If the rotating field is generated by a fixed frequency AC power source, it rotates at a fixed RPM. If the rotating field is generated by a frequency which is internally generated by the speed of the rotor, its RPM is determined by the voltages and load torques which determine the rotor speed.
There are several ways the armature's rotating field drags the other member, whether rotor or stator. 1. The dragged member may be a permanent magnet or an electromagnet powered by DC. The permanent magnet, or electro-magnet, and the rotating field lock together, north pole to south pole, and rotate together. The dragged member of an AC synchronous motor is its rotor, which has either a permanent magnet or an electro-magnet. The motor rotation is synchronous with the AC line frequency because the rotor is locked to the rotating magnetic field which is synchronous with the line frequency. The dragged member of a DC motor is its stator poles. 2. The dragged member of an induction motor is a rotor winding in which the rotating field induces a current. That current reacts with the rotating field to produce torque. To have a current induced, the rotor winding must rotate more slowly than the rotating field; the difference in speed is called slip. Slip varies with load torque. Increasing the load slows the motor until the increased current in the rotor produces an equal torque. 3. 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. Slower
motors have four poles at 0 degrees (N), 90 degrees (S), 180 degrees (N),
270 degrees (S). Such motors run at 1800 RPM,
synchronous, with 60 Hz power. Some motors are wound
with large numbers of poles for special purposes.
Typical induction motors run with 5 to 10 % slip.
Adjustable Speed
Adjustable speed is difficult to obtain with motors
whose armatures are connected to fixed frequency power
lines. HERE IS WHERE DC MOTORS SHINE. 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. This makes them particularly suitable to starting high inertia loads such as railway trains.
Speed and position of electronically controlled AC motors is done by varying the frequency and phase of the armature voltage and thus of the rotating magnetic field. The RPM of the rotating magnetic field may be adjusted all the way down to zero. Machine tool servo-motors are controlled in this manner. Stepping motors are controlled in exactly this way.
Size and Shape
Motors are made with power from milliwatts to
megawatts. They are protectively housed in open frames, dust
tight, oil tight, weather tight, and explosion proof enclosures.
Some are provided as separate rotors and stators to be built into
other machines. Some frames are made to fit exactly into their
powered machines for automatic gear alignment. Some have gear
sets built into their housings.
Generators
As with motors, all generators are AC generators. DC generators are AC
generators with rectifiers, either commutators which reverse the connections
between the output line and the winding conductors, or solid state rectifiers.
Each has either a rotating field, generated by a rotating permanent magnet or
electro-magnet, which induces voltages in stator armature windings, or a
stationary magnetic field which induces voltages in rotating armature
windings.
Laminations
In every case where a rotating magnetic field induces AC voltages or is
produced by AC current, it also induces eddy currents and hysteresis loss in
the iron core in which the armature conductors are embedded. Both are power
dissipations which not only decrease efficiency but heat the winding insulation
and limit permissible power. Therefore all armature cores are laminated of
silicon steel, an alloy developed for minimum "core loss." The laminated
structure minimizes eddy current loss.
This has been a "once over lightly" introduction to a very large art.
Other essays in this series deal with electro-mechanical actuators, amplifiers, and other devices.
For a full treatment, please see my book:
"Understanding Electro-Mechanical Engineering" published by IEEE Press. It is included by McGraw-Hill in their Electronic Engineers' Book Club. Click here.
My other books are:
"Designing Cost-Efficient Mechanisms" published by SAE Press. This is the book which introduced and explains Minimum Constraint Design. Click here.
"Real-World Engineering" Published by IEEE Press. How to be a successful engineer. Click here.
Questions? Telephone me at 619-224-3494 or e-mail ljkamm@ljkamm.com. No charge!
Lawrence Kamm, Consulting Electro-Mechanical Engineer
e-mail:ljkamm@ljkamm.com