terminals, is varied by changing the firing rate of the thyristors. The
conversion unit is required to be in the three phase, full wave configuration.
The output (DC) voltage is 135 percent of the input (AC) voltage.
Minimum protection for an electronic conversion unit is required to
consist of a line isolation transformer, transient surge suppressors, and either
current-limiting rectifier type fuses in each conductor of the AC line or static
instantaneous overcurrent circuits with sensing in each conductor of the AC line.
The controls are required to include an adjustable operating current limit capable
of limiting maximum current to not more than 50 percent overcurrent into shorted
conductors. The rectifier bridge is required to be rated, based on RMS values,
for continuous duty at a minimum of 150 percent of the highest motor current
rating that will provide full torque at all speeds, and for 300 percent of this
motor current rating for one minute. The peak inverse voltage rating is required
to be greater than 200 percent of the working peak inverse voltage. Thyristor
case temperatures are required to be less than 212 degrees in an 86 degree ambient
when delivering rated current. Parallel operation of thyristors is not permitted.
The firing pulse for thyristors is required to have a rise time of less than 1
microsecond and a peak gate current of not less than one ampere for all devices
rated above 35 amperes RMS. The maximum DC average voltage is required to be less
than 500 volts. Controlled electric braking is required to be provided by feeding
regenerated power back into the power source.
Rectifiers. In a rectifier, output voltage, and the voltage applied
across its load's terminals, is constant. The relationship between output (DC)
and input (AC) voltage for various types of rectifiers is as follows: single
phase, half wave, 0.45; single phase, full wave or bridge, 0.9; three phase, full
Minimum protection for the power diodes is required to consist of a line
isolation transformer, transient surge suppressors, and either current-limiting
rectifier type fuses or static instantaneous overcurrent tripping of the line
circuit breaker. The rectifier bridge is required to be rated for continuous duty
at a minimum of 150 percent of the load rating and for 1 minute at a minimum of
300 percent of the load rating. The peak inverse voltage rating relative to the
working peak inverse voltage is required to be greater than 200 percent for
avalanche type diodes and greater than 250 percent for other types. Diode case
temperatures are required to be less than 212 degrees in an 86 degree ambient when
the diodes are delivering rated load. When parallel operation of diodes is
utilized, each diode's actual share of the load is required to differ from its
calculated share by less than 10 percent. Purposely matched diodes are not
permitted to be used; current-equalizing devices are to be provided to force
diodes to share current. The current on the legs of the bridge circuit is
required to be balanced to within 5 percent. The DC average output voltage is
required to be less than 500 Volts. Regenerated power is required to be
automatically absorbed by resistors or other devices.
18.104.22.168 Resistors. Resistors are used to control the speed of series-wound DC
motors and, in an electro-mechanical adjustable voltage DC system, shunt-wound DC
motors. With a series-wound DC motor, the series resistors are selected to
provide evenly spaced speed points. For hoist drives, it is required that
resistances be selected so that the minimum movement of the master switch in the