Increasing the load slows the motor until the increased current in the rotor produces an equal torque. The dragged member may use magnetic hysteresis, or non-uniform shape, or other effects to generate torque between itself and the rotating magnetic field. Speed High speed motors are wound with two poles in their rotating field, North and South, at opposite ends of a diameter. Synchronous motors with a rotating field with two poles run at 3600 RPM with 60 Hz power. Such motors run at 1800 RPM, synchronous, with 60 Hz power.
Some motors are wound with large numbers of poles for special purposes.
Adjustable Speed Adjustable speed is difficult to obtain with motors whose armatures are connected to fixed frequency power lines. Their rotating field speed depends on the rotor speed itself. The speed of DC shunt motors is proportional to their armature voltage over a very wide range. Electronic speed control is done by converting AC line voltage to variable DC voltage. The speed of DC series motors varies with load, and torque varies inversely with speed.