DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.
DC motor has high starting torque.In a series motor the same current flows through the armature and the field winding. The torque is therefore proportional to the square of the current. The current is set by the supply voltage and the back-emf generated in the armature.
At zero speed there is no back-emf therefore the current is limited only by resistance, so the current and torque are high.
This type of motor is commonly used for transport because it can provide a lot of torque for starting, and the driver (e.g. in a tram) controls the starting process with a handle that places a variable resistance in series to limit the current while starting.
DC series motor has high starting torque but it is not that we cant start it without load, it is that we don’t start it without load because its high speed will severally damage the motor and may harm its surrounding by breaking the parts. It is very high speed. That is why it is used in traction where high load is given to it.