Maintenance Technology May 2015 - (Page 35)
MOTOR MANAGEMENT
THE MOST STRESSFUL TIME for an electric motor
is during starting, when the shaft speed is zero and
the motor current is at its maximum. This condition
is termed starting or locked-rotor current. Figure 1
illustrates its effects, as well as the impact of the applied
voltage on the current characteristics. As shown, the
starting current is directly proportional to the voltage
and inversely proportional to the speed. Although many
motor-performance parameters are directly proportional
to the current, the main concern here is the extra heat
produced during starting. The power lost as heat (P),
measured in kilowatts (kW), is proportional to the
square of the current flow (I2) through a resistance (R):
P = I2R
Once a motor has successfully started and reached
its load current level, its cooling circuit can dissipate
the additional heat produced by the starting current.
Restarting the motor before all extra heat has been
dissipated, however, will add more heat (kW) to the heat
already there. In that case, each subsequent start will
add even more heat, raising the motor temperature until
some component fails.
Guidance on starts
Depending on design, the thermal "weak link" for a
squirrel-cage induction motor could be the winding,
the rotor bars or the rotor shorting end rings. Thermal
protection located in the windings might not be
sufficient to prevent rotor bar or end ring damage. For
this reason, both the National Electrical Manufacturer's
Association (NEMA) and the International Electrotechnical Commission (IEC) limit the number of times a
motor can be safely started in a given amount of time, as
noted below.
NEMA Std. MG 1-2011, 12.54.1: Normal Starting
Conditions
Design A and B squirrel-cage induction motors
having horsepower ratings given in 10.32.4 and performance characteristics in accordance with Part 12 shall be
capable of accelerating, without injurious heating load,
a load inertia (Wk2) connected to the motor shaft that is
equal to or less than the values listed in Table 12-7 under
the following conditions:
a. Applied voltage and frequency in accordance with
[NEMA Standard] 12.44.
b. During the accelerating period, the connected load
torque is equal to or less than a torque that varies as the
square of the speed and is equal to 100% of rated-load
torque at rated speed.
c. Two starts in succession (coasting to rest between
starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature
not exceeding its rated load operating temperature.
MAY 2015
Fig. 1 Speed-current curve
Squirrel-Cage Induction Motors
Synchronous Speed, RPM
RPM
Hp
3600
1800
1200
900
720
600
514
53
82
118
Load Wk (Exclusive of motor Wk ) lb/ft
2
2
2
1
...
5.8
15
31
1½
1.8
8.6
23
45
77
120
174
2
2.4
11
30
60
102
158
228
3
3.5
17
44
87
149
231
335
6
6.7
27
71
142
242
375
544
7½
8.3
39
104
208
355
551
799
10
11
61
137
273
467
723
1050
15
16
75
200
400
684
1060
1540
20
21
99
262
525
898
1390
2020
25
26
122
324
647
1110
1720
2490
30
30
144
384
769
1320
2040
2960
40
40
189
503
1010
1720
2680
3890
50
49
232
620
1240
2130
3300
4790
60
58
275
735
1470
2520
3920
5690
75
71
338
904
1810
3110
4830
7020
100
92
441
1180
2370
4070
6320
9190
125
113
542
1450
2920
5010
7790
11300
150
133
640
1720
3460
5940
9230
...
200
172
831
2240
4510
7750
...
...
250
210
1020
2740
5540
...
...
...
300
246
1200
3240
...
...
...
...
350
281
1370
3720
...
...
...
...
400
315
1550
...
...
...
...
...
450
349
1710
...
...
...
...
...
500
381
1880
...
...
...
...
...
MAINTENANCETECHNOLOGY.COM | 35
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Maintenance Technology May 2015
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