Electric cable buses, more commonly known as trolley buses, are rubber-tired vehicles that receive power from overhead electrical wires using trolley poles. In essence, they function as electric buses connected to an external power source.
Here's a breakdown of how they work:
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Overhead Wires: Trolley buses draw electricity from a network of overhead wires, typically at a voltage of 600-750 volts DC. These wires are suspended above the bus route.
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Trolley Poles: The bus connects to the overhead wires using two spring-loaded trolley poles. These poles are mounted on the roof of the bus and have contact shoes or wheels that press against the wires. One pole is for the positive (+) connection, and the other is for the negative (-) connection, completing the electrical circuit.
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Electric Motor: The electricity collected from the overhead wires powers an electric motor within the bus.
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Drive System: The electric motor then drives the wheels of the bus, propelling it forward. The driver controls the speed and direction of the bus using standard controls similar to those in a regular bus or car.
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Regenerative Braking: Many modern trolley buses incorporate regenerative braking. When the driver applies the brakes, the electric motor acts as a generator, converting the kinetic energy of the bus back into electricity. This electricity can be fed back into the overhead wires (if the system allows), increasing energy efficiency, or used to charge onboard batteries.
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Maneuverability: While tethered to the overhead wires, trolley buses have some degree of lateral movement. They can typically move a lane or two to the left or right to avoid obstacles or pull up to the curb, as the trolley poles have some flexibility.
Here's a simplified table summarizing the components and their functions:
| Component | Function |
|---|---|
| Overhead Wires | Provide electrical power to the bus. |
| Trolley Poles | Connect the bus to the overhead wires, allowing it to draw electricity. |
| Electric Motor | Powers the wheels of the bus. |
| Drive System | Transfers power from the motor to the wheels. |
| Regenerative Braking | Converts kinetic energy during braking back into electricity. |
Trolley buses offer several advantages:
- Zero Tailpipe Emissions: They produce no emissions at the point of use, contributing to cleaner air in urban areas.
- Quiet Operation: Electric motors are generally quieter than internal combustion engines.
- Energy Efficiency: They can be more energy-efficient than diesel buses, especially with regenerative braking.
However, they also have limitations:
- Infrastructure Costs: Building and maintaining the overhead wire network can be expensive.
- Limited Route Flexibility: The bus is restricted to routes where the overhead wires are installed.
- Dewirements: Occasionally, the trolley poles can become detached from the overhead wires (dewirements), requiring the driver to stop and reattach them.
Despite these limitations, trolley buses remain a viable and environmentally friendly transportation option in many cities.
