The size of the motor selected is required to be based upon design
calculations. CMAA #70 formulas are required to be used for bridge, trolley, and
hoist drive motors on bridge cranes. For hoist motor sizing calculations, the
factor "Kc" is required to be greater than 1.0. For bridge and trolley drive
motor sizing calculation, the factor "E" is required to be the published gear
reducer efficiency rating(s). Calculations for hoist motors on portal, floating,
and container cranes are required to take into account the reeving arrangement.
Calculations for rotate motors on portal and floating cranes are required to take
into account wind loading; additionally, calculations for floating cranes must
also account for the effects of the list and trim of the barge on motor loading.
Calculations for travel drive motors on portal cranes are required to take into
account the unique loadings attributable to the crane starting up and travelling
through curves. Also, for both rotate and travel drives, the effects on motor
loading of drive operation with one rotate drive motor or a pair of travel drive
motors out of service must be considered. However, the travel motors on a portal
crane are required to have a combined total 60-minute horsepower rating of not
less than as determined by the formula:
HP = (534.75 X 10-9) (W)(V)
W = total weight of crane plus hook load, in pounds
V = rated travel speed of crane, in feet per minute.
The selection of the motors for container cranes is based upon a theoretical duty
cycle that describes the energizing/de-energizing and load variation with respect
to time. It permits the determination of the peak torque and the calculation of
the root-mean-square (RMS) horsepower that will indicate the proper motor rating
required motor thermal capability at constant speed. The motor must also be
capable of carrying the peak torque.
If a flux vector controller is used for a hoist drive, the drive motor is
required to be an AC single speed vector design, with blower and encoder, squirrel
cage induction type. If an adjustable frequency controller is used with a bridge
or trolley drive, the motor is required to be an AC single speed inverter duty
squirrel cage induction type. When a hoist drive motor is a two-speed squirrel-
cage motor, a mechanical load brake is required to be provided.
Motor insulation is required to be Class H, but with a Class B
temperature rise. On newer outdoor cranes, an anti-condensation heater is
required to be installed in every drive motor. It is required to be de-energized
whenever the motor rotates. All heaters are required to be fed from a separate
panel dedicated to equipment heater and battery charger circuits.
Control Equipment. The range of complexity of motor control equipment is
extensive; ease of maintainability is inversely proportional to the complexity of
the equipment. The following is a list of the various types of motor controls in
order of increasing complexity: AC electro-mechanical for squirrel cage motors, AC
electro-mechanical for wound-rotor motors, DC electro-mechanical adjustable
voltage, DC constant potential, AC adjustable frequency, AC flux vector, and DC
electronic adjustable voltage.