Friday, November 19, 2010

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.

2 comments:

  1. Hello Sajith, this is a great explanation even for someone like me who has no great electrical background.

    I do have a question for you. The Code Letter on motor name plate will help me find the locked rotor KVA for three phase motor. Can I use the same for single phase motor?

    I am using this information to size a generator for a pump station and I understand that I need the starting KW (KVA) for sizing.

    Thanks very much

    Hardik

    ReplyDelete
    Replies
    1. Hardik,

      Please read the word of caution at the beginning of the post. The letter symbol for starting current is applicable only for NEMA motors in the U.S. IEC and Indian standards don't mention this. You can take 6-times full load current as the starting current as typical.

      Delete