Page 65 - Industrial Power Engineering and Applications Handbook

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Therefore the permissible rise in temperature in a stalled
condition will be as follows:
8 for bars in cold conditions = 450 - 40 = 410°C
8 for bars in hot conditions = 450 - 150 = 300°C
8 for rings in cold conditions = 100 - 40 = 60°C
8 for rings in hot conditions = 100 - 70 = 30°C
and = J,, = current density during start in A/cmZ and
Am
2.8.2 Plotting thermal withstand characteristics
of the motor

where Calculate the thermal withstand times tSte's under cold
k = material constant for the metal; and hot conditions and also at different Ist, say 200%,
(i) for aluminium = 0.016 300% and 400% etc. of I, as shown below. After
(ii) for copper = 0.0065 determining the corresponding safe thermal withstand
(iii) for brass = 0.0276 times, according to the above formula, draw the graph
:. ts1t = - 0. - (Figure 2.16), Ist vs tSt:
I
1
o,oo65 (for copper windings)
'
J;:
Note 2 Since no system can be heat adiabatic in practice there is Stalled current late Thermal withstand time, f,,, in seconds
a certain amount of heat dissipation from the impregnated windings
to the stator core and housing. This heat dissipation is considered as % of I,
as 159 of the total heat generated as in IEC 60079-7.
200
:. actual H, = 85% of what has been calculated above 3 00
O 400
and t,,, = 5 00
Ji x 0.0065 x 0.85 600
or f\tl = e seconds(s) (2.12)
Jf x 0.00552 Iste
and for J,, (for A windings) =
. (Area of windingdtum) .Z,
Application
For safe stall conditions t,,e should be less than the thermal Note J,, is a design parameter and more details may be obtained
from the motor manufacturer.
withstand time of the motor under locked or short-circuit
condition.
Example 2.4
(i) 8 is called the permissible rise in temperature in A 250 kW motor has a cold thermal withstand time of 30
the stalled condition. seconds and a hot thermal withstand time of 25 seconds. If
(ii) For class B insulation, the maximum limiting the starting time is 7 seconds, determine the consecutive
cold or hot starts that the motor will be able to sustain safely.
temperature is 185°C and for class F 210°C (short-
30
time permissible temperature). The permissible rise Number of consecutive cold starts = - = 4.3
in temperature in class B is 80°C above an ambient 7
of 40°C. i.e. 4 starts
25
:. 8 = 185 - (40 + 80) and number of hot starts = - 3.6
=
7
= 65°C for hot conditions i.e. 3 starts
and 8= 185 -40 The period after which this can be repeated will depend upon
the heating curve and the thermal time constant of the motor,
= 145°C for cold conditions i.e. the time the motor will take to reach thermal equilibrium
after repeated starts (See Chapter 3).
(b) For the rotor
Example 2.5
To ascertain whether the stator or the rotor would fail A centrifugal compressor driven through V-belts at a speed
first during a stalled condition, the thermal withstand of 4500 r.p.m. having the torque curve as shown in Figure
time of the rotor should also be determined separately 2.18 and a moment of inertia M@ of 2.50 kgm' employs a
for the rotor bars and the end rings. The lowest values squirrel cage motor with the following parameters:
for the stator or the rotor will be the safe stall time for kW = 350
the entire motor. The limiting temperatures in rotor N, = 1485 r.p.m.
components may be considered as follows:
speed-torque characteristic as in Figure 2.18
Limiting temperature for bars 450°C GD; = 30 kgm'
Limiting temperature for rings 100°C
Operating temperature for bars 150°C Safe stall time hot - 30 S.
Operating temperature for rings 70°C Cold - 40 S.