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 MIL-HDBK-1038
contactors. They are also electrically interlocked by connecting a normally
closed contact of one contactor in series with the coil of the other contactor.
In their accelerating function, contactors are used to shunt motor current around
resistors. For example, the armature voltage of a series-wound DC motor changes
because magnetic contactors, which are used to provide distinct speed points,
usually five, shunt out or add in segments of resistance connected in series with
the motor. The rotor or secondary circuit of a wound-rotor AC motor is changed by
using contactors to shunt out or add in various segments of resistance. Timed
delay relays are commonly used to ensure that the operation of the accelerating
contactors is sequent. A timed delay relay is connected in the control circuit in
parallel with an accelerating contactor's coil and is energized simultaneously.
The contacts of the timed delay relay are connected in series with the coil of the
accelerating contactor for the next higher speed point and their closing is
delayed for a preset length of time. This prevents the accelerating contactor
from being energized for that length of time.
Normally closed contactors are arranged to establish the emergency
dynamic braking loop for DC hoist motors upon loss of power or when the controls
are in the OFF position. If the motor is being back-driven under these
circumstances by an overhauling load, it acts as a self-excited generator and
retards the load.
It is required that the requirements of NEMA Standards Publication No.
ICS 8 for controllers for DC motors used on cranes be met. ICS 8, Parts 2 and 3
have tables listing the minimum equipment for constant-potential and adjustable
voltage systems, respectively. These tables address the quantity and function of
protection equipment, contactors, and switches. Other tables list 230 VDC ratings
for DC contactors. The horsepower ratings for each size of contactor, are
approximately 50 percent higher than its comparable general-purpose rating in ICS
2, Part 5. Although the horsepower rating of contactors for adjustable voltage
controllers at voltages other than 230 VDC is proportional to the voltage,
specific ratings are given for contactor use at 500 or 550 VDC in constant-
potential controllers. The minimum size of DC contactor used is required to be
NEMA Size 3. It is required that the requirements of NEMA Standards Publication
No. ICS 8 for controllers for AC wound-rotor motors used on cranes be met; for
squirrel cage motor controllers, the requirements of ICS 2, Part 2, for general-
purpose controllers, are to be met. ICS 8, Parts 4 and 5 have tables listing the
minimum equipment for magnetic and electronic controllers, respectively. These
tables address the quantity and function of protection equipment, contactors, and
switches. Definite purpose contactors are inexpensive and have ratings assigned
to them by their manufacturer based upon relatively low duty cycles. Although
intended for use on light duty cranes, these reversing contactors may also be used
on small elevators, overhead doors, or commercial laundry equipment. The use of
definite purpose contactors, where not originally provided by the OEM, is
prohibited. Contactors manufactured to the standards of the International
Electrotechnical Commission (IEC) are not assigned sizes as are NEMA rated
contactors. IEC contactors are assigned utilization categories, based upon
defined applications, and motor rating, in kilowatts. If IEC contactors are used,
all conductors connected to them are to have a minimum temperature rating of 75
degrees (Centigrade) and Class J fuses are to be used to protect the contactors.
5.5.5.8
Thyristors. In an adjustable voltage DC system electronic conversion
unit, output voltage, and the voltage applied across the associated motor's
armature
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