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A sewing machine motor is used to turn the pulley that runs the sewing machine. There are several problems that can occur to a motor. The two main causes of motor issues relate to motor belt problems or faulty electrical parts. Before you start repairing your machine, make sure you check the machine's owner's manual. Each machine is a little different, and the manual will detail how to access the motor and recommend fixes specifically for that machine.
Check that the motor belt is not loose or broken. A worn, loose or broken belt will not turn the pulley when the motor spins. Sewing machine belts are fairly easy to access as they are frequently adjusted for sewing machine tension and timing issues. Achieve the perfect tightness with a belt that is snug but which will also "give" when you press on it gently.
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Check for worn brushes, bad wiring, shorted wiring or other normal electrical issues that can cause an inoperable motor. To check if it is a wiring problem, completely bypass the machine pedal control and plug the machine directly into an outlet. Use a multimeter to start checking all the different electrical connections and wiring to find the bad spot. Replace those faulty components.
Oil the motor belt if it is too tight. One symptom of an overly tight belt is a motor that hums but does not turn. You will need to completely access the belt, which may require some disassembly. Clean and then oil the belt.