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Functions and
characteristics
Motor protection
Motor-feeder characteristics and solutions
The trip class determines the trip curve of
the thermal protection device (inverse-time
curve) for a motor feeder.
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classes 5, 10, 20 and 30.
These classes are the maximum durations,
in seconds, for motor starting with a starting
current of 7.2 Ir, where Ir is the thermal
setting indicated on the motor rating plate.
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starting within 20 seconds (6 to 20 s) for a starting
current of 7.2 Ir.
Trip class of a thermal-protection device
The motor feeder includes thermal protection that may be built into the circuit
breaker. The protection must have a trip class suited to motor starting. Depending on
the application, the motor starting time varies from a few seconds (no-load start) to a
few dozen seconds (high-inertia load).
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setting Ir for thermal protection.
Trip class of thermal relays as a function of their Ir setting
Class
1.05 I r
(1)
1.2 Ir
(1)
1.5 Ir
(2)
7.2 I r
(1)
5
t > 2 h
t < 2h
t < 2 mn
2 s < t
y
5 s
10
t > 2 h
t < 2h
t < 4 mn
4 s < t
y
10 s
20
t > 2 h
t < 2h
t < 8 mn
6 s < t
y
20 s
30
t > 2 h
t < 2h
t < 12 mn
9 s < t
y
30 s
(1)
Time for a cold motor (motor off and cold).
(2)
Time for warm motor (motor running under normal conditions).
Currents of squirrel-cage motors at full rated load
Standardised values in HP
Rated
operational
power
Indicative values of the rated operational currents Ie (A) for
110 -
120 V
200 V 208 V 220 -
240 V
380 -
415 V
440 -
480 V
550 -
600 V
hp
1/2
4.4
2.5
2.4
2.2
1.3
1.1
0.9
3/4
6.4
3.7
3.5
3.2
1.8
1.6
1.3
1
8.4
4.8
4.6
4.2
2.3
2.1
1.7
1 1/2
12
6.9
6.6
6
3.3
3
2.4
2
13.6
7.8
7.5
6.8
4.3
3.4
2.7
3
19.2
11
10.6
9.6
6.1
4.8
3.9
5
30.4
17.5
16.7
15.2
9.7
7.6
6.1
7 1/2
44
25.3
24.2
22
14
11
9
10
56
32.2
30.8
28
18
14
11
15
84
48.3
46.2
42
27
21
17
20
108
62.1
59.4
54
34
27
22
25
136
78.2
74.8
68
44
34
27
30
160
92
88
80
51
40
32
40
208
120
114
104
66
52
41
50
260
150
143
130
83
65
52
60
-
177
169
154
103
77
62
75
-
221
211
192
128
96
77
100
-
285
273
248
165
124
99
125
-
359
343
312
208
156
125
150
-
414
396
360
240
180
144
200
-
552
528
480
320
240
192
250
-
-
-
604
403
302
242
300
-
-
-
722
482
361
289
Note:
1 hp = 0.7457 kW.
Standardised values in kW
Rated
operational
power
Standardised values in kW
currents Ie (A) for:
230 V 400 V 500 V 690 V
kW
A
A
A
A
0.06
0.35
0.32
0.16
0.12
0.09
0.52
0.3
0.24
0.17
0.12
0.7
0.44
0.32
0.23
0.18
1
0.6
0.48
0.35
0.25
1.5
0.85
0.68
0.49
0.37
1.9
1.1
0.88
0.64
0.55
2.6
1.5
1.2
0.87
0.75
3.3
1.9
1.5
1.1
1.1
4.7
2.7
2.2
1.6
1.5
6.3
3.6
2.9
2.1
2.2
8.5
4.9
3.9
2.8
3
11.3
6.5
5.2
3.8
4
15
8.5
6.8
4.9
5.5
20
11.5
9.2
6.7
7.5
27
15.5
12.4
8.9
11
38
22
17.6
12.8
15
51
29
23
17
18.5
61
35
28
21
22
72
41
33
24
30
96
55
44
32
37
115
66
53
39
45
140
80
64
47
55
169
97
78
57
75
230
132
106
77
90
278
160
128
93
110
340
195
156
113
132
400
230
184
134
160
487
280
224
162
200
609
350
280
203
250
748
430
344
250
315
940
540
432
313
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Typical motor-starting curve
Asynchronous-motor starting parameters
The main parameters of direct on-line starting of three-phase asynchronous motors
(90 % of all applications) are listed below.
Ir
b
: rated current
This is the current drawn by the motor at full rated load (e.g. approximately 100 A rms
for 55 kW at 400 V).
Id
b
: starting current
This is the current drawn by the motor during starting, on average 7.2 In for a
duration td of 5 to 30 seconds depending on the application (e.g. 720 A rms for 10
seconds). These values determine the trip class and any additional "long-start"
protection devices that may be needed.
I’’d
b
: peak starting current
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energised, on the average 14 In for 10 to 15 ms (e.g. 1840 A peak).
The protection settings must effectively protect the motor, notably via a suitable
thermal-relay trip class, but let the peak starting current through.
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version: 1.0