What is a Traction Motor in railways?

A Traction Motor is an electric motor installed in a train's bogie that converts electrical energy into mechanical energy (torque) to rotate the wheels and propel the train.

What is the difference between AC and DC traction motors?

DC motors use brushes and commutators, which require frequent maintenance. AC motors (Induction or PMSM) are brushless, more reliable, more efficient, and require significantly less maintenance, making them the standard for modern trains.

What are the benefits of PMSM traction motors?

Permanent Magnet Synchronous Motors (PMSM) are lighter, smaller, and more efficient than traditional AC induction motors because they use permanent magnets in the rotor, eliminating the need for excitation current and reducing heat loss.

The Muscle of the Train: Railway Traction Motors Explained

The muscle behind the movement. Learn how Railway Traction Motors convert electrical energy into torque, covering DC, AC Asynchronous, and PMSM technologies.

December 10, 2025 11:16 am

A Traction Motor is an electric motor used for the propulsion of a vehicle, specifically trains, trams, and locomotives. Mounted within the bogie frame or directly on the axle, it converts electrical energy (supplied by the traction inverter) into mechanical torque, which turns the wheels and drives the train forward.

Evolution of Motor Technology

The efficiency and reliability of a train depend heavily on the type of traction motor used. Over the decades, technology has shifted from heavy, maintenance-intensive designs to compact, software-controlled powerhouses.

1. DC Series Motors (Legacy)

For a century, Direct Current (DC) motors were the standard. They offer high starting torque, which is ideal for trains. However, they rely on carbon brushes and commutators to transfer electricity to the rotor. These parts wear out quickly, creating dust and requiring frequent, expensive maintenance.

2. AC Asynchronous Motors (The Workhorse)

With the advent of high-power electronics (IGBTs), AC Induction (Asynchronous) Motors became the industry standard. They have no brushes (brushless), are incredibly rugged, and are cheaper to build. Speed and torque are precisely controlled by varying the frequency of the power supply via a Variable Voltage Variable Frequency (VVVF) Inverter.

3. PMSM (The Modern Standard)

The Permanent Magnet Synchronous Motor (PMSM) represents the latest generation. Instead of using electricity to create a magnetic field in the rotor, it uses high-strength permanent magnets. This results in a lighter, smaller motor with higher efficiency, as no energy is wasted magnetizing the rotor.

Comparison: AC Induction vs. PMSM

While DC motors are largely obsolete for new designs, the choice today is usually between AC Induction and PMSM.

Feature	AC Induction (Asynchronous)	PMSM (Synchronous)
Efficiency	High (90-94%)	Very High (96-98%)
Weight/Size	Standard	Lighter & More Compact
Maintenance	Very Low (Robust)	Low (sealed design)
Cost	Low	High (Rare earth magnets)
Cooling	Usually Open/Fan Cooled	Often Fully Enclosed (Quieter)

Regenerative Braking

One of the key features of modern traction motors is their ability to act as generators. When the train brakes, the motor logic is reversed; the kinetic energy of the moving train turns the motor, generating electricity. This power is fed back into the overhead line or stored in onboard batteries, significantly reducing overall energy consumption.