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Your best choice is to use a rheostat type device. These devices are similar to a toroidal transformer except that they include a wiper that makes contact with the coil and uses a knob to rotate the wiper to adjust the output. Some of these units come with a voltage gauge printed on the outer shell that tells the voltage of the device and others have a built-in meter for voltage readout. If you use one of these devices then there would be no need to build one of the devices you need.
You might have an accidental short circuit or similar that allows the current through without controlling the amount of it. this could result from several different things there in that part of the circuit. You might have the wiper and one of the other connectors backwards or you are allowing the potentiometer to go to a point where there is no resistance in that part of the circuit. To stop this from happening again all you need to do is put a resistor in the circuit just in front of the wiper of the potentiometer. This can be as small as an 8 or 10 ohm resistor so there would never be a 0 ohm resistance there.
A potentiometer by itself is not able to control the speed of a dc motor but if it is included in a circuit with a 555 timer then it can control a dc motor's speed through the percentage of the time that the duty cycle is high.
FYI.If you need to control the speed of an AC motor you can use a rheostat type device. These devices are a high voltage inductor (coil) with a wiper that controls the amount of AC power that is output through them.To control the speed of a DC motor, you need to use a circuit that controls the duty cycle of a given motor. This works so that the voltage going into the motor is off part of the time. The more the power going into the motor is off the slower it seems to run. This is one of the easy ways to control the speed of a DC motor. There are other ways but these are more involved than a simple 555 timer would be.