Universal-motor speed changes with load because current, flux and internal voltage drops change together. Interpret the torque–speed curve under specified supply and test conditions.
Since the motor is running, when the load torque increases, the electromagnetic torque increases, and the input current also increases. At the same time, cause of the excitation current of the series motor is equal to the total current of the armature, it is equal to the input current. Therefore, when the exciting current increases, the exciting magnetic flux φd also increases. However, due to the motor speed n∝1/φd, when the input current increases, the motor speed n decreases.
In additional, due to the increase of the input current, the resistance voltage drop (ir) of the stator winding and armature winding increases accordingly. However, since the power supply voltage is constant (U=E+ir), in order to balance the power supply voltage, the induced potential E of the armature winding should be reduced, and because of E∝n, n is also reduced.
For further selection, review universal AC motor catalogue. Confirm supply, load and thermal protection for the specific configuration; a principle diagram is not an appliance wiring instruction.
The above two factors make the speed drop quickly, so the mechanical characteristics of the single-phase series motor are soft characteristics.