DC motor

Series motors are *not* supposed to be run unloaded. The nature of the thing is that without a shaft load, the motor speed continually increases. It either reaches the point that it tears itself apart, or simple parasitic drag forces build up enough to stop further acceleration.

But if you are measuring its speed *while* loaded to the design operating conditions and it runs too fast, then look for shorted field winding. This reduces the magnetic field and it must draw more current to develop the same torque (and will run faster). A light load will cause it to run fast, but not draw excess current.

Testing the field winding can be difficult as the winding normally has a very low resistance to start with so simple resistance checks aren't very useful.

daestrom