single phase series motor

When a dc drives motor equipped with a laminated field is connected  to ac single phase, the lagging reactance of the field coil will reduce the field current. While starting such a motors, armature windings connected to commutator segments shorted by the brushes look like shorted transformer turns to the filed. 

This leads to arcing and sparking at the brushes as the armature begins to turn. This problem is more when speed increases, which shares the arcing and sparking between commutator segments. The lagging reactance and arcing brushes are only tolerable in very small uncompensated series ac motors operated at high speed. Series ac motors smaller than hand drills and kitchen mixers may be UN compensated.

According to figger the circuit diagram for speed control of an ac series motor using triac. If the firing angle is increased, the voltage available across the motor will decrease, changing the speed of the motor.