Motor Starting and Running Currents and Rating Guide
A word of caution:
The following article is based on National Electrical Manufacturers'
Association (NEMA) tables, standards and nomenclature. This is somewhat
different from Indian and European practice. The class designations are
applicable only to NEMA compatible motors which are in use in the US
only. However, the logic and pattern of calculations are the same
everywhere. Hence the reader is cautioned to follow only the logical
sequence of the calculations.
Motor Starting Current
When
typical induction motors become energized, a much larger amount of
current than normal operating current rushes into the motor to set up
the magnetic field surrounding the motor and to overcome the lack of
angular momentum of the motor and its load. As the motor increases to
slip speed, the current drawn subsides to match (1) the current required
at the supplied voltage to supply the load and (2) losses to windage
and friction in the motor and in the load and transmission system. A
motor operating at slip speed and supplying nameplate horsepower as the
load should draw the current printed on the nameplate, and that current
should satisfy the equation
Horsepower = (voltage X current X power factor X motor efficiency X √3) / 746
Typical induction mo
tors
exhibit a starting power factor of 10 to 20 percent and a full-load
running power factor of 80 to 90 percent. Smaller typical induction
motors exhibit an operating full-load efficiency of approximately 92
percent, whereas large typical induction motors exhibit an operating
full-load efficiency of approximately 97.5 percent.
Since
many types of induction motors are made, the inrush current from an
individual motor is important in designing the electrical power supply
system for that motor. For this purpose, the nameplate on every motor
contains a code letter indicating the kilovoltampere/horsepower starting
load rating of the motor. A table of these code letters and their
meanings in approximate kVA and horsepower is shown in the following
table.
| Code Letter on motor name plate
|
kVA per HP with locked rotor
|
Minimum
|
Mean
|
Maximum
|
A
|
0
|
1.57
|
3.14
|
B
|
3.15
|
3.345
|
3.54
|
C
|
3.55
|
3.77
|
3.99
|
D
|
4
|
4.245
|
4.9
|
E
|
4.5
|
4.745
|
4.99
|
F
|
5
|
5.295
|
5.59
|
G
|
5.6
|
5.945
|
6.29
|
H
|
6.3
|
6.695
|
7.09
|
J
|
7.1
|
7.545
|
7.99
|
K
|
8
|
8.495
|
8.9
|
L
|
9
|
9.495
|
9.9
|
M
|
10
|
10.595
|
11.19
|
N
|
11.2
|
11.845
|
12.49
|
P
|
12.5
|
13.245
|
13.99
|
R
|
14
|
14.995
|
15.99
|
S
|
16
|
16.995
|
17.99
|
T
|
18
|
18.995
|
19.99
|
U
|
20
|
29.2
|
22.39
|
V
|
22.4
|
No Limit
|
No Limit
|
Using these values, the inrush current for a specific motor can be calculated as
Iinrush=(code letter value X horse power x 1000) /( √3 X Voltage)
An example of this calculation for a 50-hp code letter G motor operating at 460 V is shown below
Because
of the items listed above, motors that produce constant kVA loads make
demands on the electrical power system that are extraordinary compared
with the demands of constant kilowatt loads. To start them, the
overcurrent protection system must permit the starting current, also
called the locked-rotor current, to flow during the normal starting
period, and then the motor-running overcurrent must be limited to
approximately the nameplate full-load ampere rating. If the duration of
the locked-rotor current is too long, the motor will overheat due to I2R
heat buildup, and if the long-time ampere draw of the motor is too
high, the motor also will overheat due to I2R heating. The National
Electrical Code provides limitations on both inrush current and running
current, as well as providing a methodology to determine motor
disconnect switch ampere and horsepower ratings.
Table
430-152 of the National Electrical Code provides the maximum setting of
overcurrent devices upstream of the motor branch circuit, and portions
of this table are replicated below
% of Full load current
|
Motor type
|
Single element fuse
|
Dual-element time delay fuse
|
Inverse time breaker
|
Instantaneous & Magnetic trip breaker
|
Single phase motor
|
300
|
175
|
250
|
800
|
Three phase squirrel cage motor
|
300
|
175
|
250
|
800
|
Design E three phase squirrel cage
|
300
|
175
|
250
|
1100
|
Synchronous
|
300
|
175
|
250
|
800
|
Wound rotor
|
150
|
150
|
150
|
800
|
Direct current
|
150
|
150
|
150
|
250
|
For
example, a 50 hp, Design B, 460V 3 phase motor has a full load
current of 65A at 460V. The maximum rating of an inverse time breaker
protecting the motor branch circuit would be 65A x 250%, or 162.5A.
The next higher standard rating is 175A (US), so 175A is the maximum
rating that can be used to protect the motor circuit.
|
Motor Running Current
The
following figures illustrate the calculations required by specific
types of motors in the design of electric circuits to permit these loads
to start and to continue to protect them during operation.
Table of full-load currents for three-phase ac induction motors (A part of table 430-150 of NEC).
HP
|
208 V
|
230 V
|
460 V
|
575 V
|
0.5
|
2.5
|
2.2
|
1.1
|
0.9
|
0.75
|
3.5
|
3.2
|
1.6
|
1.3
|
1
|
4.6
|
4.2
|
2.1
|
1.7
|
1.5
|
6.6
|
6
|
3
|
2.4
|
2
|
7.5
|
6.8
|
3.4
|
2.7
|
3
|
10.6
|
9.6
|
4.8
|
3.9
|
5
|
16.7
|
15.2
|
7.6
|
6.1
|
10
|
30.8
|
28
|
14
|
11
|
15
|
46.2
|
42
|
21
|
17
|
20
|
59.4
|
54
|
27
|
22
|
25
|
74.8
|
68
|
34
|
27
|
30
|
88
|
80
|
40
|
32
|
40
|
114
|
104
|
52
|
41
|
50
|
143
|
130
|
65
|
52
|
60
|
169
|
154
|
77
|
62
|
75
|
211
|
192
|
96
|
77
|
100
|
273
|
248
|
124
|
99
|
125
|
343
|
312
|
156
|
125
|
150
|
396
|
360
|
180
|
144
|
200
|
528
|
480
|
240
|
192
|
Calculating Motor Branch-Circuit Overcurrent Protection and Wire Size
Article
430-52 of the National Electrical Code specifies that the minimum motor
branch-circuit size must be rated at 125 percent of the motor full-load
current found in Table 430-150 for motors that operate continuously,
and Section 430-32 requires that the long-time overload trip rating not
be greater than 115 percent of the motor nameplate current unless the
motor is marked otherwise. Note that the values of branch-circuit
overcurrent trip (the long-time portion of a thermal-magnetic trip
circuit breaker and the fuse melt-out curve ampacity) are changed by
Table 430-22b for motors that do not operate continuously.
This is illustrated with a sample problem. Consider the circuit shown.
A
40 HP, 460 V, 3 phase, Code letter G, Service factor of 1.0 is planned
for operation from a 460 V, 3 phase system. The name plate ampere is
50A. The motor is rated for continuous duty and the load is continuous.
Solve for minimum sizes of branch circuit elements?
1. Take motor full load current from table 430-150 as 52A which is higher than name plate value.
2. Determine wire size: 125% of 52A = 65A.
3. Determine inverse time breaker setting: 250% of 52A = 130A, next standard rating is 150A.
4. Determine the rating of thermal overloads: 115% of 50A (name plate current) = 57.5 A
5. Determine disconnect switch ampere rating: 115% of 52A = 59.8 A
6. Determine controller HP rating: 40 HP (same as motor nameplate HP)
The completed circuit will look like this.
NEC Torque classes and characteristics
Design Letter
|
Starting current (%FLC)
|
Relative Efficiency
|
Slip in % rpm
|
Starting torque (%FLT)
|
Stalling torque (%FLT)
|
A
|
Depends upon name plate code letter Normally 630-1000%
|
High
|
3%
|
120-250%
|
200-275%
|
B
|
Normally 600-700%
|
High
|
1.5-3%
|
120-250%
|
200-275%
|
C
|
Normally 600-700%
|
High
|
1.5-3%
|
200-250%
|
190-225%
|
D
|
Normally 600-700%
|
Medium
|
5-8%
|
275%
|
275%
|
Excerpts from EC&M's Electrical Calculations Handbook, by John M Paschal, Jr: Published by McGraw-Hill 2001.
