Different control systems may be selected for hoists
and travel in order to obtain the desired control
characteristics. The following are options for the
various control/motor combinations:
1. Alternating current Magnetic Control, ac
Squirrel Cage Motors: Squirrel cage motors have
only limited usefulness on cranes. Application is
limited to light-duty weight handling service
requiring not more than about 15 kW (20 HP) motors.
These motors are used where hook or travel speed is
slow, operation is by unskilled personnel, and
elaborate speed control and accurate positioning of
the load is not required. The main advantage of
selection of this control and motor arrangement is
low cost and simple maintenance for light capacity
cranes.
2. Alternating Current Magnetic Control, ac Wound
Rotor Type Motors: ac wound rotor type motors with
ac magnetic controls are well suited for high
capacity weight handling service (heavy duty crane
with large motors). Alternating current magnetic
control with ac wound rotor motors is generally used
when there is a restricted range of speed and load,
and there are no requirements for precision handling.
3. Alternating current Stepless Secondary Saturable
Reactor Control, ac Wound Rotor Type Motors:
Control of the system is essentially stepless and
fine control is provided. Speed is determined by
the master switch setting independent of the load
weight. Static stepless control is not suited for
extended low speed operation. Static control is
suitable for all crane services and is normally
specified when good regulated speed control is
required.
4. Alternating current Adjustable Frequency
Control, ac Squirrel Cage Motors: ac adjustable
frequency control uses ac squirrel cage motors and a
sine-coded pulse width modulated voltage to develop
a sinusoidal current waveform from a rectified
filtered ac power source. This type of control
system will provide stepless speed control with
smooth acceleration and deceleration, precise
regulated speed control, and fine minimum speed
capability.
5. Direct current Magnetic Control, dc Series Wound
Motors: dc series wound motors with dc magnetic
control are usually used in existing applications
where dc mainlines are present.
6. Direct current Variable Voltage Control, dc
Shunt Wound Motors: dc variable voltage control
uses dc shunt wound motors and a static conversion
unit to convert ac power to dc for the motor. This
type of control system will provide stepless speed
SECTION 14630A
Page 27
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