In certain applications it might be desirable to
have faster operation at high values of overload.
This can be accomplished by a change in the con-
tact setting, but the operating time at light
overloads will be reduced to a still greater extent
and thus the full overload capacity of the motor at
light overloads may be made unavailable. Fig. 7
shows time-overload characteristics for the limits
of the adjustment range when the short-circuiting
link is open and when the contact spacing has been
reduced so that the contacts will close in 15
minutes on an overload of 125 percent after cons-
tant temperature has been reached on full load.
For this combination of load and overload, at the
end of the 15-minute period the pointer should be
moved clockwise to a position where the contacts
just close. It will be possible to move the pointer
sufficiently beyond the 2.5 ampere point to obtain
a 15-minute setting for this value of full load, so
Fig. 7 shows 2.75 amperes as being one end of the
range for the accelerated operation. The upper end
of the range when the shorting link is not used has
been shown as 3.75 amperes, since the pointer
position for a 15-minute setting at 125 percent of a
3.75 ampere full load current will be near or
somewhat to the left of the 3.5 ampere scale mark-
ing.
It is not expected that these faster operating
curves will be used for general application. They
are presented to show how the relay characteristics
may be modified to meet special conditions. Since
the pointer position must be determined by test, it
could be located to give a time other than 15
minutes at 125 percent overload. If the time were
between 15 and 60 minutes, an approximate
operating curve could be estimated by interpola-
tion between Fig. 5 and 7.
Fig. 8 is similar to Fig. 7 but shows faster time-
overload curves for full load currents that require
the use of the shorting link.
It will be observed that the curves of Figs. 5
and 6 are approaching an asymptotic position at
125% of full load, and that the curves of Figs. 7
and 8 are farther from their asymptotic position at
125% of full load, as would be expected because of
the shorter time delay. Prior conditions of load
(between zero and full load) will not affect the
value of current that will ultimately close the relay
contacts, but variables such as friction and dis-
crepancies in calibration prevent precise location
of the asymptotic value. However, after making
some allowance for such variables, a value of 118%
of full load current can be considered as the max-
imum current that will not produce eventual clos-
ing of the relay contacts when settings are made as
described for Figs. 5 and 6. For settings made per
Figs. 7 and 8 this current value will be 110% of full
load.
Figs. 9 and 10 show resetting times for the type
BL-1 relay for the shorting link opened or closed.
The complete resetting time is considered to be the
time measured from the moment the relay current
is interrupted until the contacts return to the posi-
tion they would occupy for the steady state condi-
tion of 100 percent of full load current. This com-
plete time is composed of the time required for the
contacts to part and the time for them to travel
back to 100 percent position, and separate curves
are shown for these two components of the com-
plete time. The time will vary depending upon
whether the overload occurs after the motor has
been carrying 100 percent load or from a cold
start, and curves as shown for the two conditions.
The curves are shown for a relay setting at either
end of the adjustment range, and intermediate
values may be obtained by interpolation.
The ambient temperature compensation
provided in the type BL-1 relay causes its
operating time for a given current to remain ap-
proximately the same regardless of changes in the
ambient temperature at the relay. If the ambient
temperature at the motor location varies, the
motor of course will carry a higher overload safely
when the ambient temperature is low. However, a
replica type relay cannot respond to the ambient
temperature at the motor unless it has no ambient
temperature compensating means and unless it is
mounted either adjacent to the motor or, if at a
distance, in a location where there is assurance
that the ambient temperature will vary in exactly
the same way as at the motor. This condition fre-
quently cannot be met. Also, the relay temperature
at the operating point should be very close to that
of the motor at its maximum safe operating
temperature. The type BL-I relay was designed for
a minimum operating temperature much lower
than the safe operating temperature of a motor,
13
13