SOLUTION

In trains or bus, there are many compartments full of passengers & luggage, so during the starting, it requires high starting torque. DC series motor is used for the locomotive because it has very high starting torque and variable speed.

DC series motors will have a starting torque as high as 500% compared to the normal operating torque.

The DC series motor has long been the most common type of traction motor for railroad service in both electric and diesel-electric locomotives because of its superior operating characteristics. As the load on the motor increases, the field strength automatically increases, and the result is an increase in torque with a decrease in speed. Conversely, the speed increases as the load decreases. Consequently, the series motor tends to adjust itself readily to the varying tractive effort required to accelerate and power a train.