Product Manual

motors, the placement of the phase winding groups in conjunction with
the phase sequence of the power supply line produces a rotating field
around the rotor surface. The rotor tends to follow this rotating field
with a rotational speed that varies inversely with the number of poles
wound into the stator. Single-phase motors do not produce a rotating
field at a standstill, so a starter winding is added to give the effect of a
polyphase rotating field. Once the motor is running, the start winding
can be cut out of the circuit, and the motor will continue to run on a
rotating field that now exists due to the motion of the rotor interacting
with the single-phase stator magnetic field.
The development of power semiconductors and microprocessors has
brought efficient adjustable speed control to AC motors through the use
of inverter drives. Through this technology, the most recent designs of
so-called pulse width modulated AC drives are capable of speed and
torque regulation that equals or closely approximates direct current
systems.
LEESON Electric also produces permanent-magnet direct current
motors. The DC motor is the oldest member of the electric motor family.
Technological breakthroughs in magnetic materials, as well as solid state
electronic controls and high-power-density rechargeable batteries, have
all revitalized the versatile DC motor.
DC motors have extremely high torque capabilities and can be used
in conjunction with relatively simple solid state control devices to give
programmed acceleration and deceleration over a wide range of
selected speeds. Because the speed of a DC motor is not dependent on
the number of poles, there is great versatility for any constant or variable
speed requirement.
In most common DC motors, the magnetic field is produced by high-
strength permanent magnets, which have replaced traditional field coil
windings. The magnets require no current from the power supply. This
improves motor efficiency and reduces internal heating. In addition,
the reduced current draw enhances the life of batteries used as power
supplies in mobile or remote applications.
Both AC and DC motors must be manufactured with a great deal of
precision in order to operate properly. LEESON and other major
manufacturers use laminated stator, rotor and armature cores to reduce
energy losses and heat in the motor. Rotors for AC motors are heat treat-
ed to separate the aluminum bars from the rotor’s magnetic laminations.
Shaft and bearing tolerances must be held to ten thousandths of an inch.
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