Page 78 - Industrial Power Engineering and Applications Handbook

Duties of induction motors 3/59
3.6 Drawing the thermal curves Considering r = 1.5 hours
The relay may therefore be set to trip in less than 1.82 hours.
These can be drawn for temperature versus time or current
versus time as desired, depending upon the calibration
of the device, such as a motor protection relay. Below 3.6.2 From hot conditions
we provide a brief procedure to draw these curves. Similar curves can also be plotted for hot conditions
using equation (3.5) and assuming 0,) = 0, for ease of
3.6.1 From cold conditions plotting and to be on the safe side. The relay may then be
set accordingly. For brevity these curves have not been
(a) For I, I200% I, plotted here.
One may appreciate that by employing such a motor
protection relay it is possible to achieve a near thermal
image protection for all ratings, types and yakes of motors
through the same relay by setting its I- - t and 0 - t
(3.9) characteristics as close to the motor's characteristics as
possible. The O/C condition is normally detected through
(b) For I, > 200% the motor's actual heating, rather than current. for optimum
utilization. Moreover, the starting heats or the heat of the
previous running if it existed, when the motor was
(3.10) reswitched after a rest or a shutdown. are also accounted
for by measuring the thermal content.
where e,, and z are design parameter, and are provided
by the machine manufacturer. The curves can now be
plotted in the following ways. Example 3.3
The motor is operating hot, say at an end temperature of
130°C. If the motor is of insulation class B and ambient
For thermal settings temperature at 50°C then,
Ocj6,,l versus tlr different ratios of (lllI,.) as shown in Hc = 130 - 50 = 80°C
Tablc 3.l(a) and Figure 3.13(a), for l,/Zr I 200%, and and 0, = 120 - 50 = 70°C
Table 3. I (b) and Figure 3. I3(b) for 1,/11- > 200%. Since
O,,, and r arc known, f can be calculated for different
-
0:s corresponding to different motor currents. The relay .. ---= 70 114%
0,
can then be set to provide a thermal replica protection.
Referring to the curves of Figure 3.13(a) the relay
For overload settings corresponding to an overload of 150°/0 will trip in about
0
0
I,/lr versus tlzdif'ferent ratios of 0,/6,,, as shown in Table tit = 0.53, for em curves of 100% or 0.70 for 2 curve of
6"
3.2(a), and Figure 3.14(a) for i,/lr 5 200% and Table 120%.
3.2(b) and Figure 3.14(b) for I,/I, > 200%. Since I, and If 7 is taken as 1.5 hours, the motor may be set to trip in
r are known, t can be calculated for different overload about 0.60 x 1.5 hours, i.e. about 54 minutes, considering
conditions. corresponding to different temperature rises. the average value of tls as 0.60, for a 0,/6, as 114% by
interpolation. It will be noted that a setting corresponding to
The relay can then be set for optimum utilization of the a thermal curve of 100% will underutilize while corresponding
machine. to 120% will overutilize the motor, while the optimum true
utilization will correspond to Qc/O,,, as 114% only. It is therefore
Example 3.2 advisable to draw closer curves or use extrapolation whenever
For an overload of 25%, a class B motor, operating at an necessary to obtain a closer setting and plot a more true
ambient temperature of 45°C. the relay corresponding to 10% replica of the motor thermal characteristics.
over temperature rise can be set to trip as follows:
3.7 Rating of short-time motors
HC
i.e. - 1.1 = (1.25)'(1 -e-' r,
=
0,
If a short-time duty is performed on a Continuous
Maximum Rating (CMR) motor with some no-load or
idle running, the temperature rise 0 may not reach its
maximum value, e,,,, as shown in curve (c) of Figure
= 1 - 0.704 3. I I. A CMR motor therefore can be operated at higher
= 0.296 outputs on short-time duties as shown in curve (d). The
extent to which a CMR motor can be over-rated to perform
or - a particular short-time or intermittent duty is considered
- 3.378
in the following example. While evaluating the rating
.. t = T log, 3.378 for such duties, the heat during star-up and during braking
= 1.5 x 1.217 or 1.82 hours and their frequency of occurrence should be considered.