NEMA Design B motors lose only 5 percent of their synchronous speed when
rated load (torque) is applied. These motors are well suited for travel drives of
OET and underrunning cranes. NEMA Design D motors are somewhat load-sensitive,
with a speed decrease of up to 13 percent from no load to rated load torque. They
have a very high starting torque and relatively low starting current, and these
characteristics make them well suited for hoist drives. The inherent regenerative
braking of either motor design maintains their speed at virtually the same level,
whether driving or being overhauled.
Speed control of squirrel cage motors is obtained by energizing either
one of the windings of a two-speed motor, or by applying variable (adjustable)
frequency power to a single winding motor. In the two-speed motors, the motor
speed corresponds to the stator winding that is energized. In the case of
adjustable frequency control, the motor rotates at speed that is proportional to
the frequency of the AC power produced by the controller. (The controller
converts the constant-frequency AC power source into DC power and then back into
AC power at an adjusted frequency.) The adjustment of the frequency of the
applied AC power is designed to be proportional to the movement of the operator's
master switch or push button. Squirrel cage motors with adjustable frequency
controls are widely used on standard commercial crane drives.
Wound-Rotor Motor Drives. Unlike the squirrel cage motors, wound-rotor
motors have their rotors connected electrically (via slip rings) to external
(secondary) variable resistors. They have only one primary (stator) winding. The
secondary resistors of the rotor circuit serve to determine the motor speed and
other performance characteristics.
The insertion of variable secondary resistance in the rotor circuit
provides a wide range of speed and torque control. These motors are normally
started with the maximum secondary resistance in the rotor circuit, and brought up
to the desired high speed by gradually removing (shunting out) segments of the
resistors. Smooth stopping of the motors can be obtained by reversing the
process; that is, reinserting segments of the resistors. In hoist applications
they are subject to overhauling in some speed points, and there the torque due to
the hook load must be controlled by the addition of an eddy-current brake or a
mechanical (friction) load brake.
Mechanical load brakes (often referred to as "Weston type") are normally
used as an internal component of standard commercial hoist gear reducers.
Although they are entirely mechanical in nature, they must be discussed in this
section because they are integral with the electric speed control system and exert
a major influence on the behavior of the hoist in the lowering direction. The
principle behind their operation is to cause friction elements (plates on a shaft
thread or wedges on cams) to wedge and stop the downward motion of the hoist
drive. To lower, the drive motor rotates in the lowering direction and unwedges
the friction elements just enough to allow them to slip on each other. (Because
of the heat generated by the slipping friction, these brakes can only be used in
moderate service and limited lift height.) To raise, the friction elements remain
wedged but the entire brake assembly is mechanically bypassed, with the motor
driving through an undirectional ratchet-and-pawl assembly. Mechanical load
brakes are never used as independent external units, but a few manufacturers offer
them as options in their gear reducers.