DC motors running at full speed without control over it. This is caused by several reasons like loss of feedback signal from Tachogenerator, encoder, or other feedback arrangement

DC motor runs at full speed....

Motor can go to full speed due to faulty tachogenerator, faulty or loose coupling, wrong polarity of tachogenerator, wrong polarity of field or armature connections, defective reference signal, defective RAMP circuit, or even defective "firing" circuit. In case of faulty tachogenerator, a glance at speed indicator, which in many cases connected to the same tachogenerator, can give a clue. The indication will be zero.

If firing circuit has no control over armature voltage, then usually the fuses will blow first before speed increases. If fuses escape from blowing, then the speed will be attained very fast with a jerk.

Faulty reference will usually take the motor to full speed rather smoothly if everything else is okay.

If speed indication is seen, one can check the voltage divider used for scaling down the tachogenerator voltage to low value. Normally a chain of resistors is used for this purpose.

Resistor divider in a good design will make use of many small value resistors rather than a few high value resistors. ( This is because the small value resistors are more reliable than high value resistors. It is uncommon to see a hundred ohm resistor failing, whereas resitors of 1 megohm are more likely to fail. Higher value carbon resistors need a longer length of carbon track, therefore the track is delicate and likely to break fast.)

Also the variable preset potentiometer provided for adjustment of speed takes the following correct form in a good design. Note the shorting of wiper of the potentiometer with the one end. Just a small track on the PCB, but shows a real deep thinking on the designer's part. When the wiper is not shorted, as seen in the image on the right hand side, the drive will go to uncontrolled full speed when the wiper makes poor contact after a few years of use. With the arrangement as shown on the left hand side, bad contact on wiper will not take the motor to full speed. It will, on the other hand, decrease slightly and therefore not damage the mechanical parts connected to the motor.