Quadrant IV motor torque acts in the hoisting direction while the
motion continues to be in the lowering direction; that is, the motor has the
capacity to develop sufficient torque to slow down the lowering speed or bring the
hook load to a stop and, if maintained, begin hoisting.
The four quadrants of hoist motor operation are shown in Part B of
Motor Characteristic Curves. Performance characteristics of a motor are
represented graphically by the "characteristic curves". Each category, design,
and model of a motor has its own unique set of characteristic curves. Figures 30,
31, and 32 are representative examples of three types of DC drive motors used on
cranes. Figure 33 depicts the approximate characteristic curves of AC motors.
These relationships hold for squirrel cage NEMA Designs B and D motors, and for
wound-rotor motors with no resistance inserted in the rotor circuit. (The
performance of a wound-rotor motor is heavily influenced by the amount of
resistance connected to its secondary winding.)
It is important to note that each motor rating has an associated time
limit. If the motor is operated at its rated load for a longer period of time, it
will overheat. However, a motor can tolerate short periods of overloading if the
preceding operating condition was below its rated load. These characteristics
must be considered in selecting a motor for a particular crane drive.
Motor Branch Circuits. The behavior of a drive and its response to the
operator's inputs is determined by the performance characteristics of the motor in
combination with its branch circuit. Typical motor branch circuits used on crane
drives are depicted in figures 34, 35, 36, 37, 38, 39, 40, and 41; they determine
the direction of motion, the speed points, energization of the electrical braking
or speed limiting systems, and activation of the electro-mechanical holding
brakes. There are significant differences in some circuits for the same type of
motor, depending on whether they are in a hoist or travel drive.
DC Motor Branch Circuits. Figure 34 is the typical DC series-wound motor
hoist drive circuit. Figure 35 is the preferred circuit for a curved track portal
crane travel drive; it connects opposite DC series-wound drive motors in series so
that the wheels on the outer rail rotate faster than those on the inner rail and
thereby produce a "freely-rolling" condition for a crane on curved track. Figure
36 depicts a simpler variation of a travel drive suitable for a straight track
where one motor drives two opposite travel wheels through a connecting shaft.
Figure 37 shows the two options available for adjustable voltage control
of DC shunt-wound motors. The magnetic type, known popularly as the Ward-Leonard
system, requires a dedicated motor-generator set. Cranes with Ward-Leonard
systems typically use a single motor or diesel engine to drive multiple
generators; with individual generators assigned to specific drives. Because of
their bulk, their use is limited to large capacity hoists and OET bridge crane
drives that require low sensitivity to loads and where it is necessary to avoid
electronic components because of their susceptibility to interference from the
operating environment. The travel drive circuit shown in figure 38 is similar to
that of figure 35, except for the use of DC compound-wound motors, which make it
resistant to being overhauled.