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 MIL-HDBK-1038
utilize a thermal sensitive device embedded in its windings. Dynamic braking is
required to be provided. Resistors are required to be connected to the
controller's DC bus whenever motor regeneration causes the DC voltage to rise to a
predetermined unacceptable level. The resistors are required to be rated at a
minimum of 150 percent of full load motor current continuously.
The control system for a wound-rotor motor is required to be provided
with resistive or reactive secondary controls. Acceleration and deceleration is
required to be smooth. The eddy-current brake is required to be excited with
reduced voltage when the control is in the OFF position. An alternator and
rectifier are required to be used to provide reduced voltage excitation of the
eddy-current brake for emergency dynamic braking. It is required that operation
of a hoist drive on any eddy-current brake controlled point be prevented upon loss
of brake excitation.
5.5.5.6
DC Control Equipment. On electronic adjustable voltage DC control
systems, four quadrant motor control is required, with each quadrant fully rated.
A complete set of software programs and programming instructions is required to be
obtained with microprocessor-based controls. The control is required to provide
automatic regenerative braking for speed-reduction and slow down before electro-
mechanical brake setting and, for hoist drives, dynamic braking, when the control
is in the OFF position, and emergency dynamic braking to limit hook speed to a
maximum of 40 percent of rated hoisting speed with rated load under the hook, in
case of power failure or opening of any fuse or circuit breaker that affects
control of the load. Controlled electric braking is required to be provided by
feeding regenerated power back into the AC lines. All non-time delayed brakes are
required to be set upon main line contactor de-energization. For hoist drives, it
is required that the minimum movement of the master switch in the hoisting
direction will not allow a hook block or boom to lower under rated load. All
hoist drives are required to include torque proving circuitry that does not cause
"roll-up" when lowering motion is initiated. The use of a fuse in a motor loop is
prohibited. Motor direction is required to be changed by reversal of armature
voltage polarity. Weakening of motor field strength during motor operation is not
permitted. Speed regulation is required to be 1.0 percent with tachometer
feedback and 5.0 percent with armature voltage feedback. Speed regulation is
required to be flat from no-load to full-load. A 600V-0-+600V volt meter,
indicating thyristor bridge output voltage for each drive, is required to be in
the cab and on the drive's control panel. An ammeter, indicating current in the
loop circuit, is required to be on each control panel and, for hoist drives, in
the cab. Contacts of loop contactors are required to be silver-laminated copper.
5.5.5.7
Contactors. Contactors are used for line, directional, and accelerating
functions. In their line function, contactors are used to connect and disconnect
motors. Energization of the main line contactor is required to be controlled by
the POWER OFF-POWER ON pushbutton on the pendent pushbutton station or in the
operator's cab.
For DC motors, directional contactors are also used to connect the motor
branch circuit to the other side of the line and are arranged so as to cause
current to flow through the motor armature in one direction or in the opposite
direction while not changing the direction of current flow through the field; this
changes the direction of motor rotation. The direction of rotation of AC motors
is changed by reversing two of the three leads to the motor. Reversing contactors
are used for this purpose. They consist of two mechanically interlocked
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