Why DC series motor should not be started without load?

Under the no-load condition, the armature current is very less and hence the flux produced will also be less (φ α Ia). We know, the speed is inversely proportional to the flux (N α Eb/φ). So if the flux is less, high speed is produced in the motor. It is very dangerous as it will damage the motor. Thus, a DC motor should never be started without load.

Why a DC shunt motor is called a constant speed motor?

In a DC shunt motor, the field winding is connected in parallel to the armature winding and the supply voltage. Under a constant supply voltage, the flux in a shunt field winding remains constant. This constant flux will keep the motor rotating at a constant speed.