for loads with high inertia such as electric trains, electric vehicles, and large industrial machinery.
Working Principle:
- Applied voltage is reduced below the back EMF (Eb)
- As Eb is now greater than Applied Voltage, armature current (Ia) reverses direction
- Armature torque reverses, causing deceleration
- The machine operates as a generator
- Generated power is fed back to the supply line (regeneration)
- Speed decreases until back EMF falls below applied voltage
Advantages:
- Energy recovery - power is returned to the supply
- Highly efficient braking method
- Reduced heat generation
- Environmentally friendly
Applications: Electric trains, hybrid vehicles, elevators, and large industrial drives where energy efficiency is paramount.
Today, regenerative braking is extensively used in electrical vehicles. You can learn more about Regenerative Braking on wikipedia.
Comparison of Electric Braking Methods
| Parameter | Dynamic Braking | Plugging | Regenerative Braking |
|---|---|---|---|
| Energy Recovery | No (dissipated as heat) | No (dissipated as heat) | Yes (returned to supply) |
| Braking Torque | Moderate | High | Variable |
| Efficiency | Moderate | Low | High |
| Heat Generation | High | Very High | Low |
| Control Complexity | Simple | Simple | Complex |
Key Considerations for Electric Braking
- Load Characteristics: High inertia loads benefit more from regenerative braking
- Speed Control: Variable resistors allow precise braking force control
- Safety: Current limiting resistors are essential in plugging to prevent damage
- Energy Efficiency: Regenerative braking is preferred for energy-conscious applications
- Cost Considerations: Dynamic braking offers a good balance of performance and cost
Conclusion
Electric braking methods provide superior control and efficiency compared to mechanical braking systems. The choice between dynamic, plugging, and regenerative braking depends on specific application requirements including braking torque needs, energy efficiency goals, and system complexity constraints. Understanding these principles helps engineers select the most appropriate braking method for their DC motor applications.