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# ABSTRACT

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48
Table 3 shows that the magnitude of the worst case torque transient is proportional to the size
of the load inertia. As the load inertial size increases, so does the magnitude of the worst
case torque transient. In the case of the 75 hp motor with load inertia 75-3, the magnitude of
the torque transient reached a value of 32 times the measured steady state full load torque and
over 5 times the measured starting torque.
3.4 STRESS ASSESSMENT
Comparing the magnitude of the starting currents to the magnitude of the interruption
currents shows that the interruption currents are nearly twice that of the starting currents in
all cases. Although the magnitude of the interruption current is greater, the duration of this
current is much shorter than the duration of starting currents. This is due to the fact that the
motor rotor is already spinning at the instant the supply voltage returns from the interruption.
Once the back-emf and the supply voltage are realigned, acceleration to full speed doesn't
require as much time as accelerating the rotor from standstill. In comparing the length of the
starting current transients to the length of the momentary service interruption transients, it is
noted that on average, the starting transients last 10 times longer than the momentary service
interruption transients. The heating effect of the interruption currents are therefore less than
the heating effects of the startup currents and should not cause any adverse problems. [12]
reports this finding as well.
Table 3 Peak measured torque and current due to starting and due to a momentary
service interruption
Motor
Motor Rotor
Inertia
Name
Value
Peak Starting
Current
(A) / (pu)
Peak
Interruption
Current
(A) / (pu)
Peak Starting
Torque
(in-lb) / (pu)
Peak Interruption
Torque
(in-lb) / (pu)
Inertia 10-A
0.02989 kgm²
169 / 11
281 / 19
1378 / 4
-3747 / 10
Inertia 10-B
0.13289 kgm²
165 / 11
273 / 18
1998 / 6
-7232 / 20
10hp
0.0589 kgm²
Inertia 10-1
0.1489 kgm²
169 / 11
280 / 19
4531 / 13
-9369 / 26
Inertia 10-2
0.2521 kgm²
164 / 11
274 / 18
5587 / 16
-9594 / 27
Inertia 10-3
0.4583 kgm²
166 / 11
269 / 18
5812 / 16
-9535 / 26
Inertia 50-1
0.7665 kgm²
825 / 10
1507 / 19
7101 / 4
-35412 / 20
50 hp
0.3889 kgm²
Inertia 50-2
1.4271 kgm²
840 / 11
1541 / 20
8189 / 5
-41970 / 24
Inertia 50-3
2.6988 kgm²
859 / 11
1507 / 19
10204 / 6
-45553 / 26
Inertia 75-1
1.1611 kgm²
1292 / 11
2226 / 19
7165 / 3
-73287 / 28
75 hp
0.6685 kgm²
Inertia 75-2
2.2083 kgm²
1251 / 11
2184 / 19
9436 / 4
-81988 / 31
Inertia 75-3
4.3051 kgm²
1296 / 11
2203 / 19
15514 / 6
-85539 / 32

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Summary :