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.

40 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
    2. I like your blog on this topic is well-written and original

      metering pump manufacturer in india

      Delete
  2. Hi,

    It is very informative and very helpful on my research regarding seo techniques. Thanks for sharing this post.

    Induction Motor

    ReplyDelete
  3. HI Sajith
    Can we calculate the KVA rating of motor with HP Power of motor without using power factor ?

    ReplyDelete
    Replies
    1. No, Affan, you've to know the power factor to get kVA. The steps are as below
      1. Get kW from HP x 0.75
      2. Get full load pf from name plate, or assume 0.85 - 0.9
      3. Divide kW obtained in step 1 with PF in step 2 and you get kVA (shaft)(mech)
      4. Divide kVA (shaft) obtained in step 3 with motor rated efficiency from name plate or standards to get input kVA (elec)

      Delete
    2. hi sajith,

      normally motor takes 6 times more current than idle current during starting, then why overload relay not tripped the motor initially??

      Delete
    3. Pratik,

      Motor's overload relay is designed on the thermal principle. It will not operate on instantaneous overcurrent. Overload relay protects the motor from heating effect of sustained overload. If the starting current persists due to overload, it will definitely trip

      Delete
  4. Good Blog and thanks for your valuable information about Electric Motor. We are also Manufacturers and Supplier of Electric Motor in Tamil Nadu, India. Keep Blogging. Your Blog is very useful for us.

    ReplyDelete
  5. This blog is absolutely useful; I love the piece of informative writing. Brakemotors

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  6. Hmm I have got the vital information from here. Just love reading!
    electric motor

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  7. I hope you will share such type of impressive contents again with us so that we can utilize it and get more advantage. http://www.dealerselectric.com

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  8. Very useful blog, but I have a question.
    How come that the starting current of an induction motor will be 3-4 times larger when 3fases of 380V are connected in star than when they are in delta connection. (same load)

    ReplyDelete
  9. Very Impressive blog and awesome information shared. JMV (LPS) is the best Ac Surge Protection Devices manufacture in India. All types of surge product manufacture and Exporter in gulfs countries

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  10. how to calculate starting and running amps for 1.14HP
    DC motor

    ReplyDelete
  11. Thanks for taking the time to discuss that, I feel strongly about this and so really like getting to know more on this kind of field. Do you mind updating your blog post with additional insight? It should be really useful for all of us. www.acs-advancedcooling.com

    ReplyDelete
  12. i have a motor that is 40hp 460volt 3phase it has a start up 0f 220amps and running load 55 amps i have a large peak demand kw charge on electric bill how to i calculate the inrush to kw ?

    ReplyDelete
  13. Hello sir, i have a query. Am dentist and i need an inverter to back up a compressor of 800W.can you please quide, which Kva would be sufficient for me. Thanks

    ReplyDelete
  14. Hi one of the awesome article which I read. I have query about elevator that
    What will be the starting current of 5hp/1hp 3ph and 1ph motor.
    How can I calculate.

    ReplyDelete
  15. What's interesting to me is why the HP refers to the starting current instead of the nominal running current?? I wonder...

    ReplyDelete
  16. Your explanation was soo good... But I am still facing problem.. please give explanation for this nameplate details 2.8 kW, 0.85 p.f.,400V 3phase squirel cage I.M.

    ReplyDelete
  17. And also please find magnetic contactor rating

    ReplyDelete
  18. Awesome information given in detail about Motor starting and running guide.

    DOL Motor Starter Manufacturers Bangalore

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  19. hello sir.. what mean if starting current and full load current is lower than it design.. is there is any effect to motors..thanks

    ReplyDelete
  20. I pay you really special thanks for sharing this informative content.
    3 Phase motor Cambodia

    ReplyDelete
  21. hello,
    Good blog ! keep posting its helps us lot about
    Magnetic Motor Starter

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  22. Hello Sir, If i have a 2 HP motor of 12.5 Amps load. Can i start the machine with 3 KW generator?

    ReplyDelete
  23. So helpful and it helps the new comers in the electrical engineering field to understand.Great one and thanks

    ReplyDelete
  24. What is the starting current and running current of 5 hp, 3 phase ac motor ?

    ReplyDelete
  25. This is something easier for anyone how wants to learn about electric motor current.

    ReplyDelete

  26. Very usefull informaton thanks for sharing this link share for more about AC DC Full Load Current Calculation Formula Click here

    ReplyDelete
  27. Thanks for sharing useful information. can you tell me by stopping 3 phase 11KV induction motor with average current 130 amps how much power will be save for one day.

    ReplyDelete
  28. What is the normal starting current of synchonous motor with 7.5 hp, 480v 3phase, 60 Hz?

    ReplyDelete
  29. What is the normal starting current of synchonous motor with 7.5 hp, 480v 3phase, 60 Hz?

    ReplyDelete
  30. Hi , thank you for your post . we find that the no load current of an induction motor rated for 400kW , 690VAC is close to 30% of the nominal current . Will this affect the power consumption (kWH) while the motor is kept running in idle conditions we are using an inverter to run the motor,

    ReplyDelete
  31. Such a nice blog. I liked it,as we provide Three Phase DOL Motor Starter Manufacturers for more information can visit our site.

    ReplyDelete
  32. 1.5 HP MOTOR . GENERATOR CAPACITY REQUIRED?

    ReplyDelete
  33. This comment has been removed by a blog administrator.

    ReplyDelete