Rockwell Automation 1325L DC Motors, Frames C210ATZ - C440ATZ - Manual (Page 15)

DC Motors 15
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.
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, motor temperature will exceed the normal class F
rise with resulting reduction in insulation life if operated below
approximately 90% of base speed at rated voltage for prolonged
periods. Contact Allen-Bradley for torque derating curves for motors
operated continuously below base speeds.
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.
4. When contact is made, the needle will first deflect in either the up
scale or down scale direction and then return to zero. Deflection
will be in the opposite direction when contact is broken.
5. Relative polarities of the shunt and series fields are correct
(ampere-turns are cumulative) if the voltmeter needle deflects up
scale when contact is made and down scale when contact is
broken.
6. Relative polarities of the shunt and series fields are incorrect
(ampere-turns are differential) if the voltmeter needle deflects
down scale when contact is made and up scale when contact is
broken. The motor connections must be changed so that relative
polarity is correct.
If only one series field terminal is available at the controller, use it and
the available armature terminal for the test. For example, use S-2 and
A-l, if S-1 and A-2 arc connected together at the motor and not
brought to the controller.