Traction motors: applications and characteristics

A traction motor must give high starting torque, series-type speed-torque characteristic (speed falls as load rises), simple speed control, easy braking, rugged construction and good load sharing when several motors work in parallel. The main types used are listed below.

1. DC series motor

• Characteristics: torque $\propto I^2$ before saturation, so starting torque is very high; speed falls sharply with load (self-regulating); power demand on the supply varies roughly as $\sqrt{T}$, so heavy loads do not overload the line; motors share load well in parallel.
• Speed control: series-parallel control, field weakening, chopper control.
• Applications: the classic traction motor for dc railways, tramways, metro and suburban trains, and in diesel-electric locomotives.
• Drawback: commutator and brushes need maintenance; regenerative braking needs change to separate excitation.

2. DC compound motor

• Characteristics: cumulative compound gives high starting torque with a definite no-load speed, so it does not race on light load; regenerative braking is easier than with series motors.
• Applications: trolley buses and some hilly-route dc vehicles where regeneration is useful.

3. AC single-phase series (commutator) motor

• Characteristics: series-type speed-torque curve similar to dc series motor; works on low-frequency ac (16⅔ or 25 Hz) to limit commutation trouble; speed controlled by a tap-changing transformer.
• Applications: older single-phase low-frequency ac main-line railways (Europe).

4. Three-phase induction motor

• Characteristics: nearly constant speed on fixed frequency, simple and rugged, automatic regenerative braking above synchronous speed. With VVVF inverters it gets high starting torque and smooth speed control.
• Applications: modern electric locomotives, metro trains, EMUs and electric buses; earlier used on 3-phase hill railways.

5. Synchronous and other modern motors

• Linear induction motors (some metros, maglev), permanent-magnet synchronous motors (high efficiency in new trains and electric vehicles), and brushless dc motors in e-rickshaws and electric buses.