14
DC Motors
DC Motor Field Heating
Standard continuous duty DPG, TEFC and TENV stabilized shunt wound
DC motors have continuous duty fields capable of continuous excitation at
standstill (armature circuit not energized) under normal industrial
conditions.
Important:
Motors designed for forced ventilation or with a dual-cooled
heat exchanger as standard must have cooling air when fields
are excited at rated voltage. Installations having the air supply
interrupted when the motor is not operating must have field
disconnected or field voltage reduced to 50% rated by means of
field economizing resistor and relay.
Standard continuous duty self-ventilated motors are suitable for rated load
at rated speed operation at field voltages up to 110% of rated value.
However, insulation life will be reduced if operated below 60% of base
speed for prolonged periods.
Checking Relative Polarity of DC Motor Fields
Motor speed is unstable if speed increases due to an increase in load
current. As a result of instability, motor speed may hunt or overspeed. These
are unsatisfactory, possibly dangerous, drive conditions.
One of the possible causes of unstable performance of shunt wound DC
motors is incorrect series field polarity relative to the shunt field due to
improper connection. Relative polarity of the shunt and series fields can be
checked as follows:
1.
Connect a low scale (3 volt) DC voltmeter across the shunt field
terminals (F1, F2) with F1 connected to the positive (+) terminal of the
meter. At least one of the shunt field leads must be disconnected from
the controller.
2.
Use two flashlight batteries as a source of low voltage (3 volts). Hold or
connect the negative, to contact the S-2 series field terminal. Hold one
end of a wire conductor to the positive (+) center terminal so that the
other end of the wire can be used to make and break contact with the S-I
series field terminal.
3.
Watch the deflection of the voltmeter needle when contact is made with
S-I and when contact is broken.
!
ATTENTION:
To guard against motor damage caused by
inadequate ventilation, assure that motors designed for forced
ventilation as standard have cooling air when fields are excited at
rated voltage. Installations having the air supply interrupted when
the motor is not operating must have field disconnected or field
voltage reduced to 67%, rated by means of field economizing
resistor and relay. The motor insulation life can be significantly
reduced if the above precaution is not followed.
