A very cool and easy dc motor speed controller circuit could be build using a just a single mosfet a resistor and a pot as shown below.
Dc motor limit switch circuit.
The circuit uses 2 relays 2 limit switches and a dc motor.
Used to vary the pwm of the pulses at the output and which eventually is used for the speed regulation of the connected motor.
When the control switch is held in the right position the motor runs until the right limit switch opens.
The switch s1 is used to instantly reverse the motor direction whenever.
The motor should then rotate until it hits the other limit switch and then wait for the button to be pressed again to rotate in the opposite direction.
When you switch off power to an ac circuit a voltage spark or arc is created.
I have attached a schematic using push buttons instead of limit switches.
Since diode cr1 will not conduct with a negative potential across it the motor will stop.
The interlocks are specially important because if both relays are on it would be a short circuit.
Imagine you have two circuits each carrying the same current one is an ac circuit and the other is a dc circuit.