Most electric motors can be used in both directions of rotation.
The alternating current electric motors, except the universal motors, are machines that have constant speed. There is the possibility of being connected to the stator coils of an induction motor in a way that doubles the number of poles and thus reduces the speed in half. The stators can be installed with two independent windings and pole variation and reducing the number of connections can vary the synchronous speed of the motor.
Another possibility of change of speed in the induction electric motors is through the frequency inverter, which allows the control of the AC motor 174658.00 leeson by varying the frequency, but also realizes the variation of the output voltage to maintain the characteristic V / F (Voltage / Frequency) of the motor.
With winding rotor induction motors it is possible to achieve any speed from zero to approximately the synchronizing speed by varying a simple resistance attached to the rotor winding which does not involve heating because the losses in the resistance will be external to the motor.
Another method of regulating the speed of alternating current motors is by means of the rotor system with a switch, through the lag of the brushes.